Constitutive strength model for spot joints of thin-walled energy-absorbing elements

The goal of this paper was to determine the strength criterion that causes the failure of point-to-point joints of thin-walled profiles under impact load, as well as to determine the strength parameters that guarantee joint continuity during dynamic folding of crash boxes. For this purpose, extensive quasi-static and dynamic materials tests were conducted for the five most popular steel grades used in the automotive industry: DP600, DP800, DP1000, HSLA and DC01. The strength of the spot joints was investigated in a complex load state using the original instrumentation. This allowed authors to build and validate mathematical models of the load-bearing capacity of point-to-point joints and to conduct simple and efficient numerical simulations of these elements without the need to account for their geometry. Following the validation of mathematical models and load-bearing models of the joints, extensive experiments of quasi-static and dynamic crushing of thin-walled energy-absorbing top-hat (TH) structures were conducted. The data collected made it possible to calibrate the FEM model correctly and conduct numerical simulations of the dynamic crushing of energy-absorbing structures. As a result, the minimum shear strength (Fs) and cross-tension strength (Fn) of the joints were determined as a function of the sheet thickness and the parameters of Re and Rm. The creation of any spot joint enabling the simultaneous carrying of forces Fs and Fn ensures the preservation of joint continuity throughout the entire crushing process and makes it possible for manufacturers to select the type of spot joint based only on its strength.