Anisotropic fracture modelling in high-strength steel sheets using a hybrid experimental-numerical approach
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Insinööritieteiden korkeakoulu |
Master's thesis
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Authors
Date
2024-06-10
Department
Major/Subject
Solid Mechanics
Mcode
Degree programme
Master's Programme in Mechanical Engineering (MEC)
Language
en
Pages
88
Series
Abstract
The automotive industry has embraced advanced high-strength steels (AHSS) in the quest for improved fuel efficiency and safety since these materials offer an exceptional strength-to-weight ratio, allowing car manufacturers to construct lightweight body structures using thinner gauges. This not only enhances vehicle performance but also improves the structural rigidity. First-generation AHSS sheets often sacrifices ductility compared to conventional steel sheets, but this problem is solved by the third-generation AHSS like quenching and partitioning (Q&P) steels, which offer a remarkable combination of strength and ductility. The full potential of Q&P steel remains largely unexplored, and this knowledge gap hinders its widespread utilization in industry. This thesis aims to investigate QP1000 and CP800, first experimentally and then numerically under various stress states and different loading directions for anisotropy study since it is prevalent in all sheet metals and then develop anisotropic fracture models which can predict fracture in simulations corresponding to the fracture in experiments. In this study uniaxial tensile tests are performed for anisotropic material properties and fracture at stress states ranging from simple shear to plane-strain tension in three loading directions for CP800 and in seven loading directions for QP1000. While comparing the two materials, QP1000 has shown stronger and non-monotonous anisotropic behavior compared to CP800 which has a weak anisotropy. The enHill48 plasticity model is used to describe the materials’ anisotropic plasticity. The fracture behavior of the materials is characterized by both the partially anisotropic model and fully anisotropic fracture models based on the MBW model, DF2014 criterion and MMC model modified using scaling or interpolation of fracture parameters. The three criteria are compared and the MBW model performed the best. The results from the anisotropic fracture models show that CP800 has more local formability at all stress states and all loading directions as compared to QP1000 with a few exceptions. However, QP1000 exhibits more global formability. These anisotropic models are implemented in ABAQUS/Explicit simulations using VUMAT and a good correlation between the experimental and simulation results is obtained for fracture prediction by the anisotropic fracture models which are able to predict fracture exactly at the stress states and loading directions in which they were calibrated in and are able to interpolate the fracture locus and predict fracture at any arbitrary loading direction showing outstanding predictive abilities.Description
Supervisor
Lian, JunheThesis advisor
Li, ZinanKeywords
anisotropy, ductile fracture, high-strength steels, fracture modelling, plasticity