A phenomenological ductile fracture initiation model for metals is developed for predicting ductile fracture in industrial practice. Its formulation is based on the assumption that the onset of ...fracture is imminent with the formation of a primary or secondary band of localization. The results from a unit cell analysis on a Levy–von Mises material with spherical defects revealed that a Mohr–Coulomb type of model is suitable for predicting the onset of shear and normal localization. To improve the agreement of the model predictions with experimental results, an extended Mohr–Coulomb criterion is proposed which makes use of the Hosford equivalent stress in combination with the normal stress acting on the plane of maximum shear. A fracture initiation model is obtained by transforming the localization criterion from stress space to the space of equivalent plastic strain, stress triaxiality and Lode angle parameter using the material’s isotropic hardening law. Experimental results are presented for three different advanced high strength steels. For each material, the onset of fracture is characterized for five distinct stress states including butterfly shear, notched tension, tension with a central hole and punch experiments. The comparison of model predictions with the experimental results demonstrates that the proposed Hosford–Coulomb model can predict the instant of ductile fracture initiation in advanced high strength steels with good accuracy.
Using load-independent creep constraint parameters Q* and Qm*, the variations of crack front stress field and constraint level are analyzed for various crack sizes of pipelines containing internal ...surface cracks under high temperature conditions. The results show that the trends of stress changes in the axial and circumferential internal surface crack front for different crack sizes are similar. For axial internal surface cracks, when the crack depth to length ratio a/c is the same, the maximum values of crack front Q* and Qm* are shifted from the deepest part of the crack (2Φ/π = 1) to the vicinity of the free surface (2Φ/π = 0.2) as the crack depth a/t increases. As for the circumferential inner surface cracks, when the crack depth a/t exceeds 0.2, the locations where the maximum values of crack fronts Q* and Qm* appear are mainly concentrated in the deepest part of the crack (2Φ/π = 1). Based on the calculation results of the constraint parameters, the empirical formulas for the engineering calculation of the relationship between the average constraint parameters (Qavg* and Qm‐avg*) and the crack size are obtained by fitting.
•Creep constraints of crack fronts in pipes with different shapes were analyzed.•The constraints of creep crack fronts were characterized using Qm* and Q*.•CCG behaviors were consistent with the constraint parameter results.•Average constraint parameters Qavg* and Qm‐avg* for creep crack fronts had been proposed.
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The effect of stress state and loading path on the ductile fracture of aluminum 2024-T351 is characterized through tension–torsion experiments on tubular specimens. The experimental ...program includes proportional and non-proportional loading paths leading to the onset of fracture at nearly plane stress conditions at stress triaxialities between 0 and 0.6. Stereo digital image correlation is used to measure the displacements and rotations applied to the specimen shoulders. An isotropic non-quadratic Hosford plasticity model with combined Voce–Swift hardening is used to obtain estimates of the local stress and strain fields within the specimen gage section. The hybrid experimental–numerical results indicate a higher strain to fracture for pure shear than for uniaxial tension. The calibration of a Hosford–Coulomb fracture initiation model suggests that the ductility of aluminum 2024-T351 decreases monotonically as a function of the stress triaxiality, whereas it is a non-symmetric convex function of the Lode angle parameter. It is shown that a simple non-linear damage accumulation rule can describe the effect of non-proportional loading on the strain to fracture.
•Four commonly used shear test specimens were designed and optimized by FEA.•The advantages and disadvantages of each specimen were clarified to provide guidance for shear tests.•The plastic and ...failure characteristics of different specimens were compared and analyzed.•The underlying causes for the different characteristics and performances were also analyzed.
In this paper, the dynamic shear response and failure of Ti-6Al-4V using four different test specimen geometries, viz. Hat-Shaped Specimen (HSS), Flat Hat-Shaped Specimen (FHSS), Chip Hat-Shaped Specimen (CHSS) and Double Shear Specimen (DSS), are critically examined and compared. Through a combination of experiments (using the standard Split-Hopkinson Pressure Bar system), finite-element simulations and metallographic examinations of their fracture morphology, the dynamic shear characteristics (strain hardening, strain rate strengthening effect and failure strain) of Ti-6Al-4V obtained using the different specimen geometries are critically examined, compared and analyzed. It will be shown that differences in the stress/strain uniformity, the plastic deformation zone, and the stress state induced by the different specimen geometries lead to discrepancies in the measured shear response and failure that were observed. The shear stress–strain curve obtained using the DSS will be shown to be more precise than the other specimen geometries.
