Ultrasonic material test

Nondestructive material testing with ultrasonics is more than 40 years old. From the very first examinations, using ultrasonic oscillations for detection of flaws in different materials, it has become a classical test method based on measurements with due regard to all the important influencing factors. Today it is expected that ultrasonic testing, supported by great advances in instrument technology, give reproducible test results within narrow tolerances. This assumes exact knowledge of the influencing factors and the ability to apply these in testing technology. Not all influences have to be seriously regarded by the operator. In many cases some of the influences can be neglected without exceeding the permitted measurement tolerances. Due to this, the test sequence is simplified and the testing time reduced. Despite this, the future belongs to the qualified operator who carries out his task responsibly and who continuously endeavours to keep his knowledge at the latest state of the art.

1. Why use ultrasonics for nondestructive material testing?

At the beginning of the fifties the technician only knew radiography (x-ray or radioactive isotopes) as a method for detection of internal flaws in addition to the methods for nondestructive testing of material surfaces, e.g. the dye penetrant and magnetic particle method. After the Second World War the ultrasonic method, as described by Sokolovin 1935 and applied by Firestonein 1940, was further developed so that very soon instruments were available for ultrasonic testing of materials. The ultrasonic principle is based on the fact that solid materials are good conductors of sound waves. Whereby the waves are not only reflected at the interfaces but also by internal flaws (material separations, inclusions etc.). The interaction effect of sound waves with the material is stronger the smaller the wave length, this means the higher the frequency of the wave.

λ=c/f

c = Sound velocity [km/s]
f = Frequency [MHz]
l = Wave lenght [mm]

This means that ultrasonic waves must be used in a frequency range between about 0.5 MHz and 25 MHz and that the resulting wave length is in mm. With lower frequencies, the interaction effect of the waves with internal flaws would be so small that detection becomes questionable. Both test methods, radiography and ultrasonic testing, are the most frequently used methods of testing different test pieces for internal flaws, partly covering the application range and partly extending it. This means that today many volume tests are possible with the more economical and non-risk ultrasonic test method, on the other hand special test problems are solved, the same as before, using radiography. In cases where the highest safety requirements are demanded (e.g. nuclear power plants, aerospace industry) both methods are used.

2. Ultrasonic testing tasks

Is there a primary classification of tasks assigned to the ultrasonic operator? If we limit ourselves to testing objects for possible material flaws then the classification is as follows:

1. Detection of reflectors
2. Location of reflectors
3. Evaluation of reflectors
4. Diagnosis of reflectors (reflector type, orientation, etc.)

Instead of using the word "reflector", the ultrasonic operator very often uses the term "discontinuity". This is defined as being an "irregularity in the test object which is suspected as being a flaw". In reality, only after location, evaluation and diagnosis has been made, can it be determined whether or not there is a flaw which effects the purpose of the test object. The term "discontinuity" is therefore always used as long as it is not certain whether it concerns a flaw which means a non-permissible irregularity.

3. Detection of discontinuities

The essential "tool" for the ultrasonic operator is the probe. The piezoelectric element, excited by an extremely short electrical discharge, transmits an ultrasonic pulse. The same element on the other hand generates an electrical signal when it receives an ultrasonic signal thus causing it to oscillate. The probe is coupled to the surface of the test object with a liquid or coupling paste so that the sound waves from the probe are able to be transmitted into the test object.

The operator then scans the test object, i.e. he moves the probe evenly to and fro across the surface. In doing this, he observes an instrument display for any signals caused by reflections from internal discontinuities.

Every probe has a certain directivity, i.e. the ultrasonic waves only cover a certain section of the test object. The area effective for the ultrasonic test is called the "sound beam" which is characteristic for the applied probe and material in which sound waves propagate. A sound beam can be roughly divided into a convergent (focusing) area, the near-field, and a divergent (spreading) part, the far field. The length N of the near-field (near-field length) and the divergence angle is dependent on the diameter of the element, its frequency and the sound velocity of the material to be tested. The center beam is termed the acoustic axis.

The shape of the sound beam plays an important part in the selection of a probe for solving a test problem. It is often sufficient to draw the acoustic axis in order to show what the solution to a test task looks like. A volumetric discontinuity (hollow space, foreign material) reflects the sound waves in different directions.

The portion of sound wave which comes back to the probe after being reflected by the discontinuity is mainly dependent on the direction of the sound wave; i.e. it does not matter whether scanning is made with a straight-beam probe or an angle-beam probe or whether it is carried out from different surfaces on the test object. If the received portion of the reflected sound wave from the probe is sufficient then the detection of the existing volumetric discontinuity is not critical, this means that the operator is able to detect it by scanning from different directions. A plane (two-dimensional) discontinuity (e.g. material separation, crack) reflects the ultrasonic waves mostly in a certain direction.

If the reflected portion of the sound wave is not received by the probe then it is unlikely that the discontinuity will be detected. The possibilities of detection only increase when the plane discontinuity is hit vertically by the sound beam. This applies to discontinuities which are isolated within the test object.

Apparent deformation of the sound beam on a side wall
With plane discontinuities which are open to the surface of the test object, e.g. a crack running vertically from the surface into the test object, a vertical scan of the crack does not always produce the required success. In this case wave overlapping occurs (interferences) due to sound wave reflection on the side wall of the test object which seems as if the sound wave bends away from the corresponding side wall. In such cases, the probability of crack detection is very good if the angle reflection effect is used. At the 90° edge, between the crack and the surface of the test object, the sound waves are reflected back within themselves due to a double reflection. Use of the angle reflection effect is often even possible when a plane discontinuity, which is vertical to the surface, does not extend to the surface and under the condition that the sound wave reflections at the discontinuity and the surface are received by the probe.

Often in thick-walled test objects, in which there are vertical discontinuities, this condition cannot be fulfilled so that the reflected sound waves from the discontinuity and the surface of the test object do not return to the probe. In this case, a second probe is used for receiving the reflected portions of sound thus enabling detection of the discontinuity.

With this type of testing, the Tandem Technique, one probe is used as a transmitter, and the other probe is used as the receiver. Both probes are moved over the surface of the test object and are spaced apart at a fixed distance. Scanning is made for vertically positioned discontinuities at different depths of the test object, depending on the probe spacing.

Although, with angle scanning in thin test objects, there is a possibility that plane discontinuities cannot be vertically hit, the detection sensitivity is much better, especially by suitable selection of the scanning angle and the test frequency so that the user favours the single probe test as opposed to the more complicated tandem method. This is normally the case when testing welds up to a thickness of about 30 mm.

Of course the possibility of detecting discontinuities which are not vertically hit is reduced. However, this deficiency is often compensated by an additional test with another angle of incidence, or by using a probe with a lower frequency. A typical procedure can be found in the corresponding specifications (test instructions) for weld testing.