Robustness of target strength of an immersed, hollow ceramic flotation sphere of non-constant shell thickness

The target strength of an immersed spherical shell made of alumina is a sensitive function of both frequency and shell thickness. Apparent variations in target strength with orientation may be explicable in terms of variable shell thickness, which has already been observed by micrometer measurement of shards from the same broken shell of nominal diameter 91.44 mm. The thickness appears to vary over the approximate range 1.25-1.4 mm. In this study, a shell of variable thickness is modeled as a single-layer set of finite elements. Computations are presented for the target strength as a function of orientation at three selected frequencies: 10, 18.4, and 20 kHz.