2. Nonlinear Resonant Ultrasound Spectroscopy: Assessing Global Damage

The term nonlinear resonant ultrasound spectroscopy (NRUS) was first coined in the 1990s and is one of the earliest nonlinear techniques used to quantify global damage in a sample. This chapter was written as an introduction and overview for the general reader, one interested in learning more about the technique, especially its origins. As rocks are highly nonlinear, it is perhaps not surprising that the study of the nonlinearity of a material full of cracks could be applied to nondestructive testing and applications. Thus, this chapter aims to show that link. The overview of resonance techniques presented in Chap. 1 is here expanded upon and placed in a historical context with an emphasis on the experimental side, including important measurement pitfalls of applying the technique. The fact that the technique was patented early on and that the measurements are complicated by rate effects likely contributed to its lack of general use in the NDE community. After reading this chapter, we think you may agree that the technique is ready for another, closer look.