ARTICLES RESEARCH & TECHNOLOGY

Ultrasonic Test Methods, Sensors, Transducers and Techniques

ultrasonic-test-sensors

CHAPTER 3


Basic Ultrasonic Test Methods

Ultrasonic testing is a very versatile inspection method, and inspections can be accomplished in a number of different ways. Ultrasonic inspection techniques are commonly divided into three primary classifications. Pulse-echo and through Transmission (Relates to whether reflected or transmitted energy is used) Normal Beam and Angle Beam (Relates to the angle that the sound energy enters the test article) Contact and Immersion (Relates to the method of coupling the transducer to the test article).

Through Transmission Method

Two transducers located on opposing sides of the test specimen are used.  One transducer acts as a transmitter, the other as a receiver. Discontinuities in the sound path will result in a partial or total loss of sound being transmitted and be indicated by a decrease in the received signal amplitude. Through transmission is useful in detecting discontinuities that are not good reflectors, and when signal strength is weak. It does not provide depth information.

digital-display-material-thickness

digital-display-discontinuity

Figure3.2 Digital display showing loss of received signal due to presence of discontinuity in sound field

Pulse Echo Method

In pulse-echo testing, a transducer sends out a pulse of energy and the same or a second transducer listens for reflected energy (an echo). Reflections occur due to the presence of discontinuities and the surfaces of the test article. The amount of reflected sound energy is displayed versus time, which provides the inspector information about the size and the location of features that reflect the sound.

digital-display-sound-reflecting

Figure3.3 Digital display showing signal generated from sound reflecting from back surface

digital-display-reflector-miday

Figure3.4 Digital display showing the presence of reflector in midway through material with amplitude back surface reflector.

The pulse-echo technique allows testing when access to only one side of the material is possible, and it allows the location of reflectors to be precisely determined.

Resonance Method

A condition of resonance exists whenever the thickness of a material equals half the wavelength of sound or any multiple thereof in that material. Control of wavelength in ultrasonic is achieved by control of frequency. If we have a transmitter with variable frequency control, it can be tuned to create a condition of resonance for the thickness of plate under test.

This condition of resonance is easily recognized by the increase of received pulse amplitude. Knowing the resonance or fundamental frequency f and velocity V of ultrasound in the specimen the thickness of the specimen under test can be calculated.

equation-3-1

Where,

t = thickness
V = velocity
f = fundamental frequency

Sensors

Ultrasonic Probe Construction

An ultrasonic probe consists of:

(i) A piezoelectric transducer.
(ii) A backing material.
(iii) A matching transformer which matches the piezoelectric transducer electrical impedance to that of the cable to the flaw detector, to transfer maximum energy from the cable to the transducer and vice versa.
(iv) A case which is simply a holder of suitable dimensions and construction.

Piezoelectric transducer

An ultrasonic probe is generally excited by a voltage pulse of less than 10-microsecond duration. A short voltage pulse consists of a band of frequencies. Among these frequencies, the transducer vibrates with maximum amplitude at the frequency known as the resonance frequency of the transducer.

Types of Ultrasonic Probes

Transducers are manufactured in a variety of forms, shapes and sizes for varying applications. In selecting a transducer for a given application, it is important to choose the desired frequency, bandwidth, size, and in some cases focusing which optimizes the inspection capabilities.

types-ultrasonic-probes

Figure3.5 Showing types of ultrasonic probes

Contact Type Probes

Contact transducers are designed to withstand rigorous use, and usually have a wear plate on the bottom surface to protect the piezoelectric element from contact with the surface of the test article. Many incorporate ergonomic designs for ease of grip while scanning along the surface.

Contact transducers are available with two piezoelectric crystals in one housing .These transducers are called dual element transducers. One crystal acts as a transmitter, the other as a receiver. This arrangement improves near surface resolution because the second transducer does not need to complete a transmit function before listening for echoes.

contact-probe

Figure3.6 Contact probe

Dual elements are commonly employed in thickness gauging of thin materials. A way to improve near surface resolution with a single element transducer is through the use of a delay line. Delay line transducers have a plastic piece that is a sound path that provides a time delay between the sound generation and reception Interchangeable pieces make it possible to configure the transducer with insulating wear caps or flexible membranes that conform to rough surfaces. Common applications include thickness gauging and high-temperature measurements.

