Nft

RTE Akustik + Prftechnik GmbH Gewerbestr. 26 D-76327 Pfinztal/Germany Tel. +49/721/94650-0 Fax +49/721/94650-50 www.rte.de [email protected]

Welcome

We have put together some information concerning Natural Frequency Measurement

Technology: methods and procedures in eigenfrequency testing,

application reports and videos of this,

the innovative products we have developed for industrial applications in production and

development.

Separate info CDs (if not already stored on this CD) inform you about:

the firm RTE - the business areas, customers, partners all over the world and publications

about acoustic testing technology,

Solutions in the other business areas:

Noise and functional testing

Crack and material structure testing of metallic parts

Crack testing of ceramic parts

Vibration control of machines and processes

our Lynx letter Acoustic Testing Technology which appears regularly (apply for this on

www.rte.de (service center),

Know-how vibro acoustic: make use of our knowledge and potential and ask for

publications.

RTE regards itself as a partner and provides - in addition to the supplying of systems right

through to all-in-one solutions - a full service with supervision and maintenance of equipment and

parameterization.

We hope you will enjoy looking through the CD and would be pleased to hear your reaction.

Phone us on +49 721 / 9 46 50 0 or send an e-mail to [email protected].

With our best regards

Ingolf Hertlin President

Natural Frequency Testing

The Lynx (Lynx lynx)

The Lynx is the largest feline

predator in Europe. Its ele-

gance and speed are fascinat-

ing. With its acute hearing and

sharp eyesight, nothing is too

quiet or too small to escape its

vigilance.

The Lynx symbolizes our work

- quality of performance and

the ability to seize an opportu-

nity when we see it.

Make use of our potential!

Contents Please click on a tab below for direct access

Principle and methods

Testing technology

Application notes

Application videos

Publications


How does Acoustic Resonance Testing (ART) work?

Acoustic Resonance Testing (ART) is a new non-destructive testing method that allows rapid and inexpensive 100% testing of a wide range of work-pieces. It relies on the well-known physical effect that a body, after suitable excitation (e. g. through impact), oscillates at certain characteristic modes and frequencies (its natural or resonant frequencies). These oscillations are, so to speak, the specimens language (or its fingerprint): they can be measured with a microphone (airborne sound) or a laser vibrometer (body sound). The specimens geometry determines the number of its natural frequencies: a rod has few whilst a complex work-piece has many such frequencies. Typically, the information that can be obtained by acoustic resonance analysis includes cracks, structural properties, cavities, layer separation, chipping, density fluctuations etc. Damping behaviour depends firstly on the material, and secondly, on how the specimen is positioned during its excitation. In order to achieve high frequency resolution, signal duration (ringing duration) should be as long as possible (> 50 ms). Figure 1 shows examples of a specimens time-signal and resonance spectrum:

Fig. 1: Time signal (left) and resonant frequencies after impact (right) From the natural frequencies it is possible to calculate specimen-specific characteristics and assign them to quality attributes, e. g. pass / OK cracked material structure hardness deviation / partly hardened

A technical bulletin describing prerequisites, influencing factors, test parameters and reliability is available from RTE. Please contact us.

RTE Akustik + Prftechnik GmbH Gewerbestr. 26 D-76327 Pfinztal/Germany Tel. +49 (0)721/94650-0 Fax -50 www.rte.de [email protected]

Influencing factors and compensation possibilities

Influencing factors may have an impact on workpieces, which are quality relevant or

not. Influencing factors may be related to the specimen, the manufacturing process

or the test procedure. The applicaton of acoustical methods requires a systematic

procedure to evaluate the mentioned influencing factors and if necessary to com-

pensate them.

1. Temperature of the workpiece

Acoustic resonance analysis is temperature sensitive. A temperature increase results

in a shift of resonant frequencies to lower values (reduces the stiffness), a decrease

to higher values. This effect has an impact on all resonances and is linear, but de-

pends on the material and the frequency range.

Compensation can be done by heating a workpiece and then measuring the tem-

perature by a sensor (contact or contactless) repeatively and the resonance frequen-

cies. The spectrum has to be recalculated.

2. Dimension

A dimensional variation, e. g. wall thickness, changes the stiffness of the part and

has an influence which often only affects certain resonances. It is important to find

out which are sensitive and which are not.

Compensation requires to systematically identify which resonances are affected and

which are not. If possible a workpiece with maximum size is mechanically processed

to the allowed minimum value. Another method is to use a FEM and to calculate the

vibration modes at maximum and minim um value.

3. Weight

Parts with weight differences which do not result from a dimensional variation or ma-

terial structure (density variation in powder metal) have lunker or porosity. This also

changes the resonant spectrum, but this influence is often not evident. The influence

on the spectrum is non-linear and requires special compensation methods.

RTE Akustik + Prftechnik GmbH Gewerbestr.

Determination of Damping and Qua

Description of Procedure

Table of Contents 1 Introduction ................................

2 Measurement Principle ................................

3 Analysis ................................

4 Measurements ................................

4.1 Determination of the R

4.2 Determination of the Quality Q

Copyright: 2010 RTE Akustik + Prftechnik GmbH, Pfinztal / Germany

Date Author

26.11.2009 Legler, Hertlin

RTE Akustik + Prftechnik GmbH Gewerbestr. 26 D-76327 Pfinztal/Germany Tel. 0721/94650-0 Fax 0721/94650-

Determination of Damping and ality of Brake Disks


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Determination of the Resonant Frequencies ................................

Determination of the Quality Q ................................................................


Description

RTE METH Damping and Quality Brake Disks E-100518

-50 www.rte.de [email protected]

Determination of Damping and lity of Brake Disks


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100518


1 Introduction Recording the vibration mode parameters is becoming increasingly important in brake disk testing. Besides determining the natural frequencies, we also have to assess the that will enter as quality Q in the quality evaluation.

In cooperation with Daimler AG, RTE has developed and ithod, described in the following.

2 Measurement Principle

Figure 1: The measurement principle of resonance analysis

Once recorded, the resonance response is available as information in digital form and can be used for the analysis. In an orded signal shows which resonant frequencies are present. The damping and the rquality Q may be regarded as a measure of the speed at which the resonant frdown. The faster this happens, the lower the quality Q.

Figure 2: The output signal (left) and the spectral analysis (right)

Messmikrofon

Prfling

Impulshammer

Measuring microph

Spec

Impulse ha


Recording the vibration mode parameters is becoming increasingly important in brake disk ing. Besides determining the natural frequencies, we also have to assess the

ty Q in the quality evaluation.

In cooperation with Daimler AG, RTE has developed and implementedscribed in the following.

Measurement Principle

The brake disk to be examined is struck and caused to vibrate by an electromagnetically operated impulse hammer. A measuring mcrophone mounted near the point of impact records the resonance response by the body in the near field.

The measured signal contains workpiece rsonances that attenuate more orbecause of the internal damping.

: The measurement principle of resonance analysis

Once recorded, the resonance response is available as information in digital form and can be used for the analysis. In an initial approximation, the Fourier (spectral) analysis of the reorded signal shows which resonant frequencies are present. The damping and the rquality Q may be regarded as a measure of the speed at which the resonant fr

ter this happens, the lower the quality Q.

: The output signal (left) and the spectral analysis (right)

Messmikrofon

Prfling

Impulshammer

Measuring microphone

Specimen

Impulse hammer


Recording the vibration mode parameters is becoming increasingly important in brake disk ing. Besides determining the natural frequencies, we also have to assess the damping

mplemented an appropriate me-

The brake disk to be examined is struck and ed to vibrate by an electromagnetically

operated impulse hammer. A measuring mi-crophone mounted near the point of impact

cords the resonance response by the body in

The measured signal contains workpiece re-sonances that attenuate more or less quickly because of the internal damping.

