Nondestructive characterization of elastic modulus under different tensile stresses through laser ultrasound combined inversion technique

Abstract

Stainless steel (SS) is widely used in many fields including aeronautics, automobiles, marine and mechanical industries due to its outstanding feature such as good corrosion resistance and hardness. However, changes in material properties under stress, particularly changes in Young’s modulus, result in the formation of cracks, a reduction in load-bearing capacity, and fatigue damage. So, the structural integrity needs to be evaluated based on a precise measurement of mechanical properties. In this study, Stainless Steel 304 (SS-304) is used as the base material and various tensile stresses are applied ranging from 0[Formula: see text]MPa to 100[Formula: see text]MPa with increment of 10[Formula: see text]MPa in each step. Nondestructive Laser Ultrasound Technique (LUT) has been used to characterize the elastic modulus under various tensile stresses. An inverse program was developed based on the Particle Swarm Optimization (PSO) algorithms to determine material properties. Nonlinear Gauss fitting method was proposed and established the fitting equation and nonlinear curve for Young’s modulus and residual stress. The outcome of this research shows that when tensile stress is applied, the mechanical properties decrease by shifting the dispersion curve and also it is evident that the dispersion curves move toward the high-frequency-thickness while increasing the tensile stress. When the tensile stress was increased from 0[Formula: see text]MPa to 100[Formula: see text]MPa, the value of Young’s modulus decreased from 201.7[Formula: see text]GPa to 193.5[Formula: see text]GPa. Especially, the predominant changes were observed during 30–100[Formula: see text]MPa. This observation displays the bonding strength and binding energy between the atomics. Further, the proposed nonlinear Gauss fitting substantiated the experimental values and confirmed that the thickness accuracy is close to the inversion values, with an average difference of 4.32%. This research suggests a potential nondestructive method to determine the residual stress of a material by calculating the changes in the elastic modulus.