African Fusion March 2015

Parameter Frequency

Design Value

erate an effective shear wave within the range of the total refraction angle. This study considered the probe design parameters indi- vidually with the aim of designing a shear wave phased array ultrasonic probe for socket weld inspection. First, the frequen- cy was set at 3.5 Mhz, which is suitable for general stainless steel inspection, and the number of piezoelectric elements was set at 16. Efforts to minimise the pitch of the probe and radius mode transmitted along a vibrator of the probe were made in this study. As a consequence, each piezoelectric ele- ment, when vibrating, can be independent from surrounding elements. Therefore, itwas designed togenerate a strong shear wave at the edge of the piezoelectric element. Furthermore, the piezoelectric element was designed to be smaller than the wavelength so as to vibrate as a single point source. The parameters of the selected shear wave phased array ultrasonic probe are shown in Table 1. When the probe is manufactured using the selected pa- rameters, the side lobe or grating lobe should be eliminated or minimised at the used inspection angle. The grating lobe occurring in the phased array ultrasonic probe is caused by constructive interference made by sur- rounding piezoelectric elements that have identical phases and different time delays. In other words, this happens at an angle where delays match each other periodically and when signals are inphase at a refraction angle. Inparticular, a grating lobe whose beam is wide and highly sensitive is an important factor that shouldbe considered in the phased array ultrasonic probe design. Figure 2 shows the directivity for each angle calculated based on parameters in Table 1. The figure indicates that there is interference with themain beamat each refraction angle or in the absence of an adjacent sound beam. Relatively small side lobes were expected not to have an effect on the inspec- tion signal, although they are present onboth sides of themain beam. As seen in the directivity plot, as the angle increases, the size of the main beam decreases and the angle at which the main beam is generated becomes wide. The directivity illustrated in Figure 2 is the result of concentrating at a depth of 40 mm from the probe surface. Manual encoded scanner As small-bore piping socket welds are thin and the weld leg widens the gap between the probe index point and the inspection region of interest, the beam arrives at a distance of approximately two to three skips, instead of one skip. As a result, multiple reflection results in a complex signal, thus making it difficult to clearly investigate flaws with a real-time S-scan. Furthermore, in a narrow inspection space, the test instrument should be checked and inspection should be made simultaneously while maintaining contact between the small phased array ultrasonic probe and the small bore piping. Therefore, an inspector cannot concentrate on evalu- ation, which leads to low reliability of the inspection. To solve these problems, this study developed a ring-type scanner with which the probe could be fixed and clamped to the external diameter of small-bore piping. The scanner is clamped to piping in the form of two connected semicircles with an angle of 180 ° , and it adjusts the contact force of the probe and revolves in the circumferential direction. A micro-encoder that transmits the circumferential location of the probe to the phased array ultrasonic test instrument

3.5 MHz

No. of elements in primary axis

16

Primary axis pitch

0.3 mm 0.1 mm 7.0 mm 9.5 mm

Inter element spacing Width of element Total active length

Table1: Phased array probe specification.

Figure 2: Directivity plot for steering angle at 40 mm in front of Elements 4: Experiment Apparatus and Method.

PA probe Probe fixture

Encoder

Tension adjuster Figure 3: Hand driving manual encoded scanner for small-bore socket weld piping.

was attached. Figure 3 displays the manufactured manual scanner developed in this study with its design.

Flawed specimen In this study, flawed spec- imensweredesignedand manufactured todevelop the inspection technique. The material of the fa- tigue crack specimen is 304 stainless steel and two specimens with leg length of 1:1 and 1:2were designed and manufac- tured with socket weld piping with a 1.0″ nomi- nal diameter. Figure 4 shows themanufactured flawed socket weld specimens.

Figure 4: Photo of manufactured flawed socket weld specimens.

For the flawed specimens, the typically occurring types of flaw in the socket welds were analysed. Three flaws were de- signed to be included into one specimen for the flawed speci-

19

March 2015

AFRICAN FUSION