Influence Of Strain History Research Articles

Influence of Strain History on Dynamic Strain Localization and Stress State During High-Rate Tensile Loading of Titanium Alloys: Experiments, Modeling, and Analytical Methods

Abstract The determination of the mechanical response of engineering materials subjected to high loading rates plays an important role in determining their performance and application. The high strain-rate tensile response of metals is usually investigated by means of the split-Hopkinson tension bar (SHTB) apparatus. The interpretation of the obtained results is, however, subjected to analogous stress and strain uniformity challenges present during quasi-static tensile experiments. Beyond the onset of necking, strains cease to be uniform along the gauge length and localize around the necking zone. Consequently, the nominal strain rate underestimates the effective strain rate experienced by the material. The analysis of the effective strain rate and stress state beyond the onset of necking has received considerable attention in the literature. Several research efforts have focused on the optimization of the geometry of specimens to be employed for the characterization of the dynamic tensile response using the SHTB. The present work investigates, systematically, the effects of strain history and adiabatic heating on the stress state during dynamic loading. A series of monotonic and various strain history experiments were conducted and analyzed. The diameter evolution, effective strain rate, and temperature histories were measured for all conducted experiments. Numerical simulations were carried out to examine the stress state during strain localization and to accurately reproduce engineering and local thermos-mechanical variables. The effectiveness of existing postnecking corrections for high-rate experiments is assessed. A modified postnecking correlation taking into account the effects of adiabatically induced thermal softening is proposed.

Influence of Strain History and Cooling Rate on the Austenite Decomposition Behavior and Phase Transformation Products in a Microalloyed Steel

The effect of simple strain path changes as well as post-deformation continuous cooling rate during thermomechanical-controlled processing of microalloyed steel was studied using laboratory physical simulation. The phase transformation characteristics were directly analyzed by dilatometry under various cooling rates. The microstructures of the transformation products were characterized quantitatively using EBSD. The results have shown that while strain path changes impose a considerable influence on the hot flow behavior of the austenite, the cooling rate following hot deformation is the determining factor of the phase transformation mechanism and behavior which establishes the final transformation products and subsequent mechanical properties.

Influence of strain history on the mechanical and micro-fabric evolution of calcite rocks: insights from torsion experiments

Different types of deformation experiments were conducted on specimens of Carrara marble at a temperature of 1000 K, 300 MPa confining pressure, and 3 × 10−4s−1 shear strain rate. Microstructural examinations were carried out using light and electron microscopy. Fabric analysis was performed through electron backscattered diffraction (EBSD) measurements. The stress-strain relationships from the experiments indicate that the mechanical response of Carrara marble depends on the deformation history. After a first ductile deformation event the marble becomes permanently softened and is able to undergo plastic strain at relatively lower stresses compared to previously undeformed marble. After shear strain reversal of γ = 2 or less the original microstructure is restored; in highly re-strained samples (γ = ±3,4 and 5) dynamic recrystallization mechanisms create an evident foliation whose angle in respect to the shear plane defines a shear sense criterion which is in agreement with the reversed sense of shearing. After a minimum shear strain reversal of γ = 3 the pre-existing foliation is totally overprinted. The fabric produced during strain reversal experiments shows features, which are comparable with those of single-stage experiments; but the strength of the crystallographic preferred orientation (CPO) is significantly lower, indicating complex microstructural interactions during plastic reactivation. Our tests indicate that the strain weakening in Carrara marble is predominantly caused by grain size refinement through dynamic recrystallization. The development of a crystallographic preferred orientation contributes towards only about one third of the total weakening observed.

