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.

scholarly journals A Study of Yielding and Plasticity of Rapid Prototyped ABS

Mathematics ◽  
2021 ◽  
Vol 9 (13) ◽  
pp. 1495
Author(s):  
Dan-Andrei Șerban ◽  
Cosmin Marșavina ◽  
Alexandru Viorel Coșa ◽  
George Belgiu ◽  
Radu Negru
Keyword(s):  
Experimental Data ◽  
Stress State ◽  
Plastic Flow ◽  
Plane Stress ◽  
Yield Criterion ◽  
Numerical Analyses ◽  
Flow Potential ◽  
Stress States ◽  
State Configuration

In this article, the yielding and plastic flow of a rapid-prototyped ABS compound was investigated for various plane stress states. The experimental procedures consisted of multiaxial tests performed on an Arcan device on specimens manufactured through photopolymerization. Numerical analyses were employed in order to determine the yield points for each stress state configuration. The results were used for the calibration of the Hosford yield criterion and flow potential. Numerical analyses performed on identical specimen models and test configurations yielded results that are in accordance with the experimental data.

scholarly journals Stress Triaxiality and Lode Angle Parameter Characterization of Flat Metal Specimen with Inclined Notch

Metals ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 1627
Author(s):  
Jian Peng ◽  
Peishuang Zhou ◽  
Ying Wang ◽  
Qiao Dai ◽  
David Knowles ◽  
...  
Keyword(s):  
Stress State ◽  
Notch Root ◽  
Stress Triaxiality ◽  
Image Correlation ◽  
Notch Angle ◽  
Strain Evolution ◽  
Metal Specimen ◽  
Lode Angle ◽  
Stress States ◽  
Angle Parameter

The stress state has an important effect on the deformation and failure of metals. While the stress states of the axisymmetric notched bars specimens are studied in the literature, the studies on the flat metal specimen with inclined notch are very limited and the stress state is not clearly characterized in them. In this paper, digital image correlation and finite element simulations are used to study the distribution of strain and stress state, that is stress triaxiality and Lode angle parameter. Flat specimen with inclined notch was tested to extract the full field strain evolution and calculate stress state parameters at three locations: specimen centre, notch root and failure starting point. It is found that compared with the centre point and the notch root, the failure initiation point can better characterize the influence of the notch angle on the strain evolution. Conversely, the centre point can more clearly characterize the effect of the notch angle on stress state, since the stress states at the failure point and the notch root change greatly during the plastic deformation. Then the calculated stress state parameters of the flat metal specimen with inclined notch at the centre point are used in Wierzbicki stress state diagram to establish a relationship between failure mode and stress state.

Ring Specimen Geometry for Determining the Ductility of Tubulars

2016 ◽  
Author(s):  
M. A. Al Khaled ◽  
I. Barsoum
Keyword(s):  
Stress State ◽  
Plane Strain ◽  
Stress Triaxiality ◽  
Ductile Failure ◽  
Pressure Vessels ◽  
Ring Specimen ◽  
Lode Parameter ◽  
Wide Range ◽  
Failure Locus

Pressure vessels designed in accordance with the ASME BPVC code are protected against local ductile failure. Recent work has shown that local ductile failure highly depends on the stress state characterized by both stress triaxiality (T) and the Lode parameter (L). In this paper, the effect of stress state on the ductility of a tubular steel is studied. Two ring specimen configurations were optimized to allow the determination of the ductile failure locus of both tensile and plane strain loadings. The geometry of both ring specimen configurations was optimized to achieve a plane strain (L = 0) condition and a generalized tension (L = −1) condition. Notches with different radii were machined on both types to achieve a wide range of stress triaxiality. Specimens were manufactured from SA-106 carbon tubular steel and were tested to determine the ductile failure loci as a function of T and L. Failure locus of SA-106 steel was constructed based on the failure instants and was found to be independent of the variation in the Lode parameter. The ASME-BPVC local failure criterion showed close agreement with experimental results.

New Ring Specimen Geometries for Determining the Failure Locus of Tubulars

2017 ◽  
Vol 140 (1) ◽  
Author(s):  
M. A. Al-Khaled ◽  
I. Barsoum
Keyword(s):  
Stress State ◽  
Plane Strain ◽  
Stress Triaxiality ◽  
Ductile Failure ◽  
Pressure Vessels ◽  
Ring Specimen ◽  
Lode Parameter ◽  
Wide Range ◽  
Failure Locus

Pressure vessels designed in accordance with the ASME BPVC code are protected against local ductile failure. Recent work has shown that local ductile failure highly depends on the stress state characterized by both stress triaxiality (T) and the Lode parameter (L). In this paper, the effect of stress state on the ductility of a tubular steel is studied. Two ring specimen configurations were optimized to allow the determination of the ductile failure locus at both tensile and plane strain loadings. The geometry of both ring specimen configurations was optimized to achieve a plane strain (L=0) condition and a generalized tension (L=-1) condition. Notches with different radii were machined on both types to achieve a wide range of stress triaxiality. Specimens were manufactured from SA-106 carbon tubular steel and were tested to determine the ductile failure loci as a function of T and L. Failure locus of SA-106 steel was constructed based on the failure instants and was found to be independent of the Lode parameter. The ASME-BPVC local failure criterion showed close agreement with experimental results (EXP).

