Pre-strained Material Research Articles

An Experimental Study of the Evolution of Yield Loci for Anisotropic Materials Subjected to Finite Shear Deformation

This paper presents a set of experimental results concerning the evolution of yield loci for anisotropic materials in the course of finite shear deformation history. Thin walled tubes of SAE 1020 steel are tested under triaxial loading conditions – tension, torsion and internal pressure, at room temperature. The specimens were prepared from cold-drawn tubes which had a prominent fibrous grain structure. They showed considerable amount of in-plane anisotropy and orthotropic yield properties with respect to the longitudinal and circumferential axes. The as-received specimens were further pre-strained by imposing simple shear which causes the change in orientations of these elongated polycrystalline grains. Our previous experimental results [1] have verified that the principal directions of the total stretch of the current grain shape match with the directions of orthotropy, and in this work, the subsequent yield loci for the pre-strained material are determined with respect to the new reference frame fixed along these directions of orthotropic symmetry. By utilizing the unique triaxial loading scheme of our servo-hydraulic testing system it has been possible to create the principal directions of a biaxial state of stress in a specified direction. The changes in the size and shape of the yield loci in the stress space are critically analyzed in this paper.

Effect of yield stress and stress ratio on fatigue crack closure in 6063-T6 aluminium alloy

Crack closure experiments were performed on 6063-T6 Al-alloy for different stress ratios, R = 0, 0·1, 0·2 and 0·3 at different prestrains, 0%, 2% and 3%. The growth rate of fatigue crack in the prestrain material was more than that of as-received material. It was shown by crack opening displacement measurements that crack closure occurred to a lesser extent in prestrained material. On the basis of these experimental results a good correlation was obtained for da dN and ΔK eff for the single edged notched specimen as well as the centrally notched specimen using the developed equation for U. The significance of the increased yield strength for fatigue crack growth is discussed. Fatigue life decreased as the percentage of prestrain increased.

Effect of compressive prestrain on necking of round bars under a tensile load.

Several tensile stress-strain curves on virgin material corrected after maximum load have been studied, but few on compressive prestrained material have. We made tensile tests on copper round bars with maximum 80 % of prestrain. The main results obtained are as follows : (1) Tensile stress-strain curves corrected by dimensions of necking have a tendency to be equal in a large strain zone. (2) There is a point where the change of R/ρ with progress of deformation slows down and in this point, the value of ρ/R0 is approximately 2 (ρ: longitudinal radius of necking, R : the minimum sectional radius, R0 : initial radius of section) . (3) Only in the case of over 25 % prestrain are characteristic phenomena seen when compared with the case of virgin material.

Effect of Prestrain on Fracture Toughness of Rolled Mild Steel with Significant Inclusion Content

Abstract The basic method of JIc calculation using a single specimen is discussed. Dokouipil's approach for evaluating the JIc value is extended further and the effect of prestrain on rolled mild steel with significant inclusions is studied using this modified approach. Although this method does not give an accurate value of JIc, it is quite effective for a comparative study. While the fracture toughness of annealed and 7% prestrained materials are about the same, the fracture toughness of 3% prestrained material is significantly lower.

An examination of the neutral loading condition in plasticity theory

An examination is made of a number of neutral loading experiments according to classical plasticity theory. For work hardening materials the question as to whether plastic deformation occurs during neutral loading depends strongly upon the deformation produced from initial loading. Initial elastic loading with a subsequent stress path that follows the boundary of the initial vield surface is truly neutral with a wholly elastic response. However, when plastic strain is produced from initial loading then plastic flow is produced from a subsequent stress path that follows the boundary of a surface that is an isotropic expansion of the initial yield surface. Since this violates the assumption of isotropic hardening the usefulness and limitations of the rule of kinematic hardening are examined. It is further shown that the results from recent experiments are particularly relevant to the appraisal of modern developments to plasticity theory.Neutral loading without work hardening will produce plastic flow. It is shown that the corresponding Prandtl—Reuss theoretical solutions are representative of observed behaviour for both severely pre-strained materials and for others that do not harden appreciably.The concept is introduced of lower and upper bounds on the deformation that can be expected from neutral loading. These correspond to the extreme purely elastic and non-work-hardening solutions, respectively.

Effect of prestrain on fatigue crack propagation in mild steel containing non-metallic inclusions

The effect of tensile prestrain on fatigue crack propagation behaviour of commercial mild steel with significant amount of stringer inclusions has been studied. In prestrained materials the usual stable stage II crack growth region is preceded by a phase wherein a retardation in crack growth rate occurs. No such behaviour is observed in annealed material. The amount of retardation is found to increase with increase in prestrain. A mechanism for the observed retardation in crack growth rate is also presented.