Once recorded, the resonance response is available as information in digital form and can be initial approximation, the Fourier (spectral) analysis of the rec-

orded signal shows which resonant frequencies are present. The damping and the resulting quality Q may be regarded as a measure of the speed at which the resonant frequency dies


3 Analysis Actually, the aforementioned methods are only conditionally suited for determining the natral frequencies and quality with the desired precision. In its procedure RTE relies on the cicular approximation already used in the vibrational mode analysis. Here the information is represented not as a spectrum, but as a frequency response locus, and the vibrativariables of the resonant frequency and the quality are derived.

Figure 3: The determination of the vibration mode variables through circular approximation of a dampened vibrtion

This method reveals its strength particularly with resonances lying close to one another or in the case of double lines, since the parameters can be determined with sufficient preceven in a one-sided approximation (i.e. from the unaffected end).

4 Measurements

4.1 Determination of the Resonant Frequencies

The determination of the resonant frequencies is quite simple, since points of the disk and can be easily recorded with the integral measuring microphone. One to two excitations in the vicinity of the outside diameter generally suffice. The occurrence of double lines shall be considered insofar as, measurement, the lower, the upper or even both lines will have to be measured, making slight shifts in the results possible. This variation is generally low relative to the test tolerance stated in the EKB 2002, however.


Actually, the aforementioned methods are only conditionally suited for determining the natncies and quality with the desired precision. In its procedure RTE relies on the ci

imation already used in the vibrational mode analysis. Here the information is sented not as a spectrum, but as a frequency response locus, and the vibrati

nant frequency and the quality are derived.

The Fourier analysis of a signal yields two sof data, referred to as an "imaginary part" and "real part," respectively. If we now plot this imaginary part against the real part, we obtain, instead of a resoance, circles or circular segments by means of which we can determine the vibration mode variables.

By interpolating on the basis of multiple measupoints we can determine these variables with high acuracy.

: The determination of the vibration mode variables through circular approximation of a dampened vibr

This method reveals its strength particularly with resonances lying close to one another or in since the parameters can be determined with sufficient prec

sided approximation (i.e. from the unaffected end).

Determination of the Resonant Frequencies

The determination of the resonant frequencies is quite simple, since points of the disk and can be easily recorded with the integral measuring microphone. One to two excitations in the vicinity of the outside diameter generally suffice. The occurrence of

ble lines shall be considered insofar as, depending on the location of the excitation or measurement, the lower, the upper or even both lines will have to be measured, making slight shifts in the results possible. This variation is generally low relative to the test tolerance

2, however.


Actually, the aforementioned methods are only conditionally suited for determining the natu-ncies and quality with the desired precision. In its procedure RTE relies on the cir-

imation already used in the vibrational mode analysis. Here the information is sented not as a spectrum, but as a frequency response locus, and the vibration mode

The Fourier analysis of a signal yields two sequences of data, referred to as an "imaginary part" and "real part," respectively. If we now plot this imaginary part

part, we obtain, instead of a reson-ance, circles or circular segments by means of which we can determine the vibration mode variables.

By interpolating on the basis of multiple measuring points we can determine these variables with high ac-

: The determination of the vibration mode variables through circular approximation of a dampened vibra-

This method reveals its strength particularly with resonances lying close to one another or in since the parameters can be determined with sufficient precision

The determination of the resonant frequencies is quite simple, since they occur at nearly all points of the disk and can be easily recorded with the integral measuring microphone. One to two excitations in the vicinity of the outside diameter generally suffice. The occurrence of

depending on the location of the excitation or measurement, the lower, the upper or even both lines will have to be measured, making slight shifts in the results possible. This variation is generally low relative to the test tolerance


4.2 Determination of the Quality QA reproducible determination of quality requires that the resonant frequencies be detewith high precision in the method mentioned in section

The determination of the quality

1.) The quality depends on the vibration mode, i.e. it does not suffice to derive the qufrom the overall attenuation process. Like the resonant frequencies, the parameters for the quality shall be treated s

2.) The mode-specific damping and consequently the quality depends on the measurement location. Of interest is the minimum quality or the maximum damping.

3.) It shall be specified which modes must be considered, since tsegments will depend on this. In principle no limitation is imposed by the measuring technology or the method.

Figure 4: The determination of the quality on the basis of multiple measurements

Measuring along the circumferential direction yields modequality that are repeated as a function of the angle.

Depending on the mode and given the number of modes to be determined, we obtain a mesurement profile composed from a sto one another.

RTE reduces the number of measurements to a minimum, so that such a test can also be employed in production with the pace maintained. Positioning units are controlled by the test software so that during the test the disk is rotated according to the measuring program dsigned for the specific task. An impacting unit and the corresponding measuring microphone suffice for conducting this test fast and reliably.as for fully automatic operation are avai

Excitation / measurement


Determination of the Quality Q A reproducible determination of quality requires that the resonant frequencies be detewith high precision in the method mentioned in section 3.

The determination of the quality Q shall consider the following properties:

The quality depends on the vibration mode, i.e. it does not suffice to derive the qufrom the overall attenuation process. Like the resonant frequencies, the parameters for

ity shall be treated separated from the individual modes.

specific damping and consequently the quality depends on the measurement tion. Of interest is the minimum quality or the maximum damping.

It shall be specified which modes must be considered, since the number of excitation segments will depend on this. In principle no limitation is imposed by the measuring technology or the method.

: The determination of the quality on the basis of multiple measurements

ing along the circumferential direction yields mode-dependent fluctuations for the quality that are repeated as a function of the angle.

Depending on the mode and given the number of modes to be determined, we obtain a mement profile composed from a series of single measurements at a certain angle relative

RTE reduces the number of measurements to a minimum, so that such a test can also be ployed in production with the pace maintained. Positioning units are controlled by the test tware so that during the test the disk is rotated according to the measuring program d

signed for the specific task. An impacting unit and the corresponding measuring microphone ing this test fast and reliably. Suitable test apparatuses

as for fully automatic operation are available.


A reproducible determination of quality requires that the resonant frequencies be determined

Q shall consider the following properties:

The quality depends on the vibration mode, i.e. it does not suffice to derive the quality from the overall attenuation process. Like the resonant frequencies, the parameters for

specific damping and consequently the quality depends on the measurement tion. Of interest is the minimum quality or the maximum damping.

he number of excitation segments will depend on this. In principle no limitation is imposed by the measuring

dependent fluctuations for the

Depending on the mode and given the number of modes to be determined, we obtain a mea-eries of single measurements at a certain angle relative

RTE reduces the number of measurements to a minimum, so that such a test can also be ployed in production with the pace maintained. Positioning units are controlled by the test tware so that during the test the disk is rotated according to the measuring program de-

signed for the specific task. An impacting unit and the corresponding measuring microphone Suitable test apparatuses for manual as well


Acoustic Resonance Testing - advantages and limitations -

Advantages Reproducibility Very high! Maximum deviation typically equals the measurement

resolution (depending on the specimen, typically 1.5 Hz !!)

Volume testing AR evaluates the entire work-piece, independent of its size. Depending on the sensors and the excitation, both local and global quality assessment is possible

Non-destructive Clean and dry; no consumables

Sensitivity Depending on the specimens ringing duration, between 1 and 2 Hz (incl. at high frequencies)

Speed Decision possible in less than 1 second

Automation capability Inexpensive (correct specimen positioning and excitation are important)

Defect assignment Through reference objects

Limitations, prerequisites Surface defects Mostly for volume deficiencies

Structure The specimen should not be too complex, otherwise a very large number of resonant frequencies will be present

Reference objects Clearly classified - good samples are necessary in order to achieve reliable decisions

Defect size cannot be determined directly, only on basis of comparisons

Defect location normally not possible

A technical bulletin describing prerequisites, influencing factors, test parameters and reliability is available from RTE. Please contact us.


