Plastic Prestrain Research Articles

予ひずみならびに高ひずみ速度を受けるガセット継手の破壊挙動に関する研究

Fracture tests of gusset welded joint with various conditions, tensile and compressive pre-strain, static and dynamic loading, tension and bending loading, were performed and fracture characteristics were discussed. Ductile fracture strength was estimated using tensile strength of material at tested temperature. Brittle fracture strength was estimated by fracture mechanics using CTOD criteria, CTOD design curve of WES2805 was applied to estimate strain at weld toe of gusset joint. Intrinsic crack model was used to determine effective crack size parameter of gusset joint. Effect of plastic pre-strain and strain rate on fracture toughness was estimated using equivalent static temperature, considering plastic pre-strain and strain rate to temperature shift. Test results of all conditions are well expressed by above estimation method.

Influence of elastic inclusion morphology and matrix hardening behavior on bauschinger effect in metal matrix composites

Finite element modeling based on axisymmetrical cells was performed for relating the Bauschinger effect (BE) in metal matrix composites (MMCs) to the reinforcement volume fraction and shape, matrix hardening behavior, and plastic prestrain levels. It was found that elastic inclusions in MMCs introduce a significant BE, which is ascribed to the residual phase stresses in the component phases. The BE introduced by cylindrical inclusions is more significant than that of spherical ones and increases rapidly with increasing the aspect ratio of the cylindrical inclusion. The volume fraction of the elastic inclusion has a strong effect on BE. The hardening behavior of the matrix has a weak effect on BE. The BE increases with increasing plastic prestrain level.

Enhancement of the durability of structural steels and their welded joints by plastic deformation

The fatigue and corrosion-fatigue resistance of low-alloy steels increases as degree of plastic prestraining increases to 29%. The resistance of the 08kp steel prestrained and hardened by surface plastic deformation and its welded joints to fatigue and corrosion-fatigue fracture is higher than for the 08GSYuT alloyed steel. The procedures of pneumatic shot blasting and shot blasting prove to be efficient methods for increasing the cyclic and low-cycle durability of welded joints both in air and corrosive media. The corrosive medium considerably decreases the low-cycle durability of welded joints of sheet steels. The effect of the medium is more pronounced for alloyed steels and becomes weaker as the amplitude of strains increases.

Non-uniform deformation after prestrain

The deformation behaviour of prestrained metal sheets is analysed in this work. The non-uniform deformation observed during reloading in tension was studied, by following deformation in different regions of the samples. It takes into account the presence of geometrical defects in the samples and explains the importance of mechanical behaviour. A simplified analysis was used, to model the behaviour in tension of a metallic specimen with geometrical imperfection. The flow behaviour is described using a Swift law equation, which includes strain-rate sensitivity. A modified law was used for prestrained materials and this incorporates the plastic prestrain value, adjusted to the path change. The model predicts imperfection growth kinetics with strain, and strain saturation in the homogeneous region, due to the onset of necking.

Influence of Preliminary Plastic Deformation on the Low-Cycle Fatigue of a Titanium Alloy under Conditions of Deep Cooling

We consider the results of testing of PT3V titanium alloy (4.5% Al, 2.1% V, balance Ti) for low-cycle fatigue at temperatures of 293, 77, and 4.2 K, taking into account the influence of the degree (0.05, 1.0, and 3.0%) of plastic prestraining of the material (performed at the same temperatures) on durability. To simplify the procedure of testing and data processing and decrease the costs of testing, we apply the methods of mathematical design of experiments and factor analysis. It is shown that the maximum decrease in the durability of the tested alloy is attained for a degree of prestraining of 3% at a testing temperature of 4.2 K realized in liquid helium.

Development of a Three-dimensional Model for Ductile Fracture in Materials Containing Two Types of Microvoids.

There are three stages in ductile fracture process: voids initiation, void growth and void coalescence. In the final stage, ductile fracture occurs due to the void coalescence. Thomason suggested that voids begin to coalesce when a plastic limit-load condition is fulfilled for localized plastic failure of the intervoid matrix. On the basis of this critical void-coalescence condition, we developed a three-dimensional model for ductile fracture considering two types of voids with different sizes, and applied this model to a SA440 steel with and without pre-straining. With the critical strength of particle/matrix interface and the theoretical void growth strain obtained from the model, the reason for the decrease in fracture strain caused by plastic pre-strain was discussed.

