Search

This study uses a sigmoidal function to describe the plastic strain hardening of metallic materials, considering temperature and strain rate effects. The effectiveness of this approach is evaluated and systematically compared with other hardening laws. Incorporating temperature and strain rate effects into the parameters of this sigmoidal-type hardening law enables a more precise description and prediction of the plastic deformation of materials under different combinations of temperature and strain rate. The temperature effect is coupled using a simplified Arrhenius model, and the strain rate effect is coupled with a modified Johnson–Cook model. The sigmoidal-type hardening law is integrated with an asymmetric yield criterion to address complex behavior, such as anisotropy and strength differential effects. The calibration and validation of the constitutive model involve examining uniaxial tensile/compressive flow curves in various directions and biaxial tensile/compressive flow curves for diverse metallic alloys, proving the proposed model's broad applicability. [ABSTRACT FROM AUTHOR]

A non-associated/associated flow rule coupled with an anisotropic/isotropic quadratic yield function is presented to describe the mechanical responses of two distinct X65 pipeline steels. The first as a product of the cold-rolling forming (UOE) process also known as seam weld pipes and the second as a result of high temperature piercing process called seamless tube manufacturing. The experimental settings consist of a wide range of sample types, whose geometric characteristics represent different state of stresses and loading modes. For low to intermediate stress triaxiality levels, flat specimens are extracted at different material orientations along with notched round bar samples for high stress triaxialities. The results indicate that despite the existing differences in plasticity between materials due to anisotropy induced processes, material failure can be characterized by an isotropic weighting function based on the modified Mohr-Coulomb (MMC) criterion. The non-associated flow rule allows for inclusion of strain directional dependence in the definition of equivalent plastic strain by means of scalar anisotropy (Lankford) coefficients and thus keeping the original capabilities of the MMC model. [ABSTRACT FROM AUTHOR]

The investigation of high cycle fatigue (HCF) properties of materials is elaborate in experimental design. To reduce the costs, a multiscale numerical approach to predict the HCF-strength is proposed in this study. The intention is to correlate microstructural features and their related microdeformation mechanisms with macroscopic fatigue properties. The approach involves a series of numerical and mechanism-based analytical models. The microstructural features, including phase fraction, grain size and grain shape, are statistically characterized and represented in the generated representative volume element (RVE) model of the microstructure of a two phase steel. The underlying microdeformation mechanism is captured by an extended crystal plasticity (CP) model incorporating kinematic hardening on slip systems for cyclic loadings. The CP parameter set was calibrated on strain controlled low cycle fatigue tests. The numerical simulations of the cyclically loaded RVEs resulted in local plasticity fields. The highest plastically deformed grain of each RVE was identified and its grain size averaged plastic strain value extracted for further analysis. These scattered plasticity values fit extreme value distribution density functions. The parameters, which describe the shape of the functions, were used to calculate the HCF-strength. The obtained values showed a good agreement to experimental results. [ABSTRACT FROM AUTHOR]

Sandia National Laboratories have carried out the Sandia Fracture Challenge in order to evaluate ductile damage mechanics models under conditions which are similar to those in the industrial practice. In this challenge, the prediction of load-deformation behavior and crack path of a sample that is designed for the competition under two loading rates is required with given data: the material Ti-6Al-4V, and raw data of tensile tests and V-notch tests under two loading rates. Within the stipulated time frame 14 teams from USA and Europe gave their predictions to the organizer. In this work, the approach applied by Team Aachen is presented in detail. The modified Bai-Wierzbicki (MBW) model is used in the framework of the Second Blind Sandia Fracture Challenge (SFC2). The model is made up by a stress-state dependent plasticity core that is extended to cope with strain rate and temperature effects under adiabatic conditions. It belongs to the group of coupled phenomenological ductile damage mechanics models, but it assumes a strain threshold value for the instant of ductile damage initiation. The initial guess of material parameters for the selected material Ti-6Al-4V was taken from an in-house database available at the authors' institutes, but parameters are optimized in order to meet the validation data provided. This paper reveals that the model predictions can be improved significantly compared to the original submission of results at the end of SFC2 by two simple measures. On the one hand, the function to express the critical damage as well as the amount of energy dissipation between ductile damage initiation and complete ductile fracture were derived more carefully from the data provided by the challenge's organizer. On the other hand, the experimental set-up of the challenge experiment was better described in the geometrical representation used for the numerical simulations. These two simple modifications allowed for a precise prediction of crack path and estimation of force-displacement behavior. The improved results show the general ability of the MBW model to predict the strain rate sensitivity of ductile fracture at various states of stress. [ABSTRACT FROM AUTHOR]

