Introduction to Ultrasonic Inspection

Ultrasonic inspection of aerospace structures plays an importantrole in safety, quality assurance and cost. It is an extremely sensitive method of nondestructive testing. The limits are few when direct coupling (or contact) with the part can be established, and the flaw resolving capabilities are excellent. However, interpretation by trained personnel is required.

The concept is not new, consider the following:• thumping on a watermelon to judge ripeness• tapping a steel wheel with a hammer to locate hidden defects

Use of Ultrasonics

Ultrasonics employ electronically produced high-frequency sound waves that will penetrate metals, liquids, composites and other materials at speeds of several thousand feet per second. This techniques can be used for:

• flaw detection, location and classification - laps, seams, delaminations, inclusions, cracks, corrosion, porosity and other internal defects of a part

• thickness measurement

• evaluation of bond quality in bonded assemblies

• determination of elastic moduli (material stiffnesses)

What is Ultrasonics?

“Ultrasound” refers to sound of such a high frequency that it cannot be heard. The general description of such sound is “ultrasonic.”

To understand how inaudible sound is used to reveal certain conditions which are not perceptible to the normal hearing range, it is first necessary to know how sound is transmitted and received. We must also explore what equipment is required to generate and detect ultrasonic energy.

Basic Components of an Ultrasonic Flaw Detector

•A power supply to produce the various required voltages

•A rate generator or timer, to synchronize all functions

•A pulser, which generates high-voltage short-duration spike to shock a crystal into resonant vibration

•A transducer (crystal) which transmits a high-frequency sound wave into the test piece, receives the reflected echo and converts it into an electrical impulse

•An amplifier or receiver to prepare the signal for display

•A CRT or computer terminal to display the echo signals

Transducer Materials

Quartz - advantages include electrical and thermal stability, insolubility in most liquids, high mechanical strength, wear resistance, excellent uniformity and resistance to aging. A limitation is its comparatively low electromechanical conversion efficiency.

Lithium Sulfate - advantage include ease of obtaining optimum acoustic damping for best resolution, intermediate conversion efficiency, and negligible mode interaction.

Polarized Ceramics - advantages include high conversion efficiency which yields high search unit sensitivity. Because of lower mechanical strength and relatively high electrical capacitance, their use is restricted to frequencies below 15MHz. An additional limitation is coupling between various modes of vibrations.

The Piezoelectric Effect

Generation of ultrasonic vibration is due to the piezoelectric effect. Crystals, when subjected to an alternating electric charge expand and contract under the influence of these charges. Conversely, if these materials are subjected to alternate tension and compression forces they will develop alternating electric charges on their faces.Alternating voltage applied:

- + + -

crystal expands crystal contracts

Use of Couplants

A couplant is a liquid which facilitates the transmission of ultrasonic vibrations from the transducer to the test surface.

Ultrasound generated by the piezoelectric effect, for all practical purposes, will not propagate in air due to its high frequency and short wave length. A film of oil, glycerine or water is generally used. The transducer can either be in contact with the part via the couplant, or both the transducer and the part can be immersed in a couplant.

? t = 2.09?s ? t = 1.70?s

A-Scans from pulse-echo immersion tests predict existence and depth of crack.

Photomicrograph verifies existence and location of crack.

Typical A-Scan Reveals Amplitude, Shape and Time of

Flight of Ultrasonic Energy

Crack

Front Surface

EmbeddedFlaws

Back Surface

Typical B-Scan Reveals Depth and Size of Detectable Flaws

1/4" D 1/32" D

5/32" D 1/16" D Hole depth = 0.07"

1/8" D 3/32" D

Typical C-Scans Reveal Location and Size of

Detectable Flaws

2.25 MHz 5 MHz 10 MHzThrough-the-Thickness Angle Interlock Sample TI-4PWT-A1-6

Scan Size 2.5" x 2.5" (400x400), Step Size 0.00625"

Amplitude- textile pattern- in-plane waviness

- backgroundvariations

Time of Flight- thicknessvariations

Through-Transmission, 15 MHz, 3" Focal Length, Scan Size 9" x 3" (900 x 300)

Representative Textile C-Scan Reveals Strong Pattern

Original Image Threshold Morphological Filtering

Strong textile pattern evident in as-scanned images

Digital Filtering Shows Promise for Removing

Influence of Textile Pattern

A

AAmplitude Section A-A

B

B

Time of Flight Section B-B

Waves May be Identified By Scattering of Ultrasonic

Energy on the Slopes

Amplitude Sample Cross-Section

Front Surface

Back Surface

B-scans Have Had Limited Success Characterizing Fiber

Architecture

Advantages of Ultrasonics

• Versatility, access to only one side of the part is required and the test equipment may be taken to the work

• Fast response, permitting rapid and automated inspection

• Accuracy in the measurement of flaw position and size

• Greater penetrating power, allowing inspection of thick sections

• High sensitivity, permitting detection of minute defects

Limits on Application of Ultrasonic Testing

Among the factors which may limit the application of ultrasonic testing are:

• Sensitivity - the ability of the instrument to detect the small amount of energy reflected from a flaw

• Resolution - the ability of the instrument to detect flaws lying close to the surface or close to one another

• Noise discrimination - the capacity of the instrumentation for differentiating between signals from defects and unwanted noise of electrical or acoustic nature