  Ultrasound has long been used as a non-destructive tool to test for the
brittle fracture of materials. Could it be used as a similar tool to test for
ductile failure? This study reports results of local measurements of the speed
of shear waves, $v_T$, in aluminum under standard testing conditions at two
different locations on the same sample, as a function of stress. There is a
clear change in $v_T$ at the Yield stress, consistent with a proliferation of
dislocations. This measurement provides a quantitative, continuous relation
between dislocation density and externally applied stress.
