Repeated lateral impacts on steel grillage structures at room and sub-zero temperatures

This paper presents experimental and numerical investigation results for the plastic response of steel grillages, represented by one longitudinal stiffener and two transverse stiffeners of stiffened plates, used on ships or offshore structures under repeated mass impacts. The repeated mass impact scenario adopted in this paper could represent the repetition of impacted loads on marine structures due to contact with ice floes and floating objects during service. Repeated impact tests on grillage models at room and sub-zero temperatures (−50 °C) were conducted by releasing a knife-edge striker using a drop testing machine. The ends of the grillage models were firmly fixed onto a strong bed with support fixtures. To evaluate the repeated impact performance of the tested models, permanent deflections were measured after each impact test. Additionally, numerical simulations were performed using the commercial software package ABAQUS/Explicit to predict the extent of damage to the tested models caused by repeated impacts. In the calculations, the material properties of tested models were used to determine the strain hardening and strain rate hardening model using the equations recently proposed by the authors. Also, a simple analytical approach has been proposed to predict the damage evolution of the grillage subjected to repeated mass impacts. Reasonable agreement was achieved between the predictions and test results. The results showed that when the structure was repeatedly impacted, permanent deflections of the grillage significantly increased with increasing impact force. The permanent deflections tended to have certain values when the number of impacts increased. Based on the numerical results (which were substantiated with the experimental results), further calculations were then carried out to examine impact responses in greater detail and assess the effects of variations in boundary conditions and strain rate definitions on the response of grillages to repeated mass impacts.