• Enormous plasticity occurs under tension at –196 °C in a high-strength bcc steel that is brittle in toughness tests. • The transition of failure mechanisms at –196 °C shows significant stress state dependence. • A unified fracture criterion is proposed to describe cryogenic fracture covering a broad range of stress states. • A threshold triaxiality governing the transition of failure mechanisms depends on strain hardening capacity and fracture strength. The fracture properties of a high-strength steel with a body-centered cubic (bcc) crystal structure have been characterized at –196 °C by performing tensile tests with different specimen geometries, three-point bending tests using Charpy specimens, and fracture mechanics tests, covering a broad range of stress states under quasi-static conditions. Both strength and ductility of the bcc steel are significantly increased when the temperature is decreased from room temperature to –196 °C. Enormous plasticity occurs in the material during tensile tests using various specimens at –196 °C, while macroscopic brittle fracture takes place in high triaxiality scenarios. A stress state dependence of ductile to brittle transition properties is observed, as the failure mechanisms at –196 °C change from cleavage fracture to shear failure with decreasing stress triaxiality. A unified stress-state-dependent fracture criterion, which considers the transition of failure mechanisms, is proposed to describe the fracture properties of similar bcc materials at cryogenic temperatures. The threshold triaxiality at which the transition of failure mechanisms takes place is a material property that is determined by the strain hardening capacity and fracture strength. In addition, a probabilistic formulation relying on the extreme value distribution has been incorporated into the model to render the statistical nature of cleavage fracture. [ABSTRACT FROM AUTHOR]

• Obvious anisotropic plasticity and fracture behavior is found in a high-strength pipeline steel. • The plastic anisotropy evolution is decisive in the identification of the local fracture strain. • The enHill48 model predicts the correct deformation behavior. • The enHill48 model is coupled with a hybrid damage mechanics model for anisotropic fracture. • The proposed model showed excellent predictive capability of the anisotropic fracture behavior. A hybrid experimental and numerical investigation has been conducted to comprehensively characterize the anisotropic plasticity and ductile fracture behavior of a high-strength pipeline steel. Tensile tests have been performed on various flat specimens along three different loading directions to collect the experimental mechanical data covering a wide range of stress states. For numercial modeling, the anisotropic plastic deformation is described by the evolving non-associated Hill48 (enHill48) plasticity model considering anisotropic/distortional hardening and evolution of r-value. Based on the enHill48 model, in this study, an anisotropic damage mechanics model with consideration of the evolution of anisotropy and stress states has been proposed and calibrated to predict the anisotropic damage and fracture of the investigated material. It is concluded that the anisotropic hardening is critical for an accurate prediction of the ductile fracture. The proposed model has achieved good predictive capability for anisotropic fracture behavior. [ABSTRACT FROM AUTHOR]

The understanding of the influence of inclusion parameters like size, shape and surface roughness on the material’s fatigue behavior can provide important guidelines for future material design. In this study, a numerical model is used to calculate the influence of inclusions on fatigue properties. The model consists of the statistical evaluation of numerous representative volume element (RVE) calculations in combination with a crystal plasticity (CP) constitutive model. The RVEs contain inclusions which differ in size, shape, surface roughness and elastic mismatch to the matrix. These parameters have been identified to determine the fatigue lifetime for the case that inclusions are the origin of failure. For each inclusion type, up to 100 RVEs are generated. The RVEs have statistically equivalent distributed microstructural properties but differ in detail, though. The results show that the inclusion size has the biggest influence on the lifetime. The influence of shape and roughness of the inclusion surface is negligible small. The study of the elastic mismatch is matching literature findings. All in all, this study shows that the numerical model can be used to calculate the influence of inclusions on fatigue lifetime. The use of the model can help to reduce the experimental effort for specifying the minimum cleanness requirement of a steel to guarantee the structural integrity under cyclic loading. [ABSTRACT FROM AUTHOR]