Effect of Strain Rate and Plastic Pre-strain on the Ductility of Structural Steels.

Ductile fracture processes in structural steels under high strain rates have been investigated by tensile test. Under high strain rate loading, the ductility of the steels decreases. The reasons for this decrease is discussed from the analysis of ductile fracture process, as especially of void nucleation strain and void growth strain up to fracture. A three-dimensional void coalescence model for the high strain rate loading was obtained by modifying Thomason's static three-dimensional void coalescence model. The model was used to predict the void growth strain and evaluate the effect of strain rate on void coalescence. The influence of plastic pre-strain on ductility under high strain rate loading was also investigated.

Evaluation of Ductile Fracture of Structural Steels by Microvoid Model.

In the present work, the ductile fracture of structural steels and the effect of plastic pre-strain on ductility have been investigated by tensile test. The fracture process consisting of void nucleation, growth and coalescence was observed by scanning electron microscopy (SEM). The nucleation strain of microvoid was measured experimentally. The critical interfacial strength of particle/matrix interface was calculated by dislocation model and continuum analysis, and the void growth was evaluated by Rice-Tracey model. It was found that longitudinal void coalescence does not affect fracture even though large cracks exist, but particle position affects void nucleation. Thomason's plastic limit-load model was used to predict void growth strain. Through the analysis of ductile fracture process, the influence of plastic pre-strain on ductility was discussed.

A tensor function for the R-value of sheet metal

Tensile testing of sheet metals shows that both the orientation of the testpiece in the plane of the sheet and the degree of rolling reduction can influence the R-value. To explain this a quadratic and a cubic yield function are adapted to published results for a number of sheet metals. The cubic function can reproduce every dependence of R upon orientation as found from off-axis tensile tests. Present work shows that a simpler quadratic function lies within the material deviations. The theory of tensor functions enables this function to be expressed in terms of prior plastic strain, and so they provide a description of the dependence of the R-value on the processing history of a sheet metal. It is demonstrated how this junction can be applied to account for the in-plane R-variations observed when rolling a copper sheet under plane strain laboratory conditions. Of particular interest are the predictions that lead to improved drawability, i.e., the rolling reduction and sheet orientation that optimize R. A further examination is made of the stability of the R-variation pertaining to a given rolling reduction. It is shown that R remains stable under subsequent uniaxial plastic straining for any direction in the plane of the sheet. Moreover R is not altered appreciably by relatively small amounts of tensile plastic prestrain aligned with the rolling direction. That R is sensibly constant is the usual assumption made in plasticity theory.

Amplitude-dependent internal friction, microplastic strain and recovery of lead at ambient temperature

Damping properties and the influence of plastic prestrain on microplastic behaviour of polycrystalline lead have been studied by internal friction. Measurements were performed in the infraand ultrasonic ranges. Infrasonic tests were combined with high-resolution registration of the microplastic strain. The strain amplitude dependence of internal friction shows two stages characterized by different recovery times. These stages are attributed to different mechanisms: dislocation — point defect interaction at moderate oscillatory strain amplitudes and dislocation — dislocation interaction at high amplitudes. A pronounced and nonmonotonic dependence of the amplitude-dependent internal friction (ADIF) and elastic limit on prestrain is revealed. Prestraining up to 10% suppresses the ADIF in the ultrasonic range and causes a considerable increase of the elastic limit in the infrasonic range. A further prestrain up to 30% is accompanied by a drastic increase of the ultrasonic ADIF and a pronounced fast softening of lead in the micro- and macroplastic ranges. The conclusion has been drawn that ortho-and metarecovery occur intensively in lead after high prestrain. These processes are ascribed, respectively, to a decrease in net dislocation density and to a transition from a relatively homogeneous dislocation distribution to an inhomogeneous one.

Low-cycle fatigue of a metal-matrix composite: Influence of pre-straining on the fatigue life

Low-cycle fatigue tests were conducted on an Al 2O 3 particulate-reinforced metal-matrix composite. Special emphasis was placed on the effect of plastic pre-straining on the fatigue lives of subsequently performed fatigue tests. In all cases of a standard heat-treated material (T6), the pre-deformed samples showed reduced fatigue lives compared to the sample without pre-strain. The cyclic deformation curves showed initial hardening and subsequent cyclic softening for nearly the whole remaining life time. In another test series, specimens were investigated, in which the matrix was not hardened by precipitates. This material was found to be cyclically stable, showing no softening.

