Experimental and numerical studies on the vibration-based structural health monitoring of dimpled steel sheets with residual stresses

In this study, the effect of residual stresses on the free vibration response of structural steel sheets is investigated for the purpose of structural health monitoring. Residual stresses are introduced into the structural steel sheet through a prestressing process known as dimple formation. The material properties of a rimmed steel disk were initially obtained via experiment. A dimple is then formed, corresponding to an experimentally obtained drophammer impact velocity of V = 3.11 m/s for a drop angle of 30 degrees. The material properties and impact velocity are later imported into ABAQUS finite element analysis (FEA) simulation package to obtain the structural deformation before and after elastic recovery via the Dynamic Explicit Analysis. The numerically obtained residual stresses, remained after elastic recovery and caused by dimple formation, are then used as an initial prestress condition for the subsequent eigenfrequencies of free vibration. To do so, a four-step FEA structural model is developed to explore the vibration response of structural steel sheets under three conditions: (1) no dimple exists; (2) dimple exists but residual stresses are ignored intentionally; (3) both dimples and residual stresses exist. The natural frequency outcomes are then experimentally validated by using laser interferometry technique (LIT). Finally, the effect of incremental mass removal on natural frequencies in the steel sheets with or without dimple is experimentally investigated using an audio recorder. Unless the material properties are not readily available, the proposed structural model is entirely independent from any experimentation. The developed structural model demonstrates how superposition of the prestress field over the indicative stress field for each vibration mode of interest can be used to reliably predict vibration outcomes. The model can be used as a structural/mechanical design tool to explore the existence of residual stresses in steel sheets.