A Comparative Study on Various Ductile Crack Formation Criteria

2004 ◽  
Vol 126 (3) ◽  
pp. 314-324 ◽  
Author(s):  
Yingbin Bao ◽  
Tomasz Wierzbicki
Keyword(s):  
Crack Formation ◽  
Stress Triaxiality ◽  
Tensile Tests ◽  
Equivalent Strain ◽  
Fracture Mechanisms ◽  
Ductile Crack ◽  
Fracture Criteria ◽  
Wide Range ◽  
Mechanical Models ◽  
Stress States

Various fracture criteria, based on different assumptions and different mechanical models, have been proposed in the past to predict ductile fracture. The objective of this study is to assess their effectiveness and accuracy in a wide range of process parameters. A series of tests on 2024-T351 aluminum alloy, including upsetting tests and tensile tests is carried out. It is found that none of the existing fracture criteria give consistent results. Two totally different fracture mechanisms are clearly observed from microfractographs of upsetting and tensile specimens. This observation confirms that it is impossible to capture all features of ductile crack formation in different stress states with a single criterion. It is shown that different functions are necessary to predict crack formation for different ranges of stress triaxiality. Weighting functions in a wide range of stress states can be obtained by determining the fracture locus in the space of equivalent strain to fracture and stress triaxiality.

scholarly journals Strain Rate-Dependent Constitutive and Low Stress Triaxiality Fracture Behavior Investigation of 6005 Al Alloy

2018 ◽  
Vol 2018 ◽  
pp. 1-14 ◽  
Author(s):  
Yong Peng ◽  
Xuanzhen Chen ◽  
Shan Peng ◽  
Chao Chen ◽  
Jiahao Li ◽  
...  
Keyword(s):  
Flow Stress ◽  
Plastic Strain ◽  
Strain Rate ◽  
Fracture Behavior ◽  
Equivalent Plastic Strain ◽  
Stress Triaxiality ◽  
Tensile Tests ◽  
Uniaxial Tensile ◽  
Strain Rates ◽  
Stress States

In order to study the dynamic and fracture behavior of 6005 aluminum alloy at different strain rates and stress states, various tests (tensile tests at different strain rates and tensile shearing tests at five stress states) are conducted by Mechanical Testing and Simulation (MTS) and split-Hopkinson tension bar (SHTB). Numerical simulations based on the finite element method (FEM) are performed with ABAQUS/Standard to obtain the actual stress triaxialities and equivalent plastic strain to fracture. The results of tensile tests for 6005 Al show obvious rate dependence on strain rates. The results obtained from simulations indicate the feature of nonmonotonicity between the strain to fracture and stress triaxiality. The equivalent plastic strain reduces to a minimum value and then increases in the stress triaxiality range from 0.04 to 0.30. A simplified Johnson-Cook (JC) constitutive model is proposed to depict the relationship between the flow stress and strain rate. What is more, the strain-rate factor is modified using a quadratic polynomial regression model, in which it is considered to vary with the strain and strain rates. A fracture criterion is also proposed in a low stress triaxiality range from 0.04 to 0.369. Error analysis for the modified JC model indicates that the model exhibits higher accuracy than the original one in predicting the flow stress at different strain rates. The fractography analysis indicates that the material has a typical ductile fracture mechanism including the shear fracture under pure shear and the dimple fracture under uniaxial tensile.

scholarly journals A Fracture Criterion for Prediction of Fracture Initiation of Metal Materials at Various Stress States for Nuclear Waste Storage

2019 ◽  
Vol 2019 ◽  
pp. 1-14
Author(s):  
Zhihui Li ◽  
Xue Yang ◽  
Anmin Tang
Keyword(s):  
Fracture Criterion ◽  
Shear Fracture ◽  
Stress Triaxiality ◽  
Fracture Initiation ◽  
Initiation Site ◽  
Fracture Mechanisms ◽  
Nuclear Waste Storage ◽  
Maximal Shear Stress ◽  
Stress States ◽  
New Criterion

