Large Strain Conditions Research Articles

Effect of yield criteria on the formability prediction of dual-phase steel sheets

The yield criterion is the basis of the material model that can describe the plastic behavior of the material under any possible stress states. The influence of diverse yield criteria on the formability prediction was explored via analyzing the hardening behavior and the forming limit of DP590 and DP780 sheets in this paper. The potential of the yield criterion to describe the hardening behavior of sheet metal was evaluated by comparing the equivalent stress-strain curves from the cruciform biaxial tensile tests with the uniaxial stress-strain curve. To characterize the hardening behaviors of dual-phase steel sheets under large strain condition, the hydraulic bulge tests were employed to obtain the equivalent stress-strain data based on different yield criteria. Accordingly, the material models including diverse yield criteria and hardening laws were used to calculate the theoretical forming limit curves (FLCs) based on the M-K model. Through comparing the predicted FLCs and the experimental ones from Nakazima tests, it was found that the theoretical FLCs based on the M-K model largely depend on the accurate yield criterion and hardening law. In addition, the numerical simulations of two stretch tests were carried out to verify the validity of the material models and the theoretical FLCs. The results indicated that the accurate yield criterion is of great significance to the description of hardening behavior and the prediction of FLCs.

Cap inflammation leads to higher plaque cap strain and lower cap stress: An MRI-PET/CT-based FSI modeling approach

Plaque rupture may be triggered by extreme stress/strain conditions. Inflammation is also implicated and can be imaged using novel imaging techniques. The impact of cap inflammation on plaque stress/strain and flow shear stress were investigated. A patient-specific MRI-PET/CT-based modeling approach was used to develop 3D fluid-structure interaction models and investigate the impact of inflammation on plaque stress/strain conditions for better plaque assessment. 18FDG-PET/CT and MRI data were acquired from 4 male patients (average age: 66) to assess plaque characteristics and inflammation. Material stiffness for the fibrous cap was adjusted lower to reflect cap weakening causing by inflammation. Setting stiffness ratio (SR) to be 1.0 (fibrous tissue) for baseline, results for SR=0.5, 0.25, and 0.1 were obtained. Thin cap and hypertension were also considered. Combining results from the 4 patients, mean cap stress from 729 cap nodes was lowered by 25.2% as SR went from 1.0 to 0.1. Mean cap strain value for SR=0.1 was 0.313, 114% higher than that from SR=1.0 model. The thin cap SR=0.1 model had 40% mean cap stress decrease and 81% cap strain increase compared with SR=1.0 model. The hypertension SR=0.1 model had 19.5% cap stress decrease and 98.6% cap strain increase compared with SR=1.0 model. Differences of flow shear stress with 4 different SR values were limited (<10%). Cap inflammation may lead to large cap strain conditions when combined with thin cap and hypertension. Inflammation also led to lower cap stress. This shows the influence of inflammation on stress/strain calculations which are closely related to plaque assessment.

Constitutive Equations and Flow Behavior of an As-Extruded AZ31 Magnesium Alloy Under Large Strain Condition

A reasonable constitutive model is the key to achieving the accurate numerical simulation of magnesium alloy extrusion process. Based on the hot compression tests of the as-extruded AZ31 magnesium alloy, the strain-compensated Arrhenius equation, the constitutive equation taking into account dynamic recovery (DRV) and dynamic recrystallization (DRX), and the modified Fields-Backofen equation (FB) are established to describe the deformation behavior of this alloy under large strain condition (strain level greater than 1.0) and wide strain rate range (0.01 to 10 s−1), respectively. Then material parameters in each constitutive model are determined by linear fitting method. The comparison of these three kinds of equations shows that the strain-compensated Arrhenius model provides the best prediction of flow stress, and the calculated value of correlation coefficient (R) is the highest as 0.9945 and the average absolute relative error (AARE) is the lowest as 3.11%. The constitutive equation with DRV + DRX can also predict flow stress accurately, and its values of R and AARE are 0.9920 and 4.41%, respectively. However, compared to the other two constitutive equations, the modified FB equation does not give good description of hot deformation behavior for this magnesium alloy. Finally, the advantages and drawbacks of these three kinds of constitutive models are discussed and compared. Therefore, this work could provide theoretical guidelines for investigating hot deformation behavior of wrought magnesium alloys and determining the appropriate extrusion process parameters under large strain condition.

