Volume 3, Number 3		July 2004

 

 

FYI Practical Contact Ultrasonics - IIW Angle Beam Inspection

by Jim Houf* 

Figures 1-3
Figures 4-6

The most critical task prior to starting a contact angle beam ultrasonic inspection is the UT operator’s calibration of the UT equipment with respect to the specific part to be inspected. The term calibration in UT is applied to both annual scope calibration and on site or field calibration. Annual calibration determines that the scope meets horizontal and vertical linearity requirements and can be done in house by a Level III or externally by a qualified UT equipment supplier. On site calibration using the IIW block or derivatives, the focus of this article, is performed by the UT operator at the time of the inspection and consists of setting up the scope presentation and sensitivity of the inspection unit to perform a specific inspection.

The two most commonly used UT calibration methods in the US are based on either the International Institute of Welding (IIW) calibration block or a derivative that the American Society of Mechanical Engineers' Boiler and Pressure Vessel Code (ASME Code) refers to as a basic calibration block.

The IIW Block

IIW calibration blocks are 12 x 4 x 1 in. in size and are made of the same or acoustically similar material as the part to be inspected. Predominant features of an IIW block, Type 1 (Fig. 1), are two side drilled holes that are 0.06 and 2 in. in diameter and two notches. The first notch is a 0.06 in. deep curved notch with an inner radius 1 in. from the marked reference point and another 0.08 in. full width notch directly opposite the reference point. One end of the IIW block is cut to a 4 in. radius from the reference point. For illustration purposes, additional etched markings located on the long edge of the block have been shown on the same side or front of the block in Fig. 1.

The IIW block is designed to permit the operator to perform multiple functions for both straight and angle beam testing including distance and sensitivity calibration and wedge angle verification. For straight beam calibration, the transducer can be placed on the 1 in. surface at the reference point and the screen width can be set by using the 4 in. reflection from the opposite side. Block thickness can be used for 1 in. reflections. Because the reference point is directly opposite the 0.08 in. notch, resolution can also be determined.

Wedge Angle Verification

For angle beam testing, the wedge angle can be checked by placing the transducer on the 1 in. surface at point A in Fig. 2. In this example, a 70 degree point (shown on the side of the wedge) is placed over the 70 degree mark etched on the side of the block. The transducer is then moved back and forth until the return signal from the 2 in. diameter hole is maximized on the cathode ray tube (CRT) screen. The actual refracted angle can be read by determining where the exit point mark aligns with markings on the side of the block. When using 45 or 60 degree probes, the operator starts with the transducer over the corresponding mark on the block and checks the wedge angle in the same manner. It should be noted that most codes and specifications permit the wedge angle to vary slightly within ±2 degrees of the designated angle, but the the tolerance should be verified before continuing. If the wedge angle is within tolerance, the operator can proceed to distance calibration or setting screen width.

Distance Calibration

To set up a CRT screen width that represents the proper distance for the part being tested, the operator must determine the length of the sound path in that thickness of material, as was described in the previous article. Once the length of a full skip distance is calculated, the screen width can be set. In the following example we will set up a 10 in. screen.

Prior to starting the distance calibration, good operating procedure is to make sure the electrical zero, or main bang, is at or just off the left edge of the CRT screen. If not, it is possible that the operator will be working with the second reflection, which makes it impossible to calibrate the machine. A simple way to determine this is to dampen a finger with couplant and rub the bottom of the transducer face. The resulting signal can be set to the left side of the screen.

Once the operator is comfortable that the main bang is in the right place, the transducer is placed on the 1 in. block surface above the marked reference point (placement B, Fig. 2) and is aimed at the end of the block with the 4 in. radius. Using the range and delay controls (may be named differently on newer machines, see manual), maximize the signal from the 4 in. radius and set the reflector signal at the fourth major graticule on the CRT screen. Then turn the transducer around and maximize the return signal from the curved notch with the 1 in. radius. Set that signal at the first major graticule. For smaller diameter transducers, it may be necessary to move the transducer to the side of the 1 in. surface to get a good signal back from the radiused notch. Switching back and forth between the two transducer positions, the operator should continue to adjust the controls until both signals line up on the proper graticules. When this is accomplished, screen width is set to 10 in. with each major graticule representing 1 in. of sound path.

Sensitivity Calibration

Sensitivity calibration is done to provide an inspection reference level based on the amplitude (height) of a signal from a reflector of a known size. On the IIW block, that signal is generated from the 0.06 in. side drilled hole. To set sensitivity, the block is turned over and the transducer is placed on the 1 in. surface inboard of the 0.06 in. side drilled hole (placement C, Fig. 2). The transducer is moved back and forth until the signal from the hole is maximized on the CRT screen. Using the gain control, the signal amplitude is then adjusted so that the maximized signal is set at 80 percent of full screen height (FSH). The amount of gain in decibels (dB) is recorded and this gain value becomes the reference level for inspection. Note that 80 percent FSH is commonly used, but some codes and specifications may require other FSH values.

Once the UT system has been calibrated, the operator can increase the gain setting to the scanning level (dB value) dictated by the governing code or specification and perform the inspection.

Alternative Calibration Blocks

While the IIW block is a very good calibration block, its large size and heavy weight are inconvenient when carried in the field or when working out of position and up in the air. Several other calibration blocks have been designed that are smaller and lighter in weight.

