This is the eigth chapter of an eleven part article on Ultrasonics by John Drury, the Author of Ultrasonic Flaw Detection for Technicians. This article was first published in INSIGHT magazine throughout 2004/5. The chapters can be downloaded in PDF for you to build into a complete series.

To access the other chapters please use the navigation at the bottom of this page.

J.C. Drury ' BACK TO BASICS - ULTRASONICS'

Chapter8.
CALIBRATION & REFERENCE STANDARDS

As soon as one starts to carry out practical ultrasonics , it becomes apparent that neither the vertical nor the horizontal scales of the display have any absolute meaning per se. The horizontal scale can be ad­ justed to represent a great variety of different time intervals, and these, for a given material and velocity, can be translated into depth values. The vertical scale gives an indication of the amplitude of signal being detected, provided you know how much ‘Gain’ you are using, but it does not necessarily tell you much about the size of defect causing that reflection. The safest way to get more information about the specimen from the display is to compare signals from the specimen with those from specially machined blocks. These blocks we normally classify under one of two headings, depending on the function of the block.

The term ‘Calibration Block’ is defined in British Standard BS 2704 as: - "A piece of material of specified composition, heat treatment, geometric form, and surface finish, by means of which ultrasonic equipment can be assessed and calibrated for the examination of material of the same general composition." Therefore, a calibration block may be a simple step wedge in a particular material to allow the timebase to be calibrated for accurate thickness measurement, or it may be a more complex block like the A.2 block described in BS 2704 which allows calibration of timebase, plus calibration of probe index, angle, resolution etc.

The second heading, ‘Reference Block’ is defined in BS2704 as: - "An aid to interpretation in the form of a test piece of the same material, significant dimensions and shape as a particular ob­ ject under examination, but not necessarily containing natural or artificial defec­ ts". So, for example, a section of an aircraft wing forging may be prepared as a reference block so that a technician may become familiar with the standard signal patterns from the various changes in section and more easily recognise a defect quickly when examining the component on an aircraft. More usually, the block would contain artificial defects from which the sensitivity (gain) used in the test could be set.

In either case, the use of the block ensures that there is adequate timebase to display the reflecting surfaces that are of interest and that the test is carried out at a reproducible sensitivity

CALIBRATION BLOCKS

The BS 2704 A.2 Calibration Block, also known as the In­ ternational Institute of Welding (I.I.W.) block , or ‘V1 block’, is illustrated in Fig.1.The block can be used for the following assessments : -

- Calibration of the timebase in terms of thickness. - Assessment of Dead Zone.

- Checking linearity of the timebase. - Checking linearity of the Amplifier.

- Assessing overall sensitivity of probe and amplifier. - Checking Resolution.

- Determination of the angle of refraction. - Determination of probe index

- Determination of Beam Characteristics. - Finding the correct Zero Point.

Fig. 1

The A.2 block was derived from the original ‘Dutch Block’ designed by RTD Rotterdam and accepted by IIW as the ‘IIW V1 Block’. In its original form, the deep slot at the center of the 100mm radius was a scribed line and a 25mm radius slot was positioned as shown in Figure 2. This design is still used in some parts of the world, and has the advantage that shear waves can be calibrated for ranges other than multiples of 100mm. In all other respects it is the same as the A.2 block.

Fig. 2

The BS 2704 A.4 Calibration block, also known as the 'V2 block', is a more compact form of the 'V1 block' suitable for site use, although somewhat less versatile in its functions. Figure 3 illustrates the A.4 block.

Fig. 3

The Institute of Welding (I.O.W.) Beam Profile calibration block is designed primarily for beam profile measurement and has four 1.5mm diameter side drilled holes giving eight depths from two scanning surfaces. These can be examined by direct scan for probes of various angles, and at several more ranges for each probe, using in­direct scans by reflecting from the far surface. There are two series of five holes on an inclined axis to measure shear wave probe resolution and to simulate an inclined discontinuity. The block is illustrated in Figure 4.

Fig. 4

REFERENCE BLOCKS

Area / Distance reference blocks are mainly used for setting sensitivity levels and accept/ reject levels for defect sizes by reference to echo height. Blocks are produced in a range of scanning depths and each set of blocks contains the same diameter flat- bottomed hole in each block. There are three sets of blocks, a set with 3/64” diameter flat-bottomed holes, a set with 5/64" diameter holes and one with 8/64" diameter holes. The scanning depths can range from ½” to 22”, but a t shop floor level, you would only have the few blocks appropriate to your range of work. Figure 5 shows a typical block, in this case a 3 x 5 block (3" scan depth, 5/64" flat bottomed hole).

	A = Scan depth B = Hole diameter

Distance/Amplitude Correction (DAC) reference blocks are made from the same thickness and grade of material as the work piece. They contain an artificial flaw (a side-drilled hole). The change of echo height with changes in scanning distance (multiple skips) is noted and plotted on the display as a "DAC" curve so that a signal amplitude can be specified to cover all depths within the working range for reporting, acceptance, or rejection purposes. Figure 6 shows a typical ASME DAC block and Figure 7 shows a DAC curve.

Fig. 6

Fig. 7

Figure 8 shows an example of a reference block for the examination of a lug in a light alloy structure such as an aircraft fitting. It is made from the same material as the actual part to be inspected, will have the same surface finish and, in this case contains an artificial defect to aid the setting of sensitivity and to help with identification of signals during interpretation.

Reference: -

‘Ultrasonic Flaw Detection for Technicians’ - Third Edition, June 2004 by J. C. Drury