Prediction Of Ductile Fracture Research Articles

Ductile fracture modelling of steel plates under tensile and shear dominated states

The prediction of ductile fracture is highly related to the stress and strain states in the plastic stage. For steel plates in the engineering structure, despite many stress-strain equations based on the von Mises criterion developed, it is found that the finite element (FE) analysis using true stress-strain relation calibrated by the uniaxial tensile tests cannot properly describe the shear behaviour of steel plates. In this paper, the combined linear and power stress-strain law with a shear plastic factor is proposed to characterize the mechanical behaviour of steel plates under shear dominated state. Then, Bao-Wierzbicki (BW) criterion, an uncoupled damage model, is adopted to predict the tensile and shear dominated fracture of steel plates. The uniaxial tensile and shear tests of steel plates are carried out to calibrate relevant parameters in the true stress-strain relation and ductile fracture model. Furthermore, single-bolt connections subjected to tensile and shear dominated states are designed and tested. Parallel FE simulations are performed to validate the ductile fracture model with the calibrated stress-strain relations. FE results regarding these specimens show a good agreement with the corresponding experimental results. The uncoupled damage model integrated with the proposed stress-strain relation is verified to be suitable to simulate the ductile fracture of steel plates under tensile and shear dominated states.

Prediction of forming limit curve for AA6061-T6 at room and elevated temperatures

Forming Limit Curve (FLC) has been widely adopted as a practical criterion for evaluating the formability of sheet metals. Predicting a reliable FLC by a virtual methodology could lead to robust process optimization before expensive tool manufacturing. In order to increase the predictive capabilities of the virtual forming tools, an accurate modeling of the forming limit curve should be considered at room and elevated temperatures. In this work, the isothermal forming limit curves of 6000 series aluminum alloy sheet metal are predicted by performing numerical simulations of Nakajima test. A stress triaxiality and Lode angle based ductile fracture criterion is used to determine the forming limit curve. Also, the ductile fracture criterion is extended to add the impact of temperature on ductile fracture prediction. The hybrid experimental-numerical approach is used to calibrate the ductile fracture criterion. The forming limit curve of AA6061-T6 sheet metal, with a thickness of 1 mm, is predicted using the calibrated ductile fracture criterion at room and elevated temperatures. Numerical simulations are performed in 3D with the finite element code Abaqus. The limit strains are determined for specimens undergoing deformation under different strain paths. Influence of temperature on the predicted forming limit curve is discussed.

Numerical prediction of ductile fracture during the partial heating roll forming process of DP980

Numerical prediction of ductile fracture during the partial heating roll forming process of DP980

Experimental and numerical investigation of ductile fracture for AA6061-T6 sheets at room and elevated temperatures

Warm forming is largely employed to enhance the poor formability of aluminum alloys at room temperature. The knowledge of the effect of stress state and temperature on the forming limits is an important aspect for the control of warm forming operations. Therefore, the objective of this work is to investigate the influence of temperature on the ductile fracture of AA6061-T6 aluminum alloy sheet metal under different stress states, ranging from shear to biaxial tension. For this purpose, experiments and numerical simulations of uniaxial tensile tests on dog-bone shaped specimen, notched specimens with different radius, specimens with a central hole and shear specimens are conducted at room and elevated temperatures. The hybrid experimental–numerical approach is used to identify the fracture strain and the corresponding stress state parameters (i.e. stress triaxiality and Lode parameter) to derive the fracture loci at room temperature, 150∘C and 200∘C. To accurately model the material behavior, the positive strain rate sensitivity in the flow stress response at elevated temperature is considered and attention is paid to constrain the finite element model by using the real experimental boundary conditions. The strain rate dependence of fracture strain is not considered in this work. A strong dependency of the ductile fracture on the temperature and stress state is evidenced. Also, an extension of Lou’s ductile fracture criterion which includes the impact of temperature on ductile fracture prediction is proposed. The prediction accuracy of the original and extended criteria is evaluated. It is found that Lou’s ductile fracture criterion is well suited to predict accurately ductile fracture at elevated temperatures under isothermal conditions whereas the extended form is very useful to predict ductile fracture initiation under non-isothermal conditions.

Prediction of ductile damage and fracture in the single- and multi-stage incremental hole-flanging processes using a new damage accumulation law

Prediction of ductile damage and fracture in the single- and multi-stage incremental hole-flanging processes using a new damage accumulation law

Ductile Fracture Prediction During an Extended Miyauchi Shear Testing

Ductile fracture during an extended Miyauchi shear testing was predicted using an analytical model. First, an extended Miyauchi shear testing was proposed, in which the angle between the line segment that connects two neighboring notch roots and the tensile direction of the specimen was set to be an arbitrary value between zero and π/2. Next, an analytical model for the extended Miyauchi shear testing was proposed, in which the deformation of the sheet before localized necking and that after localized necking were analyzed in the region surrounding the line segment connecting two neighboring notch roots, and ductile fracture was predicted. Finally, the major strain at fracture and the minor strain at fracture obtained by the experiment for nine angles between zero and π/2 were compared with those calculated using the analytical model and those calculated using the ellipsoidal void model proposed previously by the author. Special attention was paid to the effect of prestrain which was imparted to the sheet by rolling on ductile fracture.