delay-line-wear-cap

Figure3.7 Showing membrane, delay line and wear cap

Angle beam transducers incorporate wedges to introduce a refracted shear wave into a material. The incident wedge angle is used with the material velocity to determine the desired refracted shear wave according to Snell’s Law). Transducers can use fixed or variable wedge angles. The common application is in weld examination.

angle-beam-probe

Figure3.8 Angle beam probe

Immersion Type Probe

Immersion transducers are designed to transmit sound whereby the transducer and test specimen are immersed in a liquid coupling medium (usually water). Immersion transducers are manufactured with planar, cylindrical or spherical acoustic lenses (focusing lens).

immersion-type-probe

Figure3.9 Immersion type probe

Pulse Echo Type Ultrasonic Flaw Detector

Construction and mode of operation of a pulse echo type flaw detector

Functions of the electronic elements

The cathode ray tube contains a heater coil H which heats the cathode C to make it emit electrons. These electrons are accelerated by a voltage applied between the cathode C and anode A. The resultant electron beam is focused by the focusing cylinder F to make it appear on the fluorescent screen S as a spot. As the electrons travel towards the CRT screen S they pass two pairs of deflecting plates X and Y.

A voltage applied to the X-plates deflects the electron beam horizontally while a voltage applied to the Y-plates deflects the beam vertically. The time-base generator provides a saw tooth voltage to the X-plates of CRT to move the electron beam spot from left to right across the CRT screen with a uniform speed. The speed of the spot depends on the operating time {i.e. the time in which the saw tooth voltage rises from zero to its maximum value) of the saw tooth voltage.

cathode-ray-tube

Figure3.10 Cathode Ray Tube (CRT)

principle-ultrasonic-system

Figure3.11 Working principle of an ultrasonic system

The shorter the operating time the greater is the speed of the spot. The control provided for the adjustment of the saw tooth voltage operating time and hence the speed of the spot is termed the depth range or test range control. To prevent the return of the electron beam spot to produce a trace on the screen, the time base generator simultaneously controls the brightness of the spot by means of a square wave voltage so that the spot remains bright only during the operating time of the sawtooth voltage.

Sometimes it is necessary to delay the pulse which triggers the time base (see the clock or timer below) compared to the pulse which triggers the transmitter. For this purpose, a control is provided. The control provided for this purpose is usually termed the delay control. The transmitter supplies a short electrical voltage pulse of 300 – 1000 V to the piezoelectric transducer in the probe.

The piezoelectric transducer, in turn, converts this electrical voltage pulse into an ultrasonic wave. In some instruments, controls are provided to adjust the frequency and amplitude of the electrical voltage pulse, while in others this adjustment is done automatically. The frequency and width of the ultrasonic wave pulse are controlled respectively by the thickness and degree of damping of the piezoelectric transducer in the probe.

The receiver unit consists of an amplifier, a rectifier, and an attenuator. The amplifier amplifies any voltage pulse supplied to it from the probe. The amplification is of the order of about 10. In most instruments the amplifier is of a wide band type with a frequency range of about 1 to 15 MHZ and a control to tune the amplifier to the frequency of the probe is not needed. In some it is a narrow band type and a control is provided to tune it to the frequency of the probe. The rectifier in the receiver unit rectifies the voltage signal for ease of observation. In some

In some it is a narrow band type and a control is provided to tune it to the frequency of the probe. The rectifier in the receiver unit rectifies the voltage signal for ease of observation. In some instruments, a control is provided to observe the received signal either in the rectified or unrectified state. The attenuator in the receiver unit is used to vary the signal amplitude as needed. The control provided to do this is known as the gain control and is calibrated in decibels or dB.

The clock or time circuit generates electrical pulses which trigger the time base generator and transmitter at the same time. These pulses are generated repeatedly to make the trace on the CRT screen steady and bright. The frequency with which these pulses are generated is known as the pulse repetition frequency (PRF). In some instruments, a control is provided for the adjustment of PRF while in others it is adjusted automatically with alteration in setting of the test range control.

A control known as ‘reject’ or “suppression” control is provided in the receiver unit to remove the indications of random noise, known as grass, from the CRT display. This control should not be used unless its effect on the vertical linearity is known and allowed for in any subsequent evaluation of a flaw based on reflectivity.

