Ultrasonics for Monitoring of Mining Mill Linings

Pilot Experiments

Christer Stenström

Center for Maintenance and Industrial Services (CMIS)Division of Operation, Maintenance and Acoustics

Luleå University of Technology, Sweden

April 2016

Strategic Innovation Programme for the Swedish Mining and Metal Producing Industry

PRINTED BY LULEÅ UNIVERSITY OF TECHNOLOGY, GRAPHIC PRODUCTION 2016

ISSN 1402-1528ISBN 978-91-7583-609-6 (print)ISBN 978-91-7583-610-2 (pdf)

Luleå 2016

www.ltu.se

3

Preface

The research presented in this report has been carried out at the Center for Maintenanceand Industrial Services (CMIS) and Division of Operation, Maintenance and Acoustics,Luleå University of Technology (LTU), Sweden.

The research was funded by Vinnova under grant no. 2015-01375. Vinnova is aSwedish government agency that administers state funding for research and development.The project was initiated and carried out in collaboration with Boliden AB.

The research project group consisted of Dr. Christer Stenström, Assoc. Prof. ÖrjanJohansson and Prof. Jan Lundberg at the Division of Operation, Maintenance and Acoustics,and Prof. Johan Carlson at the Division of Signals and Systems, LTU. Örjan Johanssonprovided expertise in structural acoustics, computer simulation, experimental set-ups,equipment selection and also reviewed the present report. Jan Lundberg provided expertiseregarding mining mills, experimental set-ups and maintenance engineering. Johan Carlsonprovided expertise in signal processing and ultrasonics, as well as providing equipment forlaboratory experiments, besides reviewing the present report.

5

Executive summary

Mining mill linings are continuously exposed to wear induced by the charge, and thus,mill operation has to be interrupted on a regular basis for inspections. Stoppages dueto inspections and replacement of linings lead to large production and monetary losses;especially in case of unplanned corrective maintenance. Condition monitoring duringmill operation can assist in the maintenance planning and decision process to increase theavailability and the overall performance of the material processing.

In this pilot study, the feasibility of using waterflow ultrasonics for condition moni-toring of the lining of mining mills during operation is to be investigated. The proposedwaterflow ultrasonic technique uses a water beam to transmit the ultrasound to the millshell and lining. By measuring the time of flight (TOF), it can be possible to monitorthe thickness of mill linings, without the need of interrupting operation. The method canlead to significant energy and monetary savings by reducing inspections and correctivemaintenance stoppages, and possibly increase product quality by optimised performance.

Since steel has a very high acoustic impedance in comparison to rubber, TOF mea-surements on aluminium-rubber has been used as an intermediate step towards the goalof measuring steel-rubber combinations. TOF measurements of aluminium-rubber layershave been achieved, and advancement towards the understanding of TOF measurements insteel-rubber laysers have been achieved.

The following conclusions have been reached:

• Rubber thickness can be measured through aluminium. In practice, aluminiumplugs/rivets can be mounted on the steel shell of a mill to measure TOF of rubbershell plates and lifter bars.

• Ultrasound signals can be measured through steel-rubber layers in through-transmissionand pitch-catch set-ups.

• Ultrasound signals measured in pulse-echo or pitch-catch set-up on a solid steel plateconnected to rubber, give rise to reverberations (ringing signal) that mask the rubberbackwall echoes. For avoiding reverberations of steel plate sides, an experimentalset-up requires the steel plate to be at least eight times wider and deeper (x- andy-axis) than the thickness (z-axis) of the rubber under inspection.

6

Further research could consider the following:

• Experimental TOF measurements with a steel layer eight times wider and deeper (x-and y-axis) than the thickness (z-axis) of the rubber under inspection.

• Adopt a signal processing technique for suppressing near-transducer reverberations.

• Apply a impedance matching layer between the liquid-steel interface and/or thesteel/rubber interface.

Keywords: ultrasonic testing, mining mill lining, lined pipes, NDT, condition monitoring,comminution, non-destructive testing, thickness gauge.

Abbreviations and notations

CBM Condition based maintenanceNDT Non-destructive testingTOF Time of flightUT Ultrasonic testingλ Wave length [m]ρ Density [kg/m3]ω Rotational speed [rad/s]D Diameter of transducer active element [m]f Frequency [Hz]H Hilbert transformN Near field length [m]R Reflection coefficientT Transmission coefficientv Speed of sound [m/s]vp Longitudinal speed of sound (pressure waves) [m/s]vs Transverse speed of sound (shear waves) [m/s]Z Acoustic impedance [rayl (kg/m2s)]

Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

1.1 Background 11

1.2 Project goal 13

1.3 Project objectives 13

1.4 Scope and limitations 13

1.5 Outline 13

2 Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1 Basics of ultrasonics 15

2.2 Method 162.2.1 Equipment data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

2.2.2 Material data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.2.3 Experimental set-ups . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.2.4 Feasibility of monitoring in operational environment . . . . . . . . . . . . . . . . . 19

3 Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

3.1 Objective 1 21

3.2 Objective 2 21

3.3 Objective 3 23

4 Conclusions and further research . . . . . . . . . . . . . . . . . . . . . . . 25

4.1 Conclusions 25

4.2 Further research 25

Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Articles 27

Proceedings 28

Miscellaneous 28

A Measurement through two layers . . . . . . . . . . . . . . . . . . . . . . . . 29

A.1 Contact mode set-up: 9 mm Al and 75/96 mm rubber 29A.1.1 1 MHz pulse-echo set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29A.1.2 0.1 MHz pulse-echo set-up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

A.2 Immersion set-up: 9/10 mm Al and 41/75/96 mm rubber 31A.3 Contact mode set-up: Steel and rubber 33A.4 Through-transmission set-up: Steel-rubber-steel 35A.5 Ringing signal of plate 37

B Depth of penetration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

B.1 1 MHz transducer 39B.2 0.1 MHz transducer 39

C Waterflow transducer test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

1. Introduction

Mining mill linings are continuously exposed to wear induced by the charge, and thus,mill operation has to be interrupted on a regular basis for inspections. Stoppages dueto inspections and replacement of linings lead to large production and monetary losses;especially in case of unplanned corrective maintenance. Condition monitoring duringmill operation can assist in the maintenance planning and decision process to increase theavailability and the overall performance of the material processing.