Fracture prediction in transverse fillet welded joints of high strength structural steel

Due to the huge property difference between high strength structural (HSS) steels and mild steels after welding, mechanical behavior and failure mode of fillet welded joints of HSS steels should be studied further. Numerical simulations can provide increased insight into this topic. This paper concerns finite element simulations of component tests of two types transverse fillet welded joints of HSS steels: lap-welded transverse fillet welds and cruciform type transverse fillet welds. In order to investigate the influences of triaxial stress states on fracture ductility of weld metals, a series of tests on specimens with various initial geometries are carried out. By comparing three typical uncoupled fracture models (stress triaxiality-dependent ductile fracture model (VGM), Lode angle-dependent ductile fracture model (MSSM) and combination of stress triaxiality and Lode angle ductile fracture model (BWM)), BWM gives precious prediction of ductile fracture of weld metals. The behaviors in terms of load-displacement and failure modes of transverse fillet welds are all well captured by the numerical simulations with calibrated ductile fracture models. Specifically, the location of fracture initiation and propagation of cracks obtained from simulations are both identical with real responses.

Prediction of ductile fracture for DP590 high-strength steel with a new semi-coupled ductile fracture criterion

Prediction of ductile fracture for DP590 high-strength steel with a new semi-coupled ductile fracture criterion

Numerical study of the robustness of steel moment connections under catenary effect

The local fracture will directly result in the consequent decrease in the vertical resistance of the beam-to-column assembly in the steel frame structure, which emphasizes the necessity of the ductile fracture prediction in the numerical study. In this paper, a monotonic plasticity and ductile fracture model is used to characterize the stress state and loading path dependency of the yield surface, as well as the combined effects of the shear stress and hydrostatic pressure on the damage evolution. On the basis of the validated material model, five push-down tests of the beam-to-column assemblies with different moment connections are chosen for the validation of this modeling method in terms of overall vertical force–displacement curves and the local crack initiation and propagation. Thereafter, the numerical studies of the weld access hole reveal that the gentle slope of transition cutting and larger distance between toe of weld access hole and flange weld will raise the connection ductility. The relationship between ultimate chord rotation and these two variables can be formulated by the empirical formulas. As for the bolted web-to-column connection, the deformation capacity can be improved by using compact bolt arrangement and larger bolt size. The implementation of the shear tab with long-slotted holes will also enhance the connection ductility under catenary mechanism. In general, these findings will be benefit to improve the robustness of the steel frame structure, along with the quantitative estimation of connection ductility for checking the structural safety.

Ductile Fracture Prediction in Cross-Wedge Rolling of Rail Axles.

In the process of cross-wedge rolling, axial-symmetric forgings are formed using wedge tools. These tools may be flat- or roll-shaped. This article presents two methods of cross-wedge rolling of rail axles, traditional and multi-wedge, as well as their advantages and disadvantages. Two cross-wedge rolling processes are modelled numerically using Simufact Forming. Numerical results are then verified by experiments performed on a flat wedge rolling mill. Results obtained with the two rolling methods are compared in terms of material fracture, force parameters, effective strain and thermal conditions during rolling. Results show that material fracture poses a serious problem in these rolling processes. It is found that the Cockcroft–Latham ductile fracture criterion does not predict material fracture correctly. Results demonstrate that the fracture of railway axles in cross-wedge rolling can be best predicted by the fracture criteria developed by Ayada, Brozzo, Ko, Rice and Tracey.

A high–temperature Mohr–Coulomb criterion dependent on temperature, strain rate, and stress state for ductile fracture prediction

The cracks formed in the hot deformation are typical defects caused by plastic capacity limitations. For tested steels, the fracture behavior is associated with temperature, strain rate, and stress state. In this study, a highly crack-sensitive steel with Nb micro-alloying is selected as the research material to establish the fracture criterion considering the above factors comprehensively. First, compression and tension experiments are performed, and various specimen geometries are designed to produce different stress states in the tests at different temperatures and strain rates. Second, the critical stroke of these tests is extracted, and corresponding calculations are completed using the finite element method (FEM). The loading paths of stress states are independent of temperature and strain rate; however, they differ when distinct specimen shapes are considered in the tests. Third, a high–temperature Mohr–Coulomb criterion, which is applicable at elevated temperatures, is established by considering the effects of temperature, strain rate, and stress states on fracture. Finally, some examples of compressive and tensile tests are employed for verifying the formulated fracture criterion. By comparing the FEM and experimental results, it is found that the fracture criterion could accurately describe the fracture behavior during hot deformation with distinct stress paths.

