Linear and non linear ultrasonics for material damage evaluation and health monitoring

In mechanics of material and material sciences, the early detection of damage, its non destructive characterisation and monitoring is of great importance. In this field, ultrasonic waves can be very helpful especially when nonlinear ultrasonics is considered. In this work, linear and non linear ultrasonics are used for damage characterisation. Concerning linear ultrasonics, we show that velocity and linear elastic constants are good damage indicators for composites materials. For these materials, ultrasonic velocity is measured in situ during a tensile test, at the same time acoustic emission is collected. Ultrasonic velocity decreases with damage. This decrease is very well correlated to the acoustic activity due to damage initiation and development. For other materials as metallic ones, linear ultrasonics is not sensitive enough for damage characterisation. In this case we show that nonlinear ultrasonics through the nonlinearity parameter β, is very sensitive to evaluate damage and to detect it very early. In this work we show that if linear ultrasonics through velocity measurements is a very good damage indicator especially for composite materials, nonlinear ultrasonics can be a very efficient tool for the early detection of damage and also for health monitoring. So it can greatly contribute to the understanding of damage initiation. (authors)