Cyclic Work Hardening Research Articles

Mechanical properties and deformation behavior of a refractory multiprincipal element alloy under cycle loading

In comparison with the state-of-the-art Ni-based superalloys, refractory multiprincipal element alloys (MPEAs) exhibit considerably higher strengths at temperatures above 1600[Formula: see text]C, which can be a significant potential required in the high demand for aerospace applications. However, the atomic-scale work-hardening behavior of such important materials during low-cycle loading remain unknown. Here, using molecular dynamics simulations, we study the low-cycle fatigue of nanocrystalline refractory multiprincipal element alloy with different grain sizes, to reveal the cyclic deformation, work hardening and damage mechanism. As a result, an extensive grain growth is observed during the cyclic deformation, thus driving the dynamic Hall–Petch strengthening mechanism. For the model with large grain size, the glide of partial dislocations with screw structure can be responsible for the deformation behavior under cyclic loading, and at small grain size the grain growth-coordinated deformation twinning can control the plastic process. The deformation twin boundaries generated during the cyclic loading show high stability, while the remaining high-angle grain boundaries are highly unstable. The initial softening followed by hardening depends upon the dislocation density and grain size. In particular, atomic-scale element segregation occurs after cyclic loading. This study gives a cyclic deformation micromechanism, and thus accelerates the design and development of superior fatigue-resistant refractory multiprincipal element alloy over a wide temperature range.

A simple energy‐based model for nonproportional low‐cycle multiaxial fatigue life prediction under constant‐amplitude loading

AbstractFrom the literature concerning the traditional nonproportional (NP) multiaxial cyclic fatigue prediction, special attentions are usually paid to multiaxial constitutive relations to quantify fatigue damage accumulation. As a result, estimation of NP hardening effect decided by the entire history path is always proposed, which is a challenging and complex task. To simplify the procedure of multiaxial fatigue life prediction of engineering components, in this paper, a novel effective energy parameter based on simple material properties is proposed. The parameter combines uniaxial cyclic plastic work and NP hardening effects. The fatigue life has been assessed based on traditional multiaxial fatigue criterion and the proposed parameter, which has been validated by experimental results of 316 L stainless steel under different low‐cycle loading paths.

Dependence of fatigue life of low-pressure die-cast A356 aluminum alloy on microporosity variation

The aim of the present study was to investigate the dependence of the high cycle fatigue property on the microporosity variation of a low-pressure die-cast A356 alloy. Also, it aimed to describe quantitatively the relationship between the fatigue property and monotonic tensile strength using modified Basquin’s equation which takes into account the microporosity variation. The fatigue life of the A356 alloy can be described by an exponential dependence on the variation of the fractographic porosity, in terms of the modified Basquin’s equation which is composed of the defect susceptibility of fatigue life to microporosity variation and the maximum tensile strength achievable in the defect-free condition. Using a modified form of Basquin’s equation, the maximum values of the fatigue strength coefficient and exponent in the defect-free condition are 341.5 MPa and −0.076, respectively, even though the nominal values of fatigue strength coefficient and exponent without consideration of microporosity variation are 237.6MPa and −0.048, respectively. Also, the difference between the maximum tensile strength and the fatigue strength coefficient on modified Basquin’s equation is about 120MPa, and it arises from variation in the deformation behavior due to the difference of loading condition between the monotonic and cyclic test modes such as the strain rate, Bauschinger effect, cyclic work hardening and damage accumulation on loading condition.

Description of Planer Anisotropy and Cyclic Plasticity Behavior of Aluminum Sheet Based on Crystal Plasticity Theory

In sheet metal forming, the anisotropy and the Bauschinger effect of sheets affect greatly their formability. This paper discusses how the planar anisotropy and cyclic plastic behavior (the Bauschnger effect and cyclic workhardening characteristics) correlate with the crystallographic texture based on the crystal plasticity analysis on A5052-O sheet. The analytical predictions of r-values and the cyclic stress-strain responses are compared with the experimental observations (S. Tamura et al., Materials Trans, 52-5 (2011), pp.868-875).