Brittle fracture of nuclear pressure vessel steels—II. Prediction of fracture toughness

A model for the prediction of fracture toughness for nuclear pressure vessel steels has been worked out on the basis of brittle and ductile fracture criteria formulated by the authors in an earlier paper. An analysis was performed of the stress-strain state near the crack tip in the finite strain statement. The model proposed allows one to obtain an adequate prediction for fracture toughness both at the low temperatures at which brittle fracture is realized and higher temperatures at which fracture is ductile. Also the proposed model describes the plastic prestrain effect on fracture toughness. As applied to a 2·5CrMoV steel it has been shown that fracture toughness is controlled by condition of cleavage microcrack nucleation near the crack tip but not by any propagation condition. Critical brittle fracture stress S c which is usually considered as a main parameter controlling brittle fracture, determines the temperature T tr of brittle to ductile fracture transition only but not a value of K IC.

Prediction of fracture toughness for heat-resistant steels considering specimen dimensions. Report 4. Fracture toughness after plastic prestraining of materials with cracks

The paper presents the results of an experimental investigation of the influence of warm prestressing (WPS) on fracture toughness characteristics of large-size specimens. The WPS has been found to be an efficient method for enhancing brittle fracture resistance of large-size bodies from the investigated materials and can be recommended for practical realization in nuclear reactors and other critical structures whose brittle fracture is impermissible both in the process of normal operation and in emergency situations. The optimum temperature-loading regime of the WPS is defined by both the properties of a given material and its thickness which governs the intensity of plastic deformation in the process of WPS. Based on the established mechanisms of the WPS effect, a physicomechanical model has been developed for the prediction of fracture toughness for pressure-vessel heat-resistant steels after WPS taking into account the influence of the stress state at the crack tip. The model makes it possible to predict fracture toughness for large-size bodies subjected to WPS with the given temperature and loading regimes from the results of testing small laboratory specimens. The most optimum regimes of the WPS can also be determined using this model and even those for several materials making up a structural component and subjected to the WPS.

Experimental investigation of an anisotropy in copper subjected to predeformation due to constant and monotonic loadings

Experimental analysis of the plastic properties for both an aged pure copper and the same material subjected to various types of predeformation is presented. The analysis was made by studying; the position in stress space and by determination of dimensions of the yield surfaces. The initial yield surface has been determined using the probing technique of a single specimen and this surface was used as the starting point for comparative studies of yield surfaces of the prestrained material. The material predeformations of the same magnitude were induced either by a creep process at elevated temperature or by monotonic loading at ambient temperature. After required values of plastic prestrain were attained, yield surfaces were determined by the same method as for the non-prestrained material. The anisotropic yield condition was used to create a least squares fit of the experimental data.

Thermal expansion studies of prestrained Al 2O 3/Al metal matrix composite

The present study was undertaken to investigate the effect of prestraining on the coefficient of thermal expansion (CTE) of 10 and 20 volume percent (v/o) Al 2O 3 particle-reinforced composite over various temperature ranges using thermal mechanical analysis (TMA). The CTE of the composite was shown to be dependent on the volume fraction of the broken particle, plastic yielding and flow, and other transformation processes occurring at elevated temperatures. The experimentally measured values of the CTE were found to be in good agreement with Schapery's model at various values of plastic prestrain (0–10%) and with a modified Kerner's equation, at high prestraining ( ε p > 4%) for a wider temperature range.

The two-way memory effect by the pre-strain training method in a 50Ti40Ni10Cu (at.%) alloy

The two-way memory effect (TWME) can be settled after a training of the shape memory alloy apt to stabilize the preferential growth (shrinkage) of martensite domains by decreasing (increasing) temperature. That appears as a consequence of a pattern of line defects which act as nucleation sites of selected variants growth. Though several training procedures have been suggested, a clear cut view of the most efficient one has not yet been reached. Recently a pre-strain method has been adopted to settle the TWME in TiNi alloys, with the highest reversible strain obtained for a plastic pre-strain introduced at temperatures (T) M{sub s}TA{sub f}. The TWME on 50Ti40Ni10Cu (at%) alloy has already been investigated with a training procedure based on a sequence of thermal cycles under applied stress involving the B2(parent) {leftrightarrow} B19(orthorhombic) transformation. Interest is here given to the pre-strain training TWME in a 50Ti40Ni10Cu (at%) alloy introducing the plastic prestrain either in monoclinic (B19{prime}), or in orthorhombic (B19), or in parent (B2) phase in the range of stress induced martensite.

