Multi length-scale characterisation and micro-mechanical modelling of ductile damage in advanced high strength steels

Abstract

Advanced high strength steels (AHSS) are widely used in the automobile industry to reduce body weight without compromising vehicle safety. Dual Phase (DP) steels and Transformation Induced Plasticity (TRIP) steels are the most used AHSS in vehicle structure components due to their excellent combination of mechanical properties, such as high strength, high work-hardening rate and good formability. Despite the tremendous amount of research activities devoted to further improving DP and TRIP steels’ mechanical properties by optimising phase composition and microstructure, heterogeneous microstructure causing local failure and shape imprecision remain the major problems during cold forming of DP and TRIP sheets. This disconnection between the nominal and actual formability is partially attributed to delayed development and misunderstanding of the ductile damage process in DP and TRIP steels. This study first aims to evaluate the correlation between the heterogeneous microstructure of DP and TRIP steels and their ductile damage, using both electron microscopy for local observation and 3D X-ray computed tomography (CT) for damage in the bulk sample. Secondly, the current study aims to establish a framework for building ductile damage models customized for a specific DP or TRIP steel, guided by ductile damage behaviour (void nucleation, growth and coalescence) that are experimentally measured from the bulk sample. Three DP steels with different martensite volume fractions and one TRIP steel that changes phase volume fraction continuously during straining are selected for this study. From local microstructural observation and X-ray CT 3D characterization, most of the voids found in the studied DP steels are nucleated either inside martensite grains or at the martensite/ferrite interface. DP steel with more significant amounts of martensite phase showed a greater number of voids and larger averaged void size at a given local strain during uniaxial loading. However, a remarkable difference is found in the void growth behaviour of the pre-existing void and the newly nucleated voids during staining. From in situ X-ray CT scan, numerical descriptions of voids nucleation and growth behaviour are established for each studied material. Based on these numerical descriptions of voids during plastic deformation and ductile damage, the author first revealed the discrepancy between the widely used Gurson-Tvergaard-Needleman (GTN) damage models and the experimental measurements in the current study. Following this, an updated void nucleation law and growth law that better describe void statistics changes with local strain are proposed. Additionally, a hardening law, phase composition model and shear model that reflect heterogeneous microstructures in DP and TRIP steels are proposed to build a GTN-combined (GTNC) damage model for DP and TRIP steels. All aforementioned modules are compiled into the GTNC damage model via Vectorized User Material Routine (Vumat) in Abaqus, which are validated by experimentally measured void-local strain correlation, as well as strain-stress response of each material.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Philip Withers (Supervisor) & Kun Yan (Supervisor)