Components with defects are identified from the response to strains applied at acoustic and ultrasound frequencies. The relative resonance frequency shift .vertline. DELTA.function./.function.sub.0.vertline., is determined as a function of applied strain amplitude for an acceptable component, where .function.sub.0 is the frequency of the resonance peak at the lowest amplitude of applied strain and .DELTA.function. is the frequency shift of the resonance peak of a selected mode to determine a reference relationship. Then, the relative resonance frequency shift .vertline. DELTA.function./.function.sub.0 is determined as a function of applied strain for a component under test, where fo .function.sub.0 the frequency of the resonance peak at the lowest amplitude of applied strain and .DELTA.function. is the frequency shift of the resonance peak to determine a quality test relationship. The reference relationship is compared with the quality test relationship to determine the presence of defects in the component under test.

The nonlinear elastic response of materials (e.g., wave mixing, harmonic generation) is much more sensitive to the presence of damage than the linear response (e.g., wavespeed, dissipation). An overview of the four primary Nonlinear Elastic Wave Spectroscopy (NEWS) methods used in nonlinear damage detection are presented in this and the following paper. Those presented in this paper are Nonlinear Resonant Ultrasound Spectroscopy (NRUS), based on measurement of the nonlinear response of one or more resonant modes in a test sample, and Slow Dynamics Diagnostics (SDD), manifest by an alteration in the material dissipation and elastic modulus after application of relatively high-amplitude wave that slowly recovers in time.

The objective of this thesis is to validate Resonant Ultrasound Spectroscopy (RUS) as a non-destructive evaluation tool that can be used to study effects of radiation on the mechanical properties of a material, mainly its elastic constants. RUS involves experimentally measuring the resonant frequencies of a sample and calculating the elastic constants based on these measurements. Finite Element Method (FEM) is used to get the frequencies of the modes of free vibration for the sample model. This result depends on the elastic constant values used in the FEM simulation. Studies were conducted to confirm the accuracy of the FEM model, and determine the right configuration and parameters to use for the simulation. Assuming uniform and isotropic elastic property changes, the effects of radiation damage can be quantified by obtaining a set of matching resonant frequencies between the experimental and FEM simulation results, before and after irradiating the sample. This is done by adjusting the elastic constant values used in the simulation so that the results match with the experimentally obtained resonant frequencies. With powerful enough equipment, even real time monitoring is possible in harsh environments, thus pointing out imminent failure.

Additive manufacturing (AM) is becoming increasing popular owing to its ability to manufacture geometrically complex parts and produce customer-designed parts faster than traditional machining. One of the challenges of creating high quality AM parts is that the AM process often produces defects that are difficult to detect. A number of techniques have been used to evaluate the quality of AM parts, such as traditional ultrasonic testing and x-ray micro computed tomography (micro-CT) scans. These methods are not ideal, as traditional ultrasonic testing can require multiple tests to evaluate the entire part, while micro-CT has difficulty detecting small defects in large parts. Resonance-based ultrasonic methods have the advantage of only requiring one testing configuration to evaluate the entire part. Nonlinear resonance ultrasound spectroscopy (NRUS) is a resonance-based nondestructive testing (NDT) technique for material characterization that is especially sensitive to small-scale imperfections such as microscopic cracks. Previous NRUS tests have shown correlations between the parameters measured by NRUS and the fatigue life (fatigue endurance) of a small set of samples, indicating the potential of NRUS for evaluating the build quality of AM parts as related to their performance. However, these measurements on AM metals show large variability due to the experimental setup used. Typical NRUS tests involve bonding the sample to an excitation source that induces vibration in the sample. Unfortunately, the bonding introduces artifacts in the measurements leading to the observed large measurement variability. In this study, we seek to evaluate the use of non-contact excitation sources for NRUS testing with the goal of improving the measurement repeatability. We compare the NRUS measurements using contact and non-contact excitations on wrought and AM 316L stainless steel samples with several different heat treatments. This study suggests the improved repeatability of linear resonance frequency measurements when using an air-coupled transducer. However, the intensity of resulting excitations is not sufficient for NRUS measurements, which require higher excitation voltages. We propose two additional approaches for non-contact NRUS measurements: one using a high-power laser and the other using an air cavity.

This multi-contributed volume provides a practical, applications-focused introduction to nonlinear acoustical techniques for nondestructive evaluation. Compared to linear techniques, nonlinear acoustical/ultrasonic techniques are much more sensitive to micro-cracks and other types of small distributed damages. Most materials and structures exhibit nonlinear behavior due to the formation of dislocation and micro-cracks from fatigue or other types of repetitive loadings well before detectable macro-cracks are formed. Nondestructive evaluation (NDE) tools that have been developed based on nonlinear acoustical techniques are capable of providing early warnings about the possibility of structural failure before detectable macro-cracks are formed. This book presents the full range of nonlinear acoustical techniques used today for NDE. The expert chapters cover both theoretical and experimental aspects, but always with an eye towards applications. Unlike other titles currently available, which treat nonlinearity as a physics problem and focus on different analytical derivations, the present volume emphasizes NDE applications over detailed analytical derivations. The introductory chapter presents the fundamentals in a manner accessible to anyone with an undergraduate degree in Engineering or Physics and equips the reader with all of the necessary background to understand the remaining chapters. This self-contained volume will be a valuable reference to graduate students through practising researchers in Engineering, Materials Science, and Physics. Represents the first book on nonlinear acoustical techniques for NDE applications Emphasizes applications of nonlinear acoustical techniques Presents the fundamental physics and mathematics behind nonlinear acoustical phenomenon in a simple, easily understood manner Covers a variety of popular NDE techniques based on nonlinear acoustics in a single volume