Technical Bulletin: Resonance Analysis

Updated: 3/2010

Area of application

Acoustic resonance analysis (sound testing, Acoustic Resonance Testing, ART) is a non-destructive test procedure e.g. for detecting cracks and structural faults. ART compares in a qualitative manner the resonance vibrations and loss factor of test objects with those of already known good test objects.

Prerequisites The fault influences the components vibration behaviour in a measurable way. Other faults will not be detected by ART.

Verification of suitability

By measuring typical components it is possible to test whether quality-related faults / deviations can be recognized by ART. Possible influencing factors are likewise to be recorded and evaluated. In some cases an FEM analysis for interpreting the resonance spectrum is purposeful or necessary. This is only executed on special request.

1) Quick look fundamental suitability on the basis of some typical examples of parts made available by customers.

2) Product sampling in the laboratory: 10-50 components (good and faulty parts). 3) Process sampling in manufacture 500 or more components, depending on

customer requirements concerning reliability of testing.

Influencing factors

Test objects are subject to influences which may or may not be relevant for quality and which change the positioning of resonance frequencies. Some influencing factors such as temperature and dimension can be compensated for if necessary. Fluctuations in such influencing factors, of the basic raw material (batch), of the components or of processing makes adapting the test parameters or the setting up of additional test parameters necessary.

Test parameters

RTE sets up a basic parameterisation on the basis of components made available by the customers, who are responsible for the selection. The customer is then trained in operation, evaluation and adaption of the test system. The customer must observe and optimize the test paramters while the test bench is in operation until efficiency is ensured; fluctuations of influencing factors must be compensated for if necessary by adapting the test parameters. On request RTE can provide ongoing support for these operations in the framework of a special support agreement.

Reliability The reliability of a test procedure depends on the procedures system capability, diminished by the reliability of the parameterisation and the reliability of the oberver. In the objective procedure ART the observer is not relevant for the reliability risk. But the basic principle applies for all non-destructive test procedures: there is no technical process without a residual risk. Even ART cannot guarantee 100% accuracy when differentiating between good and bad parts.

Additional documents

DGZfP Guideline US 6: Akustische Resonanzverfahren, Beuth-Verlag 2009 Deutsch, V.: NDT methods, Castell-Verlag Wuppertal, 2006 Hertlin, I.: Acoustic Resonance Analysis, Castell-Verlag Wuppertal, 2003 www.rte.de (applications, publications, product descriptions)


Acoustic Testing Systems Noise, vibration, eigenfrequencies, cracks, structure

is the family of innovative test systems optimized for application specific re-quirements and performance. Acute measurement guarantees that nothing escapes its vigilance like the lynx!

We use SonicTC.Material for the group of applications for crack and structure testing of specimen and SonicTC.NVH for the group of applications for noise, vibration and harshness testing.

Application specific versions SonicTC.Bearings bearings NVH SonicTC.Castings iron cast, grey cast, vermicular cast material SonicTC.Composites composite / layered material material SonicTC.Drives small-power motor, propulsion NVH SonicTC.EigenFrequency natural frequency measurement with age compensation material SonicTC.Engines combustion engines NVH SonicTC.Fans exhauster, ventilator NVH SonicTC.FineCeramics industrial ceramics, chinaware, porcellaine, refractories material SonicTC.Gearboxes small and medium sized NVH SonicTC.Gears toothed gears, cogged rings material SonicTC.HeayClay clay and stone ware, tiles, bricks material SonicTC.MetalForming deep-drawing, thermoforming material SonicTC.PowderMetal sintered parts material SonicTC.Pumps pumps of different size NVH SonicTC.Seats electrically adjusted seats NVH SonicTC.Tools driven units NVH SonicTC.Turbines turbines NVH SonicTC.Universal for special applications both SonicTC.Welding joining by different methods material SonicTC.WhiteGoods household devices NVH


Natural frequency measurement for production and laboratory

Task

The natural frequency of a vibrating system is that at which it will vibrate after a nonrecurring exci-

tation. If a system is subjected to external excitation at a frequency coincident with its natural fre-

quency, the system will vibrate with a particularly large amplitude, which is known as resonance.

Requirements for brake components like brake discs are defined in the EKB 2002 Technical Stan-

dard by the European Brake Noise Experts Group, to which OEMs and component manufacturers

have to adhere.

Characteristics

SonicTC.EigenFrequency conforms to the

above requirements, offers an excellent

price/performance ratio and records your fre-

quencies quickly and simply. The design of the

measuring chamber was acoustically optimized

for such measurements and meets Employers'

Liability Insurance Association regulations. The

control and evaluation unit with the 17 touch

panel displays the natural frequencies to the user within seconds and monitors them relative to

settable limits.

SonicTC.EigenFrequency offers automatic multi-position measurements and can thus guarantee

high reliability. As the micro structure of cast parts can change, the system also takes into account

the age of the specimen at the time of measurement.

SonicTC.EigenFrequency is of robust construction and can be used in severe industrial environ-

ment. The equipment can also be changed over quickly from one kind of component to another in

a few steps. In the laboratory, the SonicTC.EigenFrequency offers a variety of additional archiving

and analysis options that can provide useful information for the developer.

Benefits

Excellent price/performance ratio

Can be used on different products with minimal change-over

Permits compliance with customer specifications with minimum effort

Modular design makes equipment a sound investment

Reliable multi-position measurements of natural frequency values and ageing compensation


Selected Technical Data

Specific test parameters can be stored for product variants. The natural frequencies are displayed

on the monitor during the test.

The replaceable fixture in the test chamber can be changed easily, so that the SonicTC.-

EigenFrequency can be used for a large variety of components. The test chamber is designed for

test specimens with edges of up to 420 mm long. Larger test chambers or two test chambers (left

and right hand) are available on request.

Device variants

One or two test chambers

Test chamber on left, on right or on both sides

Analysis facilities

Time domain analysis o RMS o Statistical methods

Frequency range o Power spectrum o User-adjustable frequency bands

Ageing curve for castings

Data acquisition

Multiple excitation

Airborne sound and/or structure-borne sound (max. four channels)

24-bit resolution

51.200 samples/second

Expandability

Fixtures for brake anchors, housings, brake discs, brake drums and others

Multiple position excitation

Temperature compensation

Good/bad marking colour dot

Dimensions (chamber closed)

Approx. 500 x 964 x 500 mm (h x w x d)

RTE Akustik + Prftechnik GmbH Gewerbestr. 26 D-76327 Pfinztal / Germany Tel. +49 721/94650-0 Fax: -50 www.rte.de [email protected]

- Main features -

Application areas Natural frequency measurement with high resolution

Damping measurement

Component selection according to vibration characteristics

Quality factor calculation

Measurement Airborne noise (microphone), alternatively: structure-borne noise

Temperature

Age, casting day

Workpiece parameter acquisition for type calculation, DMC

Methods (selection)

Time domain analysis (decay)

Frequency domain analysis (FFT, DFT)

Temperature compensation

Aging influence compensation

Software Easy-to-use by the operator

Clear presentation of classification

Multiple measurements with summing up to one decision

Correlation with other values, e. g. temperature and/or casting day

Easy set-up of similar products on basis of vibration modes

Password protected parameter set-up

Optional statistical package

Fully automated testing mode without display

Measurement archive with many filter possibilities

Hardware Standard hardware for data acquisition, USB or PCI connection

Input channels for process data (e. g. mass, temperature)