A large-strain weft-knitted sensor fabricated by conductive UHMWPE/PANI composite yarns

In this paper, UHMWPE filament yarns were modified to improve their surface energy using atmospheric pressure plasma pretreatment. Then, a novel method based on in-situ polymerization was used for the continuous fabrication of conductive UHMWPE/PANI composite yarn with a conductivity of 0.87±0.1S/cm. Cylindrical plain weft-knitted fabrics with different densities were fabricated using conductive UHMWPE/PANI composite yarns, and their strain-resistance performances were studied in detail. The results showed that the resistance of the fabrics increased with increasing strain at a relatively low strain range, and it also exhibited good linearity, sensitivity, and repeatability, which were enhanced by increasing density. When the strain was relatively high, the resistance of the fabrics increased with increasing strain and then decreased when the fabrics were further stretched. The repeatability of the fabrics in the large-strain condition was worse than that in the small-strain condition. The repeatability of the fabrics was very different when comparing the first stretching and the second stretching, and the difference then declined and showed good repeatability after repeated stretching three times.

Fatigue Crack Propagation under Large Cyclic Plastic Strain Conditions

Outside the application limit of the stress intensity factor based fatigue life calculation, a substitute procedure has to be defined. Various proposals for a crack driving force parameter in elastic-plastic fracture mechanics are discussed. A preference in favor of the cyclic ΔJ-integral is elaborated, keeping its theoretical limitations in mind. Crack closure is also an important issue under large-scale yielding conditions. Experience cannot be transferred from the small-scale to the large-scale cyclic yielding regime or vice versa. The consequences for the fatigue lives under variable amplitude and multiaxial loading are shown.

Structural Changes in Tribo-Stressed Zinc Polyphosphates

The influences of pressure, shear stress and temperature on the structure of zinc orthophosphate and zinc metaphosphate was investigated. Tribological tests were performed to study the combined effect of pressure and shear stress at two temperatures. Friction tests were carried out in the boundary lubrication regime from dispersions of zinc polyphosphates in base oil. The effects of pressure alone were investigated using a diamond anvil cell in order to separate it from those of shearing. Raman spectroscopy was used to follow in situ or ex situ the structural changes of the zinc polyphosphate powders and the tribo-stressed areas. Tribofilms obtained with both polyphosphates display a partial and full depolymerization of the zinc metaphosphate at ambient and high (120 °C) temperature, respectively. The large stress and strain conditions of the tribological tests are necessary to induce a tribochemical reaction between zinc metaphosphate and iron oxide leading to a depolymerization of the phosphate in the tribofilm. The tribochemical reaction and antiwear tribofilm formation are significantly enhanced by the modest temperature increase from ambient to 120 °C. Pressure alone induces only disordering in the structure of zinc polyphosphates, with only minor changes of the chain length in phosphates and does not contribute significantly to the observed structural changes in tribofilms.

Catalogue of maximum crack opening stress for CC(T) specimen assuming large strain condition

Abstract In this paper, values for the maximum opening crack stress and its distance from crack tip are determined for various elastic-plastic materials for centre cracked plate in tension (CC(T) specimen) are presented. Influences of yield strength, the work-hardening exponent and the crack length on the maximum opening stress were tested. The author has provided some comments and suggestions about modelling FEM assuming large strain formulation.

