Cyclic Deformation Properties Research Articles

Effect of lamellar structure and plastic anisotropy on the cyclic deformation and fatigue behaviour of TiAl PST crystals

Abstract Cyclic deformation and fatigue properties of TiAl polysynthetically twinned (PST) crystals were investigated by focusing on the effect of lamellar structure and plastic anisotropy on the cyclic hardening and fatigue life. Cyclic tension/compression testing at room temperature at total strain-controlled amplitude was carried out on Ti-49.1 at.% Al and Ti-50.8 at.% Al PST crystals containing fine and coarse lamellae, respectively. Strong anisotropic behaviour was observed, depending on the angle (φ) between the loading axis and the lamellar planes. As the strain amplitude was increased, the stress amplitude rose with increasing number of cycles and the cyclic hardening became large at φ = 0, while at φ = 45° specimens exhibited weak cyclic hardening and broke without significant hardening. Although the lamellar spacing in TiAl PST crystals did not lead to much difference in the cyclic hardening rate, refinement of lamellae prolonged the fatigue life in both orientations. The lamellar boundaries act as an effective barrier to the propagation of microcracks crossing the boundaries at φ= 0. The cyclic hardening and fatigue life of specimens with φ = 0 are sensitive to the deformation substructure, particularly with regard to the number of domains in which ½<110]-type ordinary dislocations form a densely tangled and piled-up substructure. The cyclic hardening and fatigue life of crystals with φ= 0 also depends upon the rotation angle (χ) between the loading axis and the ⟨112⟩ direction on (111) lamellar planes in the γ matrix.

Basic Deformation Properties of a Polyurethane-Series Shape Memory Polymer Film.

The basic deformation properties of a polyurethane-series shape memory polymer film were investigated experimentally. The main results are summarized as follows. The modulus of elasticity below Tg is about 100 times larger than that above Tg. The loss tangent in the vicinity of Tg is about 0.6. In the case of cyclic deformation above Tg, yield stress decreases and irrecoverable strain increases with increasing number of cycles. The cyclic deformation properties vary significantly in the early cycles but vary only slightly afterwards. The strain obtained by loading at high temperature followed by unloading at low temperature is almost equal to the maximum strain. The residual strain does not vary under thermomechanical cycling, which indicates the shape fixity stability of the material under cyclic deformation. The rate of strain recovery by heating after several thermomechanical cycles is more than 95%.

Low-Cycle Fatigue Behavior of Spring Steels

Low-cycle fatigue tests were carried out on spring steels at room temperature under a triangular strain wave form to obtain basic information about low-cycle fatigue behavior of springs.The results obtained are summarized as follows:(1) Cyclic softening behavior was observed under cyclic straining with plastic strain component.(2) Fatigue life properties and cyclic deformation properties of spring steel are the same as those which obtained by extrapolating properties of tempered steels for machine structural use to higher level of hardness.(3) Estimation method of low-cycle fatigue lives from tensile properties is proposed by modifying Manson's Universal Slope Method.(4) Total damage under incremental step tests becomes 1 to 1.8 at failure. Miner's cumulative rule is applicable to fatigue life estimation under straining of randon amplitudes.(5) Total damage is very small compared with 1 at failure, when stress controlled fatigue tests are carried out after a part of strain controlled test with plastic strain component. Cyclic softening by strain cycling promotes fatigue damage due to stress cycling.

Cyclic Deformation of TiNi Shape Memory Alloy

The cyclic deformation properties of TiNi shape memory alloy wires and helical springs for the martensitic transformation and the R-phase transformation were investigated experimentally. The results are summarized as follows. For cyclic deformation associated with the martensitic transformation, the transformation stress decreases and the transformation temperature rises. The amount of variation in the transformation stress and the transformation temperature is large in the early number of cycles and is small after about the 20th cycle. For cyclic deformation associated with the R-phase transformation, the transformation stress and the transformation temperature vary little. For cyclic deformation of helical springs, the recovery deflection is very large in the R-phase transformation region. The recovery deflection and the recovery force of the helical spring under cyclic deformation vary slightly even in the martensitic transformation region.

Cyclic Deformation Properties of TiNi Shape Memory Alloy. Shape Memory Effect due to Martensitic and R-Phase Transformations.

