Ultrasonic Test : Introduction

This technique is used for the detection of internal and surface (particularly distant surface) defects in sound conducting materials.

The principle is in some respects similar to echo sounding. A short pulse of ultrasound is generated by means of an electric charge applied to a piezo electric crystal which vibrates for a very short period at a frequency related to the thickness of the crystal. In flaw detection this frequency is usually in the range of one million to six million times per second (1 MHz to 6 MHz). Vibrations or sound waves at this frequency have the ability to travel a considerable distance in homogeneous elastic material, such as many metals, with little attenuation. The velocity at which these waves propagate is related to the 'Youngs Modulus' for that material and is characteristic of that material. For example the velocity in steel is 5900 metres per second, and in water 1400 metres per second.

Ultrasonic energy is considerably attenuated in air, and a beam propagated through a solid will, on reaching an interface (e.g. a defect, or intended hole, or the backwall) between that material and air, reflect a considerable amount of energy in the direction equal to the angle of incidence.

For contact testing the oscillating crystal is incorporated in a hand held probe which is applied to the surface of the material to be tested. To facilitate the transfer of energy across the small air gap between the crystal and the test piece, a layer of liquid, usually oil, water or grease, is applied to the surface.

As mentioned previously, the crystal does not oscillate continuously but in short pulses, between each of which it is quiescent. Piezo electric materials not only convert electri-cal pulses to mechanical oscillations, but will also transduce mechanical oscillations into electrical pulses; thus we have not only a generator of sound waves but also a detector of returned pulses. The crystal is in a state to detect returned pulses when it is quiescent. The pulse takes a finite time to travel through the material to the interface and to be reflected back to the probe.

The normal method of presenting information in ultrasonic testing is by means of a cathode ray tube, in which horizontal movement of the spot from left to right represents time elapsed. The rate at which the spot moves is such that it gives the appearance of a horizontal line on the screen. The system is synchronised electronically so that at the instant the probe receives its electrical pulse the spot begins to traverse the screen. An upward deflection (peak) of the line on the left hand side of the screen is an indication of this occurrence. This peak is usually termed the initial pulse.

Whilst the base line is perfectly level the crystal is quiescent. Any peaks to the right of the initial pulse indicate that the crystal has received an incoming pulse reflected from one or more interfaces in the material. Since the spot moves at a very even speed across the tube face, and the pulse of ultrasonic waves moves at a very even velocity through the material, it is possible to calibrate the horizontal line on the screen in terms of absolute measurement. The use of a calibration block, which produces a reflection from the back wall a known distance away from the crystal together with variable controls on the flaw detector allows the screen to be calibrated in units of distance, and therefore determination of origins of returned pulses obtained from a test piece.

It is therefore possible not only to discover a defect between the surface and the back wall, but also to measure its distance below the surface. It is important that the equipment is properly calibrated and, since it is in itself not able to discriminate between intended boundaries of the object under test and unintended discontinuities, the operator must be able to identify the origin of each peak. Further as the pulses form a beam it is also possible to determine the plan position of a flow.

The height of the peak (echo) is roughly proportional to the area of the reflector, though there is on all instruments a control which can reduce or increase the size of an indication - variable sensitivity in fact. Not only is part of the beam reflected at a material/air interface but also at any junction where there is a velocity change, for example steel/slag in a weld.

Probing all faces of a test piece not only discovers the three dimensional defect and measures its depth, but can also determine its size. Two dimensional (planar) defects can also be found but it is best that the incident beam impinges on the defect as near to right angles to the plane as possible. To achieve this some probes introduce the beam at an angle to the surface. In this manner longitudinal defects in tubes (inner or outer surface) are detected.

Interpretation of the indications on the cathode ray tube requires a certain amount of skill, particularly when testing with hand held probes. The technique is, however, admirably suited to automatic testing of regular shapes by means of a monitor - an electronic device which fits into the main equipment to provide an electrical signal when an echo occurs in a particular position on the trace. The trigger level of this signal is variable and it can be made to operate a variety of mechanical gates and flaw warnings.

Since the velocity of sound in any material is characteristic of that material, it follows that some materials can be identified by the determination of the velocity. This can be applied, for example in S.G. cast irons to determine the percentage of graphite nodularity. This process can also be automated and is now in use in many foundries. A typical equipment is the 'Qualiron'.

When the velocity is constant, as it is in a wide range of steels, the time taken for the pulse to travel through the material is proportional to its thickness. Therefore, with a properly calibrated instrument, it is possible to measure thickness from one side with an accuracy in thousandths of an inch. This technique is now in very common use. A development of the standard flaw detector is the digital wall thickness gauge. This operates on similar principles but gives an indication, in LED or LCD numerics, of thickness in absolute terms of millimetres. These equipments are easy to use but require prudence in their application.

Advantages of Ultrasonic Flaw Detection:

• Thickness and lengths up to 30 ft can be tested.
• Position, size and type of defect can be determined.
• Instant test results.
• Portable.
• Extremely sensitive if required.
• Capable of being fully automated.
• Access to only one side necessary.
• No consumables.

Disadvantages of Ultrasonic Flaw Detection:

• Indications require interpretation (except for digital wall thickness gauges).
• Considerable degree of skill necessary to obtain the fullest information from the test.
• Very thin sections can prove difficult.

source : www.insight-ndt.com