Acoustic measurements, using longitudinal waves in plane specimens, based on the theory of acoustoelasticity, permit the determination of the sum of the principal stresses (σl + σ2). By automatic scanning, we are able to make such measurements throughout a region of interest. In this paper we shall be concerned with the application of this acoustoelastic stress analysis to fracture mechanics. Specifically, the energy release rates for extension and rotation of a crack will be determined experimentally (J integral for extension, L integral for rotation) followed by a numerical adjustment procedure which may be called the rescaling technique. If desired the stress intensity factors at a crack tip may also be evaluated. This procedure was applied to three different specimen configuratlons, and the results compare favorably with purely theoretical predictions.