In this pilot study, the feasibility of using water flow ultrasonics for condition moni-toring of the lining of mining mills during operation is to be investigated. The proposedflow ultrasonic technique uses a water beam to transmit the ultrasound to the mill shell andlining. By measuring the time of flight, it can be possible to monitor the thickness of milllinings, without the need of interrupting operation. The method can lead to significant en-ergy and monetary savings by reducing inspections and corrective maintenance stoppages,and possibly increase product quality by optimised performance.

To succeed, the frequency of the transducer needs to be balanced. A lower frequencyimproves penetration, which is required, but it comes with a lower resolution. Theproblem of low resolution can be solved by increasing the number of measurement points.Also, multiple echoes (reverberations) can shadow the echo from the lining, thus, signalprocessing may be required. Further issues can be found: in the interface between the shelland lining; in the correlation between low and high (lifter) profile lining elements; andin the time of flight (TOF) relative to the peripheral speed of the mill. These issues arestudied within this pilot study.

1.1 BackgroundWater flow ultrasonics was used to measure discontinuities in mining mill shell by Sten-ström et al. (2012, 2015); see Figure 1.1. This project extends the previous research onmill shells to the study of mill lining thickness.

Non-destructive testing (NDT) refers to non-invasive techniques used for character-isation and inspection of objects. NDT is used for research, manufacturing monitoringand in-service inspections, to extract information of a material’s integrity. Besides visualinspection, ultrasonic testing (UT) is one of the most used NDT techniques, together withradiography and thermography. However, radiography requires two-sided access, haslimited thickness capabilities and it is hazardous, and thermography has limited applicationwhen it comes to subsurface discontinuities.

Ultrasonic testing has high resolution, good penetration depth, one and two-sided set-uppossibilities and it is non-hazardous. A main drawback is its line-by-line scan which makesit relatively slow compared to some other NDT methods (Berger 1969; Ball and Almond

12 Chapter 1. Introduction

0.4x4 mm

0.4x2 mm

0.4x1 mm

Ø 0.8 mm

t = 15 mm

Figure 1.1: Prototype of mining mill with fabricated discontinuities and results (Stenströmet al. 2012, 2015).

1998). Another drawback is that it is hard to use UT for detecting cracks in course-grainedmaterials due to diffuse scattering and attenuation (Lundberg et al. 2011a,b). Moreover,ultrasonic inspection usually requires an experienced operator (Konshina and Dymkin2008). Nevertheless, it is a capable technique.

The literature on condition monitoring of mill shell and liners is modest. Dandotiyaand Lundberg (2011, 2012) have studied different inspection techniques for measuringwear and profile of liners to assess the remaining useful life, as significant savings dueto reduction of mill downtime can be achieved by efficient inspections, e.g. inspectionsperformed without stopping the mill. Besides evaluating various inspection techniques,power signature analysis was carried out on a mill with charge lifters of two differentsizes. Results showed a small variation of the angular velocity when the larger lifters madecontact with the charge. Several authors have used vibration analysis, acoustic emissionand embedded strain gauges for monitoring of mills which have been focused on aspectslike feed rate, angular speed, impact, stresses and such, for increasing the performance andminimise the wear of the lining (Spencer et al. 1999; Tano et al. 2003). Finite elementmethods (FEM) have also been used in some research to study mill behaviour duringoperation. Jonsén et al. (2012) combined FEM with smoothed particle hydrodynamic(SPH) modelling to study mill interactions. Furthermore, Ahmadzadeh and Lundberg(2013a,b) used artificial neural network (ANN) and process data to predict height andvolume of mill lifters. The ANN predicted the remaining height and life of the liners with

1.2 Project goal 13

accuracy greater than 90 %. The method can give large savings as mill stoppage for lininginspections can be reduced. However, the ANN needs training and is an indirect methodfor measuring lining thickness.

1.2 Project goalThe goal of this pilot study is to measure the thickness of mill linings during operationusing water flow ultrasonics.

1.3 Project objectivesTo fulfil the above goal, the following objectives have been formulated:

1. Analyse needs and equipment requirements, and carry out initial tests on transducersand equipment in laboratory.

2. Carry out ultrasonic tests on aluminium-rubber combinations.

3. Carry out ultrasonic tests on steel-rubber combinations.

Objective 1 mainly refer to selection of transducer frequencies, pulse excitation andcharacterisation of equipment performance. Test on aluminium in Objective 2 is used asan intermediate step towards the goal to measure through steel and rubber. The acousticimpedance (Z = ρv) of aluminium is about one third of the acoustic impedance of mildsteel. In Objective 3, the aluminium-rubber combination is changed to steel and rubber.The tests in Objectives 2 and 3 includes pulse-echo, pitch-catch and through-transmissioncontact mode set-ups, as well as immersion set-ups, as applicable.

Mill liner elements subjected to extensive wear and impacts are lifter bars and shellplates; often just called lifters and plates. The project goal refers mainly to measure thethickness of shell plates, but since the correlation between lifter and plate wear is notcertain, measurement regarding lifters are discussed.

1.4 Scope and limitationsThe scope is on laboratory studies of rubber mill lining in conjunction with aluminiumand steel. Since the study is confined to certain test samples and equipment, the study islimited when it comes to generalisation.

1.5 OutlineFollowing the introduction, the study is structured into chapters according to methodology,results & discussion and conclusions. Supplementary details regarding equipment, materialsamples and experiments are found as appendices.

2. Methodology

2.1 Basics of ultrasonics

Ultrasonic testing (UT) is a non-destructive testing (NDT) technique widely used in qualitycontrol and condition based maintenance (CBM). A UT system consist basically of apulser/receiver, transducer (probe), digitizer/oscilloscope and some visualisation hardwareand software.

The pulser produces high voltage electrical pulses; commonly 300-500 V, but alsohigher for highly attenuating materials, like rubbers (elastomer; thus, high ability todistribute stress) and coarse-grained materials (diffuse scattering), e.g. ceramics andaustenitic steels, such as manganese steel (Lundberg et al. 2011a,b). Common pulse typesare spike excitation (unit impulse or Dirac delta), square pulse excitation and sinusoidalexcitation.