•Experimentally investigate the effect of Lode angle on fracture strain of steels.•A new specimen type is designed to eliminate the effect of moderate triaxiality.•Asymmetry of the fracture locus ...with respect to Lode angle parameter is revealed.•The influence of stress triaxiality and yield stress on the Lode angle effect is discussed.•A fracture model is proposed allowing for the effect of both Lode angle and triaxiality.
In this paper, the effect of Lode angle on the fracture strain of steels is discussed based on experimental evidence. To isolate the effect of the Lode angle, fracture strains corresponding to stress states with constant stress triaxiality but variable Lode angle parameters are obtained from both the literature and tests performed in this study of newly designed rectangular notched specimens. From investigations of currently available experimental data, it is demonstrated that the fracture strain is significantly affected by the Lode angle, and the influence can be modeled by an exponential function of the Lode angle parameter. This implies that the fracture locus is asymmetric with respect to the Lode angle parameter, whereas many existing fracture models assume it is symmetric. Moreover, the data indicate that the effect of the Lode angle increases with a decreasing value of stress triaxiality and is more pronounced for high strength steels. Having comprehensively evaluated the effect of the Lode angle, a new fracture model termed the Lode angle Modified Void Growth Model (LMVGM) is proposed, which combines the effects of stress triaxiality T, through the Void Growth Exponential Function (VGEF), and the Lode angle parameter ξ, using exponential functions for both. The LMVGM has only three free parameters, α, β and γ, and predicts fracture strain as εf = α∙exp(-β∙T + γ∙ξ), in which β is always positive while γ can be either positive or negative depending on the material. The proposed model is compared to other published models and validated against experimental data, showing good accuracy.
•A stress-based shear ductile fracture criterion is proposed to improve the prediction accuracy of failure for lightweight metals.•The mechanical behavior of AA5182-O sheet is uncovered from tension ...to equibiaxial tension.•The direct and inverse engineering methods are adopted to calibrate the fracture parameters.•The stress state-related fracture behavior of AA5182-O is numerically simulated.•The fracture limits of AA2024-T351 alloy from compression to tension is described by the sDF2016 criterion.
Failure in lightweight metal forming is a big challenge currently and prevents their widely application in weight reduction of automobile and aerospace structures. A stress-based shear ductile fracture criterion is introduced in this study to improve the prediction accuracy of failure for lightweight metals. The criterion (sDF2016) is developed based on the DF2016 criterion thereby endowing the criterion with the precise fracture predictability under wide stress states of shear, uniaxial tension, plane strain tension and equibiaxial tension. Besides, the criterion takes into account the stress state effect on fracture in a form of the stress triaxiality, the maximum shear stress and the Lode parameter. The sDF2016 criterion is also expected to be less sensitive to strain path changing effect because sDF2016 describe the onset of fracture in stress space. For the verification of the proposed sDF2016 criterion, the experiments are carried out for AA5182-O sheet with central hole, notched, in-plane shear and bulging specimens. Plastic deformation is accurately modeled by the Swift-Voce hardening law and the Drucker yield function. The sDF2016 criterion is then calibrated both by the direct and inverse engineering approaches, which is applied to predict fracture initiation under distinct stress states. The modeled result indicates that the calibrated sDF2016 criterion based on the inverse engineering method predicts the fracture stroke with a higher accuracy than the direct approach. The sDF2016 criterion is also used to depict fracture limits of AA2024-T351 alloy from compression to tension. The application shows that the proposed ductile fracture criterion is capable of modeling the fracture limits for sheet metals under various loading conditions from shear to plane strain tension.
•Ductile fracture of X80 pipeline steel is investigated experimentally and numerically.•Three fracture modes are observed in the experiments.•Parallel computations corresponding to the experiments ...are conducted.•The newly proposed model can provide relatively more stable and accurate prediction in fracture strain.