A fracture criterion is newly proposed to evaluate fracture behavior and predict fracture initiation of metal materials in different complicated stress states for four different fracture mechanisms including quasicleavage fracture, normal fracture with void, shear fracture with void, and shear fracture without void. The dominant factors of these four different mechanisms are distinct, so it is impossible to capture all features of fracture initiation under different stress states with a single criterion, and different functions are necessary to predict fracture initiation of different mechanisms. In the new fracture criterion, different branches of the fracture criterion have been proposed corresponding to different fracture mechanisms. Quasicleavage fracture and normal fracture with void are described as a function of the principal stress, shear fracture with void is a function of the stress triaxiality and maximal shear stress, and shear fracture without void is only controlled by the maximal shear stress. The new fracture criterion is applied to predict the fracture initiation site and the fracture direction of nodular cast iron QT400-15 in combined tension-torsion tests. Predicted results are compared with experimental results to validate the performance of the new criterion in the intermediate stress triaxiality between 0 and 1/3. The new criterion is also applied to predict the crack initiation site and the direction of crack initiation of LY12 aluminium alloy and HY130 mild steel in mixed mode fracture tests to validate the performance of the new criterion in the high stress triaxiality. The new fracture criterion gives consistent results for these materials in a wide stress triaxiality range. It is shown that the new fracture criterion is a better supplement to the deficiency of fracture mechanics and also a better amendment to traditional strength theory in complicated stress states. Therefore, the new fracture criterion is recommended to be utilized to evaluate the fracture initiation of metal structures in nuclear waste storage and other engineering applications.

Calibration of Selected Ductile Fracture Criteria Using Two Types of Specimens

2013 ◽  
Vol 592-593 ◽  
pp. 258-261 ◽  
Author(s):  
Petr Kubík ◽  
František Šebek ◽  
Jindřich Petruška ◽  
Jiří Hůlka ◽  
Jan Růžička ◽  
...  
Keyword(s):  
Ductile Fracture ◽  
Tensile Testing ◽  
Process Zone ◽  
Computational Simulation ◽  
Stress Triaxiality ◽  
Fracture Criteria ◽  
Ductile Fracture Criteria ◽  
Third Invariant ◽  
Suitable Combination ◽  
Stress States

This paper presents the calibration of three universal ductile fracture criteria with the stress triaxiality and the normalized third invariant of deviatoric stress dependence. Xue-Wierzbicki, Bai-Wierzbicki and Extended Mohr-Coulomb criteria are calibrated using butterfly specimen and newly designed notched tube specimen for an austenitic stainless steel. Different stress states necessary for successful calibration are generated by suitable combination of tension/compression-shear or tension/compression-torsion loading of both specimens, respectively. Suitability of the specimens for ductile fracture criteria calibration is evaluated comparing the range of reached stress states, the homogeneity of stress in process zone and appropriateness for the computational simulation of tests. Possible combination of above mentioned results with tensile testing of standard and notched cylindrical specimens is discussed, too.

Shear Constraint and Macroscopic Fracture Criteria for Ductile Metals

1975 ◽  
Vol 42 (1) ◽  
pp. 15-24 ◽  
Author(s):  
M. A. Kaplan ◽  
G. A. Rowell
Keyword(s):  
Internal Pressure ◽  
Shear Fracture ◽  
Maximum Shear Stress ◽  
Fracture Initiation ◽  
Tensile Tests ◽  
Initiation Site ◽  
The Body ◽  
Fracture Criteria ◽  
Ductile Metals ◽  
Stress States