By performing thermomechanical cycling tests on TiNi shape memory alloy, the cyclic properties of the shape memory effect (SME) due to the rhombohedral phase transformation (RPT) and the martensitic transformation (MT) were investigated. The main results are summarized as follows. ( 1 ) In the case of maximum strain in the RPT region, SME does not vary under cyclic deformation. ( 2 ) In the case of maximum strain in the MT region, the MT stress decreases and the MT reverse transformation temperature increases under cyclic deformation. The variation in these properties is large in the early cycles but decreases thereafter and increases markedly if maximum strain is in the MT completion region. ( 3 ) In the case of maximum strain in the MT region, two-way strain, due to RPT, develops under cyclic deformation. If cyclic deformation with maximum strain in the MT completion region is repeated five times, two-way strain of the same amount as that of the RPT strain caused by loading is obtained.

Cyclic deformation property and microstructure study of HT-9 ferritic steel at elevated temperatures

In this study, cyclic deformation tests were carried out on HT-9 steel at room temperature, 300, 400, 500 and 600°C with a diametral strain range of ± 0.5%, ± 0.25% respectively. Transmission electron microscopy was presented to describe the micromechanisms of HT-9 steel under cyclic straining. The mechanical test results showed that cyclic softening behavior was observed at each testing temperature. The softening was attributed to the rearrangement of the high dislocation density structure into subgrain. In cyclically strained HT-9 specimens, carbides distributed along the subgrain boundaries. These carbides were considered to be the barriers against the subgrain to grow. The resistance of dislocation slip by carbides decreased to a less or extent at higher testing temperatures. The sharp subgrain boundaries observed in the specimens tested at higher temperatures showed a large decreasing rate in internal stress during cycling. HT-9 steel cyclically softened rapidly at testing temperatures higher than 500°C. In the case of HT-9 steel cyclically deformed at lower strain range, cyclic plastic strain was essentially accommodated by the ferrite region and strain cycling did not have obvious effects on the substructure of lath martensite.

Fatigue Behavior of a 22Cr-20Ni-18Co-Fe Alloy at Elevated Temperatures

An experimental investigation was conducted on Haynes Alloy 556 to study the fatigue behavior of the material at elevated temperatures. Fatigue tests were run at constant temperatures ranging from room temperature to 871°C with strain ranges from 0.265 to 1.5 percent resulting in lives between 102 and 106 cycles. Cyclic deformation properties were evaluated based on the fatigue data. Three fatigue life models were evaluated for their ability to predict the isothermal fatigue lives of the material. These included the Ostergren, Frequency Separation and Stress-Strain-Time models. Strengths and weaknesses of each model are discussed based on the experimental results.

Stress-Strain-Temperature Relationships of TiNi Shape Memory Alloy Suitable for Thermomechanical Cycling

The cyclic deformation behaviour in TiNi shape memory alloy under thermomechanical cycling was investigated experimentally and a constitutive equation proposed to evaluate the stress-strain-temperature relationship. The theory described well the cyclic deformation properties, such as the decrease in the transformation stresses, the rise in the transformation temperatures and the variation in these phenomena with the number of cycles.

Cyclic Deformation Properties of Shape Memory Polymer of Polyurethane Series.

The cyclic deformation properties of shape memory polymer of the polyurethane series were investigated experimentally. The main results are summarized as follows. ( 1 ) As thermal cycles progress, the residual strain increases, the recovery strain decreases and the deformation resistance increases. ( 2 ) The recovery strain in the heating proccess under no-stress conditions appears above the glass transition temperature. The temperature at which recovery of strain starts rises with cyclic deformation. ( 3 ) The recovery strain at high temperature increases significantly during the first 10 min when the temperature is kept constant, but varies slightly after that. ( 4 ) The characteristic values in ( 1 ) - ( 3 ) vary significantly in the early cycles, but vary slightly after these cycles. ( 5 ) The strain obtained at low temperature following the predeformation at high temperature does not vary under thermomechanical cycling, which demonstrates the stable shape fixity of the material for cyclic deformation.

Stress-Strain-Temperature Relations of TiNi Shape Memory Alloy Suitable for Thermomechanical Cycling.

The cyclic deformation behavior in TiNi shape memory alloy under thermomechanical cycling was investigated experimentally, and a constitutive equation was proposed to evaluate the stress-strain-temperature relation. The theory described well the cyclic deformation properties, such as the decrease in the transformation stresses of the martensitic transformation and the reverse transformation, the rise in the transformation temperature of both transformations, the increase in nonrecoverable strain, the decrease in the transformation strain range and the rate of variation in these phenomena with the number of cycles.