Opto-isolated digital process I/O (e. g. PLC)

Fieldbus connection (e. g. Profibus, EthernetIP)

Operation modes Testing: measure, classify, archive

Measurement: measure, archive

Control check: checking the correct operation of the test system

Calibration of measurement channels

Performance Feasibility studies on basis of sample parts

Product evaluation (workpiece influencing parameters)

Process evaluation (process influencing parameters)

Shipping of key-ready testing solutions

Consultancy and support

Remote service

RTE Akustik + Prftechnik GmbH Gewerbestr. 26 D-76327 Pfinztal (Germany) Phone +49 721/94650-0 Fax +49 721/94650-50 www.rte.de [email protected]

RESONANCE ANALYSIS

The newcomer in

non-destructive material testing

Every day people and machines all over the world are testing mil-

lions of products before they are used e.g. in vehicles and ma-

chines. These tests are absolutely necessary to guarantee that the

products are faultless and do not cause any damage when in use or

even endanger life. It is usually people who have to carry out the tests and within a very short

time have to make subjective decisions about quality characteristics which they cannot judge or

can judge only inadequately by assessing the surface. Interior errors such as cracks, cavities

and structural variations are important quality characteristics which cannot be detected on the

surface. In addition, the most frequently used non-destructive test procedures such as x-ray,

magnetic powder or dye penetrant inspection are expensive, elaborate and damaging to the en-

vironment.

Characteristics

Different versions of SONIC|TC are available for differ-

ent branches of industry such as foundries, coarse or

fine ceramics, powder metal or metal forming

processes.

SONIC|TC.Universal has been assembled for the la-

boratory, research and higher education area. Here you

receive the complete range of functions for making use

of all possibilities in tests. For a routine test you can

then scale down to just the functions required.

Mode of operation

The component is either made to vibrate manually or electromechanically by tapping. A micro-

phone records the sound of the component and directs it for processing to the computer.

SONIC|TC has a multitude of procedures available for analyzing and evaluating the sound.

Functions such as a post-evaluation for carrying out the what if consideration due to signals

recorded, a reference run for adapting the parameters to e.g. batch influences or various com-

pensation procedures for balancing influences on temperature or weight complete the compre-

hensive software package.


RESONANCE ANALYSIS

The newcomer in


Here the effect of frequency

shift as an assessment cri-

terion is illustrated. Due to

the defect, the propagation

of the vibration is impaired,

it differs from the normal

routine image. The position

of individual resonances is

monitored and assessed.

If an FEM analysis of the component is also available, then an al-

location of the resonances and thus of the fault too can be made

to certain component areas. The FEM analysis is useful in under-

standing the vibration behavior of the component and gives indica-

tions concerning the ideal site for stimulation and the optimal hold

positions.

Your advantage

Profit from an innovative and environmentally friendly technology - no pollution by chemicals or

radiation, no expendable materials. The requirements for a laboratory measurement are few.

Very good results can be achieved even with simple resources. Thus the technology is also ex-

cellently suited for practical laboratory exercises.

On the other hand, there are still many effects for which there is still no mathematical descrip-

tion. Or we measure acoustic effects for which we have no explanation. The smaller the test

objects, the more problematic the stimulation of the components becomes. So for many re-

search areas there are still excellent topics for a thesis or dissertation.


RESONANCE ANALYSIS

The newcomer in


Users

RTE maintains good contacts with higher education colleges and institutes in the region and

has also expanded internationally in the past years. The following institutions use our systems

in lectures, laboratories and in research (selection):

University of Miskolc Hungary

Brno University of Technology Czech Republic

University of Applied Sciences Karlsruhe

University of Applied Sciences Cologne

Fraunhofer IZFP Saarbrcken

Leibnitz Institute for Agricultural Engineering

Potsdam

Federal Waterways Engineering

and Research Institute Karlsruhe


RESONANCE ANALYSIS

The newcomer in


Who are we?

RTE Akustik + Prftechnik GmbH (Acoustic + Testing Technology) was

founded in 1986 as a technology-oriented company with the objective of pro-

viding products and services in the area of acoustic testing technology.

RTE counts amongst its customers chiefly firms in the automotive industry,

foundries, forges, in housing and medical technology or in the ceramics in-

dustry.

Our customers can profit from our 25 years experience.

RTE supplies acoustic measuring and testing technology for production in

the areas

Noise testing and functional testing

Noise measurement, NVH, operating characteristics, safety characteristics

Acoustic materials testing

Crack testing and microstructure testing by sound analysis, resonant frequency mea-

surement, natural frequency measurement (eigenfrequencies)

Acoustic process monitoring

Assembly process, machining process, filling process, joining process

Services

Service and Support (24/7), feasibility analyses, design service, training

Address

RTE Akustik + Prftechnik GmbH

Gewerbestr. 26

D-76327 Pfinztal

Phone: +49 (0)7 21 9 46 50-0

Fax: +49 (0)7 21 9 46 50-50

info rte.de

www.rte.de

www.rte-acoustics.com

RTE Akustik + Prftechnik GmbH Gewerbestr. 26 D-76327 Pfinztal/Germany Tel. +49 721/94650-0 Fax -50 www.rte.de [email protected]

Customers using Material Testing (cracks, structure, eigenfrequencies)


Partners worldwide

Country RTE area Company Internet

Brasil Material testing

RAIMECK Ltda Sao Paulo www.raimeck.com.br

China Material testing De Hua Materials Testing Co. Kowloon, Hong Kong www.dehua.com.hk

India Material testing

NDT Technologies Ltd. New Bombay

www.ndttechnologies.com

Iran Material testing Hami Fanavaran Yekta Ltd. Tehran www.hami-yekta.com

France Noise testing

Material testing Vibration testing

RTE Bureau de Liason France Colmar www.rte.de

Austria Material testing

Mittli KG Wien www.mittli.at

Indonesia Material testing

Panairsan Jarkata www.panairsan.com

Mexico Noise testing


Grupo CTT, S.A. de C.V. Aguascalientes www.grupoctt.com.mx

Korea Material testing EuroHitec Co., Ltd. Anyang-city, Kyunggi

Croatia Material testing

IDEF d.o.o. Zagreb www.idef.hr

Slovenia Noise testing


TEAM TRADE d.o.o. Kamnik www.team-trade.si

Turkey Material testing

TMM NDT Inspection Co. Bursa www.tmmndt.com

Czech Republic Material testing

Testima, spol. s.r.o. Praha www.testima.cz

Spain Material testing Vibration testing

EUROMAQUINA S.A. Madrid www.euromaquina.com

Taiwan Material testing AI-KWANG Tech Co., Ltd. Ren-Wu Township www.ai-kwang.com.tw

Thailand Material testing

NDT Instruments Co., Ltd. Bangkok www.ndtithai.com

Venezuela Material testing S.D. International C.A Caracas


The First Step to Profit We examine your parts

Acoustic materials

testing

allows non-destructive, fast analysis of the condition of metallic and

ceramic materials as well as of composites, by using the Acoustic

Resonance Testing (ART).

The resulting sound (or vibrations) in the excited test specimen make it

possible to derive information about its composition. Thus one can de-

tect, in simple fashion, whether it exhibits e. g. cracks, structural de-

fects, porosity or adhesion faults.

The language of the

test specimen

It is necessary to understand the preconditions, which permit success-

ful application of the very powerful acoustic methods. We need to ana-

lyse the way in which a perfect specimen differs from a faulty one. The

type of defect must also be established, as unambiguously as possible.

