Abstract
To meet requirements for reduced fuel consumption of cars, the use of components made of sheets of high-strength steel instead of conventional steel has been on the rise. However, low ultimate elongation of high-strength steel often causes problems during plastic deformation and more research is needed to improve failure predictability. Ductile failure models available in commercial finite element analysis (FEA) packages require the material’s tensile strength and failure strain for failure prediction. For stress states that are more complex than the uniaxial case, accurate prediction of the how, when, and where failure occurs has been become problematic and it has been investigated by numerous researchers. In this study, we investigate the prediction of failure in DP980 sheets under triaxle stress states. We first determine the shapes of specimens using certain triaxial stress states, such as pure shear, uniaxial tension, biaxial deformation, which are induced by corresponding tensile tests. When failure occurs, equivalent strain at the failure locus is obtained by means of digital image correlation (DIC) and then plotted against triaxiality and Lode angle, based on which the triaxiality failure diagram (TFD) is established to implement in the FEA program of LS-DYNA. Validation is made by comparing the numerical results with burring test data. Good agreement was found for failure locus and strain distribution at the time of failure.
Highlights
In the context of fuel consumption, weight reduction plays an ever-increasing role in the car industry, which means that conventional steels are being replaced by high-strength steel (HSS) sheets showing elevated tensile strength
The failure limit of metal sheets is defined as the elongation at failure in tensile testing, and failure is determined based on the forming limit diagram (FLD), which is a plot of major against minor strains [6]
When we look in positive η-direction, the two curve sections are V-shaped, with the first—the region between uniaxial compression and uniaxial tension (− 1/3 ≤ η ≤ 1/3)—hitting its low at η = 0 whereupon it increases toward uniaxial tension (η = 1/3) and the second—the region between uniaxial tension and biaxial tension (1/3 ≤ η ≤ 2/3)—having its minimum at plane strain
Summary
In the context of fuel consumption, weight reduction plays an ever-increasing role in the car industry, which means that conventional steels are being replaced by high-strength steel (HSS) sheets showing elevated tensile strength. The higher tensile strength of such HSS sheets is usually at the expense of ultimate elongation, leading to frequent failure under plastic deformation. This issue calls for in-depth research, both numerical and experimental, on how, when, and where failure occurs and how this can be predicted accurately. The failure limit of metal sheets is defined as the elongation at failure in tensile testing, and failure is determined based on the forming limit diagram (FLD), which is a plot of major against minor strains [6]. During sheet metal forming, FLD becomes inapplicable (or at least limited in its applicability) once the deformation path departs from its original one or if the stress state is not (purely) tensile. The failure strain at the failure locus cannot be deduced accurately from the Elongation [%]
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