Properties of soft clay under static and cyclic loading : Andersen, K H Proc International Conference on Engineering Problems of Regional Soils, Beijing, 11–15 August 1988P7–27. Publ Beijing: International Academic Publishers, 1989

This paper presents soil properties for foundation design of structures on soft clay subjected to static and combined static and cyclic loads. The foundation design aspects include bearing capacity, cyclic displacements, equivalent soil spring stiffnesses for use in dynamic structural analyses, soil stresses against structures, and settlements due to cyclic loads. The properties include static, post-cyclic static and cyclic shear strengths and deformation properties, pore pressures generated by cyclic loads, and post-cyclic recompression behaviour. It is shown how these properties can be determined from laboratory tests, and the paper presents numerical values of soil properties from triaxial and direct simple shear tests on Drammen clay. Numerical values of other soft soils are also included in some cases. The laboratory tests on Drammen clay aimed at determining soil properties for foundation design of offshore platforms, but the data can be used in foundation design of structures subjected to cyclic loads in general.

The bovine pericardial xenograft: III. Effect of uniaxial and sequential biaxial stress during fixation on the tensile viscoelastic properties of bovine pericardium.

Our previous two articles have shown that glutaraldehyde-fixed bovine pericardium is nearly isotropic, whether fixed without constraints, with tethering, or with pressure. In this study, we have used uniaxial stress during fixation to produce bovine pericardial material with marked tensile anisotropy. Rectangular and cruciate pericardial samples have been mechanically examined after one of four treatments: (i) fixation under 88-kPa uniaxial stress, (ii) fixation under 176-kPa uniaxial stress, (iii) 3 h of 176-kPa uniaxial stress in saline followed by 24-h fixation under the same stress, (iv) fixation under 176-kPa uniaxial stress followed by a second fixation under 176-kPa stress in a direction normal to the first. Strips of material were cut at 0 degree, 30 degrees, 60 degrees, and 90 degrees to direction of the initial stress, and tested for response to cyclic loading, stress relaxation, plastic deformation, and fracture properties. Fixation under uniaxial stress produced anistropy similar to that seen in porcine aortic valve leaflets; however, the overall extensibility of the material depended on the applied stress and the aspect ratio of the stressed sample. While loading in saline produced no change, the sequential biaxial stressing produced a reduction in anisotropy, suggesting exposure of additional crosslinking sites. Uniaxial stress during fixation may be a useful method for construction of anisotropic heart valve leaflets.

Low-cycle fatigue properties and surface crack growth behavior under intermittent over-straining.

Low-cycle fatigue properties and surface crack growth behavior were investigated under two-step intermittent over-straining (ISO) of various strain ratios λ on low-carbon steel S 15 C and austenitic stainless steel SUS 304 specimens. The results obtained were summarized as follows : (1) The plastic strain range Δεp as a cyclic deformation property did not change after IOS for S15C, but theΔεp decreased after IOS for SUS304 because of remarkable strain hardening. (2) A retardation phenomenon in surface crack growth at low values of λ after IOS and cycles to failure Nf were observed for both the S 15 C and the SUS 304 specimens. When the value of λ increased, the retardation effect decreased and crack growth curves for S 15 C were at the front or rear of the curve of a constant amplitude test, but for SUS 304, the Nf decreased more clearly than that of the constant amplitude test because of the remarkable acceleration phenomenon after ISO. (3) When we arranged the surface crack growth rate da/dN with J integral range J under the IOS condition, the data lay in the range of a factor of two for the da/dN-J relations of the constant amplitude tests, in spite of repeated retardation or acceleration of crack growth after IOS.

Cyclic deformation behaviour of Ni3Ge single crystals at room temperature

Abstract Fully reversed strain-controlled tension-compression tests have been performed at room temperature to study cyclic deformation properties of Ni3Ge single crystals. The cyclic deformation was performed with the stress axis parallel to the [245] direction at a strain rate of 1·0 × 10−45−1 and at a total strain amplitude between 5·0×10−4 and 1·5 × 10−3. Cyclic strain-hardening occurred to saturation over the range of the employed strain amplitudes. The cyclic stress amplitude was found to be higher in compression than in tension at a plastic strain amplitude larger than 1×10−5, where mew dislocations are operative. This stress asymmetry became more prominent (i.e. the mean stress amplitude became larger in magnitude) as the plastic strain amplitude became larger. At a plastic strain amplitude less than 1 ×10 −5, where edge dislocations are operative. the stress amplitude was found to be symmetric. A similar stress asymmetry was also observed for monotonic flow stress. The cyclic hardening is explained by considering an interaction between the screw dislocations.

