Papers by Keyword: Ductile Damage

Abstract: Forming processes significantly influence the product properties of a formed workpiece. Next to the effects of work hardening and residual stresses, the influence of ductile damage determines the final performance of a formed component. Thus, precise damage models are crucial for designing new forming process sequences. In general, this is achieved by modelling the evolution of damage as a function of hydrostatic and deviatoric stress, characterized by the stress triaxiality and the Lode-parameter. However, calibrating damage models to the effects of triaxiality and the Lode-parameter is not trivial, since experiments usually represent a combination of both influences. A recent experimental approach by the authors offers the possibility to vary the Lode-parameter in extrusion experiments while keeping the triaxiality constant. This paper aims to use this data of the isolated deviatoric effect on damage to calibrate a damage evolution equation. The model is calibrated to void area fraction measurements obtained by scanning electron microscopy of extruded case-hardening steel 16MnCrS5. For validation, the model predictions for non-constant Lode-parameter histories are compared to corresponding experiments. The model and experiments are in good agreement.

Abstract: Due to the emerging relevance of topics such as climate change or scarcity of resources, the requirements for energy efficiency, emissions and resource conservation are increasing. In this context, components manufactured by metal forming offer a high potential for lightweight construction, cost effectiveness and resource efficiency. The defects resulting from forming processes e.g. in form of micropores and their growth are currently not taken into account. A commercial design is usually based on mechanical material properties and additional safety factors. The knowledge of the ductile forming-induced damage in the component design enables an improved design. In this study, the influence of different forming process parameters during full forward rod extrusion on the structural damage and the fatigue properties were investigated for the case-hardened steel AISI 5115 (16MnCrS5, 1.7131). The intention was to compare the fatigue properties of different damage states under cyclic axial and axial-torsional loading including the identification and separation of underlying damage mechanisms. A significant effect of superimposed cyclic torsional loading on cyclic axial properties and mechanisms was found, which was associated with a decrease of 38 % in the lifetime. Axial-torsional fatigue tests were conducted at various test temperatures to determine the effect of forming-induced damage and test temperature on the fatigue strength. In addition, differences in microstructure as a result of forming-induced and fatigue-induced damages were validated by using scanning electron microscopy.

Abstract: Both experimental method and numerical method are used to analyze the large variation in the material ductility of high pressure die casting (HPDC) Aural-2 alloy in the present work. The X-ray tomography (XRT) technique is used to characterize and reveal the significant variation of the internal porosity for the investigated material. The Mises plasticity model in conjunction with a mixed Swift-Voce hardening law, and a stress state dependent fracture initiation criterion are used to accurately describe the deformation response of the material. Very good agreement with the experimental results is obtained in the predicted average force-displacement responses for the calibrated stress states. A probabilistic damage mechanics model is put forward to depict the apparent stochastic ductile fracture behavior over a wide range of stress states. The 5^th and 95^th percentiles of the fracture initiation locus are recalibrated based on the proposed probabilistic ductile fracture model, which could provide an almost perfect prediction of the maximum and minimum bounds of force-displacement curves.

Abstract: Aim of this study is to describe the ductile damage of metastable austenitic steels which show TRansformation Induced Plasticity (TRIP). Therefore, a criterion for the austenite to martensite transformation, the caused additional hardening and evolution equations for the TRIP-strain are incorporated into the damage model of Rousselier. As a first approach, the model is calibrated against unit cell simulations of the porous material for different stress triaxialities.

Abstract: The ingot forging process is numerically simulated applying both the Shima-Oyane porous plasticity model as a coupled damage model and the uncoupled normalized Cockcroft & Latham criterion. Four different cases including two different lower die angles (120^o and 180^o) and two different sizes of feed (400mm and 800mm) are analysed. Comparison of the simulation results with recommendations in literature on ingot forging, indicates the normalized Cockcroft & Latham damage criterion to be the most realistic of the two.