RTE can examine test pieces in its own lab or directly on the cus-

tomers premises. RTE offers its customers different levels of service:

Service level 0:

Quick look

With a few representative samples RTE analyses whether the acoustic

nesonance method appears promising.

Are vibrations measurable? Has the defect an influence on the vibration?

This service is free of charge for the customer!

Service level 1:

Product evaluation

Typical test samples of the same batch are analysed in RTEs lab

(number has to be agreed - typically 10 perfect samples and some

faulty ones per typical defect) to analyse the typical differences be-

tween good and bad parts. This includes reproducibility, excitation and

sensor position and characteristics for the classification.

We charge our customers at a flat rate for equipment and documenta-

tion which includes the investigations results in the form of a compre-

hensive report. Please ask for an offer.

The results obtained at this level will be of direct benefit during the fol-

lowing collaborative stages.

Service level 2:

Process evaluation

(measuring on site)

Sometimes, because of the test object size or due to special process

conditions, it is necessary to carry out measurements on site. RTE

staff, using mobile measurement and analysis equipment, carry out

tests on the customers premises for a fixed charge which includes all

associated costs (includes a detailed written report on the results).

FORM RTE Checklist analysis E-1107 RTE Akustik + Prftechnik, Germany

Check list part analysis (quick look, product analysis)

Customer: Drawn up by:

Place: Date:

Acoustic resonance testing (ART) is, like ultrasonic, a volume-related method with which flaws can be recognized in the work piece (in contrast to the surface-related method such as magnetic particle testing or

eddy current). It is likewise a comparative method: the characteristics are compared with those of good parts.

With a Quick look (QL) it should be possible to test whether ART is in principle applicable to recognize the faulty parts according to the customer's requirements. The Product evaluation with more parts follows if the QL is positive.

1. Product description

No. Questions Description

P 1 Name of the product

P 2 What kind of material is being dealt with?

(elastic-modulus, density, basic material etc.)

P 3 In which area is the product used?

(e.g. brake system, gears)

P 4 Number of variations / types?

P 5 Processing condition of the parts?

(e.g. stripped, blasted, ground, finished condition,...)

P 6 Error description

(e.g. crack, difference in hardness, porosity, nodularity)

P 7 Cause of error

(by the processing machine, from the raw material...)

2. Selection

No. General Remarks

S 1 Number: to be agreed (QL: 3 OK, up to 3 NOK) Specimen should only differ according to the defect

S 2 Marking: clear marking imperative! NOK parts with faulty section marked

S 3 Batch: according to S1 all from one batch!

Comparative method! If not possible: mark batch parts accordingly

S 4 State of processing: all in the same condition (comparative method!)

S 5 Results / information from other test procedures?

(Manufacturer / method)

3. Description Manufacturing Process


M 1 Production steps before testing?

M 2 Production steps after testing?

M 3 Present testing?

Manual, automatic)

M 4 Quality of raw material?

(Blanks from supplier, bar stock, tin etc.?)

M 5 Which processing steps are automated? How?

M 6 Tested parts / hour?

4. Customer data


C 1 Analysis report: to whom?

C 2 Contact partner for RTE (responsible for project)?

C 3 What happens to tested objects? (freight collect or scrap?)

C 4 Responsible for production planning?

C 5 Responsible for investment decision?

C 6 In-line testing planned? (Manual loading / unloading or fully

automatic?)

C 7 Urgency for in-line testing?

C 8 Cost expectation for overall solution?

FORM Checklist Customer Requirements E0312.docCopyright: RTE Akustik + Prftechnik Stand: 06/02

Customer: Author: Check list Customer Requirements City: Date:

Area Subject Result Comment

Task Aim of testing

Current status / testing

Is a test specification available?

Test object Description We appreciate photos!

(TO) Where is the TO used

Description of defects We appreciate photos!

Different types? (table with dim. / weight?)

Process Cycle time

Part feeding (manual, automatic)

Part storing (manual, automatic)

Handling time

Type of production control (PLC)

Marking/labeling of parts after test?

Test conditions

Operation Where is the location planned?

Environment conditions: noise:

temperature:

humidity:

dust, dirtiness:

Design Delivery conditions available? Mechanics:

Electric:

Pneumatic:

Electrical feeding (V)

Pneumatic feeding (bar)

Remote service possible?

Modem / ISDN / Internet?

Integration into local network planned?

Test result presentation? Lights red/green?

Presentation of test details on monitor?

If so: language?

Archive of test results on hard disc?

Statistic information wanted? offline/online

Documentation of test bench language:

number:

complexity:

Contact Responsible for project

person Production

Quality management

Purchase

Project ma- Preferred delivery date

nagement Payment terms: advance payment

delivery

installation

acceptance

Time for payment (days)

Warranty (month, shifts)

Spare-part package requested?


Application Areas of Acoustic Resonance Testing

Acoustic resonance testing can be applied to all workpieces that sound.

Industry Material Products (examples) Testing for Casting iron casting calipers, steering knuckles nodularity, cracks, inclusions, cementite

grey casting camshafts, brake discs perlite, cracks, natural frequencies

malleable cast iron strut plate cracks, structure

Ceramics clay roof tiles, industry ceramics cracks, surface, dimensional stability

catalytic converter cracks

Die-casting Aluminium pedals, steering wheel cracks

steering gear housing

Forge conrods, stub axles cracks

synchronous rings cracks

Glas bottles cracks

plates cracks

Ingot mould chassis parts, cylinder head cracks, inclusions

Metal forming air bag parts cracks (smilies)

Sintered metal powder metal chain wheels, drive pulleys cracks

miscellaneous products cracks

planetary carrier cracks, welding

Steel iron drive shafts cracks, natural frequencies

steering racks cracks, hardness

rims cracks

RTE Akustik + Prftechnik GmbH Gewerbestr. 26 D-76327 Pfinztal/Karlsruhe Tel. 0721/94650-0 Fax 0721/94650-50 www.rte.de [email protected]

Natural Frequencies Testing of Brake

Components in Laboratory

Task

All components of a brake system (disc, caliper, holder and

pads) must harmonize in that way that the resonance

frequencies of the single parts do not interfere with the other

components. Otherwise the brake may squeeze and rattle.

For testing brake calipers and holders a test bench is needed

that allows a flexible testing of right and left versions with

minimum changeover effort. RTE developed a test bench which

allows a alternating testing with two fixtures. For different types

the complete fixture can be replaced by another one.

Solution

Double test bench with manual loading and unloading

Automatic type recognition and parameter loading

Type specific fixture with sensor and excitation

Changeover by replacing the fixture

Different products can be tested at the same time

Benefit

Reliable and objective measurement

Easy changeover to different types

Test bench controls correct bad part deposit

Documentation and retraceability

Testing Technology

Test system SonicTC.EigenFrequency

Data archive and statistical evaluation

Easy to set-up and operate

Frequency resolution < 1 Hz



Components in Production

Task





For testing brake calipers and holders a test bench is needed

that allows a flexible testing of right and left versions with

minimum changeover effort. RTE developed a test bench which

allows a alternating testing with two fixtures. For different types

the complete fixture can be replaced by another one.

