The impact resonance method involves applying a gentle impact to the outside surface of a structure and measuring the near field response. An impact hammer is used to mechanically excite the material under test, causing vibrations to propagate through the structure. Immediately after the impact, the vibrations are recorded by an acoustic broadband receiver. Information from both the time domain and the frequency domain are then analyzed to determine the material’s state. By observing the resonant frequency peaks, impact-induced residual noise, and signal level, damaged and undamaged composite materials can be differentiated.
Non-linear acoustic methods seek to determine how an ultrasonic waveform changes when it propagates through or over the surface region of a medium. These changes are directly related to the stress-strain relationship and the hysteretic properties of a material, and are not unduly affected by the ray path. In damaged materials, the stress-strain relationship does not obey Hooke’s Law of Elasticity; stress is not proportional to strain; it is not linear. In addition, these materials often have a stress-strain relationship that is non-symmetric, so the reaction to compressional forces will have different properties to that of tensile forces. This is a result of the cracks opening and closing under tensile or compressional loads.