Pre-strained Specimens Research Articles

Anisotropic strain localization in tensile prestrained sheet steel

Multiple forming operations are often needed to stamp complex shapes out of sheet metal. Large changes in strain path can occur from such operations. This study examined the effects of a particular strain path change, tensile-tensile, on the mechanical properties of an aluminum-killed steel. Large tensile specimens were prestrained various amounts in one direction followed by machining smaller tensile specimens at 0, 45, and/or 90 deg to the prestrain direction. The smaller samples were then pulled to failure. For samples pulled in the same direction as the prestrain, the residual strength and ductility were equivalent to those obtained from an interrupted tensile test. In contrast, both the 45 and the 90 deg prestrained specimens showed a larger than expected flow stress and an abrupt change in the nature of the residual ductility at prestrains of 7.5 pct and larger. At 7.5 pct prestrain, the uniform strain, as measured by the maximum-load point on the load vs elongation tensile curve, decreased abruptly. The decrease was accompanied by a corresponding increase in the post-uniform strain. This unusual behavior is explained in terms of a rapid increase in strain-hardening with strain.

Effects of pre-strain on plane stress ductile fracture in ?-brass

The effects of pre-strain on plane stress ductile fracture in a 70/30 alpha brass Austral 207 have been studied using the deep-edge-notched tension (DENT) specimens. The amount of pre-strain varies between 5 and 35%. It is found that both the specific essential work of fracture (w e) and the critical crack opening displacement (δc) decrease with increasing pre-strain. A simple theory for estimating the specific essential work of fracture in the presence of pre-strain is suggested and it gives good agreement with experimental results. Elongations to fracture in the DENT specimens are also predictable from a simple deformation analysis which considers the plastic elongations due to crack initiation, crack propagation and final stretch of a ligament that has reached a necking strain equal to that in a simple plain tension test. Micro-hardness measurements show that the strain localization is more intense near the fracture surface as the pre-strain level is increased and this is suggested to be an explanation for the low δc values obtained in pre-strained specimens.

ＳＵＳ３０１ステンレス鋼の応力腐食割れ感受性に及ぼす加工誘起α′‐マルテンサイトの影響

The present investigation has been carried out to make clear the effect of strain-induced α'-martensite on stress corrosion cracking (SCC) susceptibility of austenitic stainless steel.The specimens were prepared from SUS 301 stainless steel sheets with metastable austenite, and were given the prestrain of 10, 20 and 40% at the temperature -80 to 80°C. Then the SCC susceptibility of these specimens was evaluated in boiling 35% MgCl2 solution under the applied stress of 35kg/mm2.The change of α'-martensite content depending on prestrain and prestraining temperature gave a remarkable effect not only on the mechanical properties but also on the SCC susceptibility. It was found that the SCC susceptibility of prestrained specimens extremely decreased when α'-martensite content was within a specific range: e.g., about 10∼90% on 20% prestrained specimens. On the other hand, the fracture surface morphology was considerably influenced by the change of α'-martensite content. In the case of small α'-martensite content, the fracture mode changed from transgranular cracking with typical fan-shaped pattern to mixed intergranular-transgranular cracking. In the case of large α'-martensite content, however, transgranular cracking was observed, which seemed to be caused by the dissolution of martensite-lath. Based on these results, the possibility of hydrogen cracking was discussed in the SCC behavior of 20% prestrained specimens under anodic and catholic polalization.

304 ステンレス鋼の高温水中における応力腐食割れに及ぼす予ひずみの影響

The effect of pre-strain on intergranular stress corrosion cracking (IGSCC) susceptibility of Type 304 stainless steel has been studied using a constant load tensile specimen in 288°C water containing 26ppm dissolved oxygen. The results obtained are summarized as follows:(1)The time to failure of the specimens pre-strained at room temperature or 288°C are longer than those of non pre-strained specimens.(2) The stress-strain behaviour of materials changes by pre-straining. The effect of pre-strain on IGSCC initiation could be explained in terms of a relation between the rupture of surface oxide film and the stress-strain behaviour of materials.

引張りおよび圧延加工を加えたα黄銅の低温焼鈍硬化現象

The low temperature anneal-hardeninig of α-brass polycrystals was studied. The special aim of the present work is to make clear the relation between the degree of anneal-hardening and the amount of cold working. The main results are as follows. The increase in hardness with increasing the amount of cold-working was not continuous, but the discontinuous and abrupt increases of hardness were observed at certain strains. In tensile prestrained specimens, the Bauschinger effect measured by compression test was pronounced with increasing the prestrain as a whole, but it became inconspicuous transitorily at the prestrains where the abrupt increases of hardness were observed.In both tensile test and rolling these special strain values where the abrupt increases of hardness were observed coincided with the strains at which the work-hardening characteristics changed. Especially in rolling, the hardening additional to the usual three stage hardening in FCC metals, were further observed at higher strain ranges, corresponding to the occurrence of deformation twins, macroscopic shear bands, respectively.In these prestrain values at which the abrupt increases of hardness appeared, the amount of annel-hardening were smaller than the intermediate prestrain of each stage. Moreover, the compressive yield stress of tensile prestrained specimens also showed minimum values after annealing. These behaviors were discussed from the view point of the stability of dislocation structures.