Microstructure evolution of gold thin films under spherical indentation for micro switch contact applications

RF MEMS (Radio Frequency Micro Electro Mechanical System) switches are promising devices but their gold-on-gold contacts, assimilated for this study to a sphere/plane contact, represent a major reliability issue. A first step toward understanding failure mechanisms is to investigate the contact metal microstructure evolution under static and cyclic loading. After static and cyclic loading of sputtered gold thin films under spherical indentation, high-resolution Electron Back Scatter Diffraction (EBSD) is used to investigate the contact area. Grain rotation against {111} fiber texture of 1-μm-thick sputtered gold thin film is a signature of plastic deformation. Grain rotation is observed above 1.6 mN under static loading using a spherical diamond indenter with 50-μm tip radius. The heterogeneity in grain rotation observed corresponds to a greater plastic deformation in the middle of the indent than at the edge. A 30° grain rotation due to cyclic work hardening is observed for a half-million mechanical cycles under 300 μN load using a spherical gold tip (20 μm radius). The same test in hot switching mode induces a grain growth in the contact area. Therefore, thermal effects occurring during hot switching are underlined.

Modélisation du durcissement cyclique de l’aluminium produit au Cameroun

Modelling of the cyclic hardening of aluminium produced in Cameroon Strain-hardening is a very significant parameter to traduce the elastoplastic behaviour of materials, particularly for the simulation of their working by plastic deformation. This paper presents a model of cyclic hardening of two nuances of aluminium produced in Cameroon, the 1200 and the 5005. The experimentation was led under quasi-static or variable stresses with constant amplitude. The results obtained allow to determine the static mechanical characteristics in tension and compression of the studied nuances and to establish their monotonous and cyclic strain-hardening curves. The amplitude of stress is a linear function of the plastic deformation in a logarithmic scale, which allows a simple modelling of the law of cyclic work hardening of studied materials.

A Study of Surface Integrity when Machining Refractory Titanium Alloys

In this paper, the surface integrity is studied when machining the aeronautical titanium alloys. Surface roughness, lay, defects, microhardness and microstructure alterations are studied. The result of surface roughness judges that the CVD-coated carbide fails to produce better Ra value than the uncoated. Lay is characterized by cutting speed and feed speed directions. Feed mark, tearing surface, chip layer formation as built up layer (BUL), and deposited microchip are the defects. Microhardness is altered down to 350 microns beneath the machined surface. The first 50 microns is the soft sub-surface caused by thermal softening in ageing process. Microstructure alteration is observed in this sub-surface. Down to 200 microns is the hard sub-surface caused by the cyclic internal work hardening and then it is gradually decreasing to the bulk material hardness. It is concluded that dry machining titanium alloy is possible using uncoated carbide with cutting condition limited to finish or semi-finish for minimizing surface integrity alteration.

Surface integrity of dry machined titanium alloys

The study is focused on the machined surface integrity of titanium alloy under the dry milling process. Roughness, lay, defects, microhardness and microstructure alterations are investigated. The result of surface roughness shows that the CVD-coated carbide tool fails to produce better Ra value compared to the uncoated tool. Lay is found to be dependent on cutting speed and feed speed directions. Microhardness is altered down to 350 μm beneath the machined surface. The first 50 μm is the soft sub-surface caused by thermal softening in the ageing process. Down to 200 μm is the hard sub-surface caused by the cyclic internal work hardening and then it gradually decreased to the bulk material hardness. It was concluded that for titanium alloys, dry machining can be carried out with uncoated carbide tools as far as cutting condition is limited to finish and/or semi-finish operations.

