Determination of mechanical property profiles for improved crash performance of a dual phase automotive steel

The most important objective of the automotive industry in the current decade is to design light weight vehicles in order to be environmental friendly. However, in the meantime, also safety requirements during collisions have to be met. Crashworthiness is mainly concerned with the controlled absorption of crash energy. Parameters such as internal energy absorption and crush force efficiency are key constants in evaluating a component’s crash worthiness and its safety. In the design of automotive structures for crashworthiness, there is a strict requirement for null strain sub modules that provides a rigid space of survival area for the occupants and deformable sub modules which can dematerialize the kinetic energy effectively. The front crash box of a car is the one of the most important deformable components which dissipates kinetic energy during frontal crash, which is the most pronounced crash situation that affects the entire automotive structural behavior. A comprehensive experimental program considering the material plasticity and fracture behavior under various loading conditions experienced in automotive crash situations are designed and carried out. A commercial dual phase automotive steel, DP-K1000, with a martensite phase fraction of 45% is considered for the study. The material is available in the form of rolled sheets with a final thickness of 1.5 mm. To determine the intrinsic mechanical properties of the dual phase steel under consideration, tests are performed at wide range of loading speeds, thereby covering the quasi-static, intermediate and dynamic strain rates (0.0001 to 1000 s-1) and temperatures (-40 to 400˚C). An Instron material testing machine is used to perform experiments at static and medium strain rates whereas the dynamic loading rates are achieved with the aid of split-Hopkinson tensile bar setup. A set of specially designed and optimised fracture specimen geometries are employed for damage and fracture characterization of DP1000 steel. Each specimen thus designed help in achieving a characteristic stress state (for example a central hole specimen for uniaxial tension, three notched dogbone samples R4.5, R1.8 and R0.4, a plane strain and shear specimen) and reveal a wide range of damage mechanisms exhibited by these kinds of advanced high strength steels. A hybrid extended modified Bai-Wierzbicki (eMBW) damage plasticity model is then implemented to properly describe the non-negligible impact of damage in strength and ductility and dependence of stress state on damage accumulation. The material model constitutes a plasticity formulation to characterize the material behavior before the initiation of damage followed by a phenomenological criterion to indicate the damage initiation. A phenomenological formulation of ductile damage is used for a user friendly and simplified input of material parameters. Finally, a damage induced softening part incorporating a damage mechanics approach is adopted to characterize the post damage behavior. The performed experimental campaign provide a reliable data set to derive the material model parameters of the adopted damage plasticity model. The calibrated material model is thus used to investigate the crash response of this dual phase automotive steel both qualitatively and quantitatively. The constitutive model is imposed on the analysis for the dynamic response of auto-body crash box utilizing the explicit finite element scheme Abaqus. The proposed eMBW material model is implemented into Abaqus Explicit via a user material subroutine (VUMAT). A full scale model with 3D solid elements and sufficient mesh refinement is adopted to accurately predict the localization and damage phenomena within the crash box. The concurrence of the derived crashworthiness experimental results and deformation behavior from finite element scheme is thus evaluated. Total energy absorption of the crash box can be due to the coupled effects of geometry and material property. The effect of geometry is decoupled from the material effects so that the energy absorption contribution from the material effects as a function of plastic deformation and damage properties of the material can be solely investigated. The study aims at the identification of relevant mechanical properties of the automotive material which are important to evaluate and improve its crash performance, as well as in-depth understanding of the influence of the material behavior on the component performance. The component performance is quantified by the energy absorption during a crash test on a crash box made of DP-K1000 steel. A Taguchi optimization procedure based on a concrete design of experiments was performed to assess the individual contribution of the material mechanical properties to the overall component performance. This technique is adopted on the component scale crash box model with the optimized set of material parameters. Material mechanical property sets are then varied within a predefined range in accordance with the strategy for design of experiments such that all possible combinations of material properties linked to the crash performance can be systematically analyzed. Yield strength, ultimate stress, hardening coefficient, damage initiation strain and failure strain were some of the parameters considered in the optimization routine. Additionally, relative contribution of each of the mechanical properties were addressed. The outcome of this optimization approach was a sorted list of relevant mechanical property profiles in order of their significance of contribution towards higher energy absorption. The interaction between the contributing mechanical material properties is quantified for different strain rates, stress states and temperatures. Peak crushing forces will result in increased accelerations transmitted from the frame to the passenger compartment. Various material property sets were evaluated based on the common crashworthiness criteria to identify the effective design (combination of material mechanical parameter set) which provide low peak force and trigger the regular collapse behavior. Force versus displacement curves were plotted in each case, providing detailed insight into the force variation during deformation. Results from the analysis suggests that the material yield stress has the most significant contribution in bringing about a change in the energy absorption characteristics, followed closely by ultimate tensile strength ratio. Yield stress, ultimate strength and hardening rate together have a strong positive effect on the energy absorption characteristic while failure strain has a weak contribution. Crush efficiency increases with increase in yield strength primarily when the hardening capability of the material is higher. By the use of a material with optimized mechanical property profile, a performance improvement in the component level can be aimed at, using the same geometry and material thickness as in the original automotive crash box design