Effect of Prestrain on Stress Corrosion Cracking of Type 304 Stainless Steel by Means of the Stress Relaxation Method

Abstract The relationship between the susceptibility of stress corrosion cracking (SCC) and the amount of deformation structures in Type 304 stainless steel in an H2SO4/NaCI solution system at room temperature was investigated by means of the stress relaxation method. SCC was observed in the prestrained specimens, whereas no cracking occurred in specimens without prestraining. The number of cracks was affected by the amount of deformation structures, which varied according to the annealing temperature after prestraining or according to the prestraining temperature. The results of magnetic measurements on the prestrained specimens as a function of the annealing temperature showed that the cracking region was in good agreement with the range of existence of strain induced α-martensites. Transmission electron microscope observation showed that ε- and α-martensites formed by prestraining at temperatures below 413 K (140 C) dissolved in preference to the austenite matrix. A strain induced martensite in Type 304 stainless steel seems to precede cracking in this system.

積算塑性ひずみ損傷則による冷間加工の疲労き裂伝ぱ速度に及ぼす影響の検討

Fatigue crack propagation test were performed on single edge-notched pre-strained 7075-T6 aluminum alloy specimens. The similarity between the effect of pre-strain and that of stress ratio on fatigue crack propagation rate was confirmed by taking the acccumulated plastic strain rate, which involves not only the accumulated plastic strain in the monotonic plastic zone but also the plastic pre-strain. An analytical expression was proposed to explain the effect of pre-strain (i. e., the effect of cold work) quantitatively together with the effect of stress ratio. The applicability of this expression was evaluated by examining the effects of cold work and stress ratio on fatigue crack propagation rate of 7075-T6 aluminum alloy and various alloys determined previously.

H<SUB>2</SUB>SO<SUB>4</SUB>-NaCl溶液中におけるオーステナイト系ステンレス鋼の応力腐食割れにおよぼすニッケルおよび窒素の影響

Stress corrosion cracking (SCC) susceptibility was investigated by stress relaxation tests in a H2SO4-NaCl solution for a series of vacuum melted 18%Cr-xNi stainless steels (x=8∼30%Ni) and SUS 316 steels with varying nitrogen contents.As the nickel content of 18%Cr-xNi steels was increased, the time to failure increased adruptly and SCC was not observed in the steels containing 20∼30%Ni for 2500 h. The time to failure of SUS 316 steels containing 0.03∼0.18%N slightly decreased with increasing nitrogen content.Among the deformation structures only a strain induced α′-martensite reveals a magnetic effect. Magnetic measurements were, therefore, performed to examine the existence of α′-martensite. The permeability of the steels containing 8∼11%Ni became higher with an increase in the degree of prestraining. The same behaviour was not observed on other steels. The time to failure of SUS 316 steels, which did not show any change in permeability, depended on the degree of prestraining. Transmission electron microscopic observations showed the existence of ε-martensite in SUS 316 steels prestrained 10% in elongation. With the increased nickel content, the deformation structures of 10% prestrained specimens changed successively in the order of α′ and ε-martensites (8%Ni, 11%Ni), ε-martensite (13.87%Ni), band structure (14.06%Ni), bundle structure (20%Ni) and cellular structure (30%Ni).It is thought that the addition of nickel in austenitic stainless steel increases the stacking fault energy and makes the formation of strain induced martensite difficult. This should decrease the transgranular SCC susceptibility of the high Ni steels in a H2SO4-NaCl solution.

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.

The thermal stabilities of In situ composites in a temperature gradient—I. The Al-CuAl 2 eutectic

A study has been made of the thermal stability of the aligned lamellar Al-CuAl 2 eutectic when subjected to a temperature gradient of about 10 3Kmm −1. After only a few hours instabilities formed on those lamellae that were roughly perpendicular to the heat-flow direction, and which were above a temperature of ~0.9 T e . These instabilities rapidly pinched off from the parent phase and subsequently migrated through the adjacent eutectic plates. At a more macroscopic level, extensive diffusion of Al up the temperature gradient, with attendant thinning of α lamellae, was observed. Experiments involving the orthodox isothermal annealing of strain free and pre-strained specimens confirmed the role of thermal diffusion as responsible for the present observations. It is anticipated that, over typical turbine-component lifetimes of several thousand hours, such effects will be important at much lower homologous temperatures and temperature gradients. Practical in situ composites of the lamellar type may thus need closer scrutiny before they can be unreservedly recommended for typical aero-engine applications.