Solution


Automatic type recognition and parameter loading

Type specific fixture with sensor and excitation

Changeover by replacing the fixture

Different products can be tested at the same time

Benefit



Test bench controls correct bad part deposit


Short cycle time for volume testing near production

Testing Technology






Application Note

Natural frequency measurement of brake discs for production and laboratory

Task

The natural frequency of a vibrating system is that at which it will vibrate after a nonrecurring

excitation. If a system is subjected to external excitation at a frequency coincident with its

natural frequency, the system will vibrate with particularly large amplitude, which is known as

resonance. Requirements for brake discs are defined in the EKB 2002 Technical Standard

by the European Brake Noise Experts Group, which is supported by OEMs like Daimler, Au-

di, VW, Fiat, BMW, Ford and Porsche and all main component manufacturers have to ad-

here.

Characteristics

SonicTC.EigenFrequency from RTE conforms to the above requirements, offers an excel-

lent price/performance ratio and records your frequencies quickly and simply.

SonicTC.EigenFrequency offers automatic multi-position measurements and can thus guar-

antee high reliability. As the micro structure of cast parts can change, the system also takes

into account the age of the specimen at the time of measurement.

SonicTC.EigenFrequency is of robust construction and can be used in severe industrial en-

vironment. The equipment can also be changed over quickly from one kind of component to

another in a few steps. In the laboratory, the SonicTC.EigenFrequency offers a variety of

additional archiving and analysis options that can provide useful information for the develop-

er.

Test system cabinet version

The control and evaluation unit for harsh environment with

the 17 touch panel displays the natural frequencies to the

user within seconds and monitors them relative to settable

limits.

This cabinet rack version is movable and can be connected

to the specific automation system or fixture via a Harting

connector.


Test system chamber version

The replaceable fixture in the test chamber can be changed

easily, so that the SonicTC.EigenFrequency can be used

for a large variety of components. The test chamber is de-

signed for test specimens with edges of up to 420 mm long.

Larger test chambers or two test chambers (left and right

hand) are available on request.

The chamber version can be used for audit testing in a lab

and is movable.

Test system manual production version

The test bench is loaded and unloaded manually by a

worker. The test bench PLC determines the type of the test

specimen automatically by dimensional characteristics and

moves the excitation devices and the microphone, which are

mounted on linear slides, automatically to the measurement

position by servomotors.

A light barrier safeguards the worker access during meas-

urement.

Test system automatic production version

The design and manufacturing of a test

bench for integration into the production flow

is customer specific and can include decol-

lating, testing, marking of good parts and

bad part sorting.

The test bench can determine the test

specimen type automatically using a camera

or gets this information from the line PLC.

Excitation devices and the microphones,

which are mounted on linear slides, can

automatically be moved to the measurement

position by servomotors. In addition it is

possible to measure the damping (Q-factor)

of the brake disk.


Random (low volume) Natural

Frequencies Testing of Brake Discs

Task





For testing brake discs within the production a compact fixture

is necessary which allows a fast changeover to different brake

discs types. As an option the device should allow the Q factor

calculation by turning the workpiece for multiple measurements.

Solution

Portable all-in-one solution for random inspection

Suitable for all dimensions and shape of brake discs

Manual adaption to different product types

Upgrade with turntable for quality factor measurement

Benefit



Portable device includes all necessary functions


Mode specific Q and overall Q with turntable

Testing Technology







Discs in Production (manual operation)

Task





For high volume testing brake discs within the production RTE

developed a manually loaded test bench without changeover to

different brake discs types. As an option the device allows the

Q factor calculation by turning the workpiece for multiple

measurements.

Solution

All-in-one solution for high volume inspection


Manual loading and unloading automatic testing

Servo-motor driven automatic adaption of sensors and ac-tors to different types

Light barrier protection of operator

PLC controlled operation


Benefit


Fully automatic changeover to different types




Testing Technology







Discs in Production (automatic operation)

Task





For high volume testing brake discs within the production RTE

developed a line integrated test bench without changeover to

different brake discs types. As an option the device allows the

Q factor calculation by turning the workpiece for multiple

measurements.

Solution

Fully automatic test bench for integration in production


Type identification by camera (dot matrix code) and auto-matic adaption of sensors and actors by servo-motors

Double excitation and acquisition by microphones

Reference disc / master part for control of settings

PLC controlled operation


Benefit


Fully automatic changeover to different types




Testing Technology






Natural Frequencies Testing of

Steering Columns in Production

Task

Steering columns propagate vibrations from the front axle to the

steering wheel. The wheel vibrates when the excites frequency

and the natural frequency are nearly the same. When driving

the main vibrations come from the tires, elsewise from the

engine.

By natural frequency measurement and simulation of the

steering wheel mass the columns should be grouped. This

grouping allows to decide whether the steering columns can me

mounted in a car with benzine engine or a diesel engine.

Solution


Test fixtures for manual and electric steering columns

Structure-borne vibration measurement in two directions

Simulation of built-in condition of a car

Different products can be tested alternately

Benefit


Easy grouping of columns according to their vibrational condition

Realistic test conditions according to car situation


Testing Technology





CD EF Videos E-120411

To watch the application videos please click on the corresponding tab.

Brake discs:

Semi-automatic natural frequency testing

Brake discs: Fully automatic natural frequency testing

Acoustic Resonance Analysis

Order: copiesAcoustic Resonance AnalysisA series of publications for non-destructive testing. NDT Compact and understandable, Vol. 5, Castell-Verlag, 2003, 58 pages.

12,00 plus postage and 7 % VAT:

Germany: 13,66 Europe: 14,90 Worldwide: 17,90

20 % reduction for members of the DGZfP e.V. or DGaQs e.V.

Fa.

RTE Akustik + Prftechnik GmbHGewerbestr. 26D 76327 Pfinztal

Germany

Company:

Name, first name:

Street:

ZIP code, city:

Country:

Tel.: Fax:

E-Mail:

Member of DGZfP e.V. DGaQs e.V.

Place, date Signature

Fax: +49 (0) 721/94 65 0-50E-Mail: [email protected]

Acoustic resonance analysis, often also called sonic testing is one of the oldest non-destructive testing methods. An impact is applied to the test piece to make it vibrate and the sound is assessed by ear or through a microphone. Using this method the tester or a testing device can recognize cracks and inclusions or can assess hardness and grain structure.

The book Acoustic Resonance Analysis by Ingolf Hertlin is the first one on this subject and has been published by the Castell Verlag, Wuppertal, as Volume 5 in the series Publications on Non-Destructive Testing - NDT Compact and understandable. The author is the Managing Director of RTE Akustik + Prftechnik GmbH, Pfinztal. The book conveys not only the fundamentals of acoustics but also the principle of resonance analysis and also gives examples of solutions in industrial series testing.

Contents:1. Introduction2. Physics3. Resonance analysis4. Testing techniques5. Applications6. Resonance analysis within the NDT7. Appendix (concepts, designation, literature)


Natural Frequency Measurement

'Frequently Asked Questions'

F 1 What is the motivation for 'natural frequency testing'?

There are different applications which make use of this kind of test:

A: Checking the microstructure of the castings

It is a fast way to continuously keeping track of material characteristics like

hardness and modulus of elasticity in a casting process. Cast iron proper-

ties are influenced by the added alloying elements, which change the mi-

crostructure of the cast. This causes a change of the wave propagation

within the test specimen. The method is used to check/distinguish

for/between grey iron (GJL), ductile iron (GJS), vermicular iron (GJV) or

malleable iron (GJMW or GJMB).

B: Assuring part acoustic behavior/influence during operation

A brake system consists of multiple components - most of them are cast-

ing parts. These parts can be easily made sounding by striking. This ability

to sound (or vibrate) is also observable in the assembled condition. If two

parts are mounted together having a similar or the same natural frequen-

cy, the part vibration will be amplified - they are getting into "resonance".

This situation must be avoided.

