Nondestructive residual stress measurements in composites

The use of fiber reinforced composites as structural material continues to increase. One concern for structural composites is failure initiated by residual stresses. Therefore, nondestructive techniques must be developed to characterize the state of residual stress. The technique which demonstrates significant potential uses ultrasonic surface waves. To detect residual stresses, the velocity changes of ultrasonic Rayleigh surface waves were measured as a function of residual stress in graphite-epoxy composites. The samples were carefully prepared to obtain three different levels of compressive residual stress. The Rayleigh wave velocities were found to directly depend on the magnitude of the residual stress. In addition, Rayleigh wave velocity changes were found to vary as a direct function of the tensile residual stress induced in the near surface region of composite and epoxy samples that were prepared with slight variations in the cure process. The tensile residual stresses were induced by the thermal energy given off during the cure process, causing the near surface region to contract against a fully cured core of the thick composite and epoxy samples. With the completed work, the potential for using ultrasonic surface waves to characterize the state of residual stress in fiber reinforced composites has been demonstrated.