Papers by Keyword: Stress Triaxiality

Abstract: Despite remarkable advances in additive manufacturing (AM), the uncertainty in direction-dependent strength and fracture behavior of metallic components still poses major challenges for their reliable structural application. The layered nature of laser powder bed fusion (LPBF) produces highly anisotropic textures and microstructure architectures that influence both plastic flow and fracture. While numerous studies have characterized tensile anisotropy, the coupling between build-induced anisotropy and stress-state-dependent fracture remains largely unresolved, yet it governs the structural integrity of AM parts under multi-axial loading. In particular, the extent to which anisotropy alters the ductile-to-brittle transition or fracture locus is still unknown. This study addresses this gap by combining experiments and advanced constitutive fracture modelling for two typical AM metals, austenitic 316L stainless steel and AlSi10Mg aluminum alloy. The goal is to formulate a unified, physically based description of anisotropic plasticity and fracture that is applicable across various material classes. LPBF samples of 316L stainless steel and AlSi10Mg were built at multiple orientations between 0° and 90° relative to the build direction. Uniaxial tensile tests were carried out with digital image correlation to capture full-field strain evolution and to determine r-values as a measure of plastic anisotropy. Complementary fracture tests under different stress states ranging from simple shear to plane strain tension were designed to evaluate the fracture dependence on stress states and anisotropy. It can be concluded that both alloys exhibit orientation-dependent flow and r-value during plastic deformation. The fracture strain decreases with rising triaxiality, yet its rate of decrease depends strongly on orientation, demonstrating a clear coupling between anisotropy and stress state.

Abstract: This work is devoted to the analysis of the influence of the triaxiality factor and the Lode parameter on the ductile fracture of a stainless steel tube. A micromechanical-based model incorporating several deformation mechanisms and formulated in the framework of the dislocation density theory is chosen to model the viscoplastic behavior of the 316L stainless steel. After adaptation of the implementation of the model into the finite element code Abaqus 2020 and the calibration of the model parameters with experimental available results, simulations of healthy and notched tubular specimens were carried out. In order to vary the triaxiality and Lode angle, we used specimens of different sizes and notch shapes. The results showed the capacity of the model to reproduce the experimental results of tubular structures. It was found that the strength and ductility of the specimens depend on the Triaxiality Factor and Lode Parameter.

Abstract: Flanges are commonly used in aircrafts to provide stiffness and support for the assembly Incremental Sheet Forming (ISF) processes have been approached to produce both stretch and shrink flanges as a low-cost alternative in the fabrication of a small number of parts and prototypes. This work analyzes stretch and shrink flanges of AA2024-T3 sheet with different geometries manufactured by Single Point Incremental Forming (SPIF). The numerical simulation using Finite Elements of the flanges allows evaluating the stress in successful and failed flanges. On the one hand, the formability of stretch flanges is usually evaluated in terms of principal strains within the Forming Limit Diagram (FLD). However, this approach does not seem to capture all the physics to explain the enhancement in formability observed in SPIF over the conventional forming. A formability analysis is performed in the field of stress triaxiality versus equivalent plastic strain, discussing the differences between successful and fractured specimens. On the other hand, for shrink flanging, the appearance of wrinkles is analyzed in terms of the compressive stresses along the flange during the incremental forming. This allows to determine a critical limit stress of winkling to predict the failure in practice for a given geometry and forming condition.

Abstract: One of the problems of studying the rheological properties and plasticity of metals and alloys from the results of tensile tests of cylindrical specimens is the need to determine the stress triaxiality value, which depends on the shape and size of the neck formed. An analytical description of the neck profile makes it possible to increase the accuracy of experimental measurements of its dimensions, in particular, the radius of curvature in the smallest cross-section of specimen. This paper is devoted to searching a universal neck profile equation that allows calculating the radius of neck curvature regardless of the nature of the material hardening curve and the stage of strain localization. The exact surface equation is established and its accuracy is estimated for hardening and softening material.

