SIGNAL PRESENTATION

4.3.1. Echo amplitude and its control

A convenient way of measuring changes in amplification or attenuation of ultrasonic waves is in terms of decibel (dB). A decibel is l/l0 th of a bel which is a unit based on logarithms to the base 10. In physics, the term “power” is used to mean the rate at which work is done. If two powers are taken as ‘P1’ and ‘P2’, they are said to differ by ‘n’ bels when

𝑃1/𝑃2 = 10 or

𝑛 = log 𝑃1/𝑃2 bels

Hence, since 1 bel is equal to 10 decibels the above equation can be rewritten as:

n = 10 log P1/P2 decibels

Acoustical power is proportional to the square of the amplitude and hence for comparison of amplitudes Al and A2, we have:

(4.1) n = 10 log(A1/A2)

As the amplitudes of the reflected ultrasonic waves are proportional to the heights of the echoes displayed on the CRT screen, therefore Equation 4.1 can be modified as:

(4.2) 𝑛 = 20 log 𝐻1/𝐻2 𝑑𝐵

Where,

H1 = echo height proportional to ultrasonic wave amplitude A1 H2 = echo height proportional to ultrasonic wave amplitude A2 The advantages of decibel unit are:

(a) large echo height ratios can be given in small figures, e.g.

1000 : 1 = 60 dB 1000,000 : 1 = 120 dB

(b) a reversal of the echo height ratios only requires a change of sign, e.g.

100,000 : 1 = +80 dB , and 1 : 100,000 = –80 dB

(c) multiplication of the echo height ratios corresponds to the simple addition of a dB value, e.g.

gain factor 2 +6 dB gain factor 10 +20 dB gain factor 100 +40 dB 4.3.2. A-scan presentation

The most commonly used presentation is the A-scan presentation. In this presentation, the horizontal line on the screen indicates the elapsed time and the vertical deflection shows the echo amplitude. From the location and amplitude of the echo on the screen the depth of the flaw in the material and an estimate of the size of the flaw can be made. A typical A-scan system is shown in Figure 4.15.

FIG. 4.15. A-scan presentation (Basic display).

4.3.3. B-scan presentation

B-scan display is a plot of time versus distance, in which one orthogonal axis on the display corresponds to elapsed time, while the other axis represents the position of the transducer along a line on the surface of the testpiece relative to the position of the transducer at the start of the inspection. Echo intensity is not measured directly as it is in A-scan inspection, but is often indicated semiquantitatively by the relative brightness of echo indications on an oscilloscope screen. A B-scan display can be likened to an imaginary cross section through the testpiece where both front and back surfaces are shown in profile. Indications from reflecting interfaces within the testpiece are also shown in profile, and the position, orientation, and depth of such interfaces along the imaginary cutting plane are revealed.

4.3.3.1.System Setup

A typical B-scan system is shown in Figure. 4.16. The system functions are identical to the A-scan system except for the following differences.

First, the display is generated on an oscilloscope screen that is composed of a long-persistence phosphor, that is, a phosphor that continues to fluoresce long after the means of excitation ceases to fall on the fluorescing area of the screen. This characteristic of the oscilloscope in a B-scan system allows the imaginary cross section to be viewed as a whole without having to resort to permanent imaging methods, such as photographs. (Photographic equipment, facsimil recorders, or x-y plotters can be used to record B-scan data, especially when a permanent record is desired for later reference.)

FIG. 4.16 Typical B-scan setup, including video-mode display, for basic pulse-echo ultrasonic inspection.

Second, the oscilloscope input for one axis of the display is provided by an electromechanical device that generates an electrical voltage or digital signals proportional to the position of the transducer relative to a reference point on the surface of the testpiece. Most B-scans are generated by scanning the search unit in a straight line across the surface of the testpiece at a uniform rate. One axis of the display, usually the horizontal axis, represents the distance traveled along this line.

Third, echoes are indicated by bright spots on the screen rather than by deflections of the time trace. The position of a bright spot along the axis orthogonal to the search-unit position axis, usually measured top to bottom on the screen, indicates the depth of the echo within the testpiece.