The Influence of Different Stress States on 16MnR Deformation Capacity

2013 ◽  
Vol 716 ◽  
pp. 590-594
Author(s):  
Shi Lei Zhao ◽  
Yi Liang Zhang ◽  
Gong Feng Jiang
Keyword(s):  
Plastic Deformation ◽  
Stress State ◽  
Equivalent Stress ◽  
Great Influence ◽  
Equivalent Strain ◽  
Complex Stress State ◽  
Biaxial Stress ◽  
Deformation Capacity ◽  
Engineering Structures ◽  
Stress States

16MnR is the typical material of pressure equipment which worked under complex stress state in engineering application. In order to be close to the actual combined tension-shearing stress state and explore the relationship of deformation capacity and different stress state, many groups of combined tension-torsion tests on 16MnR specimens were designed and the equivalent stress-strain relation under different stress state was obtained. The concept of stress triaxiaty (TS value) was cited to characterize the different stress state and the result showed different stress states have a great influence on the material plastic deformation capacity, TS value turns larger, the plastic deformation weakened; 16MnR has a strongest plastic deformation capacity in pure torsion; the level of tensile stress had no significant effect on the maximum stress in the biaxial stress state, but has a significant inverse relationship with the maximum equivalent strain .At last, the mathematical relationship between maximum equivalent-strain and stress triaxiaty could be found. If the stress state of one point in the engineering structures is certain, the maximum equivalent-strain can be estimated.

On a class of non-coaxial plasticity models for granular soils

2005 ◽  
Vol 462 (2067) ◽  
pp. 725-748 ◽  
Author(s):  
H.S Yu ◽  
X Yuan
Keyword(s):  
Simple Shear ◽  
Shear Loading ◽  
Granular Soils ◽  
Strain Rates ◽  
Principal Stresses ◽  
Constitutive Theories ◽  
Strain Behaviour ◽  
Plastic Potential ◽  
Stress States ◽  
Soil Behaviour

The non-coaxiality of the directions of principal stresses and principal plastic strain rates in granular soils under stress rotations has long been observed and recognized in soil tests using both simple shear and hollow cylinder apparatuses. A few constitutive theories have also been proposed in the literature to account for the effect of stress rotations and the subsequent non-coaxial soil behaviour, particularly in the context of shear band analysis. However, the lack of corresponding general numerical methods makes it difficult to investigate the influence of non-coaxial stress–strain behaviour on the results of geotechnical boundary value problems. This paper presents a numerical evaluation of a class of non-coaxial, elastic–plastic models that are developed by combining the conventional plastic potential theory and the double shearing theory. The general non-coaxial constitutive theories are first formulated and then a finite element implementation of the theories is carried out. To evaluate the non-coaxial theories, the problem of simple shear of soils is chosen to investigate the predicted behaviour of soils under simple shear loading conditions where the axes of principal stresses rotate. In particular, the influence of initial stress states and the degree of non-coaxiality are examined. It is found that the numerical results predicted using the non-coaxial model are in general agreement with the experimental observations reported in the literature.

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.

The Sensitivity to the Lode Parameter in Ductile Failure of Tubular Steel Grades

2018 ◽  
Vol 140 (3) ◽  
Author(s):  
I. Barsoum ◽  
M. A. Al-Khaled
Keyword(s):  
Stress State ◽  
Ultimate Strength ◽  
Stress Triaxiality ◽  
Ductile Failure ◽  
High Strength ◽  
Pressure Vessels ◽  
Electron Microscopic ◽  
Lode Parameter ◽  
Steel Grades ◽  
Failure Locus

Ductile failure in steels is highly controlled by the stress state, characterized by the stress triaxiality (T) and the Lode parameter (L). The ASME Boiler and Pressure Vessel Code requires pressure vessels to be designed to resist local ductile failure. However, the standard does not account for the Lode parameter dependence in its failure locus. In this study, the influence of the stress state, characterized T and L, on the ductility of ASME tubular product steel grades is investigated. Two seamless pipes of midstrength carbon steel SA-106 Gr. B and high-strength superduplex steel SA-790 were considered. Ring specimen geometries for plane strain (PS) stress state (L = 0) and tensile stress (TS) state (L = −1) are utilized to establish the ductile failure locus in terms of T and L for the two steels. The experimental results (EXP) show that the effect of the Lode parameter on the failure locus for the SA-106 Gr. B steel is insignificant, whereas for the SA-790 steel, the effect is rather significant. A parameter SL is introduced in order to quantify the sensitivity of the failure locus to the Lode parameter. It is found that for materials with ultimate strength lower than about 550 MPa, the sensitivity to L is insignificant (SL ≈ 1), whereas for materials with ultimate strength higher than 550 MPa, the sensitivity to L could be significant (SL > 1). Scanning electron microscopic (SEM) analysis of the fracture surfaces revealed that the sensitivity to L is closely associated with the rupture micromechanisms involved.

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