Abstract: This paper assesses various newly developed ductile fracture criteria including modified Mohr-Coulomb (MMC), DF2012, DF2014, DF2016, Hu-Chen and Mu-Zang, which were all proposed in the last decade. The AA2024-T351 is used for the assessment by comparing the predicted fracture limits to the experimental results both in strain and stress spaces. Fracture loci are also constructed by these criteria to evaluate their characteristics. The evaluation demonstrates that the Lode parameter and stress triaxiality should be properly coupled for reasonable modeling of ductile fracture in wide loading conditions. This study also shows that the coupling of the Lode parameter can also be realized by introducing the effect of the largest shear stress in fracture criteria.

Abstract: A stress-based model is developed to describe shear ductile fracture of lightweight metals. The proposed function couples the effect of the maximum shear stress and the stress triaxiality on fracture limits of metals during plastic deformation. Effect of the maximum shear stress in the proposed fracture model is correlated with the influence of the Lode parameter on fracture limits. The proposed fracture model is applied to depict the fracture locus of AA2024-T351. The predicted fracture locus is compared with experimental results of the alloy. The comparison demonstrates that the proposed fracture model reasonably characterizes the fracture stress in various loading conditions of compression, shear and tension.

Abstract: Computational modeling of three-dimensional crack propagation in very ductile materials is still a challenge in fracture mechanics analysis. In the present work a new stress-triaxiality-dependent cohesive zone model (TCZM) is proposed to describe elastic-plastic fracture process in full three-dimensional specimens. The cohesive parameters are identified as a function of the stress triaxiality from ductile fracture experiments. The predictions of TCZM show good agreement with the experimental results for both side-grooved C(T) specimen and rod bar specimen.

Abstract: This paper analyzes the changes in the state of stress in the plastically deforming structural steel which are attributable to the evolution of defects in the microstructure of the material. Numerical simulations were conducted to study the behaviour of a structural element under tensile stress using the Gurson-Tvergaard-Needleman material model for porous media. The model can be employed to assess the effects of microstructural phenomena on the strength of a material under plastic deformation. The main aim of the analysis was to determine the relationship between the changes in the triaxial state of stress and the changes in the volume fraction of voids, characterizing the material porosity.

Abstract: In this work, 3D ductile fracture locus was determined for the advanced high strength (AHS) steel sheet grade DP780 using a hybrid approach between experiment and FE simulation. Tensile tests of different sample geometries were performed for the investigated dual phase steel, by which varying stress triaxiality (η) and lode angle (θ) values developed in the material during loading were introduced. During the tests, the direct current potential drop (DCPD) method and digital image correlation (DIC) technique were applied for identifying crack initiation on the micro-scale and fracture of the specimens due to local plastic deformation. Obtained force and displacement curves were correlated with the electric potential curves. Then, the moments of crack onset were determined for various states of stress. In parallel, the most critical areas of deformed samples before fracture were observed by the DIC method. Subsequently, FE simulations of the tensile tests were carried out and calculated local stresses and strains were gathered. The stress triaxialities, equivalent plastic strains and lode angles were evaluated for the corresponding detected areas. These threshold variables obtained from different specimens were plotted as the 3D failure locus for defining crack initiation and fracture occurrence in the DP steel.

Abstract: There have been many efforts to investigate and develop a numerical damage and failure models during metal forming process of lightweight alloys. Due to the difficulties experienced during experimental determination of the incurred damage during forming of lightweight alloys, many researchers have sought to predict the damage, failure and forming limit curves using numerical simulations. Conventional finite element analysis of metal forming processes for lightweight parts which have been subjected to a nonlinear strain history often breaks down due to numerical difficulties. Many scientific research works have attempted to use different mathematical methods to model the damage progression and failure of alloying material under large deformation. The damage initiation, progression and also failure of alloys are a result of accumulated damage under plastic deformation [1-3]. These models (single and multi-damage parameters) are generally based on energy and constitutive equations to simulate the fracture and failure processes in metal alloys. However, these methods have serious short comes in predicting the damage and failure in metal forming process with strain rate effects. In the present study, following the in-depth study of damage initiation and progression in lightweight alloys, a frame work has been setup to develop a numerical model for damage accumulation during forming process. Based on the existing damage theory, a mathematical extension for damage initiation and also damage accumulation under wide range of stress triaxiality (including near pure shear) has been developed. An experimental program has also been carried out using samples made from lightweight alloys. One of the main contributions of this paper is to show the advantages of using plasticity-based modified damage models to investigate the damage accumulation in cast aluminium alloys.