Ductile fracture prediction of additive manufactured Ti6Al4V alloy based on an extended GTN damage model

Selective Laser Melting (SLM) is one of the revolutionary additive manufacturing (AM) technologies to produce metallic mechanical parts. To thoroughly understand the damage and fracture behavior of SLM-fabricated Ti6Al4V alloy under complex stress states, a number of tensile, compressive and torsional experiments covering various stress states were conducted with different types of the specimens. A modified GTN (Gurson-Tvergaard-Needleman) model with dual damage variables characterizing the void growth mechanism and void shear mechanism was adopted to describe and predict the ductile fracture behavior of the SLM-fabricated Ti6Al4V alloy under complex stress states. A new stress state-dependent function was introduced to extend the application of the model to negative stress triaxiality state. A finite element (FE) based inverse analysis strategy was proposed to calibrate the model parameters. The simulation results showed that the damage and fracture behavior of SLM-fabricated Ti6Al4V alloy can be well described by the modified GTN model. The morphologies of the fracture surfaces were successfully predicted by using the developed damage constitutive model, in which the variable of void fraction dominates fibrous zone failure based on the void growth mechanism, and the shear damage variable facilitates shear fracture based on the void shear mechanism.

Quantitative damage evaluation of LY225 steel under monotonic tensile loading based on acoustic emission entropy

The purpose of this paper is to establish a quantitative relationship between acoustic emission (AE) signal acquisition data and the void growth model (VGM) of low yield point steel LY225 under monotonic tensile loading, to realize the real-time evaluation of plastic damage and the prediction of ductile fracture of materials. A series of material tests, finite element analysis and scanning electron microscope (SEM) tests were carried out to calibrate the VGM fracture parameters of LY225 steel, including the characteristic length l∗ and the critical value of VGI. It is found that the AE signals under monotonic tensile loading have a prominent two-stage characteristic and the cumulative AE entropy of AE signals in LY225 steel is proportional to the volume of material involved with plastic deformation. Then a bilinear function model between cumulative AE entropy and void growth index VGI is proposed and calibrated with monotonic tensile test results from a set of standard rectangular section specimens, and the effectiveness of the proposed model in more complex stress and strain state is verified by tests of a set of variable cross-section flat specimens.

The modified GTN-Thomason criterion for modelling of ductile fracture considering shear factor and size effect in micro-scaled plastic deformation

Coarser grains lead to fewer voids on the fracture surfaces of miniaturized workpiece via affecting the void nucleation, while finer grains result in a higher flow strength by influencing void growth. Furthermore, finer grains make the occurrence of normal tensile fracture difficult, but they accelerate the happening of shear-dominated fracture. These insights help the development of ductile fracture criterion by considering the fracture micro-mechanism in micro-scaled plastic deformation. Shear factor and size effect are thus introduced into the newly modified GTN-Thomason criterion to predict the ductile fracture in micro-scaled plastic deformation of copper sheets with different microstructural grain sizes and deformation stress states. The developed criterion is implemented in simulation of deformation process and well validated to be efficient by corroboration of the experimental results and the simulated load-stroke responses and deformation profiles of five sheets with different grain sizes and stress states. According to the accurate simulation results, the influences of stress triaxiality, the normalized third invariant and grain size on void evolution are further analysed. The results show that the increase of the normalized invariant prevents void shear and leads to a better ductility, while the increase of stress triaxiality facilitates the growth of voids to result in a worse ductility. When the deformation with a relatively high normalized invariant has an ignored void shear, the increase of grain size accelerates the onset of void coalescence and the rate of void growth, leading to a rapid failure. However, when the deformation with a relatively low normalized invariant has an unneglected void shear, finer grains increases the rates of void nucleation, growth and shear, causing a smaller fracture strain. These findings and the modified GTN ductile fracture criterion enhance the understanding and prediction of ductile fracture in micro-scaled plastic deformation, respectively, and further facilitate the development of microparts using by deformation-based micro-manufacturing process.

