11.7 2-D PLANAR PWAS PHASED ARRAY STUDIES

In the previous section, we have explored the capabilities of 1-D linear arrays of PWAS transducers and have shown that they can successfully detect the presence of cracks by performing large area scanning and imaging with guided Lamb waves. However, we have also encountered some of major shortcomings of 1-D linear arrays, such as

1. the degradation of beamforming properties at angles close to 0 and 180

2. the half-plane mirror effect, which does not allow discriminating between a target placed above the array and a target placed below the array (Fig. 11.41).

Figure 11.41: 1-D linear array: (a) 1-D linear array along a straight line; (b) beamforming of an 8-PWAS linear array showing beamforming symmetric about the array itself.

We have also seen that various array optimization techniques can be employed to alleviate the first issue. However, the second issue is an inherent basic property of the 1-D linear array and cannot be overcome through array optimization. Because of its inherent geometrical limitation, the 1-D linear array beamforming is naturally symmetric about the array itself, as illustrated in Fig. 11.41b. The image in Fig. 11.41b shows a mainlobe directed at 60 , and also a mirrored lobe shows up symmetrically at 60 . This inherent mirror symmetry results in the inspection area being limited to, at most, 180 .