Distance Sensitivity Calibration Block.  The most commonly used alternative block is the distance sensitivity calibration (DSC) block (Fig. 3). The DSC block measures 4 x 1 x 2.5 in.  which is considerably smaller and lighter than the IIW block and will fit into a pocket. This block has a flat scanning surface with a 1 in. radius at one end and a 3 in. radius at the other end. The 3 in. end of the block has a machined 0.375 in. deep, 0.031 in. wide flat bottomed notch with a 2.625 in. radius from the reference point. Manufactured commercially, these blocks can be purchased from many UT suppliers.

When using a block that has a radius on both ends, it is important to remember that most of a sound beam will reflect from an interface, so when the sound reflects from the 1 in. radius, most of the sound returning towards the transducer will reflect from the scanning surface and travel down towards the 3 in. radius. This sound will then return to the scanning surface but since it hits the scanning surface at the wrong angle to enter the probe , it reflects downward towards the 1 in. radius and then returns to the transducer, creating a second signal. As a result, it is important to note that the distance between the back wall signals is the sum of the distances from the radii to the reference point and the second back wall (and all others) will show on the screen at 4 in., the sum of 3 + 1 in., after the preceding back wall.

To perform a distance calibration for a 5 in. screen, the transducer is placed at the reference point and aimed at the end with the 1 in. radius. The first return signal is maximized and placed on the second major graticule. Two additional signals should be on the screen; a short signal from the notch and a taller signal from the 3 in. radius. The signal from the 3 in. radius should be placed on the tenth major graticule and using the delay and range controls, the 1 in. and 3 in. signals should be adjusted until both fall on the proper graticule. When this is done, the scope should be set for a 5 in. screen. The signal locations are represented by the green signals shown on the 5 in. screen (Fig. 4a). To confirm calibration, the transducer is reversed and aimed at the 3 in. radius. If calibration is correct, the first signal on the screen will be the notch (just past the fifth graticule) and the next will be the 3 in. radius signal (sixth graticule). These positions are shown by the signals shown in purple on the 5 in. screen presentation (Fig. 4a). As mentioned above, since the signal from the 1 in. radius occurs 4 in. later, it would come up at 7 in., which cannot be seen on a 5 in. screen.

To perform a distance calibration for a 10 in. screen, the transducer is again placed at the reference point and aimed at the end with the 1 in. radius, but this time the first return signal is maximized and placed on the first major graticule. Because the distance between back walls is the sum of the radii, the operator should also see back wall signals at 5 in. and 9 in., with notch signals 0.375 in. before each back wall signal, shown in green in the 10 in. screen presentation (Fig. 4b). Again, the delay and range controls should be used to position the back wall signals in the proper places. To verify calibration, the transducer is again reversed and aimed at the 3 in. radius. The operator should then see back wall signals at 3 in. and 7 in., with notch signals slightly before each back wall signal (signals shown in purple on 10 in. screen presentation in Fig. 4b).

In either calibration, if the second set of signals (shown in purple) do not come up on the screen where they should be, the operator should use the delay control to determine that the main bang is where it should be (at or off left side of screen). If not, and if a first back wall is at that location, move the main bang to the left edge of the screen and start over.

Sensitivity calibration using a DSC block is performed in the same manner for either a 5 or 10 in. screen. Once screen width has been set, the transducer is aimed at the 3 in. radius and the signal from the notch is maximized and set to 80 percent full screen height (or as detailed in governing documents). The gain setting for this signal amplitude is used as the reference level for the inspections. When using a 10 in. screen, there will be two notch signals, at screen locations of 2.625 in. and 5.25 in. The 2.625 in. signal should be set to 80 percent FSH (or as required), and if the operator is permitted to do so, peaks of the two signals can be connected to create a rudimentary distance amplitude correction (DAC) curve.

There is a correlation between the IIW block and the DSC block. The signal amplitude from the notch of an accurate DSC block should be within ± 2 dB of the signal created by the 0.06 in. side drilled hole in the IIW block, and this should be checked at regular intervals.

Distance Calibration Block.  The distance calibration (DC) block (Fig. 5a), often called a 1-2 block, is similar in shape to the DSC block but has a 1 in. radius and a 2 in. radius. It does not have a notch for sensitivity calibration. For this reason, distance calibrations can be performed but the operator must carry a separate block to set the sensitivity level of the equipment

Half Round Distance Calibration Block.  Another DC block is the half round block (Fig. 5b). Like the 1-2 block, it can be used for distance calibration but a separate sensitivity block is required. The advantages to the half round block are that that they can be readily manufactured by any machine shop and, by using various radii, very narrow screen widths can be set up.

IIW Hit Block.  The IIW hit block is a small portable sensitivity calibration block that is a 1 in. thick piece of material representing the corner of the IIW block that contains the 0.06 in. side drilled hole. The block is 4 x 2 x 1 in. with an 0.06 in. side drilled hole 0.6 in. down from the 1 in. scanning surface and 1.4 in. in from the end of the block (Fig. 6). By using the hit block and a DC block mentioned earlier, calibration for both distance and sensitivity is achieved. Distance is done as mentioned above, and sensitivity is done using the hit block in the same manner as is done on a full size IIW block.

 

*Jim Houf is Senior Manager of ASNT’s Technical Services Department and administers all ASNT certification programs.  (800) 222-2768 X212, (614) 274-6899 fax, <jhouf@asnt.org>.

 

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