STRAIN EVOLUTION AND DAMAGE DEVELOPMENT DURING TIGHT-RADIUS BENDING OF ADVANCED HIGH STRENGTH STEELS

Improved vehicle fuel efficiency and driving safety requirements have promoted the development of Advanced High Strength Steels (AHSS) in the last few decades. The mechanical performance of AHSS is commonly characterized by the product of the ultimate tensile stress and total elongation. However, tensile elongation is not suitable for predicting the performance of a material under complex forming operations. This work aims to investigate the effect of the steel microstructure on bending performance and to make a parallel between strain partitioning and damage nucleation in tension and bending.
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THE INFLUENCE OF TRANSFORMATION INDUCED PLASTICITY IN THIRD-GENERATION ADVANCED HIGH STRENGTH STEELS

Considerable research has been invested in developing thin sheet Advanced High Strength Steels (AHSSs) and to metastabilize phases at ambient temperatures; however, little has been done to determine the extent to which the transformation from austenite to martensite (TRIP), can suppress/delay damage. The damage processes that lead to fracture in AHSSs are complex and understanding them requires a careful assessment of the strain partitioning amongst the phases, the evolution of microstructure with strain and how damage accumulates in the form of voids and microcracks. This can only be accomplished by applying a range of methodologies tracked as deformation proceeds, including micro-Digital Image Correlation (µDIC), Electron Backscattered diffraction (EBSD), X-ray microtomography (µXCT) and synchrotron-sourced High Energy X-ray diffraction (HEXRD). Such experiments have also been applied to notched specimen to further understand the response of AHSSs at different states of stress. Data will be presented on a range of ultrahigh strength AHSSs with and without TRIP-assistance (dual phase (DP), quench & partition (Q&P), and Medium-Mn steels). The data suggests that grain refinement, TRIP and decreased mechanical heterogeneity amongst phases can be used to suppress damage. It remains a challenge to quantify these effects separately, opening new avenues for experimental and modeling investigations.
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A MODIFIED J-Q CONSTRAINT APPROACH TO ASSESS EFFECTIVE NOTCH FRACTURE TOUGHNESS

This paper uses a modified constraint-based fracture mechanics approach to estimate the effective notch fracture toughness . A modified J-Q constraint correction approach is proposed to evaluate the role of the notch tip acuity on the severity of the stress field accounting for two main characteristics, i.e. spread and maximum stress dependence of notch tip acuity. The methodology uses standard pre-cracked specimen toughness and the constraint-based approach in BS7910/R6 procedures to estimate mean values of notch fracture toughness. Experimental notch tests for S355 specimens at -140oC show good correlation with the model predictions
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FRACTURE ANALYSES OF THIN-DUCTILE MATERIALS USING CRITICAL CTOA AND TWO-PARAMETER FRACTURE CRITERION

The critical crack-tip-opening angle or displacement (CTOA/CTOD) fracture criterion is one of the oldest fracture criteria applied to metallic materials. Improved computer-aided photographic methods have been developed to measure CTOA during the fracture process; and elastic-plastic, finite-element analyses (ZIP2D) with a constant CTOA and a plane-strain core have been used to simulate fracture of laboratory specimens. The fracture criterion has been able to link the fracture of laboratory specimens to structural applications. This paper analyzes fracture of cracked thin-sheet 2219 aluminum alloy over an extremely wide range in width, crack-length-to-width ratio, and applied loading. The results from the critical CTOA fracture analyses on the thin-sheet material showed that the stress-intensity factor at failure (KIe) was linearly related to the net-section stress (Sn), as predicted by the Two-Parameter Fracture Criterion (TPFC).
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ANALYSES OF DUCTILE FRACTURE USING HUNNY THEORY

We present a theory with a structure that enables analyses of ductile fracture under any type of loading. The theory builds on the standard concept of homogeneous yielding and further proceeds from the concept of unhomogeneous yielding on a (yield) system that depends on the spatial distribution of voids. Depending on the desired level of refinement in analysis, a given simulation employs one or more yield systems with the isotropic limit being reached for an infinite number. We illustrate the predictive capabilities of the theory by considering simulations of three-dimensional crack initiation and growth in a round notched bar, a shear specimen and a compression pin.
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1-TO-1 COMPARISON OF SEM-DIC TO CP STRAIN FIELDS OF ULTRATHIN STEEL FILMS TO UNRAVEL PLASTICITY TO DAMAGE INITIATION

