ABSTRACT
Many materials, such as metals, behave linearly at low stress levels while the nonlinear behavior is observed when it is subjected to high stresses. This chapter argues that nonlinear acoustic parameters are noticeably affected by very small defects, such as fatigue generated dislocations at the grain boundaries, and micro-cracks that are much smaller than the wavelength of the interrogating ultrasonic waves. Nonlinearity in the stress–strain relation can appear due to various effects – one popular belief is that as waves propagate through the damaged mat erial crack surfaces near the crack tips open and close causing variations in stiffness and temperature of the material in that region. Fibers give the composite mortar specimens higher fracture toughness and also introduce some nonlinearity in its response, probably due to the additional interfaces between the fibers and the mortar matrix. The acoustic nonlinearity decreased monotonically as the heat treatment condition changed from A to C.