Predictions for weak mechanical ignition of strain hardened granular explosive

Predictions are given for the coupled bulk and grain scale response of initially unstressed, strain hardened granular HMX (C4H8N8O8) due to mild piston impact (impact speeds <100 m/s). Importantly, this response depends on the material’s strain history as the stress necessary for bulk inelastic compaction (crush up) increases with the solid volume fraction. Although the quasistatic compaction behavior of HMX is well characterized, the influence of strain history on the bulk and grain scale dynamic loading response has largely been unexplored. In this study, the initial solid volume fraction of the unstressed material is varied over the range of φf⩽φ0⩽1, where φf=0.655 is its free pour value. A Hugoniot analysis for the bulk material identifies three dispersed compaction wave structures that depend on the impact speed and initial solid volume fraction, and are analogous to elastic-plastic waves in dynamically loaded solids. For increasing impact speed, these structures consist of (1) a single viscoelastic wave; (2) a leading viscoelastic wave and a trailing viscoplastic wave; and (3) a single viscoplastic wave. It is shown that the resulting localized heating near intergranular contact surfaces can trigger sustained combustion of the material. Predictions for the grain scale thermochemical response indicate that significant bulk viscoplastic heating is required for ignition of materials with φf⩽φ0⩽0.88, whereas bulk viscoelastic heating leads to the ignition of denser materials (φ0>0.91). Both viscoelastic and viscoplastic heating are predicted to be important for ignition of materials having 0.88⩽φ0⩽0.91. Within this transition range there is predicted a sharp increase in impact sensitivity as the power input needed for ignition rapidly decreases to a value close to that for the free pour density (0.40 MW/cm2) before increasing again. This result is important for assessing the impact sensitivity and deflagration-to-detonation transition of damaged explosives.

Time-dependent tests on intact rocks in uniaxial compression

Abstract The influence of strain history on rock specimen deformation during multi-level loading and unloading cyclic uniaxial compression creep tests is studied with a creep testing machine. An experimental data processing method for such creep tests is suggested. The correction formulas to determine the rheological model parameters are derived for the case when load relaxation is considered. Creep and relaxation tests under uniaxial compression on four types of rocks are conducted using an electronic hydraulic servo-controlled stiff testing machine. The creep and relaxation laws of the different rocks are compared. The complete stress–strain curves for red sandstone specimens are obtained at nine strain rates from 2.43×10 −6 to 4.38×10 −3 /s. The effects of strain rates on rock strength and limit strain are discussed. Empirical equations to evaluate the strain rate dependence of rock mechanical properties are presented.

The influence of strain history on ductile failure of steel

By comparing the micromechanisms of ductile fracture of metallic materials (void nucleation, void growth and void coalescence) and the loading conditions in terms of local stresses and strains, the fracture behaviour may be described in terms of local properties as, for example, void volume fraction, to find out the damage development during plastic deformation and the conditions of crack initiation. For the damage development due to void growth the stress triaxiality during deformation is thought to be an important parameter. There are different models for void coalescence considering a critical combination of stress triaxiality and plastic strain, a critical void volume fraction and a critical amount of energy added. Experiments on notched tensile specimens had been done to determine the plastic strain at crack initiation. FEM-calculations were used to calculate the parameters stress triaxiality and equivalent plastic strain during deformation at the point of void coalescence taking into regard the loading history. Strongly varying stress triaxiality during deformation was achieved by employing one- and two-step experiments with tensile bars of different notch radii. To find a criterion for void coalescence the following two models are applied: (1) Void growth model: By using this equation to model the damage process a critical size to describe crack initiation by void coalescence is sought. However, it is found that based on this model void coalescence may be described only by a two-parameter-criterion, which in this case is geometrical. (2) Damage work model: To find a one-parameter-criterion the “damage work model” is applied. It takes into account the major influence of stress triaxiality on damage development and the stress-strain-curve of the material. Based on this model crack initiation by void coalescence may be described by one parameter which in this case is an accumulated local energy.