低炭素25Cr-20Ni系鋼におけるσ相生成におよぼす引張変形の影響

In order to investigate the σ phase formation in a series of 25 Cr-20 Ni steels, the effect of prestrain up to 20% by tension on the σ phase formation at 800°C was studied for low carbon 25 Cr-20 Ni steel with and without an approximately 3% addition of Mo or Si.The results obtained are as follows:(1) The effect of prestrain on the σ phase formation was observed in all of the three specimens and the σ phase was formed in the early stages of heating in the prestrained specimens.(2) The morphology of the σ phase thus formed was not the widmanstätten type, but the σ phase existed along the annealing twin boundaries and its amount, slightly increased with increasing prestrain.(3) In the strongly prestrained specimens, the σ phase formation also occurred on the slip bands in the vicinity of grain boundaries.(4) In the undeformed specimens with Si and Mo, there were denuded zones where the σ phase formation was hardly observable.(5) It seems probable that the σ phase formation in the prestrained specimen is enhanced by the deformation-induced crystal defects, while that in the undeformed specimens depends on the point defect concentration.

アルミ合金の腐食疲労強度に及ぼす引張予ひずみの影響に関する研究

In order to investigate the effect of tensile prestrain on the corrosion fatigue strength, high cycle fatigue tests were performed on aluminium alloy (2017) sheet specimens prestrained by pulling under completely reversed plane bending stress in laboratory air and in salt water (3.0% NaCl). The conclusions obtained are summarized as follows;(1) Corrosion fatigue strength for the prestrained specimens decreased in comparison with that for the as-received specimens, and it decreased monotonously with increasing prestrain regardless of the applied stress amplitude level.(2) Cyclic work hardening and subsequent softening behaviours were observed repeatedly twice for the as-received and 3% prestrained specimens during the corrosion fatigue process. In the specimens prestrained above 3%, the saturation of work hardening took place at about 20% of the fatigue life and thereafter the work softening occured continuously until fracture under subsequent stress cycling.(3) The number of cycles to crack initiation in corrosion fatigue decreased with increasing tensile prestrain. On the other hand, the number of cycles in the crack propagation period was independent of the tensile prestrain.(4) It is suggested that the experimental results obtained in this test can be explained from a consideration that the barriers to the motion of dislocations are easily relaxed by the corrosive dissolution of the specimen surface prestrained during the stress cycling and the plastic deformation of grains easily occurs near the specimen surface.

Effect of Hydrostatic Pressure on Deformation and Fracture of Pre-Strained Specimen : Mechanics of Ductile Fracture in Tension of Carbon Steel

Effect of Hydrostatic Pressure on Deformation and Fracture of Pre-Strained Specimen : Mechanics of Ductile Fracture in Tension of Carbon Steel

Effect of prestrain at 77 K on the plastic deformation of Zircaloy - 2

The tensile properties of unirradiated and neutron-irradiated Zircaloy-2 specimens have been studied after a pre-straining treatment at 77 K. The prestrain was found to increase the rate of work hardening and the uniform and total elongations of unirradiated transverse specimens, as observed by Reed-Hill and Dahlberg in the case of zirconium. Prestrain at 77 K had a small effect on the stress-strain behaviour of irradiated specimens. Metallographic examination showed extensive 11 2 1-twinning in all pre-strained specimens.

Polishing, thinning and etching of polycarbonate

A chemical polish for poly-(bisphenol-A-carbonate) has been developed based on methanolic caustic potash. Two modes of interfacial degradation can be distinguished: etching, where a surface crack is progressively deepened; and polishing, where it is progressively removed. Both mechanisms operate at all concentrations of solution, etching preceding polishing. The depth and extent of etching are a function of concentration and crack width, as is the polishing period which follows. Micropolishing always precedes macropolishing and the overall rate of polishing is strongly dependent on concentration. Polishing damaged samples leads to an increase in ultimate tensile strength by removal of Griffith cracks, and ultrathin films may be prepared by a simple jetting technique. Polishing prestrained specimens produces etch pits which may correspond to crazes formed in the bulk state.

The Effect of Torsional Prestrain on the Tensile and Low-Cycle Fatigue Properties of Mild Steel

The effect of torsional prestrain on tensile and low-cycle fatigue properties of a medium carbon steel was studied. A series of small-sized tensile and fatigue specimens were prepared from large-sized prestrained specimens in order to ensure good homogeneity of the torsional prestrain in the cross-section of the small-sized specimens. Three modes of prestraining were selected in this study; unidirectional, twistback, and single- or cyclic-reversed torsion. Fracture ductility decreased almost linearly with increasing unidirectional prestrain, while the decrease of fracture ductility in the other modes of prestrain was much less than that in the unidirectional one. A transitional decrease in fracture ductility, which has been reported for inhomogeneously prestrained specimens, was not observed in this study. A slight trace of recovery in fracture ductility due to twisting back the unidirectional prestrain to the macroscopically undistorted original state was observed, in contrast with the almost complete recovery for the inhomogeneously prestrained specimens. The low-cycle life of the unidirectionally prestrained specimens was reduced by an amount which would be predictable from the decrease in fracture ductility of those specimens. For other modes of prestrain, however, the influence on fatigue life was undistinguishably small in this study.