Cyclic Plastic Deformation of Stainless Steel SUS304 Subsequent to Plastic and Creep Prestraining.

Effects of plastic and creep prestrain on cyclic plastic deformation are experimentally investigated. Thin walled tubular specimens of stainless steel SUS304 are subjected to combined axial and torsional loads. Prestraining is conducted at room temperature and 600°C, and subsequent cyclic loading under constant strain amplitude is performed at room temperature and 600°C. The cyclic stress-strain curves become saturated with increasing number of cycles. The properties of cyclic loading are examined based on the maximum stress values of the saturated cyclic stress-strain curves. The saturated cyclic stress subsequent to creep straining is influenced by testing temperature as well as strain amplitude. Small strain amplitudes result in large saturated stress values comparing with cyclic loading without creep prestraining. The effects of cyclic loading under combined stress state are also examined using the equi-plastic strain curves. Tensile creep straining expands the equiplastic strain curves in the directions of tensile and torsional stresses. Cyclic loading subsequent to tensile creep straining result in the expansion of the equi-plastic strain curves in the directions of tensile and compressive stresses.

Kinetics of F.c.c. → b.c.c. heterogeneous martensitic nucleation—II. Thermal activation

A critical analysis of the experimental data for the activation energy for f.c.c. → b.c.c. martensitic nucleation in FeNi, FeNiC, FeNiCr and FeNiMn alloys has been performed employing the theory of dislocation discrete-obstacle interaction. It is shown that the normalized activation energy Q ( T)/ μ( T) vs normalized driving force Δg n /Δ g ̂ data for isothermal martensitic nucleation follows the general form proposed by Kocks, Argon and Ashby [in Thermodynamics and Kinetics of Slip, Progress in Materials Science, Vol. 19. Pergamon Press, Oxford (1975)] for the kinetics of conventional slip deformation in solid solution strengthened alloys. Curvature of the Q( Δg) function defines a temperature dependent activation volume for isothermal martensitic nucleation. The analysis is consistent with rate control by the interaction of solute atoms with the thermally-assisted motion of the martensitic interface. The contribution of a solute to the thermal component of interfacial friction is found to scale with its athermal contribution. The analysis incorporates kinetic data for the effects of applied elastic stress and plastic pre-strain, providing a quantitative estimate of the interfacial friction from dislocation forest hardening.

UNSTABLE FATIGUE CRACK PROPAGATION AND FATIGUE FRACTURE TOUGHNESS OF STEELS

AbstractThis paper presents the results of fatigue crack growth and fatigue fracture toughness studies of a high‐pressure vessel steel with particular emphasis on the influence of heat treatment, low temperatures, plastic prestraining, the stress ratio and specimen dimensions.It has been shown that steels in an embrittled state, caused primarily by thermal treatment and low‐temperatures, exhibit unstable fatigue crack growth which is characterized by alternate crack jumps (cleavage zones) and zones of fatigue crack growth. The fatigue fracture toughness, which corresponds to the first crack jump, and final fracture can be appreciably lower (i.e. up to 50%) than the static fracture toughness under plane strain conditions at the corresponding temperature.An analysis has been performed of unstable and stable fatigue crack growth and a model of unstable crack propagation is proposed which accounts for the observed experimental behaviour.

The Bauschinger effect in compression-tension of sheet metals

It is important to know how sheet metals behave under complicated loading conditions, since yielding and hardening of a metal are actually dependent upon the stress state and deformation history. An example is the Bauschinger effect, which is manifested as a loss of yield strength during reverse loading upon plastic prestraining. Quantitative analysis of the Bauschinger effects is required for better understanding of this problem. In this study, experiments carried out by subjecting sheet metals to uniaxial compression-tension tests show that the Bauschinger effect varies with the material properties and the magnitude of prestrain. On the basis of the experiments and the continuum theory of plasticity, parameters defining the Bauschinger effect are proposed, which interpret the combined effect of work hardening and work softening of a material during forward and reverse straining, and a model revealing the Bauschinger effect on the subsequent yield surface is finally presented.