Operation of a pulse echo type flaw detector

The simultaneous triggering of the time-base generator and transmitter by the clock, initiates an ultrasonic pulse from the probe at the same time as the electron beam spot starts to move across the cathode ray tube. When a single crystal probe is used, the electrical voltage pulse supplied by the transmitter to the probe is also fed to the receiver unit and is thus amplified and displayed as indication ‘a’ on the CRT screen.

The indication, ‘a’ is known as the “transmission echo”, “transmission pulse”, “initial pulse “or “main bang “. The electron beam spot continues to move across the CRT screen as the ultrasound from the probe moves through the specimen.

When the ultrasound reaches the reflecting surface b, a part of it is reflected via the probe and receiver unit to register an indication l b’ on the CRT screen. The other part which is carried to the far surface ‘C’ of the test specimen is reflected by it to be displayed as indication 1C’ on the CRT screen. The indication from the reflecting surface ‘b’ and the far surface or backwall 1C’ of the specimen are known as the ‘defect echo’ and back-wall echo “or ” bottom echo ” respectively.

The pulse repetition frequency (PRF) should, therefore, be high enough to make the pattern bright enough to be visible to the human eye. On the other hand for a 500 mm thick specimen, the time required for the whole operation to complete is about 6oo microsecond. If a high repetition frequency is used in this case, confusion would arise because the probe will send a second ultrasonic pulse before the first one is received.

Depending upon the thickness of the test specimen, in most instruments, the PRF can be varied between 50 pulse per second (PPS) to 1250 PPS. This is done automatically in the modern instruments with the setting of the testing range control.

In twin crystal and transverse wave probes there is a perspex delay block fitted between the piezoelectric transducer and the surface of the specimen. The ultrasonic waves travel for some time before entering the test specimen. To stop the electron beam spot traveling a distance proportional to this traveling time in the Perspex delay block, the delay control is used. The delay control makes the time – base generator wait for a period equal to the perspex traveling time before making the spot start from the zero position.

Scan presentation

Information from ultrasonic testing can be presented in a number of differing formats.
Three of the more common formats include:

A-scan
B-scan
C-scan

Each presentation mode provides a different way of looking at and evaluating the region of material being inspected. Modern computerized ultrasonic scanning systems can display data in all three presentation forms simultaneously.

A-scan presentation

The A-scan presentation displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and the elapsed time (which may be related to the sound energy travel time within the material) is displayed along the horizontal axis.

Most instruments with an A-scan display allow the signal to be displayed in its natural radio frequency form (RF), as a fully rectified RF signal, or as either the positive or negative half of the RF signal. In the A-scan presentation, relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector. Reflector depth can be determined by the position of the signal on the horizontal sweep.
scan-presentation

Figure3.12 A-scan presentation

B-scan presentation

It is a graphical presentation method of the results of a series of thickness measurements that shows, in scale, the cross-section of the component or the inspected part. The B-Scan thicknesses tracking allows the location, identification, and sizing of thickness losses and cavities, providing a permanent record using a printer or a video tape, and a mapping by means of planned view sketches.

b-scan-presentation

Figure3.13 B-scan presentation  

C-scan presentation

The C-scan presentation provides a plan-type view of the location and size of test specimen features. The plane of the image is parallel to the scan pattern of the transducer. C-scan presentations are produced with an automated data acquisition system, such as a computer controlled immersion scanning system.

Typically, a data collection gate is established on the A-scan and the amplitude or the time-of-flight of the signal is recorded at regular intervals as the transducer is scanned over the test piece. The relative signal amplitude or the time-of-flight is displayed as a shade of gray or a color for each of the positions where data was recorded. The C-scan presentation provides an image of the features that reflect and scatter the sound within and on the surfaces of the test piece.

Use of A-scan in conjunction with C-scan is necessary when depth determination is desired.

c-scan-presentation

Calibration of the test system

Calibration is an operation of configuring the ultrasonic test equipment to known values. This provides the inspector with a means of comparing test signals to known measurements.

Calibration standards come in a wide variety of material types, and configurations due to the diversity of inspection applications. Calibration standards are typically manufactured from materials of the same acoustic properties as those of the test articles. The following slides provide examples of specific types of standards.

