Interpretation of side-mounted bender element results using phase shift and group velocity

The use of bender elements has become increasingly popular for the assessment of the small strain shear stiffness G(max). This may be attributed to the simplicity of the test (i.e. both test equipment and procedure) and the fact that it is non-destructive. The test yields results that have often been reported to compare well, qualitatively, with laboratory measurements using other test methods, although doubts over the accuracy of absolute values have remained, and continuing efforts have been made to improve both the bender element technique and the interpretation of its results. This paper provides a preliminary report of further developments. A continuous sine wave signal and low-frequency spectrum analyser has been used to obtain the phase velocity between applied and measured voltage of a conventional bender element configuration, over a wide range of frequency. Bender elements have also been installed on the side of the specimen, so that both group and phase velocities could be measured between sensors remote from both the source and from the rigid specimen ends. The results suggest that repeatable and consistent determinations of G(max) may be achieved by measuring the group velocity of a single-pulse sine wave between side-mounted bender elements.