• Cleavage fracture occurs after significant plastic deformation at room temperature in an advanced high strength steel (Q&P steel) with superior tensile properties. • Different failure patterns under sheet metal forming conditions are demonstrated using three types of advanced high strength steels. • The forming limit framework is extended to account for cleavage fracture, which is revealed to be relevant for metal forming of high strength materials. • A sensitivity analysis on failure patterns concerning material properties and stress state is performed to provide insights into developing high strength metallic materials. • The proposed framework provides a comprehensive guide for forming of various types of sheets. The forming limits and failure properties of three distinct advanced high-strength steels (AHSS) have been investigated under various stress states in tensile tests with optimized specimen geometries. In addition to the commonly observed failure patterns governed by localized necking and ductile fracture for two of the AHSS, after substantial plastic deformation at room temperature, cleavage fracture occurs for a large range of stress states in a laboratory quenching and partitioning steel with superior tensile properties. The competition between failure patterns, encompassing ductile and cleavage fractures with and without necking, is governed by the mechanical properties of materials and the stress states, as a transition of failure mechanisms occurs with increasing triaxiality. The forming limit framework is, therefore, further extended to seamlessly integrate cleavage fracture in this study, where the competition between various failure mechanisms is demonstrated using three AHSS in the space of critical strain and principal stress. These findings shed light on the importance of considering cleavage fracture strength as a parameter besides the strength-ductility synergy in advanced high-strength metallic materials, and the proposed framework also gives a more comprehensive guide in designing and conducting the sheet metal forming processes. [Display omitted] [ABSTRACT FROM AUTHOR]

• A method to derive the isothermal hardening curves for steels at higher strain rates is demonstrated. • With the proposed method, the Taylor–Quinney coefficient variation at the intermediate strain rates range is calculated. • The eMBW model is formulated to reflect the temperature, strain rate and stress state influences on plastic deformation and fracture behavior. • The model is fully calibrated and validated by the lab-scale tests and successfully predicts the crashworthiness characteristics of the tube crushing tests. • The crack formation mechanics in the crushing tests is shearing and second-level bending caused by the self-contact between formed folds. Experimental and numerical investigations at both lab and structure scales on the plasticity and failure behavior of an automotive dual-phase steel sheet (DP1000) are performed in this study. On the lab level, an extensive experimental program considering the temperature and strain rate effects on the plastic deformation behavior and the stress state dependency of the ductile fracture behavior is designed. On the structure level, dynamic square tube crushing tests are performed on a drop tower. The extended hybrid damage mechanics model is formulated in the study to describe the deformation and damage/fracture behavior of DP1000 considering the influence of temperatures, strain rates, and loading histroy. The plasticity and fracture description at the lab scale is used to calibrate the material parameters of the model, which is further applied to predict the crashworthiness of the square tube crushing tests. The proposed model has been proven to show very good predictive capability at both lab and structure scales. It is further found that for the modern high-strength steels short cracks could be developed in the tube crushing tests and the crack formation mechanics is shearing and local second-level bending caused by the self-contact between folds. Image, graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • An evolving non-associated Hill48 plasticity model was formulated. • The model accurately accounts for the anisotropic hardening as well as the r-value evolution. • Under non-associated flow rule, both the yield function and flow potential determine the occurrence of localisation. • The evolving feature of the anisotropic properties contributes to the good prediction of the forming limits. Abstract Experimental and numerical investigations on the characterisation and prediction of cold formability of a ferritic steel sheet were performed in this study. Tensile tests and Nakajima tests were conducted for the plasticity characterisation and the forming limit diagram determination, respectively. For the plasticity behaviour description, an evolving non-associated Hill48 anisotropic plasticity model was formulated to accurately characterise the anisotropy evolution under monotonic loading, including anisotropic hardening as well as the r-value evolution. The detailed model parameter calibration procedure was also demonstrated. Eventually the model was applied to the forming limits prediction in conjunction with the modified maximum force criterion. A systematic and detailed study was performed addressing the impacts of the evolving and non-associated characteristics of the model formulation on the forming limits prediction by comparing the proposed model with the classical ones. Both the plasticity model and its application to the formability prediction were validated by the experimental results. Graphical abstract [ABSTRACT FROM AUTHOR]

The aim of this paper is to develop a micromechanical model to quantitatively upscale the damage initiation of dual-phase steels under various stress states from micro to macro and reveal the underlying mechanisms of the damage initiation dependency on stress states from a microstructural level. Finite element (FE) model based on the real microstructure of a DP600 steel sheet is employed by representative volume element (RVE) method. Several numerical aspects are also discussed, such as mesh size and discretisation feature of the phase boundary. The plastic strain localisation theory is applied to the RVE modelling without any other damage models or imperfections. Three typical stress states, uniaxial tension, plane-strain tension and equibiaxial tension, are considered to investigate the influence of the stress state on damage initiation. The quantitative evaluation of the damage initiation for three stress states obtained from the RVE simulation shows the dependency on both stress triaxiality and Lode angle. The results are further compared to the experimentally calibrated damage initiation locus (DIL) and a fairly good agreement is achieved. From this study, the general physical understanding of the effect of stress states on damage initiation is explored and the method for quantitative analysis of the damage initiation in a microstructural level is also established. The microstructure heterogeneity is considered as the key factor that contributes to the damage initiation behaviour of the dual-phase steel. [ABSTRACT FROM AUTHOR]