モリブデンの脆性におよぼす予歪の効果について

The effect of prestraining by tension on the brittleness of electron beam melted pure molybdenum was studied and the following conclusions were obtained:(1) The brittleness of the recrystallized pure molybdenum sheets can be improved considerablly below room temperature by prestraining about 5∼10% in the ductile range. The effect of prestraining increases with the amount of prestrain and changes with the temperature at which prestrain was given.(2) The prestrained specimens containing many edge dislocations show better ductility at low temperature than the prestrained specimens containing many screw dislocations.(3) Strain ageing due to interstitial impurities is negligible below 300°C.(4) As an important mechanisum, free dislocations and some Frank-Read sources introduced by the prestraining release the stress concentration when the prestrained specimens were tensile-tested at lower temperature, and the brittleness can be improved.(5) Ageing at 500°C after prestraining at 300°C decreases the ductility of prestrained specimens at low temperature. This is due to the dislocation locking caused by the interstitial impurities.

The influence of pre-strain on deformation twinning in niobium single crystals

Single crystals of niobium were grown by the electron beam floating zone technique and purified by annealing in ultra high vacua. Crystals orientated for either single or double slip were predeformed at room temperature or 158°K in order to investigate the effects of various dislocation distributions on the twinning behaviour. It was found that pre-strain always raises the twinning stress and that more twins form in prestrained specimens. Pre-strain at 158°K in both double and single slip orientations and at room temperature in double slip orientations, is effective in suppressing twinning at temperatures down to 77°K, but not at 55°K and below. The twinning stress continues to increase slightly with decreasing temperature down to 20°K and there is evidence that twinning is always accompanied by some slip. In very pure pre-strained crystals, twinning is not restored by annealing. It is concluded that a homogeneous distribution of dislocations is the main requirement for effective twin suppression and that dislocations which intersect the twinning plane are more effective than those which are parallel to it. A possible nucleation mechanism is the dissociation of screw dislocations, formed by micro-slip, at or below the macroscopic yield stress at which these dislocations move.

Effect of Pre-Strain on the Fatigue Limit of 7:3 Brass Specimens

Either with the aim of studying the effect of the pre-strain on the fatigue strength of materials, or for the purpose of increasing fatigue strength, there have been investigators so many who have tested many materials with various amounts of pre-strain under repeated stresses, such as push-pull, rotary or reversed bending, and reversed torsion with some of those having mean stress, and the results from these tests indicate that fatigue limit is generally increased by pre-strain. On the other hand, there have been reports that the fatigue limit of specimens that have been subjected to a small amount of pre-strain (just above yield point) drops below that of the original material.The above-mentioned facts are ascertained in this paper by the experiments with 7:3 brass specimens with electro-polished and pre-strained surface. The amounts of pre-strain are 1.3, 5.3 and 8.7%. With 1.3% pre-strain, the fatigue limit decreases 8.3%. This phenomenon is explained by the fact that slips occurred in relatively weak grains on the surface of the specimen. The damage is retained and there is no artificial strengthening of the material. On the other hand, in the largely pre-strained state, the improvement by artifice is greater than the damaging effect due to the slip, and the fatigue limit is increased. By this consideration, the same phenomenon about notched prestrained specimens can also be explained.

Snoek peaks and their stability in annealed and deformed tantalum

The internal friction behaviour of electron-beam-refined tantalum containing oxygen, nitrogen and carbon has been investigated during constant-rate heating and cooling cycles in the range from 20° to 400°C. The height of the Snoek peaks remained unchanged in specimens which had been slowly cooled prior to testing and only slight change in peak height occurred as the result of vacuum quenching. However, the height of the oxygen peak decreased and that of the nitrogen-carbon peak increased appreciably on heating pre-strained specimens; on cooling the reverse occured. An exchange mechanism between oxygen atoms at dislocations and nitrogen and/or carbon atoms in free solution is proposed to account for these variations. Isothermal measurements at the Snoek peak temperatures have shown that the peak heights decrease with time, after straining, at a rate which obeys the t 2 3 law proposed by Cottrell and BILBY. The data obtained from these investigations has been used to calculate dislocation densities in 6% pre-strained tantalum-oxygen solid solutions and dislocation atmosphere densities in the fully-strain-aged condition at 150°C.