Finally, to ensure that echoes are recorded as bright spots, the echo-intensity signal from the receiver-amplifier is connected to the trace-brightness control on the oscilloscope. In some systems, the brightnesses corresponding to different values of echo intensity may exhibit enough contrast to enable semiquantitative appraisal of echo intensity, which is related to flaw size and shape.

4.3.3.2.Signal Display.

The oscilloscope screen in Figure.4.16 illustrates the type of video-mode display that is generated by B-scan equipment. On this screen, the internal flaw in the testpiece shown at left in Figure 4.16 is shown only as a profile view of its top reflecting surface. Portions of the testpiece that are behind this large reflecting surface are in shadow. The flaw length in the direction of search-unit travel is recorded, but the width (in a direction mutually perpendicular to the sound beam and the direction of search-unit travel) is not recorded except as it affects echo intensity and therefore echo-image brightness. Because the sound beam is slightly conical rather than truly cylindrical, flaws near the back surface of the testpiece appear longer than those near the front surface.

4.3.3.3.Applications.

The chief value of B-scan presentations is their ability to reveal the distribution of flaws in a part on a cross section of that part. Although B-scan techniques have been more widely used in medical applications than in industrial applications, B-scans can be used for the rapid screening of parts and for the selection of certain parts, or portions of certain parts, for more thorough inspection with A-scan techniques. Optimum results from B-scan techniques are generally obtained with small transducers and high frequencies.

4.3.4. C-scan presentation

C-scan display records echoes from the internal portions of testpieces as a function of the position of each reflecting interface within an area. Flaws are shown on a readout, superimposed on a plan view of the testpiece, and both flaw size (flaw area) and position within the plan view are recorded. Flaw depth normally is not recorded, although it can be measured semiquantitatively by restricting the range of depths within the testpiece that is covered in a given scan. With an increasing number of C-scan systems designed with on-board computers, other options in image processing and enhancement have become widely used in the presentation of flaw depth and the characterization of flaws. An example of a computer-processed C-scan image is shown in Figure 4.17, in which a graphite-epoxy sample with impact damage was examined using time-of-flight data. The depth of damage is displayed with a color scale in the original photograph.

FIG. 4.17.Time-of-flight C-scan image of impact damage in graphite-epoxy laminate supported by two beams (arrows).

4.3.4.1. System Setup

In a basic C-scan system, shown schematically in Figure 4.18, the search unit is moved over the surface of the testpiece in a search pattern. The search pattern may take many forms; for example, a series of closely spaced parallel lines, a fine rastor pattern, or a spiral pattern (polar scan). Mechanical linkage connects the search unit to x-axis and y-axis position indicators, which in turn feed position data to the x-y plotter or facsimile device. Echorecording systems vary; some produce a shaded-line scan with echo intensity recorded as a variation in line shading, while others indicate flaws by an absence of shading so thateachflaw shows up as a blank space on the display Figure 4.18.

FIG. 4.18. Typical C-scan setup, including display, for basic pulse-echo ultrasonic immersion inspection.

4.3.4.2.Gating

An electronic depth gate is another essential element in C-scan systems. A depth gate is an electronic circuit that allows only those echo signals that are received within a limited range of delay times following the initial pulse or interface echo to be admitted to the receiver-amplifier circuit. Usually, the depth gate is set so that front reflections and back reflections are just barely excluded from the display. Thus, only echoes from within the testpiece are recorded, except for echoes from thin layers adjacent to both surfaces of the testpiece. Depth gates are adjustable.

By setting a depth gate for a narrow range of delay times, echo signals from a thin slice of the testpiece parallel to the scanned surface can be recorded, with signals from other portions being excluded from the display.

Some C-scan systems, particularly automatic units, incorporate additional electronic gating circuits for marking, alarming, or charting. These gates can record or indicate information such as flaw depth or loss of back reflection, while the main display records an overall picture of flaw distribution.

4.4.RECORDING INSTRUMENTS 4.4.1. Automatic monitor

Many instruments are equipped with a monitor function which facilitates observing the flaw in the expectancy range. The start and end of the flaw expectancy range can thereby be marked by means of a step on the base line of the screen or an additionally displayed bar on the screen.

If now an echo appears within this range then this releases a visible and/or audible alarm signal.