The role of material constraint in the shear fracture of ductile metals is investigated by means of a series of torsion and internal pressure tests on aluminum alloy tubing combined with analytical descriptions of the stress states at fracture. These descriptions include work hardening and, in the case of a tube under internal pressure, account for the nonaxisymmetric deformations that always precede fracture. The results, which indicate that shear fracture initiation depends on conditions at points far removed from the initiation site when the maximum shear stress vector there is directed through the interior of the body, are supported by the results of tensile tests on plates with shallow notches. These latter experiments are also used to show that tensile and shear fracture are governed by independent fracture criteria. A continuum fracture theory for ductile metals, based on the concepts of material constraint and independent fracture criteria, is proposed. The theory predicts fracture in terms of the stress state. A critical analysis of the theory is provided along with examples of fracture phenomena which the theory predicts, but which are not explained by existing theories.

scholarly journals Dynamic fracture of a dual phase automotive steel

2018 ◽  
Vol 183 ◽  
pp. 02047
Author(s):  
Sarath Chandran ◽  
Patricia Verleysen ◽  
Junhe Lian ◽  
Wenqi Liu ◽  
Sebastian Münstermann
Keyword(s):  
High Strain ◽  
Image Correlation ◽  
Fracture Mechanisms ◽  
Dual Phase ◽  
Strain Rates ◽  
High Strain Rates ◽  
Damage Initiation ◽  
Damage And Fracture ◽  
Wide Range ◽  
Stress States

Dynamic testing of sheet metals has become more important due to the need for more reliable vehicle crashworthiness assessments in the automotive industry. The study presents a comprehensive set of experimental results that covers a wide range of stress states on a dual phase automotive sheet steel. Split Hopkinson bar tensile (SHBT) tests are performed on dogbone shaped samples to obtain the plastic hardening properties at high strain rates. A set of purpose designed sample geometries comprising of three notched dogbone tension samples is tested at high strain rates to characterise the dynamic damage and fracture properties under well controlled stress states. The geometry of the samples is optimised with the aid of finite element analysis. During the tests, high speed photography together with digital image correlation are implemented to acquire full field measurements and to gain more insight into the localisation of strains at high strain rates. An experimental-numerical approach is proposed to effectively determine the fracture characteristics of the dual phase steel under extreme conditions. A modified Bai-Wierzbicki model is implemented to assess the damage initiation and subsequent failure. Additionally, the fracture mechanisms are studied utilizing scanning electron microscopy.

scholarly journals Influence of Strain and Stress Triaxiality on the Fracture Behavior of GB 35CrMo Steel during Hot Tensile Testing

2018 ◽  
Vol 2018 ◽  
pp. 1-11 ◽  
Author(s):  
Zheng Li ◽  
Yajun Zhou ◽  
Sanxing Wang
Keyword(s):  
High Temperature ◽  
Tensile Testing ◽  
Crack Formation ◽  
Fracture Strain ◽  
Stress Triaxiality ◽  
Fracture Morphology ◽  
Tensile Tests ◽  
35Crmo Steel ◽  
Size Growth ◽  
High Temperature Tensile

To better understand cavitation nucleation and crack initiation in 35CrMo steel during high-temperature tensile processing and the effect of stress triaxiality on its fracture behaviors, uniaxial and notch high-temperature tensile tests were performed. The microstructure, fracture morphology, fracture strain, and stress triaxiality of the tested 35CrMo steel were then characterized and discussed. The results showed that crack formation in 35CrMo steel included stages of nucleation, growth, and microcavity aggregation. Scanning electron microscopy and energy-dispersive X-ray spectroscopy demonstrated that crack formation was closely related to the presence of steel inclusions. High-temperature tensile testing of samples with different notch radii showed that the fracture strain of 35CrMo steel was decreased with increasing stress triaxiality, that is, increased stress levels corresponded to decreased material plasticity. In addition, the recrystallization degree was decreased with increased stress triaxiality, and the grain size growth was slowed. The failure of 35CrMo steel occurred via ductile fracture, and low stress triaxiality, and high temperature conditions induced large and deep dimples on the fracture surface.