Elastic–plastic fracture analyses for pipeline girth welds with 3D semi-elliptical surface cracks subjected to large plastic bending

Offshore pipelines are subjected to large plastic strain conditions, such as bending accompanied by high internal pressure, during the installation and operation processes. These pipelines are usually constructed by the girth welding method and weldments may contain naturally occurring surface and embedded planar and/or volumetric imperfections and/or defects. It is widely recognized that the existing fracture assessment procedures which are based on the load-controlled method, are not explicitly designed for situations with large plastic deformation. The main objective of the current paper is to find a routine to make fracture assessment for a pipeline subjected to plastic bending. In this paper, based on 3D elastic–plastic finite element analyses, the influences of various parameters on the fracture responses (CTOD) of the cracked pipelines are investigated and a CTOD estimation formula is proposed. Furthermore, comparison with fracture assessments made by BS7910:2005 code and finite element analyses indicates that the former gives over-conservative predictions. Based on the proposed formula, a failure assessment diagram (FAD) for strain-based fracture, is presented for the specific geometry and material properties of the particular pipe with semi-elliptical surface cracks under large plastic bending, where the global strain is up to 3%.

Model for large strain consolidation under constant rate of strain

SUMMARYA numerical model, called CCRS1, is presented for one‐dimensional large strain consolidation under constant rate of strain loading conditions. The algorithm accounts for vertical strain, general constitutive relationships, relative velocity of fluid and solid phases, changing compressibility and hydraulic conductivity during consolidation, and an externally applied hydraulic gradient acting across the specimen. Soil compressibility is rate independent, and as such, the current model is most appropriate for less‐structured clays. Verification checks show excellent agreement with analytical and numerical solutions for small and large strain conditions. A series of numeric examples indicates that compressibility and hydraulic conductivity constitutive relationships can have an important effect on constant rate of strain consolidation response. Results also indicate that analytical solutions obtained using small strain theory can be in significant error for large strain conditions with changing coefficient of consolidation. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Catalogue of the numerical solutions for SEN(B) specimen assuming the large strain formulation and plane strain condition

In this paper the values of the maximum opening stress and its distance from crack tip determined for various elastic–plastic materials for single edge notched specimens in bending (SEN(B)) are presented. The influence of the yield strength, the work-hardening exponent and the crack length on the maximum opening stress was tested. The numerical results were approximated by the closed form formulas. In this paper some comments and suggestions about modeling FEM assuming large strain formulation are given.

Study on Rolling Processing Evaluation Characteristics Using Sub-DIC Method

It is extremely important to evaluate the rolling deformation and large strain distribution conditions in the surface of the processed materials after machining. On the other hand, the sub-pixel digital image correlation (Sub-DIC) method enables to measure the full-field strain distribution only using the photographs taken by the commercially available digital camera. This study aims to develop the rolling deformation and the large strain distribution conditions using the Sub-DIC method. Here, it is well known that the measurement accuracy deteriorates in large strain conditions when the conventional Sub-DIC method is used. In this paper, we proposed the facility technique for measuring the large strain distribution by the Sub-DIC method in which the sub-set area are applied to the strain regarding the reduction in thickness of rolling processing. And, we demonstrated the measurement characteristics using the facility Sub-DIC method through a series of experiments from the viewpoint of changes in the test piece surface strain distribution conditions. Moreover, we evaluated the curve of test piece after rolling processing with a radius of curvature. Finally, we discussed measurement characteristics of the steady/ unsteady conditions in the rolling processing using the facility Sub-DIC method.

Dynamic numerical simulations of void growth and coalescence with stress triaxiality maintained constant—Application to ductile solids with secondary voids

AbstractDynamic explicit finite element analysis is used to investigate void growth and plastic collapse of an axisymmetric unit cell model with a primary spherical void imbedded in a porous matrix material. The Gurson–Tvergaard–Needleman homogenized model is used to describe the plastic behaviour of the matrix material. The simulations are performed under large strain conditions for varying secondary void volume fractions and quasi‐static loading controlled by constant stress triaxiality. The proposed accomplishment of constant stress triaxiality associated with dynamic explicit computations provides a method allowing to trace the collapse of the unit cell from the onset of coalescence to practically its final failure. Consistent with experimental and theoretical results available in the literature, the obtained results substantiate the sensitivity of coalescence to the presence of secondary voids. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Mesure de la déformabilité des sols in situ à l'aide d'un essai de chargement statique d'une pointe pénétrométrique