Numerical Prediction of Ductile Fracture in Sn-5Sb Alloy Hot Deformation Based on Phenomenological Models and Finite Element Analysis

Numerical Prediction of Ductile Fracture in Sn-5Sb Alloy Hot Deformation Based on Phenomenological Models and Finite Element Analysis

Formability enhancement in hot spinning of titanium alloy thin-walled tube via prediction and control of ductile fracture

The damage and fracture in hot spinning of titanium alloy is a very complex process under the combined effects of microstructure evolution and stress state. In this study, their dependences on processing parameters were investigated by an integrated FE model considering microstructure and damage evolution, and revealing the effects of microstructure and stress states on damage evolution. The results show that the inner surface of workpiece with the largest voids volume fraction is the place with the greatest potential of fracture. This is mainly attributed to the superposition effects of positive stress triaxiality and the smallest dynamic recrystallization (DRX) fraction and β phase fraction at the inner surface. The damage degree is decreased gradually with the increase of initial spinning temperature and roller fillet radius. Meanwhile, it is first decreased and then increased with the increases of spinning pass and roller feed rate, which can be explained based on the variations of β phase fraction, DRX fraction, stress state and tensile plastic strain with processing parameters. In addition, the dominant influencing mechanisms were identified and discussed. Finally, the thickness reduction without defect in the hot spinning of TA15 alloy tube is greatly increased by proposing an optimal processing scheme.

Prediction of ductile fracture in skew rolling processes

This paper begins with a review of the literature regarding the modelling of skew rolling processes, with a special focus on the problem of material fracture. Numerical modelling is performed to compare stresses and strains in the cross sections of bars produced by skew rolling with the use of two and three rolls. It has been found that the two-roll skew-rolled bars are more prone to the occurrence of fracture in their axial zone. It is proposed that cracking in skew rolling processes be predicted with the use of classical fracture criteria. The accuracy of crack prediction by these criteria depends on the correct determination of the critical damage. The critical damage in skew rolling processes can be determined by two tests: rotary compression of a tapered specimen (when a process is performed with two tools) or rotary compression of a disc-shaped specimen in a cavity between the tools (when a process is performed with three tools). Six fracture criteria and a new test of rotary compression of a tapered specimen are employed to investigate fracture in a two-roll skew rolling process for producing cylindrical bars made of 100Cr6 grade steel. It has been found that stress-based criteria are more suitable for modelling fracture due to their lower sensitivity to temperature variations in the axial zone of rolled bars.

Application of Anisotropic Ductile Fracture Model for Prediction of Diagonal Cracks in Outer Rim of Flange-Shaped Parts

Inclined rotary forming (IRF) combines bending and axial compression with rotation and is employed for forming flange shaft-structured parts from round rod billets. Although the forming load is smaller than that in conventional processes, cracks may develop during this process. However, it is difficult to predict the onset of cracks using conventional ductile fracture prediction equations. In this paper, the ductile fractures were predicted using the anisotropic ductile damage equations that the present authors previously developed. The proposed equation was expressed as a second-rank tensor resulting from the product of the stress tensor and the strain increment tensor. Referring to previously published results on the compression of cylindrical specimens, the difference between diagonal and longitudinal sidewall cracks was assumed by considering the anisotropy of ductile damage. The proposed equation was then applied to the IRF process, where diagonal cracks developed at the outer rim of the flange. Consequently, it was found that the proposed anisotropic damage model can predict the onset of diagonal cracks on the outer rim and the effect of the lubrication condition on the onset of these cracks.

Effect of non-associated flow rule on fracture prediction of metal sheets using a novel anisotropic ductile fracture criterion

The ductile fracture behavior of anisotropic materials was investigated and modeled by the uncoupled ductile fracture criterion for aluminum alloys 6016-AC200. The ductile fracture model takes into account micro-mechanisms of void nucleation, void growth, and evolution of void coalescence. The anisotropic yield function and non-associated flow rule were applied to increase the accuracy of the ductile fracture prediction. Series of uniaxial tensile tests, equi-biaxial tension test, and hydraulic bulge test were utilized to identify coefficients of the plasticity model. Various standard fracture tests covering a wide range of stress triaxiality from negative to moderate and high-stress triaxiality were used in order to calibrate the ductile fracture parameters for AA6016-AC200. The strain field on the surface of specimens was captured by the non-contact measurement DIC system for all tests. The fracture surface, fracture locus, and fracture forming limit diagram constructed by the proposed ductile fracture criterion were plotted. These basic fracture tests and a square cup drawing test were simulated by the ABAQUS/Explicit commercial software to verify the efficiency of the proposed fracture criterion and the advanced plastic model in predicting the onset of ductile fracture behavior. The results indicated that the proposed ductile fracture criterion can be utilized for predicting the onset of the anisotropic ductile fracture behavior in sheet metal forming.

Elastoplastic Damage Models for Ductile Materials

The application of the Elasto plastic-damage model is used to describe the ductile fracture model. Few parameters are responsible for ductile damage and its mechanism can be revealed by using Elasto plastic models. An overview of different models is described in this paper. Parameters responsible for high-stress triaxiality and low-stress triaxiality are also described in this paper. The objective of this article is to provide a review of Elasto-plastic damage models, which can be used for prediction of ductile fracture. Stress triaxiality is the ratio between mean stress and equivalent vonmisses stress.