The Use of Acoustoelastic Measurements to Characterize the Stress States in Cracked Solids

The theory of acoustoelasticity predicts that a plane longitudinal acoustic wave passing through a solid which is already in a deformed state will propagate with a velocity (v) which is different from the (v0) of the same wave propagating through the undeformed medium. It may be shown that Δv/v0 = (v-v0)/v0 = B(σ1+σ2) where σ1 and σ2 are the principal stress in the plane normal to the wave propagation direction and B is the acoustoelastic constant. Wave transit time measurements allow the relative velocity change Δv/v0 to be determined, so that contours of constant principal stress sum (σ1+σ2) may be mapped by acoustically scanning a stressed solid. We have used the technique described above to characterize the states of stress in cracked and notched aluminum panels. A method for extracting crack stress intensity factors from the acoustic data is proposed and illustrated for center-cracked panel specimens. The results indicate that the technique may offer a promising method for nondestructive testing and evaluation