Experimental characterization and numerical modeling of micromechanical damage under different stress states

The use of HSLA steels for the manufacture of automotive components is interesting from an engineering point of view. This family of steels, while possessing high strength, also has good formability and can be used in forming manufacturing processes. In some forming processes such as blanking, shear strain localization occurs, which causes damage and results in the final fracture of the material. This paper presents an experimental study based on in situ tests to understand and identify the physical mechanisms of ductile damage under two stress states: tension and shear. Different macroscopic tests were performed to calibrate a damage model based on a micromechanical approach. This damage model is based on the Gurson–Tvergaard–Needleman theory and presents recent improvements proposed by Nahshon and Hutchinson and by Nielsen and Tvergaard so as to better predict fracture under a wide range of stress states, especially with low levels of stress triaxiality. These extensions have made the identification of the material parameter more complicated. In this work an identification strategy has been proposed using tests on specimens with different shapes. The identified parameter values are validated and the fracture model show good predictive capability over a wide stress state range

Numerical integration of an advanced Gurson model for shear loading: Application to the blanking process

A new extension of the Gurson damage model has been proposed recently to predict ductile fracture under shear dominated loads. The aim of this work is to verify the ability of this approach to simulate, in an accurate way, the damage evolution in shearing processes. An implicit stress integration algorithm is then developed to implement the new model in a finite element code. The numerical procedure is checked through simulations of shear and uniaxial tension tests on a single elements. The extended Gurson damage model is tested and applied to the punching process to compare its predictive ability with the original approach. The obtained numerical results are in good agreement with experimental results of the punching process, showing better ductile fracture predictions compared to the original Gurson model

Experimental and numerical analysis of micromechanical damage in the punching process for High-Strength Low-Alloy steels

Sequential sheet metal forming processes can result in the accumulation of work hardening and damage effects in the workpiece material. The mechanical strength of the final component depends on the “evolution” of these two characteristics in the different production steps. The punching process, which is usually in the beginning of the production chain, has an important impact on the stress, strain and damage states in the punched zones. It is essential that the influence of these mechanical fields be taken into account in the simulation of the forming sequence. In order to evaluate the evolution of each phenomenon, and in particular damage accumulation in the forming process, it is essential to characterize the punching process. The objective of this work is to understand and identify the physical damage mechanisms that occur during the punching operation and to establish relevant numerical models to predict the fracture location. The effect of the punch–die clearance on mechanical fields distribution is also discussed in this work

Safety part design optimisation based on the finite elements method and a genetic algorithm

This paper deals with a numerical approach for improving the mechanical properties of a safety belt anchor by optimizing its shape and the manufacturing process by using a multi-objective genetic algorithm (NSGA-2). This kind of automotive component is typically manufactured in three stages: blanking, rounding of the edges by punching and finally bending (90°). This study focuses only on the rounding and bending processes. The numerical model is linked to the genetic algorithm (GA) in order to optimize the shape of the part and the process parameters. This is implemented by using ABAQUS© script files developed in the Python programming language and CATIA© script files in VBScript. The algorithm modifies the part’s design parameters in the CAD system, imports the model in STEP format into ABAQUS CAE and starts the Finite Elements Analysis (FEA) automatically. The material behaviour is modelled using a specific Lemaitre material damage formulation implemented in ABAQUS© via a FORTRAN user subroutine. The influence of two process parameters (the die radius and the rounding punch radius) and five shape parameters on the component behaviour is investigated. The search for the optimum component design depends on three objective functions which are (i) the material damage state at the end of the forming process, (ii) the von Mises stress field and (iii) the maximum von Mises stress in the folded zone. A global optimisation is finally performed in order to improve the ultimate unbending load and the volume of the safety part. This work has two major areas of innovation: (a) the improvement of the genetic algorithm NSGA-2; and (b) the development of an integrated numerical procedure including “Computer aided design” and “mechanical finite element simulation” controlled by the genetic algorithm.DEVILL

Failure prediction and validation of a steel automotive safety part

The objective of this work was to investigate thefailure behaviour of a HSLA steel component subjected to a bending unbending operation. This study was conducted through a combined experimental and numerical approach to better understand the limitations of current models and generate more suitable data that can be used to further optimize the manufacturing process. The numerical approach was carried out using two finite element codes (ABAQUS and PAM-STAMP) and two fracture models (Gurson and CrachFEM). The material and fracture data to be used in the numerical simulations was obtained through mechanical tests (tensile and shear tests) conducted to identify the monotonic and damage behaviour of $500 MC. Furthermore, bending and unbending tests were performed with the purpose of validating the models and study the operating performance of this material when subjected to these kinds of loading. The final purpose of this work is to present a numerical approach that can accurately predict the mechanical and failure behaviour of a real automotive safety component.Bourse CIFRE DEVILLE S

Development of a microscopic damage model for low stress triaxiality

Sheet Metal 2011This work deals a contribution to ductile damage of High-Strength Low-Alloy (HSLA) steel steels under low stress triaxiality. This work is based on micrographics observations and in situ shear tests to examine the evolution of microstructure in this kind of loading and to identify the damage process associated. Numerical simulations by finites elements has been performed to simulate the material behavior of nucleation mechanism and the interaction between cavities during the coalescence phase, as well as the effect of the relative position of the inclusions in the shear plane. The model used as a reference in this work is the Gurson-Tvergaard- Needleman (GTN) model. It has been recently improved in order to take into account the effects of low triaxiality during shearing. The implementation of this model in a finite element code is in progress.Devillé S