Abstract: Titanium-based alloys are commonly applied to aerospace, medicine and energy due mainly to their high specific mechanical properties and high corrosion resistance. Reinforcement with particles further improves their specific strength and stiffness. In previous studies, the hot formability of both unreinforced Ti6Al6V2Sn alloy, and reinforced with 12%vol of TiC particles was analyzed by hot compression tests carried out by means of Gleeble device and metallography. It was observed that the hot workability of these materials is limited at given forming conditions by non-desirable shear bands, voids, as well as micro- and macro cracks, especially in the composite. In this work, damage during hot deformation is predicted by damage models coupled to FEM. Therefore, the flow localization parameter α described by thermal and microstructural softening and the strain rate sensitivity are computed and implemented in DEFORM^TM 2D to describe the localization of the plastic flow. The results show intense flow localization as a combination of low dynamic restoration (given by small m values) and temperature gradient. The damage analysis combined with the Cockcroft and Latham continuum cumulative stress model.

Abstract: Deformation response and failure process of a spot welded joint are investigated in this study. For this purpose, a cross-tension spot welded joint sample made of dual phase steel sheets (DP600) is prepared and tensile tested to failure. Complementary FE simulation of the test is performed. The FE model acknowledges the variation of properties across the spot welded region. Rice-Tracey ductile damage model is approximated and employed in the simulation. Close comparison of load-displacement curves and deformed shape with measured values serve as validation of the FE model. Results show that FE simulation with damage-based model adequately predicts tensile deformation and failure of the spot welded joint. Tensile failure of the joint is confined to the heat affected zone and heat affected/fusion zone interface of the joint. Localized through-thickness necking of the sheet metal is captured. In addition, the predicted fracture of the spot welded joint is accompanied by localized extensive plastic deformation.

Abstract: The objective of this study is to evaluate how the inclusion sizes influence the damage value and the maximum hydrostatic stress during multi-pass dry drawing process. It was well known that non-metallic inclusions can lead to material fracture during metal forming process, and be harmful to the quality of the final product. In multi-pass dry wire drawing, the temperature rise during deformation greatly decreases the quality of final product. In this study, the pass schedule in which initial diameter of 3.55 mm is reduced to final diameter of 2.115 mm was isothermally designed for the high carbon content steel. Spherical non-metallic inclusion of Al₂O₃ is located in the center of a steel rod. Influences of inclusion size, 5 μm, 10 μm, 20 μm and 50 μm in initial diameter 3.55 mm on ductile damage was investigated by FE-simulation in which material fracture was estimated using normalized Cockcroft and Latham criterion.

Abstract: Tensile tests on smooth and notched axisymmetric specimens were carried out to determine the large strain work-hardening curves and the ductile fracture characteristics of an AA6060 aluminium alloy for three different processing routes. The alloy was processed in three subsequent steps: 1) casting and homogenization, 2) extrusion, and 3) cold rolling and heat treatment to obtain a recrystallized grain structure. After each processing step, the material was tested after natural ageing for more than one week. A laser-based extensometer was used to continuously measure the average true strains to failure in the minimum cross-section of the specimens and the true stress-strain curves were calculated. Since these curves are influenced by necking, they do not represent the correct work-hardening of the material. Accordingly, finite element (FE) simulations of the tensile tests on the smooth axisymmetric specimens were conducted to determine the work-hardening curves to failure, using an optimization tool that interfaced with the nonlinear FE code and the experimental stress-strain curves as objectives. The microstructure of the alloy was characterized after the three processing steps by optical and scanning electron microscopy, and fractography was used to investigate the failure mechanisms.

Abstract: Gamma titanium aluminides are promising alloys for low-pressure turbine blades. A significant disadvantage of such intermetallic alloys is failure induced during forming processes due to ductile damage and flow instabilities. Previous investigations on a gamma titanium aluminide alloy (TNM), have shown ductile damage due to tensile stress components and instabilities such as shear bands, pores and micro-cracks at low temperatures and high strain rates. The main part of the current work is to delineate damage and unstable regions in the low temperature region. Hot deformation experiments are conducted on a Gleeble®3800 thermomechanical treatment simulator to obtain flow curves to be implemented in a finite element method (FEM) code. Instabilities in the material are described by existing instability criteria as proposed by Semiatin and Jonas and implemented into FEM code DEFORM^TM 2D. Predictions of ductile damage models and the instability parameter are validated through detailed microstructural studies of deformed specimens analysed by light optical- and scanning electron microscopy.