The transducer includes a piezoelectric element, and thus, converts the electricalpulse to a mechanical displacement, i.e. sound. The thickness of the active element,i.e. the piezoelectric element, corresponds to half the desired wavelength, which in turngives a certain central frequency. Behind the active element, i.e. opposite to the face ofthe transducer, is a backing material, which decides the damping characteristics of thetransducer according the acoustic impedance (Z) difference between the damping materialand the active element. The damping characteristics affects the transducer’s sensitivity andthe duration of vibration (decrease of amplitude). High damping (ZBacking ≈ ZActive element)gives less ring down (multiple reflections in a thin material, i.e. reverberations) time of theactive element, shorter echoes (higher axial resolution, i.e. resolving power) and highersensitivity (ability to detect weak signals) due to wider bandwidth. On the other hand, lowdamping gives a longer waveform duration (more energy), narrower bandwidth, greatermaterial penetration, but less sensitivity.

An impedance matching layer is often found on the face of the transducer. If theimpedance matching layer is made one forth of the wavelength thick and given an acousticimpedance between the active element and the item under inspection, the waves reflect inphase and maximum energy is transmitted. However, effective matching is limited to thecentre frequency of transducers.

Penetration and resolution are highly dependent on transducer frequency and diameter.Higher frequencies give higher axial resolution, but attenuate more due to material mi-crostructure, e.g. grains (crystallites) and concrete aggregates. Lower frequencies attenuateless but diverge more, i.e. larger beam spread, which in turn depends on the active elementdiameter, and thus, diffraction. The sound field can then be divided into near field (N) andfar field, which are characterised by complex interference effects to a more uniform wavefront. The near field length is given by N = D2/4λ , where D is the active element diameter

16 Chapter 2. Methodology

and λ is the wavelength.Transducers can be found as longitudinal wave or shear wave transducers. In a longitu-

dinal wave, or pressure wave, the particle motion are parallel to the wave direction, and ina shear wave, or transverse wave, the motion is perpendicular to the wave direction. Thestudy of this report pertains to longitudinal wave transducers.

Common UT set-ups are pulse-echo, pitch-catch and through-transmission. A singleactive element is used for both transmitting and receiving waves in pulse-echo set-ups. Inpitch-catch set-ups, two transducers, or a two-element transducer, is used for transmissionfrom one element to receive in another element, where the both elements are on the sameside of the sample under test. A through set-up differs in the placement of the elements;the transmission and receiving is on the opposite side of the sample under test.

Received ultrasound echoes go back to the receiving side of the pulser/receiver, wherehigh/low pass filter is applied to the signal, followed by digitisation and display. Commongraphical representations are A-scan, B-scan and C-scan. An A-scan shows the voltage(arbitrary unit – a.u.) as a function of time or distance (t = vt/2) at a measurement spot.A B-scan is constructed from A-scans along a line, which graphically gives the cross-section data of a sample. A C-scan is then constructed from lines of A-scans (a grid ofmeasurement spots), which graphically is presented as if the sample is seen from the above.

The acoustic impedance, given by Z = ρv, is important in determining the transmissionand reflection of sound at medium interfaces. The share of reflected energy is given byR = {(Z2−Z1)/(Z2 +Z1)}1/2. However, in case of a layer, the transmitted energy also dependson the layer thickness i relation to the wave length, as discussion above.

2.2 Method

2.2.1 Equipment data

Requirements for laboratory tests concern mainly the pulser/receiver, transducers andexperimental set-up. High voltages and low frequencies are desired due to layered materialsunder test and high attenuation.

Three different pulser/receivers have been used in the experiments: GE USM Go,Matec PR 5000 and JSR Ultrasonics DPR300. The GE USM Go is on 300 V amplitudeand can produce spike and square wave excitation. The Matec PR5000 is on 600 Voltamplitude, 2500 W RMS and produces a sinusoidal tone burst. The DPR300 is on 900Volt amplitude and produce a spike excitation.

Two different transducer frequencies have been used; 1 MHz and 0.1 MHz. For thepurpose of the present study, 1 MHz may be the upper limiting range and 0.1 MHz maybe the lower limiting. Transmission and returning echoes attenuate with distance anddepends on the transducer frequency. Thus, material thickness can be adjusted accordingto transducer frequency. Nevertheless, the mill shell, lining plates and lifter are often tensof centimetres thick, so 1 MHz or lower seems appropriate.

The transducers are: one GE K1SC, one GE K1S and two GE K0.1G (GE 2004); seeTable 2.1.

For digitising the analogue signal, ADQ214 from SP Devices are used. The ADQ214has 14 bits resolution and a sampling rate of 400 MS/s.

2.2 Method 17

Table 2.1: Transducers data.

K1SC K1S K0.1G #1 K0.1G #2

Frequency (f) [MHz] 1 1 0.10 0.11

Diameter (D) [mm] 24 34 34 34

Waveform duration [µs] ≈ 2.5-3.5(1) ≈ 2.5-3.5(2) ≈ 30(3) ≈ 30(3)

Relative bandwidth at -6 dB 100±35 % 80±20 % 66 % 52 %

Near field length (N) [mm] 22.7±3.4 41±6 – –

Focal width at -6 dB [mm] 2.9±0.1 4.2±0.1 – –

Measured at: 1) polystyrene, 2) poly(methyl methacrylate) and 3) polyvinyl chloride.

2.2.2 Material dataThe longitudinal speed of sound has been measured for the rubber, aluminium and steelunder tests. The thickness of the test pieces was measured by taking the average of threemeasurements. Using the thickness measurements, the longitudinal speed of sound wascalculated by measuring the TOF between front and back wall echoes in a echo set-upusing 1 MHz K1SC transducer, i.e. no end correction was applied. See Table 2.2. Tablevalues are for example provided by Olympus, GE and Class Instrumentation Ltd.

Table 2.2: Measured values and table values of TOF.

Material Measured vl [m/s] Table values [m/s]

Rubber of mill lining 1 566 1 600–1 800(1)

Aluminium 6 315 6 300

Steel, low carbon – 5 900

Water – 1 480

1)1 600 m/s for polybutadiene and 1 800 m/s for butyl.

Coefficients of reflection and transmission are given by:

R =ρ2v2−ρ1v1

ρ1v1 +ρ2v2=

Z2−Z1

Z2 +Z1(2.1)

and

T =2ρ2v2

ρ1v1 +ρ2v2=

2Z2

Z1 +Z2, (2.2)

18 Chapter 2. Methodology

where:

R = Reflection coefficientT = Transmission coefficientρ = Density [kg/m3]v = Speed of sound [m/s]Z = Characteristic (acoustic) impedance [rayl]

The numbers (1 and 2) in Eq. 2.1–2, refer to the media of the incident wave and themedia of the transmitted wave, respectively. Using water and steel as and example, i.e.ZSteel = 45.5× 106 rayl and ZWater = 1.48× 106 rayl, for water-to-steel R = 0.937 andT = 0.063. For steel-to-water, R = −0.937 and T = 0.063; the minus signs indicatesthat the phase is shifted 180°. For aluminium-water, these figures are R = 0.840 andT = 0.160. Similar values are found for when water is replaced with the rubber in question,i.e. aluminium-rubber gives R = 0.815 and T = 0.185 and steel-rubber gives R = 0.926and T = 0.074. Moreover, ZAl = 17.0×106 rayl and ZRubber = 1.74×106 rayl.