This study pays attention to the ductile fracture of X80 pipeline steel using both experimental and numerical approaches. First, nineteen experimental cases are tested, consisting of the normalized uniaxial tension, the plain strain tension, pure shear, and shear plus tension/compression. Three fracture modes are observed throughout the experiments. A hybrid testing-FE method is applied to determine the post-necking strain hardening curve of the X80 pipeline steel. Then, parallel computations are conducted in the Abaqus to obtain ductile fracture parameters. The simulated results indicate that the determined strain hardening curve has applicability for all experiments. According to the ductile fracture parameters, three models, viz. the Hosford-Coulomb model, the DF2016 model, and a newly proposed model, are calibrated based on the hold-out strategy. Fracture prediction of the X80 pipeline steel is conducted by the three calibrated models. The results show that the newly proposed model outperforms the other two in terms of stability and accuracy.
•The fourteen different shapes and notch specimens of TC4 titanium alloy are designed for uniaxial tensile and compression experiments during wide stress triaxiality.•The Swift-Voce hardening model ...criterion is well used as the plastic parameter in FE model simulation for TC4 titanium alloy under uniaxial tensile and compress loading.•The experiment and numerical results indicate that the DF2016 fracture model can provide the most accurate prediction of fracture behavior for forged TC4 titanium alloy.
It is necessary to determine the fracture criteria and parameters that conform to the plastic deformation and fracture mechanism of titanium alloy under wide stress triaxility. Fourteen different shapes and notch specimens were designed for uniaxial tensile and compression experiments. Based on the analysis of plastic behavior and fracture mechanism, the Swift-Voce hardening model, CPB06 yield criterion and DF2016 fracture criterion were respectively established. The fracture parameters of the ductile fracture model were calculated by the analytic equation solving method. The fracture behavior of forged TC4 titanium alloy was successfully predicted by finite element software. The research results showed that the dimples of fracture morphology become larger and deeper with the increase of stress triaxiality, and the ductile fracture mechanism changes from shear fracture to dimple fracture mechanism. The comparison of experiment and simulation indicated that the Swift-Voce hardening model and CPB06 yield criterion reasonably simulate the plastic deformation. It also proved the DF2016 model can provide the most accurate prediction of fracture behavior for forged TC4 alloy under different stress states.
•A ductile fracture model is proposed.•The fracture model is stress triaxiality and Lode angle dependence.•The effects of various parameters are considered.•The validity of the fracture model is ...verified.•The asymmetry of the fracture locus and the unified stress space are discussed.
In this study, the ductile fracture of the alloy metal is analyzed. First, the key factors affecting the initiation of ductile fracture are investigated. Then, a new uncoupled ductile fracture model is established in the stress space by considering the volume change work and the distortion work simultaneously. The proposed ductile fracture model is not only dependent on stress triaxiality but also dependent on the Lode angle parameter. Subsequently, the proposed ductile fracture model is transformed to the mixed stress-strain space. Experimental tests are adopted to verify the validity of the fracture model. The comparative study with four typical ductile fracture criteria, viz., the pressure modified maximum shear stress criterion (PMMS), the extended Mohr-Coulomb criterion (EMC), the extended Hosford-Coulomb criterion (EHC), and the extended unified strength theory criterion (EUST), indicates that the proposed fracture model predicts the fracture strain with good stability and accuracy in a wide range of stress states, especially in the range of the high stress triaxiality. Finally, the asymmetry of the fracture locus and the realistic stress space are discussed.
•A new phenomenological ductile fracture criterion considering micro mechanisms of void nucleation, void growth, and evolution of void coalescence is presented in this paper.•The secondary voids band ...and rotation of voids effect are considered in the new ductile fracture criterion.•The new ductile fracture criterion can be utilized for predicting initial fracture in sheet metal forming with lager range of stress triaxiality.
A new phenomenological ductile fracture criterion that is proposed. The proposed model is associated with the micro mechanisms of void nucleation, void growth, and evolution of void coalescence. The secondary voids band and rotation of voids effect are considered in the new ductile fracture criterion. A series of upsetting test results of aluminum 2024-T351 and TRIP RA-K40/70 steel are used to construct and compare the accuracy of fracture locus proposed by new ductile fracture criterion, Modified Mohr-Coulomb criterion and extend Lou-Huh criterion. The fracture locus constructed using the proposed criterion is close to the experimental data points over a wide stress state range from uniaxial compression to balanced biaxial tension. Then, a series of upsetting tests and square cup drawing tests are conducted with Al6014-T4 to evaluate the accuracy of the proposed criterion. All results indicate that the proposed ductile fracture criterion can be utilized for predicting initial fracture in sheet metal forming.
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