Cyclic Deformation Behavior of Ni3Ge Single Crystals At Room Temperature

ABSTRACTFully reversed strain-controlled, tension-compression tests were performed at room temperature to study cyclic-deformation properties of Ni3Ge single crystals. The cyclic deformation is performed with the stress axis parallel to the [1 1.94 2.69] direction at a strain rate of 1.0 × 10-4 s-1 and at a total-strain amplitude between 5.0 × 10-4 and 1.5 × 10-3. Cyclic strain-hardening occurs to saturation over the range of the employed strain amplitudes. The cyclic-stress amplitude is higher in compression than in tension at a plastic-strain amplitude larger than 1 × 10-5 where screw dislocations are operative. This stress asymmetry becomes more prominent (i.e., the mean stress-amplitude becomes larger in magnitude) as the plastic-strain amplitude becomes larger. At a plastic-strain amplitude less than 1 × 10-5 where edge dislocations are operative, the stress amplitude is symmetric. A similar stress asymmetry is also observed for monotonic flow stress. The cyclic hardening is explained by considering an interaction between the screw dislocations.

Effect of Grain Size on Low Cycle Fatigue in Low Carbon Steel

Low cycle fatigue tests were performed on a low carbon steel of 0.1 percent carbon content, which has various grain sizes of the range from 18 to l63μm in diameter. The cyclic softening is followed by a stationary state in cyclic deformation behavior in the fine grain material, while the coarse grain one exhibits a rapid cyclic hardening in the early stage, which is followed by a saturation hardening. Such a difference in cyclic deformation property can be interpreted well by considering that in the yielding behavior between the fine and the coarse grain materials. The cyclic stress-strain property is not influenced, by the grain size. The coarse grain material of 163μm in diameter has about 0.4times shorter fatigue life than that in the fine grain one of 18μm. The grain size dependent fatigue life of this kind is valid for both processes of crack initiation and propagation.

Polyurethanes. IV. Effect of poly(propylene glycol) tacticity on the properties of block copolyether‐urethanes

AbstractBlock copolyèther–urethane–ureas containing isotatic polypropylene glycols were synthesized and their dynamic mechanical properties compared to the analogous atactic structures. At poly(propylene glycol) molecular weights below 1000, tacticity appears to have little effect on the stress–strain properties of the block copolymers. At higher molecular weights, the ability of the isotatic polyglycol segments to crystallize under strain can effect cyclic deformation properties.

The effect of strain-rate on the cyclic deformation properties of α-iron single crystals

The effect of strain rate on the cyclic hardening behaviour and the dislocation pattern of α-iron single crystals deformed at constant plastic strain amplitude at room temperature was investigated. Strain rates of ϵ ̇ pl = 10 −3 s −1 led to typical “low temperature features” (high thermal s component, long drawn-out screw dislocations), whereas with ϵ ̇ pl = 2.5 × 10 −5 s −1 the behav resembled that of f.c.c. metals (high athermal stress component, dense edge multipole bundles). The cyclic stress-strain curve, as determined at constant strain rate, could be divided into regions of cyclic micro- and macrostrain, characterized by limited and extensive motion of screw dislocations respectively. Implications regarding the nature of the fatigue limit and the effect of strain rate on fatigue failure are discussed.

Evaluation of Deformation Phenomena of Metals for Fatigue Analysis

AbstractSimple calculations for estimating extremes in fatigue life of metals, with or without accounting for cycle-dependent hardening, softening, and mean stress relaxation, are presented. These calculations aid in assessing the importance of cyclic deformation properties in cumulative damage procedures. An SAE 1045 quenched and tempered steel is considered for illustration. In general, cycle-dependent deformation phenomena may be important only in intermediate and long life situations. The sensitivity of predicted life to cyclic hardening or softening depends on the degree of hardening or softening a material exhibits, whereas in the case of mean stress relaxation it depends on the shape of the cyclic stress-strain curve or the strain hardening exponent. However, the need for a proper accounting of cyclic history, recognized as the “cycle counting problem” in fatigue literature, is the most important requisite of a cumulative damage analysis. Two alternative approaches of dealing with the cycle counting problem are discussed.

Some effects of retained austenite on the fatigue resistance of carburized steel

Cyclic deformation properties have been measured for high carbon steels that simulate the surface layers on carburized AISI 4027 steel. The influence of retained austenite is manifested in cyclic hardening and the development of mean compressive stresses. These responses, which are found to vary in direct proportion to the initial retained austenite content, are attributable to deformation-induced transformation. The importance of such cyclic changes is demonstrated by strain-life relations for carburized 4027; specimens with large amounts of retained austenite in the surface layers possess superior fatigue resistance to otherwise identical specimens that are more completely transformed. It is also shown that the benefits of retained austenite diminish as the intrinsic ductility of the fully transformed material increases.