scholarly journals On Phenomenological Failure Loci of Metals under Constant Stress States of Combined Tension and Shear: Issues of Coaxiality and Non-Uniqueness

Metals ◽  
2019 ◽  
Vol 9 (10) ◽  
pp. 1052
Author(s):  
Cliff Butcher ◽  
Armin Abedini
Keyword(s):  
Stress State ◽  
Plane Stress ◽  
Simple Shear ◽  
Stress Triaxiality ◽  
Failure Surface ◽  
Equivalent Strain ◽  
Shear Loading ◽  
Stress Conditions ◽  
Failure Locus ◽  
Stress States

The present study investigates how the choice of characterization test and the composition of the stress state in terms of tension and shear can produce a non-unique failure locus in terms of stress triaxiality under plane stress conditions. Stress states that are composed of tensile and simple shear loadings result in a loss of proportionality between the cumulative strain and stress such that the principal frames become non-coaxial despite a constant stress triaxiality. Consequently, it is shown that the conventional interpretation of a failure locus in plane stress is based upon an implicit assumption of proportional coaxial loading. The use of simple shear tests along with traditional in-plane tensile tests for fracture characterization is only one “path” that can be taken in terms of the stress triaxiality, which may produce a bifurcation at uniaxial tension while the tension–torsion path does not. In general, the failure locus in terms of the equivalent strain is a failure surface and must consider the composition of the stress state that produces a given triaxiality. A comprehensive review of phenomenological fracture loci within a modified Mohr-Coulomb (MMC) framework is performed to highlight how the choice of stress states obtained using different characterization tests can change the apparent fracture locus of a material. The finite strain solutions for the work conjugate equivalent strain are derived for various loading paths that produce the same stress triaxiality. It is then shown that accounting for non-coaxiality leads to equivalent failure strains that are even higher than previously reported in tension–torsion tests within the literature. The equivalent plastic strains integrated from finite-element simulations are work-conjugate by definition. The equivalent strains estimated from the cumulative principal strains using DIC strain measurement depend upon a coaxial or non-coaxial assumption. Finally, an analytical solution for the onset of diffuse necking that accounts for the stabilizing influence of shear loading against a tensile instability is considered. Even under plane stress conditions, a failure surface arises in terms of the equivalent strain at necking, the stress triaxiality, and the severity of shear loading.

Material Behavior of Two High Grade Pipe Steels Under Different Triaxility Levels

2010 ◽  
Author(s):  
S. Cravero ◽  
R. Bravo ◽  
L. Mantovano ◽  
H. Ernst
Keyword(s):  
Hydrostatic Stress ◽  
Stress Triaxiality ◽  
Yield Condition ◽  
Tensile Tests ◽  
Material Behavior ◽  
Triaxial Stress ◽  
Von Mises Stress ◽  
Ductile Material ◽  
Stress States ◽  
Von Mises

Particular geometries and loading conditions may have important effects on the stress fields of a given component promoting complex triaxial stress states and modifying the hydrostatic stress level. The yield condition of a ductile material is represented by the von Mises stress. However, the triaxial stress states have important effects on material toughness and ductility. This work presents a study of the effects of stress concentrators (different triaxial stress states) on material rupture. The aim is to determine the effects of hydrostatic stresses on the strain at failure in two low alloy high strength mill steels employed in field well and linepipe applications. Cylindrical specimens with different notch radius were tested to obtain different hydrostatic to von Mises stress ratios during tensile tests (h = σh/σVM). The considered notch radii were 2.0, 0.8, 0.4 and 0.25 mm. The notched specimens were loaded in tension and applied load vs. reduction of transversal area data were recorded during the tests. Numerical simulations of the tensile tests allow reproducing the test in the numerical model and calculating the stress and strains fields during each stage of the applied loading. Finally, tables of strain at failure vs. stress triaxiality are obtained for both steels that allow determining the most appropriate material for critical applications.

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