2.2.3 Experimental set-ups

Experimental tests have been carried out in increments to estimate where the limitationof ordinary UT is, regarding measuring rubber thickness through metal. In this context,ordinary UT refer to UT without extensive signal processing or use of impedance matchinglayers, or other more sophisticated solutions. Planned experiemental set-ups are as follows:

Test 1: Test of penetration capability of transducers and pulser/receivers.Test 2: Pulse-echo measurement of Al-rubber in contact mode.Test 3: Pulse-echo measurement of Al-rubber immersed in water.Test 4: Pulse-echo measurement of steel-rubber in contact mode.Test 5: Pulse-echo measurement of steel-rubber immersed in water.Test 6: Comparison of detectability between waterflow and immersion set-ups.Test 7: Frequency response test of mill liner lifter bar.

In case measurement of rubber thickness through metal, aluminium or steel, cannot bereached, the following additional tests follow:

Test 8: Test with varying thickness of metal layer with reference to wavelength andalso with liquid (honey) between the layers.

Test 9: Through transmission test of metal-rubbber-metal to assess energy through-put.

Metals that have been used are aluminium and steel. The aluminium thicknesses are 9, 10and 31.5 mm, and the steel thicknesses are 5.9, 10 and 29.5 mm. The 5.9 mm steel equalsthe wavelength at 1 MHz, and the 29.5 mm steel equals the ½-wavelength at 0.1 MHz.The 31.5 mm aluminium equals the ½-wavelength at 0.1 MHz. The used steel samples areshown in Figure 2.1.

2.2 Method 19

(a) Lifter bar samples. 75 and 96 mm of rubber measuredfrom the aluminium bar.

(b) 60 and 133 mm thick rub-ber samples.

(c) 5.9, 29.5 and 10 mm mild steel samples. Holes are madeto hinder edge reflections.

(d) Clamp used for couplingof metal and rubber.

Figure 2.1: Samples of lifter bars, rubber and steel layers, and clamp for fastening.

2.2.4 Feasibility of monitoring in operational environment

A cross-sectional sketch of a mining mill section is shown in Figure 2.2. The steel shell is2-3 cm thick, the shell plates are less than 10 cm thick and the lifter bars may be 10-30 cmthick/high.

The feasibility of monitoring mining mills with waterflow ultrasonics have been dis-cussed in the preceding project by Stenström et al. (2015). Considering an mill of 10 m indiameter and an ultrasonic waterflow beam with an diameter of 10 mm, it gives a versineof x = r− sqrt

(r2− chord2/4

)= 2.5 µm, i.e. not considerably large. However, since the

object is cylindrical, all measurements are affected in the same way.Another issue is the TOF from transmitting sound to receiving the rubber backwall

echo. Since the mill is rotating, their might be a chance that the point of measurementspatially moves away from the waterflow too far. For calculating the tangential speed atthe mill shell periphery, two mills are considered (Dandotiya and Lundberg 2012). Millone has a diameter of 5.7 m and an operating angular speed of 8 rpm. Mill two has adiameter of 10.6 m and operates at 10.4 rpm. The speeds are then given by v = ωr = 2πr f ,resulting in 2.4 ms-1 and 5.8 ms-1 for mill one and two, respectively. However, the speedcan be varied and mills are commonly not operated at full speed. If it is assumed that wehave 100 mm of water (v = 1 480 m/s), 30 mm of steel (v = 5 900 m/s) and 70 mm ofrubber (v = 1 566 m/s), and TOF = l/v, the TOF back and forth equals 235 µs. A tangentialspeed of 5 m/s = 0.005 mm/µs then gives a rotation of 1.2 mm in 235 µs. With referenceto a waterflow beam diameter of 10 mm, 1.2 mm shift can be accepted. However, a larger

20 Chapter 2. Methodology

Rubber shell plate

Rubber lifter bar

Aluminium bar

FasteningSteel shell

Figure 2.2: Cross-sectional sketch of mill shell and lining section.

waterflow beam diameter might be required due to diffraction and depth of penetration.This may also be required in case of thicker rubber, e.g. lifters of 20 cm or more.

For good acoustic coupling between the steel shell and lining, it is important that thesteel-rubber interface is free from air bubbles. Placing the UT equipment underneath themill, where high mill charge load is found, is then optimal considering acoustic coupling.Also, the water from the waterflow transducer will fall down, and if needed, it is easier tosuck up. However, this also improves the acoustic coupling between the mill lining andcharge, and may also deform the steel-rubber interface, which in turn gives smaller echoreflection. Alternatively, the UT equipment can be placed on the side or top of the mill.

Mining mills shells consist of several pieces bolt joined, which results in up to about10 cm high flanges stretching along the axis of rotation of mills. Sound does not attenuatemuch in water, but a water beam of 10 cm in free air require somewhat high pressure, andthus, more water than desired. A possible alternative is to use a pneumatic system to shiftthe transducer radially for passing flanges.

Considering cost, designing a UT system for mill monitoring is a significant investment.The experience within the present project, is that the cost of unplanned stoppage of a millcan cost between e10 000-40 000 per hour. However, costs are out of the scope of thisstudy.

For a brief review of mining mills and their condition monitoring, see the precedingproject by Stenström et al. (2015).

3. Results and discussion

3.1 Objective 1Objective 1 is to ‘Analyse needs and equipment requirements, and carry out initial tests ontransducers and equipment in laboratory’. With reference to the thickness of common millliner components, 1 MHz transducer was reasoned as the upper limit for measuring liningthickness through metal, if possible. 0.1 MHz transducer was chosen as the lower limitconsidering spatial resolution and results from computer simulations. See Appendix B fortransducer tests.