Thickness calibration standards may be flat or curved for pipe and tubing applications, consisting of simple variations in material thickness. Distance/Area Amplitude standards utilize flat bottom holes or side drilled holes to establish known reflector size with changes in the sound path from the entry surface. There are also calibration standards for use in angle beam inspections when flaws are not parallel to entry surface.

These standards utilized side drilled holes, notches, and geometric configuration to establish time distance and amplitude relationships.

stander-test-blocks

Figure3.15 Stander test blocks

Qualification standards differ from calibration standards in that their use is for purposes of varying proper equipment operation and qualification of equipment use for specific codes and standards.

Calibration methods

Calibration refers to the act of evaluating and adjusting the precision and accuracy of measurement equipment. In ultrasonic testing, several forms of calibrations must occur. First, the electronics of the equipment must be calibrated to ensure that they are performing as designed.

This operation is usually performed by the equipment manufacturer and will not be discussed further in this material. It is also usually necessary for the operator to perform a “user calibration” of the equipment. This user calibration is necessary because most ultrasonic equipment can be reconfigured for use in a large variety of applications. The user must “calibrate” the system, which includes the equipment settings, the transducer, and the test setup, to validate that the desired level of precision and accuracy are achieved.

reference-block-calibration

Figure3.16 Reference standards blocks for calibration

In ultrasonic testing, reference standards are used to establish a general level of consistency in measurements and to help interpret and quantify the information contained in the received signal. The figure shows some of the most commonly used reference standards for the calibration of ultrasonic equipment.

Reference standards are used to validate that the equipment and the setup provide similar results from one day to the next and that similar results are produced by different systems. Reference standards also help the inspector to estimate the size of flaws.

In a pulse-echo type setup, signal strength depends on both the size of the flaw and the distance between the flaw and the transducer. The inspector can use a reference standard with an artificially induced flaw of known size and at approximately the same distance away for the transducer to produce a signal. By comparing the signal from the reference standard to that received from the actual flaw, the inspector can estimate the flaw size.

The material of the reference standard should be the same as the material being inspected and the artificially induced flaw should closely resemble that of the actual flaw. This second requirement is a major limitation of most standard reference samples. Most use drilled holes and notches that do not closely represent real flaws. In most

In most cases the artificially induced defects in reference standards are better reflectors of sound energy (due to their flatter and smoother surfaces) and produce indications that are larger than those that a similar sized flaw would produce. Producing more “realistic” defects is cost prohibitive in most cases and, therefore, the inspector can only make an estimate of the flaw size.

Reference standards are mainly used to calibrate instruments prior to performing the inspection and, in general, they are also useful for:

1. Checking the performance of both angle-beam and normal beam transducers (sensitivity, resolution, beam spread, etc.).
2. Determining the sound beam exit point of angle beam transducers
3. Determining the refracted angle produced
4. Calibrating the sound path distance
5. Evaluating instrument performance (time base, linearity, etc.)

Introduction to Some of the Common Standards

IIW Type US-1 Calibration Block

This block is a general purpose calibration block that can be used for calibrating angle-beam transducers as well as normal beam transducers. The material from which IIW blocks are prepared is specified as killed, open-hearth or electric furnace, low-carbon steel in the normalized condition and with a grain size of McQuaid-Ehn No. 8. Official IIW blocks are dimensioned in the metric system of units.

The block has several features that facilitate checking and calibrating many of the parameters and functions of the transducer as well as the instrument where that includes; an angle-beam exit (index) point, beam angle, beam spared, time base, linearity, resolution, dead zone, sensitivity, and range setting.

clibration-block

Figure3.17IIW Type US-1 Calibration Block

Miniature Angle-Beam Calibration Block

The miniature angle-beam block is used in a somewhat similar manner as the as the IIW block, but is smaller and lighter. The miniature angle-beam block is primarily used in the field for checking the characteristics of angle-beam transducers.

With the miniature block, beam angle and exit point can be checked for an angle-beam transducer. Both the 25 and 50 mm radius surfaces provide ways for checking the location of the exit point of the transducer and for calibrating the time base of the instrument in terms of metal distance. The small hole provides a reflector for checking beam angle and for setting the instrument gain.

miniature-angle-beam-clibration-block

Figure3.18 Miniature Angle-Beam Calibration Block