Transient extensional rheology and the influence of strain history

This paper presents new experimental results on the measurement of transient extensional viscosity (TEV) and on the three-dimensional (3D) surface formed when TEV, strain and time of deformation data are plotted. In order to test the validity of the hypothesis that such 3D surfaces are unique for a particular polymer at a given temperature, a terpolymer of 2-ethyl hexyl acrylate, n-butyl acrylate and acrylic acid has been extended at varying rates including step changes in rate of extension. These experiments also throw light on the influence of strain history on extensional viscosity. It is shown that a step drop in rate of extension, produces a step increase in TEV which then relaxes back on to the 3D surface. A step increase conversely causes a step fall in TEV with a subsequent rise back to the original surface. The results support the view formed from previous experimental results that these surfaces are a valid representation of TEV although alternative methods of forming the plots should also be considered. Prior straining is shown to be significant only during the period in which the fluid is relaxing from previous deformation modes.

Influence of Strain History on Ductile Failure of Steel

Notched tensile bars of a high strength structural steel were loaded to void coalescense. The results show that the damage curve determined using radially loaded specimens is not valid for specimens under non-radial loading. The results of the experiments with different load histories of notched tensile bars show, that a single parameter criterion like a damage parameter is not sufficient to describe the initiation process of void coalescense. The shape of the voids and the triaxiality at initiation exhibit a strong influence on the initation values. There are two ways to describe the initiation. A simple one considers a critical combination of void growth rate, triaxiality and plastic strain at initiation. The other one uses a shape factor to describe the shape of the voids and calculates the initiation values. Both models work with an accuracy in the order of the experimental scatter.

ELEVATED TEMPERATURE LOW‐CYCLE FATIGUE BEHAVIOR OF HK40 ALLOY

Abstract— By conducting creep tests and creep‐fatigue tests at temperatures of 800°C, 900°C, and 1000°C, on centrifugal cast HK40 alloy (which is used for example for the steam reformer tubes of fuel cell plants) and varying the strain rate during the loading and unloading process, the influence of strain history on the life of the material has been clarified. Furthermore, the data obtained from these tests were subjected to analyses by means of life evaluation methods utilizing the life fraction rule, strain range partitioning method, and damage rate approach, and comparative studies were conducted on each life evaluation method. As a result, problematic points concerning the application of the life fraction rule, strain range partitioning method, and damage rate approach have been clarified and data which would be useful in establishing design guidelines for structures to be used under conditions such as involves the interaction of creep and fatigue have been obtained.

The chemical and mechanical processes of thermal fatigue degradation of an aluminide coating

The influence of strain history on the oxidation and mechanical degradation of an aluminide coating was examined by induction heating of stepped-disk specimens. The coating was applied to a single-crystal Ni-base superalloy (RENE N4) by pack aluminization. The anisotropic elasticity of the single-crystal substrate allowed simultaneously subjecting the aluminide coating to different strain amplitudes. Two distinct modes of coating degradation were observed for tests performed in air between temperature limits of 520 °C and 1080 °C: scalloping (spatially periodic surface oxidation and roughening) and cracking. The degree of scalloping became more severe as the compressive strain imposed on the coating was increased. Six thousand cycles between peak strains of -0.20 and 0.007 pct produced uniform surface oxidation, without scalloping, whereas 6000 cycles between peak strains of -0.56 and 0.01 pct gave oxidation and scalloping to 80 pct of the coating thickness. Cracks along coating grain boundaries were observed after 6000 cycles between peak strains of -0.45 and 0.16 pct. The depth of scalloping was found to correlate approximately with peak compressive substrate strain. Based on this correlation, a mechanism for scallop initiation and growth involving cyclic breakdown of the surface oxide and irreversible cyclic creep of the coating is proposed. Cracking along coating grain boundaries is attributed to tensile strains applied below the transition temperature of the coating. The results obtained from this study indicate that cyclic strain history is an important variable which should be included when determining the oxidation rate of coatings and alloys.