The penetrometers allow one to obtain a global parameter, which is the cone resistance; this one is related to the soil failure and not to its deformability. The idea is to use a penetrometer to measure in situ the deformability properties of the soils. A new method of soil deformability measurement is then presented in this article: a monotonic static loading test using a penetrometer. The main objective is to measure particularly the three mechanical parameters, which are Young's modulus E, the cohesion c, and the friction angle ϕ. This article describes the testing equipment and the tests performed by precise procedure. An interpretation of the obtained monotonic experimental curves is also presented. It is based on a mathematical regression by three coefficients. According to this study, conducted on a purely frictional soil, the test allows one to obtain by a simple, fast, and economic way, a relevant evaluation of the soil deformability properties and its resistance. An axisymmetric finite element model of the experimental test is proposed to identify the correlation between the three mathematical coefficients of the experimental curve and the three rheological parameters E, c, and ϕ. The initial conditions before the test, which are crucial, are simulated by performing an unloading–reloading loop. Interesting results are obtained with this simplified, but realistic, finite element model. However, a simulation considering large strain conditions is still needed.Key words: penetrometer, deformability, monotonic static loading, deformation modulus, finite element simulation.

Plastic deformation behaviors and dynamic recrystallization mechanisms of ZK60 magnesium alloy

Hot deformation behaviors of a ZK60 magnesium alloy were investigated by hot compressive tests on a Gleeble-1500 thermal simulation test machine at temperatures ranging from 473 K to 723K and strain rates of 0.01-1s-1. Microstructures evolution of the alloy as well as dynamic recrystallization mechanisms at different strains with the strain rate 0.1s-1 at 623K were examined under optical microscope (OM) and transmission electron microscope (TEM). The relationships between flow stress, strain rate and deformation temperature were analyzed, and the deformation activation energy at elevated temperatures (573-723K) were calculated. The results indicated that under the present deformation conditions, dynamic recrystallization happened in the alloy and fine recrystallized grains tended to form ductile shear zones under the large strain condition; and there were different dynamic recrystallization mechanisms at different strains; within the temperature range from 573K to 723K, deformation activation energy changed with the deformation temperature and strain rate.

Multiaxial fatigue life prediction for a natural rubber

This study deals with the fatigue crack initiation under multiaxial non-proportional loadings in a natural rubber. Push–pull, torsion and tension-compression with a superimposed static torsion fatigue test results are presented. A short presentation of some important features concerning short fatigue crack growth is given. Two fatigue crack criteria are proposed, the first one based on the first and second invariant of the Cauchy stress tensor, the second, based on the micromechanisms of crack initiation, consist of a critical plane approach under large strain conditions using a micro to macro approach. The second criterion was found to give the best results, by predicting the fatigue lifes, crack orientations and location, even in cases with internal crack initiation.

Crack initiation and propagation under multiaxial fatigue in a natural rubber

The ever growing use of elastomers and polymers in structures leads to the need of pertinent multiaxial fatigue life criteria for such materials. Thus, the understanding of the fatigue crack initiation micro-mechanisms and their link to the local stress and/or strain history is essential. Scanning electron microscopy and Energy Dispersive Spectroscopy (EDS) have been used to investigate those micromechanisms on a natural rubber. Rigid inclusions were systematically found at the crack initiation. Depending on the type of inclusion (identified by EDS), cavitation at the poles or decohesion are the very first damage processes observed. Cracks orientations are compared to local principal stress orientation history, the later being obtained from finite element calculations (FE). It is shown that if large strain conditions are correctly taken into account, cracks are found to propagate systematically in the direction given by the maximal first principal stress reached during a cycle, even under non-proportional loading. A fatigue life criterion is proposed.

