Development and Application of Nonlinear Ultrasonic Based Nondestructive Testing Techniques for Monitoring Metal Fusion Process

Abstract

Steel is one of the most widely used materials in modern society for its numerous advantages. Steel is heavily used for construction of civil structures and machine parts. Metal fusion is an important manufacturing technique in steel industry. Metal fusion processes such as welding and Additive Manufacturing (AM) are commonly used in industries; however, inevitable occurrence of defects degrades the quality of materials. Such defects are the weak spots that can lead to catastrophic structural failures; therefore, timely detection of these defects is important. Ultrasonic Nondestructive Testing and Evaluation (NDT&E) techniques can be used to detect these defects. The conventional linear ultrasonic techniques are mostly used in today’s industries. Unfortunately, linear techniques often fall short of detecting small defects. Various nonlinear techniques have been and are being developed in order to successfully detect micro-scale defects. Higher Harmonic Generation (HHG) and Nonlinear Wave Modulation Spectroscopy/Frequency Modulation (NWMS/FM) techniques are two most well-established techniques. However, there are some restrictions when applying these techniques for certain types of structure where elastic waves propagate as guided waves. Recently developed nonlinear ultrasonic techniques, called Sideband Peak Count (SPC) and Sideband Peak Count - Index (SPC-I) techniques, have some inherent advantages over other nonlinear techniques. In this dissertation the effectiveness of SPC-I technique is first examined through Finite Element Analysis (FEA) and then validated experimentally. Although the SPC-I technique is shown to be robust, the complexity in controlling the variables makes it difficult for inexperienced users to apply this technique. A new Sideband Peak Intensity (SPI) technique is proposed here to make the technique more attractive to a wide range of users. The advantages of SPI technique is demonstrated by comparing its performance with the SPC-I technique; two techniques are compared conceptually by illustrative examples and then verified experimentally.