In advanced high strength steels, crack propagation and fracture is preceeded by damage initiation and propagation, yet, the nature of the plasticity mechanisms leading to damage are debated. To fully unravel the plasticity-to-damage mechanisms, we present a novel integrated experimental-numerical nanomechanical framework for testing ultra-thin specimens, yielding (i) full 3D reconstruction of grain/phase shapes and orientations, (ii) front&rear-sided, high-resolution, microstructure-correlated SEM-DIC strain fields, and (iii) one-to-one comparison to numerical strain fields computed with (advanced) crytal plasticity. Results on martensite ‘bridges’ show that limited plasticity results in martensite damage whereas significant plasticity prevents damage; analysis reveals the key role of ‘substructure boundary sliding’ in martensite on damage initiation.
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INFLUENCE OF LARGE STRAIN REVERSE LOADING ON DYNAMIC STRAIN LOCALIZATION AND FAILURE OF DUCTILE METALLIC RODS

A bespoke real time strain control setup is constructed to apply the reverse loading directly to the gauge section of 304L stainless steel specimen up to a maximum strain level of ±0.16. The subsequent tensile tests of the reverse loaded specimens are performed from quasi-static to high strain rates of 1000 /s. A higher strain reverse loading significantly influences the development of necking instabilities, with smaller strain to necking inception, higher local stress in the necking zone, and higher local strain rate up to failure. An analysis of the local stress-strain relationship indicates that the reverse loaded 304L rod shows good impact energy absorption up to failure, which agrees with the ductile fracture surfaces of the 304L materials with reverse loading.
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A GURSON-TYPE LAYER MODEL FOR DUCTILE POROUS SOLIDS CONTAINING ARBITRARY ELLIPSOIDAL VOIDS WITH ISOTROPIC AND KINEMATIC HARDENING

Extensions of Gurson’s model for porous ductile materials have been done by Madou and Leblond (2012) for general ellipsoidal cavities made of rigid-plastic materials, and Morin et al. (2017), for spherical voids with rigid-hardenable matrices. The aim of this work is to provide a homogenized criterion for porous ductile materials incorporating both void shape effects and isotropic and kinematic hardening. A sequential limit-analysis is performed on an ellipsoidal representative volume made of some rigid-hardenable material, containing a confocal ellipsoidal cavity. The overall plastic dissipation is obtained by using the velocity field proposed by Leblond and Gologanu (2008) and that satisfies conditions of homogeneous strain rate on an arbitrary family of confocal ellipsoids. The heterogeneity of hardening is accounted for by discretizing the cell into a finite number of ellipsoids between each of which the quantities characterizing hardening are considered as homogeneous. The model is finally assessed through comparison of its predictions with the results of micromechanical finite element simulations. The numerical and theoretical overall yield loci are compared for various distributions of isotropic and kinematic pre-hardening with a very good agreement.
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A NON-LOCAL GURSON MODEL WITH TWO FRACTURE-MECHANISM ASSOCIATED LENGTH SCALES: SUPPORTED BY NUMERICAL ANALYSES AND EXPERIMENTS

An extension of Gurson’s porous plasticity model capable of preventing pathological strain localization, and describing crack initiation and propagation under both shearing and tension is investigated. This paper separates the progression of shear failure and flat dimple rupture based on the assumption that these two failure mechanisms are governed by different characteristic length scales, a deviatoric and a dilatational length scale, respectively. A set of numerical analyses is presented which brings out the effects of these length scales on the development of e.g. cup-cone and slant fracture. Guided by the outcome of the numerical study, a set of tests has been designed and carried out for calibration of these length scales.
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THE FOURTH SANDIA FRACTURE CHALLENGE – PREDICTING PUNCTURE IN A METAL STRUCTURE [Keynote]

The fourth Sandia Fracture Challenge (SFC4) investigated the puncture of aluminum structures through comparing various computational predictions to physical experiments. Five teams, internal to Sandia National Laboratories, submitted predictions with mixed success. Qualitatively, many teams were able to predict the deformation and failure modes at the critical velocity for puncture, but the extent of damage was underpredicted by all. Quantitatively, predictions for critical velocity varied widely, though were in the correct order of magnitude. The SFC4 highlighted difficulties in modeling damage and fracture in shear-dominated loading cases.
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