A main concern regarding measuring rubber through metal is the multiple echoeswithin the metal layer that give rise to a ringing effect, i.e. reverberations. If the ringing hasnot declined enough before the returning rubber backwall echo, it may be hidden withinthe ringing signal. Increasing the voltage would increase the returning echo energy, at thesame time as it would increase the ringing signal. High voltage is therefore only neededfor penetrating through the thick rubber. The ability to penetrate rubber with the chosentransducers have been tested in Appendix B. The tests in Appendix B concerns 300 V spikeand square pulse excitation using GE USM Go and 600 V sinusoidal wave using MatecPR5000. The GE USM Go and Matec PR5000 showed similar results using transducerK1SC. The JSR Ultrasonics DPR300 at 900 V gave 1-2 more echoes compared than whatis seen in Figures B.2 and B.3.

Since ringing can be a major concern, it might be advantageous to use two transducersin a pitch-catch set-up. Thus, pitch-catch set-ups are to be tried out using two transducersof the same frequency, 1 and/or 0.1 MHz. Furthermore, two transducers of the samefrequency can be used in a through set-up to measure the outgoing energy.

Besides that signal processing may be required to be able to measure rubber thicknessthrough metal due to ringing, another solution can be to use impedance matching layers.Such layers can be used in the water-metal and metal-rubber interfaces. Total transmissionand zeros reflection can be reached for the centre frequency of a transducer betweendifferent materials using a matching layer that is a quarter-wavelength thick and hasan impedance of Z0 =

√Z1Z2 (Callens et al. 2004). Besides increasing the energy into

the rubber, it would also reduce the ringing effect. However, the result depends on thepulse excitation, and it can also be hard in practice to find a matching layer of a specificimpedance. However, signal processing and impedance matching layers are out of thescope of the present pilot experiments.

3.2 Objective 2Objective 2 is to ‘Carry out ultrasonic tests on aluminium-rubber combinations’. Tests havebeen carried out firstly in contact mode and secondly in water immersion (See Appendix C

22 Chapter 3. Results and discussion

for comparison of waterflow and immersion set-ups performance).In contact mode, 1 MHz K1SC transducer has been used on a lifter of 9 mm Al

vulcanized to 75 mm rubber, and on another lifter of 9 mm Al vulcanized to 96 mm rubber.Rubber backwall echo from 9 mm Al vulcanized to 96 mm rubber is shown in Figure 3.1.The measurements coincide well with theoretically calculated TOF values using the vl ofTable 2.2. See Appendix A for measurement on 9 mm Al vulcanized to 75 mm rubber.

(a) 9 mm Al + 96 mm rubber giving ≈125 µs TOF. Theo-retical TOF equals 126 µs.

(b) Test piec and 1 MHz transducer.

Figure 3.1: Al + rubber pulse-echo measurement using 1 MHz transducer at 300 Volts,500 ns, square wave pulse. The x-axes in (a) is of µs.

In water immersion, 1 MHz K1SC transducer has been used on lifters of: 10 mm Alvulcanized to 41 mm rubber, 9 mm Al vulcanized to 75 mm rubber, and of 9 mm Alvulcanized to 96 mm rubber. The rubber thickness of all three samples was accuratelymeasured. Figure 3.2 shows the result of 76 mm water + 9 mm Al + 76 mm rubber. Theresults from the other two lifter samples are given in Appendix A.

(a) 9 mm Al + 75 mm rubber giving ≈100 µs TOF. Theoreti-cal TOF equals 99 µs.

(b) The experimental set-up corre-sponding to the A-scan to the left(a).

Figure 3.2: 76 mm water + 9 mm Al + 76 mm rubber pulse-echo measurement using 1MHz transducer at 300 Volts, 500 ns, square wave pulse. The x-axis in (a) is of µs.

3.3 Objective 3 23

3.3 Objective 3Objective 3 is to ‘Carry out ultrasonic tests on steel–rubber combinations’. The acousticimpedances of aluminium and steel are 17 and 45 MRayl, respectively, due to the largedifference in material density. The transmission coefficient of aluminium–rubber and steel–rubber then turns out to be 0.185 and 0.074, respectively. It is therefore a considerable stepto go from aluminium to steel.

Various UT measurements on steel and rubber clamped together have been tried out.Some measurements were promising but consistent results could not be reached. Examplemeasurements are shown in Figures A.7-A.9.

In a pulse-echo set-up of steel-rubber, the sound passes through the steel layer twotimes, which is similar to steel-rubber-steel through-transmission. To see if there is anyenergy that comes through the layers, measurements on such an set-up was carried out(Figures A.10). Moreover, to compare the amount of energy that comes through aluminium-rubber and steel-rubber, another set-up was carried through.Two set-ups where studied:31.5 mm aluminium with 54.5 mm rubber, and 29.5 mm steel with 54.5 mm rubber. Thethicknesses of the aluminium and steel corresponds to 5 wavelengths of 1 MHz frequency.The measurement results from using 1 MHz K1SC and K1C are shown in Figure 3.3. Thefigure shows the envelope of the signal, given by:

y(n) = |x(n)+ iH {x(n)}| (3.1)

where H {x(n)} denotes the discrete Hilbert transform of x(n). The area underneath thecurves is then proportional to the energy throughput. Using trapezoidal integration inMatlab, the ratio between of the integrated curves becomes 1.08. The value from integrationis eight percent larger for aluminium in relation to steel. By study the amplitudes, it isfound that the amplitude ratio of the first arriving sound wave is AAl/AFe = 2.51, and of thesecond arriving sound wave is AAl/AFe = 1.27. However, this value would differ more incase of two layers of metal instead of one.

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Figure 3.3: Envelope of through-transmission in steel-rubber and al-rubber using 1 MHzK1SC and K1S.

The difficulty of measuring rubber thickness through steel is found to be a combinationof ringing signal in the steel and weaker returning signal from the rubber. As the widthand depth of the steel plates (≈ 160×100) are on a scale close to the rubber under test and

24 Chapter 3. Results and discussion

transducer wavelengths, a large number of reflections (reverberation) appear from plateedges and drill holes, which in turn give rise to a slowly decaying noise of both pressureand shear waves. This effect is shown in Figure A.13; it can somewhat be compared to anecho chamber. With a larger steel plate, the sound can continue to propagate away fromthe transducer, and in combination with a larger rubber lining sample, a greater dampingcan be achieved. Therefore, a more realisic experimental set-up is where the steel plates isat least eight1 times larger than the thickness of the rubber under inspection.

