Nonlinear ultrasonic testing for evaluation of fracture toughness in 4130 steel

Abstract

An estimation of plane strain fracture toughness [endif]--> is essential to assess the operational safety of fracture-critical systems; loss of [endif]-->can lead to catastrophic failure. However, it is not possible to quantify [endif]-->in situ; the laboratory testing to determine [endif]-->is destructive. Here, we investigate nonlinear ultrasonic testing as a nondestructive method to evaluate [endif]-->. This is based on a hypothesized correlation between the nonlinear ultrasonic parameters and [endif]-->characteristics of a material given their mutual dependence on microstructure. Using Second Harmonic Generation (SHG), surface and bulk waves are used to measure the (relative) nonlinearity parameter for tempered 4130 steel samples. Coupons made of the same materials are also tested for their [endif]-->characteristics using Charpy V-Notch testing, providing a direct comparison between destructive and nondestructive measures. Our results show correlations between ultrasonic parameters and [endif]-->characteristics of 4130 steel samples, but different relations are observed for bulk and surface waves. The observed differences are possibly due to sample heterogeneity and different wave structures of the two wave modes and should be further investigated. We demonstrate the potential of nonlinear ultrasonic testing for [endif]-->estimation. The ultimate goal is developing a nondestructive method to assess [endif]--> in situ.