Assessing the Collision Damage to Steel Structures by Means of Nondestructive Evaluation

Abstract

Over height vehicle collisions with bridges are a common occurrence throughout the world. Accidental over height collisions can cause damage to a bridge superstructure and the full extent of the structural consequences from these collisions is not yet understood. This study examines the applicability of acoustic emission and nonlinear ultrasonic testing to assess structural damage on steel girders induced by over height vehicle impact. A numerical study was conducted in Ansys Explicit Dynamics that simulated an over height truck collision on a full-scale steel girder. Parametric variables of stiffness constraints, impact velocity, and impact mass were analyzed. The resulting contact force, damaged girder shapes, and plastic deformation was studied. Experimental studies on A572 Gr. 50 steel coupons were evaluated with acoustic emission and nonlinear ultrasonic testing. These studies provided data on the quantity of plastic deformation that could be detected using NLUT and AE characteristics from impact. Tensile testing was completed on ten steel coupons to specified elongations. Prior to tensile testing, the coupon samples were stress relieved in a furnace and developed an oxide layer. AE testing was performed in-situ on samples with and without the oxide layer. Distinct changes in acoustic emission energy were correlated to elastic and plastic deformation. Oxide layer breakage characteristics were analyzed using PCA clustering. NLUT and Rockwell B hardness testing was performed before and after tensile testing. Higher harmonics were observed during the nonlinear ultrasonic testing and acoustic nonlinearity parameters were found to increase with increasing plastic deformation. The third harmonic acoustic nonlinearity parameter was especially responsive to plastic deformation. Metallography on the samples was performed and an analysis of the increasing grain elongation with strain was conducted. To study acoustic emission response to impact, an impact test was completed on a steel beam using an impulse hammer and acoustic emission in-situ. A correlation between AE characteristics from impact and structural response was discussed. Plastic strain on the girder from the numerical simulations was found to be above a detectable threshold determined from the NLUT experimental results. Critical areas for inspection of the girder were identified from the numerical model.