Macro-Mechanical Analysis for Symmetric and Asymmetric Fully Plastic Crack Growth

Asymmetries like welds or shoulders, may eliminate one of the two shear zones of symmetrically fully plastic cracked parts and thus give crack propagation along the remaining active slip band through pre-strained material instead of through the relatively unstrained region between two shear bands of the symmetric case. One thus expects a reduced ductility in the single shear band asymmetric case. A macro-mechanical analysis provides a physical basis for explaining the development of deformation in both geometries, as observed from tests on several alloys. The asymmetric case is approximated as Mode II shearing, with the crack extension occurring by sliding off along a single slip plane and fracture. In the symmetric Mode I case, the crack is assumed to extend by alternating shear on two symmetric slip planes and fracture.

Fully plastic asymmetric crack growth near a single shear band

An asymmetric configuration may arise if a crack is near a weld or shoulder. In this case, loading into the plastic range can give a single asymmetric shear band extending from the crack tip instead of the two shear zones of the symmetric case. The resulting crack propagation near the active slip band, into previously pre-strained material, may give less ductility than the typical symmetric case where the crack advances into relatively unstrained material between the two shear zones. An approximate solution for the growth of such asymmetric cracks in regions dominated by an HRR type strain singularity is presented. The effect of strain hardening on crack growth is investigated and the predicted growth rate is compared with test data on several alloys.

A brittle to ductile transition in NiAl of a critical grain size

The current investigation presents a breakdown analysis of the elastoplastic behavior of commercially pure aluminum pre-strained via severe plastic deformation (SPD) and tested in tension. The tensile samples selected, owing to their prior SPD, had the gradient microstructure from the fragmentation stage as well as a homogeneous equiaxed structure from the steady-state regime. Except for the regions of low deformation and steady-state of SPD, the microstructures of pre-strained material were largely dominated by the presence of geometrically necessary boundaries. During tensile straining, dislocation strengthening contributed more to the overall material strength in fragmentation stage samples, while grain boundary strengthening played a major role in the steady-state stage samples. The dislocation density evolution rate-based approach predicted a more active role for the dynamic annihilation/recovery events in the SPD material when compared to the fully recrystallized condition. This could explain the drastic drop in tensile elongation of the pre-strained material as well as the enhancement in uniform and post-necking elongation with the gradual increase in the amount of pre-strain. A plastic instability condition based on the dislocation density evolution approach successfully accounted for the observed stable deformation limits in the coarse- and fine-grained material.

両振り引圧疲労におけるき裂の発生と進展に及ぼす過大応力の影響

Alternating two-load level fatigue tests were performed using annealed S40C steel to investigate the influence of overloads on the fatigue damage in crack initiation and propagation stages. The base block consisted of axial reversed strain cycling Δεp=1.1% as overloads and constant stress cycling σa=22kgf/mm2 as a baseline loading. Load levels were changed carefully so that no macroscopic residual strain could remain in the specimen.The load level at which the microcrack initiated in the first block determined the future crack growth behavior: In the case of large number of overloads in one block, microcracks initiated during overload cycling and the damage was cumulated linearly in the next base loading until cracks grew 0.1mm at the specimen surfaces. On the other hand, in the case of small number of overloads, microcracks initiated during next base load cycling and grew at higher rate than the normal; 0.1mm crack initiation life was reduced about 1/3 of the life for no overload and was about the same as that of the pre-strained materials. In the crack propagation stage, overloads caused crack growth retardation and a greater number of overloads resulted in a longer propagation life.

Effect of injection temperature, 5% pre-strain and long time aging on the high-temperature mechanical properties of helium injected 316 stainless steel

The helium embrittlement of 316 stainless steel, helium injected to concentrations of 3 ~ 10 at ppm at 573 ± 50 or 1073 ± 20 K, was measured by tensile tests at 1023 K and was completely suppressed by the 5% pre-straining. This improved effect was lost by a post-injection again of 1000 h at 1123 K or 3000 h at 1023 K. It was concluded from the TEM observations that in the solution treated material helium bubbles were concentrated on the grain boundaries and caused embrittlement, but in the 5% pre-strained material helium bubbles nucleated on the dislocations and grew large enough to be unable to be dragged to the grain boundaries by the dislocations during the tensile test. The embrittlement after the long time aging was due to the gathering of the large helium bubbles at the σ-phase boundaries by a sweepout mechanism due to dislocation climb during the long time thermal recovery process.