Further actions for estimating the TOF in rubber lining include impedance matchinglayers and signal processing. However, the matching capability drops quickly for frequen-cies higher or lower than the centre frequency, and it might also be hard to find a matchinglayer with the required impedance. The logic behind impedance layers is briefly discussedin Section 3.1. Signal processing can then be a more feasible approach. The reverberationsin the steel shell mask the echoes from the rubber lining. In order to estimate TOF in thelining, some technique for suppressing near-transducer reverberations can be adopted, e.g.the echo-cancellation method described by Carlson et al. (2002).

The present study is focused towards measuring thickness of rubber shell plates ofmill liners. However, the correlation between lifter bars and plates wear is not certain.It is therefore desired to measure the thickness of lifters. As the rubber is considerablythicker when it comes to lifters, a lower frequency is likely required. From the tests inFigures 3.1 and 3.2, 0.5 MHz can be a suitable frequency for larger lifters. However, sinceit is an aluminium bar vulcanised into the lifter, it is most likely impossible to assess lifterthickness using TOF measurements. A possible solution is to study the frequency responseinstead of study the time domain. Excitation may then be done through a fastening screw(Figure 2.2) using a stationary wireless self-powered sensor. Another solution, wich isknown to work from the present study, is to mount an aluminium plug on the steel shellthat stretches in to the lifter bar and measure with a waterflow transducer.

1The speed of sound in steel is about four times the speed in rubber, and the transducer is placed in themiddle of the steel plate.

4. Conclusions and further research

4.1 ConclusionsThe following conclusions have been reached:

• Rubber thickness can be measured through aluminium (Figures 3.2). In practice,aluminium plugs/rivets can be mounted on the steel shell of a mill to measure TOFof rubber shell plates and lifter bars.

• Ultrasound signals can be measured through steel-rubber layers in through-transmissionand pitch-catch set-ups (Figures A.13 and A.12b).

• Ultrasound signals measured in pulse-echo or pitch-catch set-up on a solid steel plateconnected to rubber, give rise to reverberations (ringing signal) that mask the rubberbackwall echoes. For avoiding reverberations of steel plate sides, an experimentalset-up requires the steel plate to be at least eight times wider and deeper (x- andy-axis) than the thickness (z-axis) of the rubber under inspection (Section 3.3 andFigure A.13).

4.2 Further researchFurther research could consider the following:

• Experimental TOF measurements with a steel layer eight times wider and deeper (x-and y-axis) than the thickness (z-axis) of the rubber under inspection.

• Adopt a signal processing technique for suppressing near-transducer reverberations.

• Apply a impedance matching layer between the liquid-steel interface and/or thesteel/rubber interface.

Bibliography

ArticlesAhmadzadeh, F. and J. Lundberg (2013a). “Application of multi regressive linear model

and neural network for wear prediction of grinding mill liners”. In: Internationaljournal of advanced computer science and applications 4.5, pages 53–58. DOI: 10.14569/IJACSA.2013.040509 (cited on page 12).

— (2013b). “Remaining useful life prediction of grinding mill liners using an artificialneural network”. In: Minerals engineering 53, pages 1–8. DOI: doi:10.1016/j.mineng.2013.05.026 (cited on page 12).

Ball, R. and D. Almond (1998). “The detection and measurement of impact damage in thickcarbon fibre reinforced laminates by transient thermography”. In: 31.3, pages 165–173.DOI: 10.1016/S0963-8695(97)00052-2 (cited on page 11).

Berger, H. (1969). “Ultrasonic imaging systems for nondestructive testing”. In: The journalof the acoustical society of america 45.4, pages 859–867. DOI: 10.1121/1.1911560(cited on page 11).

Callens, D., C. Bruneel, and J. Assaad (2004). “Matching ultrasonic transducer using twomatching layers where one of them is glue”. In: NDT&E International 37.8, pages 591–596. DOI: doi:10.1016/j.ndteint.2004.03.005 (cited on page 21).

Dandotiya, R. and J. Lundberg (2011). “Evaluation of abrasive wear measurement devicesof mill liners”. In: International journal of COMADEM 14.2, pages 3–17 (cited onpage 12).

— (2012). “Economic model for maintenance decision: A case study for mill liners”. In:Journal of quality in maintenance engineering 18.1, pages 79–97. DOI: 10.1108/13552511211226201 (cited on pages 12, 19).

Jonsén, P., B. I. Pålsson, and H.-Å. Häggblad (2012). “A novel method for full-bodymodelling of grinding charges in tumbling mills”. In: Minerals engineering 33, pages 2–12. DOI: 10.1016/j.mineng.2012.01.017 (cited on page 12).

Konshina, V. N. and G. Y. Dymkin (2008). “Modern approaches to the certification ofultrasonic testing techniques”. In: Russian journal of nondestructive testing 44.2,pages 77–85. DOI: 10.1134/S1061830908020010 (cited on page 12).

Lundberg, J., A. Bohlin, and M. Syk (2011a). “Blindfold tests on manganese crossing inrailway application”. In: International journal of system assurance engineering andmanagement 2.2, pages 169–182. DOI: 10.1007/s13198-011-0073-6 (cited onpages 12, 15).

— (2011b). “Capacity test of ultrasonic equipment used for crack detection in railway ap-plication”. In: International journal of system assurance engineering and management2.2, pages 163–168. DOI: 10.1007/s13198-011-0074-5 (cited on pages 12, 15).

28 Bibliography

Stenström, C., J. Carlson, and J. Lundberg (2015). “Condition monitoring of cracks andwear in mining mills using water squirter ultrasonics”. In: International journal ofcondition monitoring 5.1, pages 2–8. DOI: 10.1784/204764215814981585 (cited onpages 11, 12, 19, 20, 41).

ProceedingsCarlson, J., F. Sjöberg, N. Quieffin, R. K. Ing, and S. Catheline (2002). “Echo-cancellation

in a single-transducer ultrasonic imaging system”. In: The Institute of Electrical andElectronics Engineers (cited on page 24).

Spencer, S. J., J. J. Campbell, K. R. Weller, and Y. Liu (1999). “Acoustic emissionsmonitoring of SAG mill performance”. In: Intelligent processing and manufacturing ofmaterials, 1999. IPMM ’99. proceedings of the second international conference on.Volume 2, pages 939–946. DOI: 10.1109/IPMM.1999.791509 (cited on page 12).