Structure evolution of carrageenan/milk gels: effect of shearing, carrageenan concentration and nu fraction on rheological behavior

Carrageenans, sulphated polysaccharides extracted from red seaweeds, are widely added to milk systems because of the gelation they promote and for their ability to recover a structure at rest after shearing. The gelation of iota-carrageenan is closely related to the helix-coil transition they undergo at approximately 48 °C in milk. Carrageenan gels (e.g. iota with and without nu precursors) were formed in milk or permeate by decreasing the temperature from 60 to 10 °C following various mechanical treatments, e.g. gelation and ageing of the gel at rest, gelation under shearing and ageing at rest. Gels were characterized rheologically during cooling and ageing, performed in the dynamic mode, to follow the mechanical recovery of the mixed systems at rest. In many cases, a strain-hardening behavior was observed using strain sweep measurements: above the upper limit of the linear domain, stress increased more than proportionally to the strain. Two types of iota-carrageenan were compared: iota with and without nu precursors. The content of the precursor affects the ability to form helical structures. Samples without precursor gave the highest level of helical structures. The carrageenan concentration effect was tested in the range 0.1–0.5 wt%. Differences in sheared and unsheared gel properties, particularly in large strain conditions, are discussed in terms of gelation process, phase separation, carrageenan/milk proportion and carrageenan type. The industrial interest in using either type of carrageenan is discussed.

Molecular dynamics investigation of the mechanical properties of gallium nitride nanotubesunder tension and fatigue

This study adopts a classical molecular dynamics (MD) simulation with the realistic Tersoffmany-body potential model to investigate the mechanical properties of gallium nitride(GaN) nanotubes. The investigation focuses primarily on the mechanical properties of(n,0) and (n,n) GaN nanotubes since these particular nanotubes represent two extreme cases. The presentresults indicate that under small strain conditions, mechanical properties such as Young’smodulus are insensitive to the wrapping angle. Conversely, the wrapping angle has a significantinfluence upon these mechanical properties under large strain conditions. It is demonstrated that(9,0) GaN nanotubes are far less resistant to bond rotation. Under large tensile strain conditions, due tothe unfavourable bond orientations induced by Stone–Wales (SW) transformation, the bonds in(n,0) GaN tubes quickly degenerate. Moreover, the present results suggest that the tensilestrength of a nanotube is strongly sensitive to the temperature and strain rate. Regardingthe fatigue test, this study uses a standard theoretical model to derive curves of amplitudestress versus number of cycles for the current nanotubes. The results demonstratethat the fatigue limit of GaN nanotubes increases with increasing temperature.

The coupled radial–axial deformations analysis of flexible pipes conveying fluid

AbstractThis paper presents large deformation analysis of pipes conveying fluid in which two complicated behaviours are taken into consideration. The first is the coupling between radial and axial deformations of pipe wall, and the other is the interaction between a deformed pipe and transported fluid having the variable internal flow velocity. The coupled radial–axial deformation theory of the pipes and the continuity theory of flow inside the moving deformed pipes are developed to undertake these coupling behaviours. All strong and weak forms of governing equations are obtained by carrying out the virtual work formulation. The hybrid‐finite element method is used to solve the highly non‐linear static problems, which configure the initial large deflection and large strain conditions of the pipes. The state‐space finite element model for use in analyses of non‐linear vibration and system stability is established as well as the suggested numerical solution procedures. The numerical studies of the pipes under circumstances of intense radial loads such as deep‐water risers demonstrate that even a slight change of the radial deformation has a significant effect in increasing non‐linear responses, and reducing stabilities of the pipes. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Three dimensional elastoplastic contact analysis at large strains with enhanced assumed strain elements

In this paper, a model for elastoplastic contact analysis under large strain conditions is presented. Consideration of the impenetrability constraints follows through a penalty formulation. Due to the presence of large strains and to the impenetrability restrictions, special care by the choice of the finite element formulation is required. Here, the enhanced assumed strain concept is employed. Since this concept is based on the enhancement of the strain fields with no interelement continuity requirement imposed, the simple global structure of the displacement based model remains unchanged.