The response threshold of the monitor is also variable so that an echo indication only releases the alarm when it has reached a certain height. This mode of operation is called “coincidence”

mode. Some systemse are fitted with monitor which can also operate in the “anticoincidence”

mode, i.e. an echo indication only releases the alarm when it has fallen below a certain threshold value. The anticoincedence mode is usually used for monitoring the backwall echo height. This monitoring enables the operator to check whether sufficient ultrasonic energy is being transmitted into the test specimen or not.

In addition to the monitor function, most of these instruments have a control output which can be used to further process the information. As soon as an echo appears within the monitor threshold a voltage is fed to the control output which is proportional to the echo height and which can be immediately used for automatic recording. By means of this monitor function together with a path pick-up which is fixed into the probe, C-scans of workpieces can be easily printed on an X-Y recorder.

4.4.2. Computer interfacing

The signal acquired through a conventional ultrasonic flaw detection system can only be processed by a computer if it is first converted into a digital signal. The digitization of the analogue signal is carried out in a special electronic circuit called the analogue-to-digital converter (ADC) which samples the incoming analogue signal at a fast rate of more than 20 MHz. This process of connecting the output of an ultrasonic flaw detector through an ADC to the computer is called as computer interfacing. The ADCs used in ultrasonic detection are either of successive approximation type or of flash type of 8 or 12 bit resolution and sample rate (or conversion rate) of 20 MHz or above.

ADCs of 8 to 12 bit resolution and a sampling rate of 200 MHz or more are available on boards which can be mounted into an empty slot provided in a personal computer. These boards will then digitize and store the output of the flaw detector which can then be processed by the computer using appropriate software packages.

4.4.3. Recorders, printers and colour markers 4.4.3.1.Recorders

Almost any type of recorder can be used in ultrasonic test systems. The type of recorder is usually related closely to the mechanical scanner used, since search unit position is often one or two of the variables to be recorded. Some of the pertinent advantages of various recorders follow.

Strip chart recorders

These recorders are usually the least expensive type. Chart paper flows through the recorder at a constant rate while a pen (or pens in a multi-channel recorder) moves back and forth across

the paper. They are useful where the search unit is carried over the test piece at a constant rate so that its position can be determined easily from the chart.

The helix-drum type recorder is commonly used to make C-scan recordings. In this recorder a chemically treated paper is passed between a bar and a drum with a helix of wire wrapped around it. The printing bar has a narrow edge and is connected to the output of an alarm or proportional output gate. The other electrical terminal is the helix. As the drum rotates, the contact point between it and the bar scans across the paper. Electric current passing through the paper between the bar and the helix darkens the paper. The scanning of the probe over the work piece can be directly coupled to the scan of the spot across the paper. At the end of each scan the search unit and the paper are both indexed along. C-scan recordings showing many shades of gray are possible, each shade representing different amplitude.

X-Y recorders

In an X-Y recorder the pen moves in two orthogonal directions and the paper remains stationary.

In many models the pen can be lifted from the paper automatically allowing its use as a C-scan recorder. Also in many models a constant scan speed along one axis can be chosen if desired.

Magnetic tape recorders

Tape recorders have been used for recording the A-scan display of the CRT screen directly, although the tape must be played back on another CRT device. However, this scheme does allow retention and re-examination of the signal pattern seen during an inspection.

4.4.3.2.Printers

Digital recorders are new to ultrasonic testing. They print rows of numbers on a paper tape rather than drawing lines. Each column or group of columns of numbers can record several different variables simultaneously. The numerical print out is often much easier to use than trying to read values from a scale on a strip chart. However, electrical data must be supplied to the recorder in a digital form. This means that analogue to digital converters must be used thus increasing the cost of the system, or the system must be designed to produce data in a digital form originally.

4.4.3.3.Colour markers

Colour markers are devices which are triggered by the monitor gates to mark the location of flaws on the test specimen surface. These devices are usually used in automatic inspection of plates, bars, billets, pipes, etc.

In immersion testing the location of flaws is either marked by using a pneumatically operated lipstick or by running a grinding belt which is pressed against the specimen at positions of the flaw.

5. CHAPTER 5. CALIBRATION OF THE TESTING SYSTEM