Nondestructive Evaluation of Fracture Toughness in 4130 Steel Using Nonlinear Ultrasonic Testing

The knowledge of ‘plane strain fracture toughness’(K_IC) is essential to the operational safety of fracture-critical systems. However, it is not yet possible to quantify K_IC in-service due to the destructive nature of K_IC testing. Here, we investigate nonlinear ultrasonic testing (UT) as a nondestructive alternative. We hypothesize a correlation between the nonlinear ultrasonic parameters and K_IC of a material due to their mutual dependence on materials’ microstructure. Using second harmonic generation, both surface and bulk wave modes are used to estimate the classical nonlinearity parameter (β) for tempered 4130 steel samples. We also report wave velocity and exponent, a new parameter describing the relationship between the second and fundamental harmonic amplitudes. Corresponding coupons are tested for their K_IC characteristics using Charpy V-Notch (CVN) testing, providing a novel direct comparison between destructive and nondestructive tests. Results of nonlinear bulk wave testing indicate a monotonic relation between β and CVN absorbed energy values. The surface wave test results show a different non-monotonic trend. Bulk wave speed and exponent show no correlations with absorbed energy, while surface wave speed and exponent show similar relations. The differences between bulk and surface wave test results are attributed to sample heterogeneity and different wave structures of the two wave modes. Our findings demonstrate the potential of nonlinear UT for in-situ K_IC estimation.