Stenström, C., J. Carlson, and J. Lundberg (2012). “Mining mill condition monitoringusing water jet ultrasonics”. In: IEEE international ultrasonics symposium. Springer,pages 1913–1916. DOI: 10.1109/ULTSYM.2012.0480 (cited on pages 11, 12).

Tano, K., B. Pålsson, and S. Persson (2003). “Continuous monitoring of a tumbling mill”.In: Proceedings of the XXII international mineral processing congress. Edited by M.Castro, J. Remmerswaal, M. Reuter, and U. Boin. Southern African Institute of Miningand Metallurgy (cited on page 12).

MiscellaneousGE (2004). Coarse grain probes. General Electric (cited on page 16).Matec (2000). PR5000 pulser/receiver operator’s manual. Matec Instrument Companies

(cited on pages 30, 39).

A. Measurement through two layers

A.1 Contact mode set-up: 9 mm Al and 75/96 mm rubberA.1.1 1 MHz pulse-echo set-up

Backwall echoes (thickness measurements) from 9 mm Al vulcanised to 75/96 mm rubberis shown in Figure A.1. The measurements coincide well with theoretically calculatedTOF values using the vl of Table 2.2. The transducer used was a 1 MHz K1SC and theinstrument was a GE USM Go at 300 Volts square pulse.

(a) No contact; only ringing signal. (b) 9 mm Al + 75 mm rubber giving ≈100 µsTOF. Theoretical TOF equals 99 µs.

(c) 9 mm Al + 96 mm rubber giving ≈125 µsTOF. Theoretical TOF equals 126 µs.

(d) Test pieces and 1 MHz transducer.

Figure A.1: Al + rubber pulse-echo measurement using 1 MHz transducer at 300 Volts,500 ns, square wave pulse. The x-axes in (a)–(c) are of µs.

To compare if there is any possible difference in acoustic coupling between rubber vulcan-ised against aluminium and rubber clamped against aluminium, another set-up was triedout. The clamp shown in Figure 2.1 was used together with 10 mm aluminium and 60 mm

30 A. Measurement through two layers

rubber fastened together; the rubber was compressed to 58 mm. The 1 MHz K1SC wasused together with Matec PR5000 pulser/receiver. See Figure A.2; no liquid was added inthe aluminium-rubber interface.

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Figure A.2: 10 mm Al + 60 mm rubber compressed to 58 mm, giving ≈76 µs TOF.Theoretical TOF equals 77 µs. Without any liquid in the aluminium-rubber interface.

A.1.2 0.1 MHz pulse-echo set-up

A similar measurement as in the previous section (Figure A.1 and A.2) is seen in Fig-ure A.3, using 0.1 MHz GE K0.1G, with sinusodial pulse excitation of Matec PR5000pulser/receiver (Matec 2000). The pulser/receiver were as follows: freq. equals 100 kHz,pulse width equals 2.5 µs, amplitude equals 50 %, gain equals 20 dB and high/low passfilters disabled.

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Figure A.3: 9 mm Al + 75 mm rubber pulse-echo measurement using 0.1 MHz transducer.3-4 backwall echoes are visible. See Figure A.1d for the sample under test.

A.2 Immersion set-up: 9/10 mm Al and 41/75/96 mm rubber 31

A.2 Immersion set-up: 9/10 mm Al and 41/75/96 mm rubberThree aluminium rubber samples have been tested immersed into water. The samplesare: 10 Al + 41 mm rubber, 9 Al + 76 mm rubber and 9 Al + 96 mm rubber. The rubberbackwall echoes can be seen in all three samples; see Figures A.4-A.6. The transducerused is 1 MHz K1SC together with GE USM Go.

(a) 10 mm Al + 41 mm rubber giving ≈55 µsTOF. Theoretical TOF equals 55.5 µs. The x-axis ranges from 0-120 µs.

(b) The experimental set-up corresponding tothe A-scan to the left (a). The water distanceequals 71 mm.

(c) 10 mm Al + 41 mm rubber giving ≈55 µsTOF. Theoretical TOF equals 55.5 µs. The x-axis ranges from 0-80 µs.

(d) The experimental set-up corresponding tothe A-scan to the left (a). The water distanceequals 49 mm.

Figure A.4: 49/71 mm water + 10 mm Al + 41 mm rubber pulse-echo measurement using1 MHz K1SC at 300 Volts, 500 ns, square wave pulse. The x-axes in (a) and (c) are of µs.

32 A. Measurement through two layers

(a) 9 mm Al + 75 mm rubber giving ≈100 µsTOF. Theoretical TOF equals 99 µs. The x-axisranges from 0-150 µs.

(b) The experimental set-up corresponding tothe A-scan to the left (a). The water distanceequals 76 mm.

Figure A.5: 76 mm water + 9 mm Al + 76 mm rubber pulse-echo measurement using 1MHz transducer at 300 Volts, 500 ns, square wave pulse. The x-axis in (a) is of µs.

(a) 9 mm Al + 96 mm rubber giving ≈125 µs TOF. TheoreticalTOF equals 126 µs. The x-axis ranges from 0-150 µs.

(b) The same measurement as shown in the fig-ure above (a), but without probe delay. Thex-axis ranges from 0-260 µs

(c) The experimental set-up corresponding tothe A-scans in (a) and (b). The water distanceequals 94 mm.

Figure A.6: 94 mm water + 9 mm Al + 96 mm rubber pulse-echo measurement using1 MHz transducer at 300 Volts, 500 ns, square wave pulse. The x-axes are of µs.

A.3 Contact mode set-up: Steel and rubber 33

A.3 Contact mode set-up: Steel and rubberVarious tests on steel and rubber has been carried out but consistent results have not beenreached. Some of the tests are shown in Figures A.7. Steel and rubber were clampedtogether as show in Figure A.11 but with one layer of steel instead of two. The marked outtimings in the A-scans of the figures does not necessarily have to come from the rubberbackwall. Tests were carried out both with and without honey in the steel-rubber interface;no visible differences in the results were seen (see Figure A.2).

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Figure A.7: 5.9 mm steel + 93 mm rubber pitch-catch measurement using 0.1 MHztransducers and Matec PR5000 at 2.5 µs pulse width and a frequency of 0.1 MHz.

34 A. Measurement through two layers

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Honey

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29.5 mm

89.2 mm

(b) Experimental set-up with trans-ducers not touching each other andtheoretical TOF.

Figure A.8: 29.5 mm steel + 93 mm rubber compressed to 89 mm with pitch-catchmeasurement using 0.1 MHz transducer and Matec PR5000 at 2.5 µs pulse width and afrequency of 0.1 MHz.

