The reflection coefficient of a Rayleigh wave at a surface crack is measured in both the long wavelength and the short wavelength limits. from the long wavelength measurement, the maximum value of the stress intensity factor is evaluated. Using glass samples, we have formed surface cracks, measured their reflection coefficient, and predicted failure stress with an error of less than 5% from the measured fracture stress. In short wavelength limit, the reflection coefficient is measured versus frequency. Length and depth resonances of the crack are observed and used to estimate the crack geometry with an accuracy of 10-20% for EDM notches in steel.