The experimental verification of the evolution of kinematic and isotropic hardening in cyclic plasticity

S imple plastic shear, plastic torsion, cyclic plastic shear, cyclic plastic tension-compression and out of phase tests of 18G2A and 21CrMoV57 steel were performed. The specimens were loaded monotonically and cyclically with constant effective strain rate under controlled amplitude of plastic strains at room temperature. Using the technique of successive unloadings, the standard physical quantities and the stress jumps corresponding to opposite directions of plastic straining were measured. For the Huber-Mises yield surface such results enable the evolution of kinematic and isotropic hardening for such programs to be followed. The results obtained show new effects concerning the influence of strain history on cyclic plasticity and can be useful for verification of existing theoretical approaches and stimulation of new theoretical ideas.

The influence of strain history and strain gradient on confined concrete

This paper outlines the results of tests performed to study the effect of strain gradient, strain history, and initial precracking on the efficiency of confinement in reinforced-concrete columns. To this end, five reinforced-concrete specimens with closely spaced ties have been tested under combined flexure and axial load.Two of the specimens were used as control specimens, subjected only to a strain gradient. The remaining three specimens were precracked using different loading schemes and then tested under a combination of flexure and axial load. Two of these specimens were tested with repeated loading.The test results revealed that precracking lowered both the flexural rigidity and the ultimate capacity of the specimens. No significant differences were observed due to the type of loading used to initially crack the specimens. Repetition of loading did not seem to affect the ultimate capacity of the specimens.Comparison of the test results with four analytical models for confined concrete showed that the models that predicted the behaviour of the control specimens accurately, overestimated the capacities of the precracked specimens. The Thompson and Park concrete model accurately predicted the behaviour of the specimens tested with repeated loading. Key words: reinforced concrete, confined concrete, confinement, columns, cyclic loading, precracked column, experimental.

The dependence of the flow stress of armco iron on strain history

This paper presents experimental results concerning the influence of strain history on the yield stress of Armco iron. Uniaxial tensile tests were performed over the temperature range of 213 to 1023 K and the strain-rate range of ϵ ̇ = 2.77 × 10 −5 s −1 to ϵ ̇ = 2.77 × 10 −2 s −1 To obtain qualitative information concerning the structure of the deformed material, additionally observations were made using a transmission electron microscope. Experimental results show that the value of the flow stress depends not only on strain ϵ, strain rate ·/ϵ and temperature T, but also on the history of deformation and the temperature and time-dependent thermally activated phenomena of ageing and recovery. These phenomena cause changes in structure, and therefore also in the plastic properties of material. On the basis of experimental results, an attempt has been made to separate the effects of deformation from those of the accompanying phenomena of ageing and recovery, depending on the temperature and duration of the processes.

The Changes in Misorientation during the Plastic Deformation of Polycrystalline Metals

In a series of studies on the influence of strain history on the plastic deformation of polycrystalline metals, the following experiments were performed in order to make clear the mechanism of plastic deformation of the material subjected to low-cycle tensile pulsating stresses. The measurement of misorientation was made by the use of back-reflection Laue X-ray method, employing the coarse-grained sheet specimens which were constructed with polycrystalline structure and monocrystalline structure respectively. From the viewpoint of crystal plasticity, it will be considered that the aggregate plastic deformation of the polycrystalline metals subjected to low-cycle tensile stressing is mainly interpreted upon the changes in the structure of the material and statistical inhomogeneity of microscopic stress and strain distributions in the material space. From the present experimental examinations, the following conclusions have been derived.(1) The measured values of misorientation in the polycrystalline structural grain under repeated tensile stressing have been found as larger than those in those in the monocrystalline structural grain. Moreover the farmer are dependent on the direction angle of principal strain in the grain.(2) Inhomogeneity in distribution of misorientation is more remarkable in the grains of polycrystalline structure than in those of monocrystalline structure.(3) The difference between the plastic deformation of polycrystalline structural sheet specimen under low-cycle tensile pulsating stresses and that of monocrystalline structural sheet specimen will be interpreted upon the model of mobility of lattice defects which is considered as the changes in structure of the material subjected to the repeated tensile stresses.