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(b) Experimental set-up.

Figure A.9: 5.9 mm steel + 60 mm rubber compressed to 57 mm in echo mode using1 MHz transducer and Matec PR5000 at 0.5 µs pulse width and a frequency of 1 MHz.

A.4 Through-transmission set-up: Steel-rubber-steel 35

A.4 Through-transmission set-up: Steel-rubber-steelSince consistent results were not reach in steel-rubber pulse-echo mode, it is interestingto see if there is any energy that comes through the layers. The sound passes throughthe steel layer two times in a pulse-echo set-up, which is similar to steel-rubber-steelthrough-transmission. Figure A.10 shows A-scans from through-transmission set-up using1 MHz K1SC and 1 MHz K1S together and two 0.1 MHz K0.1G together. The layersare 5.9 mm steel, 56.6 mm rubber and 29.5 mm steel. Multiple echoes from the two steellayers are seen in Figure A.10a. In Figure A.10b, the marked out 84 µs corresponds fairlygood to the time it takes for the sound to reflect back and fourth in the rubber (2×56.6mm additional distance); theoretical TOF equals 72 µs. The clamping set-up together withthe 0.1 MHz transducers are shown in Figure A.11.

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(a) Through transmission using 1 MHz transducers ( GE K1SC and K1S). The29.5 mm steel echos are visible and equals 10 µs.

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Figure A.10: Through-transmission from 5.9 mm steel, through 56.6 mm rubber and 29.5mm steel.

To compare how much energy that comes through aluminium-rubber and steel-rubber,further measurements were carried out. Two set-ups where studied: 31.5 mm aluminiumwith 54.5 mm rubber, and 29.5 mm steel with 54.5 mm rubber. The thicknesses of thealuminium and steel corresponds to 5 wavelengths of 1 MHz frequency. The measurement

36 A. Measurement through two layers

Figure A.11: Experimental set-up for through-transmission, from 5.9 mm steel, through56.6 mm rubber and 29.5 mm steel.

results from using 1 MHz K1SC and K1C are shown in Figure A.12. The figure shows theenvelope of the signal, given by:

y(n) = |x(n)+ iH {x(n)}| (A.1)

where H {x(n)} denotes the discrete Hilbert transform of x(n). The area underneath thecurves is then proportional to the energy throughput. Using trapezoidal integration inMatlab, the ratio between of the integrated curves becomes 1.08. The value from integrationis eight percent larger for aluminium in relation to steel. By study the amplitudes, it isfound that the amplitude ratio of the first arriving sound wave is AAl/AFe = 2.51, and for thesecond arriving sound wave is AAl/AFe = 1.27. However, this value would differ more incase of two layers of metal instead of one.

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Figure A.12: Envelope of through-transmission in steel-rubber and al-rubber using 1 MHzK1SC and K1S.

A.5 Ringing signal of plate 37

A.5 Ringing signal of plateTo illustrate the ringing signal of a plate, the 5.9 mm steel plate in Figure 2.1c was partlycut in the middle, measured, cut completely, and measured once again. See Figure A.13.The four edges and the drill holes causes a large number of reflections (reverberation) thatgive rise to a noise with a long decay time (Figure A.13c). It can somewhat be comparedto an echo chamber.

(a) 5.9 mm plate partly cut. (b) The experimental set-up with inclined trans-ducers. The 5.9 mm steel plate is not visible.

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(d) Pitch-catch A-scan of partly completely cutsteel plate. Rubber backwall echo at ≈100 µs.

Figure A.13: Test for illustration of ringing signal in plate using 1 MHz K1SC and K1S.

B. Depth of penetration

This appendix shows K1SC and K0.1G (Table 2.1) transducers ability to penetrate rubber(Figure B.1).

B.1 1 MHz transducerFigure B.2 shows multiple backwall echoes of a 60 mm rubber sample. The measurementsgive an indication of penetration depth in this kind of rubber using 1 MHz K1SC at300 V spike and square pulse excitation. In Figure B.2a, using spike pulse excitation, fivebackwall echoes are easily seen and an indication of a sixth echo can be seen as a changein the otherwise continuous noise. In Figure B.2b, using square pulse excitation of 500 nswidth, six echoes are easily seen, compared to five easily seen with the spike excitation.

B.2 0.1 MHz transducerFigure B.3 shows multiple backwall echoes of a 132.8 mm rubber sample. The transducersused were two 0.1 MHz K0.1G, with sinusodial pulse excitation from a Matec PR5000pulser/receiver (Matec 2000). Echoes corresponding to about 90-100 cm of rubber is seenin Figure B.3. The pulser/receiver were as follows: freq. equals 100 kHz, pulse widthequals 2.5 us, amplitude equals 50 %, gain equals 20 dB and high/low pass filters turneddisabled.

Figure B.1: 60 and 133 mm thick rubber samples together with K1SC and K0.1G trans-ducers.

40 B. Depth of penetration

(a) Spike pulse excitation, givig 5–6 backwallreflections; equals 30–36 cm.

(b) Square pulse excitation, givig 6 backwallreflections; equals 36 cm

Figure B.2: Depth of penetration measurement using a 1 MHz K1SC transducer at 300 V.The x-axes are of millimetres.

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Figure B.3: Depth of penetration in 132.8 mm rubber, using 0.1 MHz K0.1G transducers.

C. Waterflow transducer test

The aim of measuring mill linings condition during operation requires a waterflow set-up.Stenström et al. (2015) used waterflow ultrasonics to monitor the steel shell of miningmills, but any study of waterflow transducer performance was left out. This appendixprovides comparison of waterflow and water immersion transducer performance. Theexperimental set-ups are shown in Figure C.1, and the measurement results are shown inFigure C.2. The test sample is a piece of steel. As the speed of sound in water (1 480 m/s)is very high in comparison to the waterflow, any time a sound wave goes through the water,it can be compared to water that stands still, i.e. similar to water immersion. Ultrasonictesting can be done as long as the waterflow is fairly steady.

(a) Set-up for waterflow test. (b) Set-up for immersion testwithout waterflow.

(c) Set-up for immersion testwithout nozzle.

Figure C.1: Set-ups for test of 5 MHz GE H5KF waterflow transducer.

42 C. Waterflow transducer test

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Figure C.2: Results of 5 MHz GE H5KF waterflow transducer test.