To do so, each part of the brake system has to be designed to its own

natural frequency signature. The result is, even if one part is getting into

oscillation, the other components do not care about and stay quiet. Brake

squeaking can be reduced to a minimum. The automotive industry has de-

fined natural frequencies as part of the specifications and drawings.

C: Component sorting

Steering columns propagate vibrations from the front axle to the steering

wheel. The wheel vibrates when the excited frequency and the natural fre-

quency are nearly the same. When driving the main vibrations come from

the tires, elsewise from the engine.

By natural frequency measurement and simulation of the steering wheel

mass the columns should be grouped. This grouping allows deciding

whether the steering columns can be mounted in a car with benzine en-

gine or diesel engine.


F 2 What is the principle of the 'natural frequency test'?

The test is based on the acoustic resonance analysis. This method uses the

known physical effect that a body having been suitably excited (e. g. by tap-

ping), vibrates in certain characteristic forms and frequencies.

These vibrations, also called 'natural frequencies', are so to speak the

'language of the work piece', which can be measured by a microphone (air-

borne sound) or by a laser vibrometer (structure-borne sound) and digitally ana-

lyzed. The decisive factor for the number and spread of natural resonances of a

work piece is its geometry: a bar has few natural resonances; a complex work

piece has multiple natural resonances.

F 3 How is the 'natural frequency test' practically performed?

First, the part has to be placed on soft tips to avoid damping effects. To stimu-

late the parts oscillation, a tiny electrically driven metal hammer is used to cre-

ate an impact on the parts surface. This mechanical impulse makes the part vi-

brating in its characteristic forms and frequencies. A microphone mounted close

to the test specimen surface is collecting these frequencies and converting

them into electrical signals. Finally a data acquisition system is converting the

signals into numerical values, which are processed by the computer. A special

algorithm called 'Fourier Analysis' (FFT) is used to present the typical finger-

print of the tested part. Each vertical peak represents one natural frequency

(Fig. 1). The higher the level, the stronger the oscillation and - in the opposite

direction - the more sensitivity the part has at this specific frequency.

Fig. 1: The acoustical finger print of a test part


F 4 What are the different excitation methods?

To stimulate the oscillation of the part, different methods are possible:

Fig. 2: Excitation methods

The excitation has to stimulate the test specimen to vibrate in its resonances.

These can then be measured by a microphone, a laser vibrometer or an accel-

erometer (see F 5).

The impulse modal hammer consists of a quartz force sensor mounted on the

striking end of the hammer head to measure the excitation force. As the excita-

tion force has only an influence on the resonance frequency amplitude, but not

on the value, it is not necessary to use a modal hammer for resonance fre-

quency evaluation.

An electrodynamic hammer is designed to have a short contact time, can be

adjusted to the necessary excitation force and allows constant amplitudes. This

device is sufficient for natural frequency testing and used in automation applica-

tions.

Manual excitation is used in initial tests. As the excitation force is not constant

the level of the resonances varies.

Piezo-electric actors vibrate at a specific frequency depending on the supplied

voltage. When you contact such a device with the test specimen it will vibrate

with the same frequency as the actor. If the actor frequency meets a resonance

frequency of the part you can measure this with a second sensor (accelerome-

ter). The advantage is the high frequency range, the disadvantage is that it is

necessary to sweep the induced frequencies. This is time consuming and the

contact quality and position have a great influence on the result.

F 5 Which sensor is the best for measuring the resonance frequencies?

Resonance frequencies can be measured contactless by a microphone or laser

vibrometer or contacted by an accelerometer. Whereas a laser vibrometer and

an accelerometer measure the resonance frequency and amplitude at a single


position the microphone is the only device that can measure integral, i.e. all

frequencies regardless of the position. To measure all vibration modes with

significant amplitude two or more microphones can be used.

F 6 How long does it take to get the natural frequencies of a part?

This depends on the number of impacts and the part itself. When testing a

brake disc, a caliper or a bracket, typically two or three excitations are used to

induce the natural frequencies. After the excitation, it takes less than 1 second

to read and analyze the sound. So in total, within two seconds or even less all

natural frequencies are available for evaluation.

F 7 What about the environmental conditions to perform this test?

In most cases nothing. This measurement is not influenced by environmental

noise and vibrations. The sensors are directional microphones which are locat-

ed close to the part surface. If necessary, simple damping elements are placed

in the area surrounding the specimen to ensure a good signal-to-noise ratio.

F 8 Which influences have an impact on the 'natural frequencies' of the part?

There are two different types of influences:

A: Static influences:

Static influences are based on the production process and the part materi-

al itself. They are invariant over the time and primary responsible for the

position of the natural frequencies. These are mainly geometrical dimen-

sions, coefficient of elasticity, density, hardness, cast structure and some

others.

B: Temporary influences:

Temporary influences are work piece temperature and age of the part. The

following example explains the temperature influence (Fig. 3):

If a part temperature is 50 C and the reference values are typically

measured at 20 C we can expect a frequency shift of about 0.75 %.

This is a respectable value compared to a tolerance range of 3 %.

In fact, by measuring the part temperature, this effect can be com-

pensated by the software.

The age of the part is responsible for a shift of natural frequencies within a

time period of e. g. 30 days (Fig. 4). During this time the natural frequen-

cies of the part are shifted to higher frequencies following an exponential

curve. This effect is caused by changes in the microstructure as well as by

reduction of material tensions (especially with grey iron).


The reference values in a drawing are referred to 'aged' parts. A very the-

oretical solution would be to wait 30 days before performing the measure-

ment. The more efficient way is to input the age of the part into the meas-

urement software which uses this information to calculate the values as

they would be at an age of 30 days.

All types of compensation methods mentioned above are part of RTE's

test software SonicTC.EigenFrequency.

Fig. 3: Influence of the part temperature Fig. 4: Influence of the part age

F 9 Measure cast blank or machined part?

Both - with different focus. In the casting area the main issue is to check the

averaged values over one batch. For brake discs the tolerances of raw parts

are often defined as acceptable within 5 % and for machined parts within 3 %

of the reference values. The measured statistical mean values should be as

close as possible to the reference values to avoid increased reject later on. Ma-

chining changes the natural frequencies (see above). Batch inspection by sam-

ples may be sufficient, but depends on customer requirements.

F 10 Is this technology applicable in laboratory and shop floor?

Yes - as long as the measurement conditions follow the rules mentioned above.

It can be used for low volume/laboratory (Fig. 5), optimized for brake discs),

medium volumes in labs/shop floors (Fig. 6) and manufacture integrated as ful-

ly automated units (Fig. 7, Fig. 8).

Fig. 5: SonicTC.Cube: manually loaded test

device (low volume, special fixture, manual

type adaption, optimized for brake discs, fly

wheels etc.)


Fig. 6: Manually loaded test

benches (medium volume,

type-specific work piece

carriers, optimized for cali-

pers, brackets, knuckles)

Fig. 7: Manually loaded test bench

(high volume, automatic changeover,

optimized for brake discs)

Fig. 8: Inline 100 % test bench (high volume,

automatic changeover, optimized for brake discs)

F 11 Are there any specifications concerning 'natural frequency test'?

Yes - there are some. Common specifications concerning brake parts are the

EKB Technical Standards from the European Brake Noise Experts Group (EKB

2001/2002 for brake discs, EKB 4001 for brake calipers). It contains some hints

regarding theory and practical application of that test. In addition the brake

manufacturers and OEMs often have proprietary specifications.

Any further questions? Feel free to contact RTE the experts in

acoustic testing for production and development.