An X-Ray Study on the Plastic Deformation of Polycrystalline Metals Subjected to Cyclic Stressing

A series of studies have hitherto been made on the influence of strain history on the plastic deformation of polycrystalline metals, and as an instance thereof is given in the present paper, a report of investigation that was made from the viewpoint of crystal plasticity, on the behavior of plastic deformation of metals subjected to low-cycle pulsating stresses. In order to discuss the structural change of the material under low-cycle stressing, the experimental measurement of misorientation was performed in the course of low-cycle tensile stressing on the coarse-grained sheet specimen by application of X-ray back-reflection Laue method. From the present analytical and experimental studies, the following conclusions have been obtained:(1) The measured value of misorientation in the coarsed grain of the specimen has increased with advanced number of cycle of low-cycle stressing. Moreover, the change of misorientation in the polycrystalline structural grain under cyclic stressing is larger than that in the monocrystalline structural grain.(2) The misorientation in the polycrystalline structural grain is dependent not only on the orientation factor of each diffraction plane but also on the Schmid factor of the grain at an early stage of cyclic stressing. But these dependency has decreased at the stage of an advanced cycles.(3) The variation in distribution of misorientation in the polycrystalline structural grain is remarkable at the stage of an advanced cycles in its favor in comparison with that in the monocrystalline structural grain.

Plastic Deformation of Polycrystalline Metals Subjected to Cycle of Stress

It is the aim of the present study to elucidate the plastic deformation of polycrystalline metals, consequent on the cycle of stress to which the material was subjected, with respect to its strain history, as part of the series of studies to be made on the influence of strain history on the plastic deformation of polycrystalline metals. It will be considered that the aggregate plastic deformation of the material subjected to low cycle of stress is to be mainly interpreted from redistribution of stress field in the material. The experimental examination of this proposition was made by using the strain measurement apparatus specially designed. From the present analytical and experimental studies on low carbon steel at room temperature, the following conclusion has been made.(1) The yield function of Eqs. (1) and (2) is adaptable to the representation of shape of yield surface of the material under cyclic stressing.(2) The plastic deformation mechanism of polycrystalline metals under a cycle of stress is interpreted from an assemblage of single models as shown in Fig. 3.(3) The cycle-dependent plastic deformation occurs in a different manner of each part of the test specimen, because of the difference in each part of the material structure and its structural change during the stress cycle. It is to be ascribed to the anisotropy of the material, to Bauschinger effect and also to redistribution of yield stress of each part of the structural element.

Study on the Influence of Strain History on Creep of Polycrystalline Metallic Materials at Elevated Temperatures

The present study is the sequel of the authors' previous papers on the influence of strain history on metallic creep at elevated temperatures, with further examination of the materials and detailed discussion on them in a series of studies on the laws of plasticity in creep of polycrystalline metals. From the present analytical and experimental studies on low carbon steel at 450°C, the following conclusions have been made.(1) In regard to the experimental examination of initial isotropy of the material tested, the determination of anisotropic parameter A in the yield function of Eqs. (1) and (2) is simpler and more reasonable than that of usual Hill's parameter.(2) No remarkable development of anisotropy of the material crept under fixed principal stress axes has been observed.(3) In general loadings, in which the principal stress axes are rotated and the level of the equivalent stress is varied during the creep, the creep curve under the condition fairly deviates from the analytical curves based on the strain-hardening hypothesis. It is considered that in such loadings the development of anisotropy of the material crept must be more remarkable than that under the fixed principal stress axes, and that it is therefore dependent on strain history.(4) In the creep tests unber combined constant axial tension and repeated torsion reversals, it does not necessarily follow that the transient increase in shearing strain rate drops immediately after the stress reversal with repetition of the reversal. It is considered that the influence of Bauschinger effect on metallic creep is remarkable when the stress reversal is imposed on the material of which anisotropy is heavily developed.