ジルカロイ管の集合組織変化と降伏曲線に及ぼすひずみ履歴の影響

Fully-annealed Zircaloy-2 tubing was strained by balanced axial stress σx and circumferential stress σθ(stress ratio: a=σ2/σθ). Then, texture and subsequent yield loci of these prestrained materials were measured.Results of texture measusement after prestraining showed that (0002) poles tend to move towazd the radial tube direction under a=0, 0.5 and 1, but toward the circumferential tube direction under a=2 and ∞. Specimens highly prestrained under a=0 and 0.5 have extremely concentrated texture. Such texture changes can be explained by a deformation model in which <c+a> type slip system was assumed as one of the deformation system. The yield strength of most prestrained materials is higher than that of starting material, however, the material prestrained under a=∞ shows lower yield strength than starting material under test condition of a=0. It was observed that the texture change had an important influence on subsequent yield behavior. Typically, the material highly prestrained under a=0.5, which had concentrated basal poles, gave the yield locus chazacterized by remarkable "texture hardening".

An Analysis of the Yield Criteria and the Stress-Strain Relationships of a Pre-Strained Commercially Pure Aluminium

An analysis of the stress path and the strain trajectory of a pre-strained engineering material in the deviatoric plane is presented. The deviatoric stress vector, which for a pre-strained material is initially at an angle to the strain trajectory, is shown to coincide with it after the ‘recoverable’ energy is consumed. Experimental work is carried out on the yield criteria and the stress—strain relationships of a pre-strained commercially pure aluminium. The significance of the isotropic hardening rule in relation to a metal's straining memory is demonstrated. It is shown that beyond a stress level associated with an isotropic hardening rule based on the Mises yield criterion, the representative stress—strain curves of a pre-strained specimen run parallel to that of an annealed material.

Initiation and propagation of fatigue crack in pre-strained material

Experimental investigations of the effect of pre-straining on the crack initiation and propagation in fatigue have been carried out on mild steel and stainless steel. X-ray analysis of the material has clearly revealed that pre-straining of the material introduces surface compressive residual stress. With increasing degree of pre-strain the crack nucleation period is delayed and the rate of crack propagation is decreased. The maximum stress intensity factor K max is related to the rate of crack propagation by a relation da/dN=CK max q , where the value of the exponent q is approximately 2.5. The constant C decreases with increasing pre-strain. Thus it has been observed that the increase in fatigue life is contributed by the delay in crack nucleation and a decrease in the rate of crack propagation. The increase in the endurance limit appears to bear a linear relation with the degree of pre-strain.

Research Note: The Influence of Surface Preparation on Machining Forces and Chip Geometry

In machining practice, repeated passes result in the presentation of pre-strained material to the cutting edge. The effect of such strains on machining forces and chip geometry is investigated in a series of machining experiments performed on α-brass using a variety of cutting conditions. It is found that both thrust force and chip side spread depend on surface preparation, but cutting force and chip length ratio are not significantly affected by surface preparation under the conditions of the experiments. Surface preparation is also found to affect the scatter of results.

The effect of pre-strain on fatigue crack growth and crack closure

A single pre-strain ( ϵ = 0.03) applied to 2024-T3 material raised the static yield strength from 428 to 480 MN/m 2. The growth rate of a fatigue crack in the pre-strained material was about twice as large as in the material that was not pre-strained. A pre-strain followed by 1000 high, but still elastic pre-stress cycles did not further increase the growth rate. It was shown by COD measurements that crack closure occurred to a lesser extent in the pre-strained material as compared to the original material. The significance of the increased yield strength for fatigue crack growth is briefly discussed.

The Effect of the Rotation of the Stress Axes on the Yield Criterion of Prestrained Materials

The yield locus of a prestrained material can be based on different systems of combined stress, such as biaxial tension and combined tension and shear. No matter what the combined stresses may be, however, the deviatoric yield stress is a function of only two variables, the characteristic index, which represents the stress ratio, and the stress direction, or the inclination of the principal stress axes. It is shown that, when tubular specimens are tested under combined torsion and axial tension, the results show the mixed effects of the characteristic index and the stress direction on the yield stress. In such tests, the two effects can, however, be separated if the strain vectors as well as the yield stresses are known. The theory is applied to several groups of experimental results on the yield locus of prestrained materials, including Taylor and Quinney’s results.