International EURO PM2009, Copenhagen

Copyright 2009 - RTE Akustik+Prftechnik GmbH, Germany page 1 of 7

Compensation-based non-destructive automatic quality control using acoustic methods

Ingolf Hertlin RTE Akustik + Prftechnik GmbH

Gewerbestr. 26, D-76327 Pfinztal, Germany www.rte.de

Abstract Cracks, porosity and density differences can occur during the manufacturing process. The non-destructive testing method acoustic resonance testing (ART) allows fast quality control of each single part integrated in the production flow. The method evaluates in particular the mechanical stiffness of the test specimen. The temperature of the part, dimensional variations and mass / density variations have an influence on the measurement results (resonant frequencies) in the same way as cracks and porosity and may mask them. Compensation methods allow the - permitted - product-specific variations to be distinguished from defects. In this way the method is able to detect capillary cracks reliably. The presentation describes the procedural method, the type and nature of compensation and the results of industrial applications.

Keywords Non-destructive testing (NDT), quality control, acoustic, resonance analysis, crack testing, process compensation, acoustic resonance testing

Introduction Acoustic sound analysis is a well-established non-destructive method for components that radiate sound, especially in metal and ceramic industry. The method is used in high-volume production to scan the parts fast and cost-effective within the production flow. Not only the raw materials, but also the manufacturing process affects the product characteristics and determines how a specimen sounds. Whereas human testers can easily adapt to this, an automatic inspection system relies on references and compares the measured sound to decide, whether the part is good or bad. In the past, solutions came up which did not cope with the complexity of this task: a) The product material is in general a mixture of different raw materials. Rheological

additives and a heating process influence the stiffness of the part. Inoculation during the cast process influences the graphite structure.

b) Inhomogeneity of the microstructure or density variation causes an acoustical variation.

c) Dimensional variation changes the vibration modes of the part. d) Part temperature affects the elastic modulus and density and thus the resonance

frequencies of the part. Many non-destructive inspections for the quality control use surface-related methods and do not assess the inner structure or defects like cracks as volume-related procedures do. These reasons and low performance of test systems at times resulted in low confidence in the assessments of fully automated final inspection systems. To cover the allowed process and part variation it is mandatory to eliminate, i. e. compensate, this impact on the acoustic behaviour of the part. Consecutively powerful classification methods can be used to find defects like capillary cracks or structural defects.



NDT method Acoustic Resonance Testing Dynamic systems can be characterized in terms of one or more natural frequencies [1]. The natural frequency is the frequency at which the system would vibrate if it were given an initial disturbance (stimulus) and then allowed to vibrate freely. The physical model of a natural frequency is a mass hanging from a spring. The mass represents the part being tested and the spring represents its stiffness. The mass m vibrates up and down at its resonant frequency f, which is proportional to the stiffness k divided by the mass (undamped system).

m

kf ~ (1)

The wave propagation in solids consists of longitudinal and transversal waves with the sound velocity

)21()1(

=

EcL

)1(2 +=

EcT

(2) with E elastic modulus, density and Poissons ratio. Elastic modulus and density are both temperature dependent. Any part has an infinite number of resonances, each determined by a specific combination of material properties and dimensions. Sound testing makes use of this effect: After suitable stimulation, e.g. by striking, the body vibrates in well-defined characteristic patterns and frequencies. A sound is a mixture of many single frequencies (in this case resonances) at a certain level and is not limited to the audible range but also includes the ultrasonic area. The oscillations, registered with a microphone, a laser vibrometer or an accelerometer, are so to speak the "language" of the test object, its distinctive fingerprint. They provide information about the entire test object and not just locally at the site of the sensor. Acoustic resonance testing (ART) [2] is a volume-related, non-destructive testing procedure, which makes the quick and cost-saving 100 % testing of test objects possible (Fig. 1). Sound testing in line production is an approved technology.

Fig. 1: Non-destructive testing methods (selection) The principal difference between ART and the classic NDT methods is that ART is sensitive for strength change: a structural defect reduces the stiffness and lowers the resonant frequency. Different defect types can be evaluated with one measurement, e. g. cracks, material deviations and structural deviations (inhomogeneity, stiffness / hardness, porosity). The main objective is that the defect influences the acoustical behaviour of the part and is significant with relation to the production scatter.



The sensitivity (defect size, defect location) is also related to the part size and shape. Experimental and theoretical evaluations show that the size of a defect as well as the defect location has an impact on the frequency values [3]: A larger defect results in the shift to lower peak frequencies (same defect location) and a deeper defect in the shift to higher peak frequencies (same defect size). The first influence is higher than the latter and means that the resonance method is more sensitive for the expansion of a defect than its location. The decision for the applicability of the acoustic method requires systematic engineering and tests prior to automation.

Why compensation? Various influences the radiation and measurement of the sound and hence the resonance frequencies (Table 1). In general there are influences from the manufacturing process (called process-related influences) and influences that come from the test bench and / or the test system set-up and measurement conditions (called test-related influences). Especially the excitation to generate the resonance frequencies is important: Position and direction are responsible for the level of a resonance frequency. The strength of the excitation can cause a linear or non-linear response from the specimen resulting in frequency changes and additional frequencies in the spectrum. The contact time is responsible for the frequency range and the damping of frequencies.

Influence Effect Compen sation

Method

Process-related influence Aging hardness change (e. g. in castings) yes calculation Blowholes (minimal) disturbance of wave propagation no test objective Crack size / location wave propagation disturbance, flexural & longitudinal resonances no test objective Density wave propagation no test objective Dimension flexural & longitudinal vibration modes yes characteristics Hardness flexural resonances yes test objective Porosity wave propagation no test objective Raw material e-modulus and density, wave speed yes characteristics Temperature change of e-modulus and density yes measurement Weight see density / raw material yes measurable

Test-related influence Human classification set up of characteristics, quality assessment no training strategies Excitation direction resonance stimulation yes engineering, layout Excitation force linear / non-linear part behaviour yes engineering, layout Excitation position resonance stimulation yes engineering, layout Excit. contact time resonance spectrum, damping yes engineering, layout Sensor charact. resonance measurement, resonance spectrum yes engineering, layout Sensor direction resonance measurement yes engineering, layout Sensor position resonance measurement yes engineering, layout Sensor type resonance measurement, resonance spectrum yes engineering, layout Support material wave propagation, damping, degree of freedom yes engineering, layout Support position wave propagation, damping, degree of freedom yes engineering, layout Test characteristics classification power yes engineering Test environment signal-to-noise ratio yes engineering, layout Test parameters measurement resolution, filter parameters and others yes engineering

Table 1: Influence on the acoustical behaviour of a specimen Taking all this into account the challenge is to evaluate the impact of these process and test influences according to a specific test object and test objectives.



Compensation techniques As explained above the (accepted) process variation results in the change of specimen behaviour and thus variation of the resonance frequencies To make the test parts comparable it is necessary to eliminate this influence as best as possible by adapting the resonance sprectrum. Within this contribution we explain first some of the process and then some test related influences. Precondition of applying frequency compensation is an extremely accurate measurement. Fig. 2 shows the relative standard deviation related to the vibration mode of a part with the first vibration mode at 8,5 kHz and the tenth at 78,3 kHz. ART has an excellent measurement precision of about 0.0035 % at the low frequency and 0.0006 % at higher frequencies [4] and depends only on the frequency resolution independent of the specific equipment.

Experimental standard deviation (rel.)

0,00000000,00000500,00001000,00001500,00002000,00002500,00003000,00003500,0000400

1 2 3 4 5 6 7 8 9 10

Vibration modes (1: 8,5 kHz, 10: 78,3 kHz)

Rel

. de

via

tion

Fig. 2: Standard deviation dependency

Process-related influence Temperature

Temperature measurement precision depends on a nu

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