Fracture of Ni with grain-size from nanocrystalline to ultrafine scale under cyclic loading

A two-step fatigue loading scheme was developed to study the fracture characteristics of nanocrystalline and ultrafine-grained Ni, with grain-sizes of 23 and 260 nm, respectively. Cyclic work-hardening, measured by the mid-loop width during cycling, was observed in both types of material in the first loading step. The loop width increases rapidly as the load is upgraded in the second step. Crack initiation and striation have been observed for the first time in nanocrystalline Ni.

Use of CDM in Materials Modeling and Component Creep Life Prediction

Physically based continuum creep damage mechanics (CDM) has been reviewed and shown to provide a unifying framework for some seemingly diverse methods of predicting design and remanent creep lifetimes. These methods—theta projection, omega parameter, Larson-Miller parameter, and Robinson’s life fraction rule—exhibit certain strengths in common with CDM, but also weaknesses which CDM identifies and avoids. CDM consists of sets of coupled rate equations for inelastic strain, internal stress, and microstructural evolution (damage) which can then be integrated under boundary conditions appropriate to the test or service operating conditions: constant load/temperature for creep; constant total strain for stress-relaxation, variable stress/temperature, etc. Other state-variable approaches to creep and cyclic plasticity (for example, those due to Bodner, Miller, Chaboche, and Robinson), differ from CDM mainly in concentrating on the primary/secondary stages of creep (or cyclic work-hardening) and/or by their introduction of damage in an empirical Kachanov manner. The application of physically based CDM to LCF/thermal fatigue and its potential for predicting lifetimes of welded joints are also discussed. [S0094-9930(00)00903-3]

High Cycle Fatigue Properties of SUS 304 Stainless Steel under Load-Increasing Conditions.

High cycle fatigue properties of austenitic stainless steel SUS 304 were investigated for various conditions of cyclic stress amplitude σa-increasing tests under rotating bending and tension-compression loadings. Fatigue tests were also performed using specimens subjected to cyclic preloading. X-ray diffraction methods were utilized to measure the amount of martensitic transformation and residual stress. A marked effect of coaxing was observed under rotating bending tests and tension compression tests at 2 Hz. Stress-strain response revealed that the coaxing effect was caused by cyclic work hardening due to cyclic loading below the fatigue limit, but cyclic work softening due to temperature increase took place during stress cycling at the final step. Martensitic transformation was detected for some rotating bending specimens but not the tension compression specimens, which suggests that it is not the controlling behavior for the cyclic work hardening. Compressive residual stress was measured at the surface of the specimens exhibiting coaxing effect, which was considered to play an important role on the coaxing effect.

Study of Fatigue Damage on Low Cycle Fatigue.

To study fatigue damage on low cycle fatigue, tensile tests after fatigue were carried out using cylindrical specimens, and the relationship between the change of residual fracture ductility and the cycle ratio was investigated. The main results obtained are as follows : (1) Until the last stage of fatigue life (N/Nf=0.9 ; crack length 1.5 mm), the residual fracture ductility eFR did not decreased remarkably and the tensile fracture mode did not vary from cup & cone type. These results differ from previous studies using hourglass specimens. (2) The exhaustion of eFR is caused by two factors : One is the internal damage due to cyclic work hardening and the other is the surface damage due to small cracks. The degree of these damages is bath small. (3) Most of the fatigue damage on low cycle fatigue depends on the length of surface cracks.

Statistical models for cyclic strain hardening of copper single crystals

Models based on testing of copper single crystals and on the statistical theories of flow and cyclic work hardening are developed to explain the measurements of plastic strain amplitude as a function of acting shear stress amplitude, flow strength and number of cycles. The penetration probability of mobile dislocations through the microstructural obstacle distribution is analogous to the Weibull distribution depending on the ratio of stress amplitude to flow strength and other physical parameters. For the easy glide orientation crystals the results support the strain hardening model where new obstacles are stored as Orowan type loops and for multiple slip orientation crystals the forest cutting and the long range stress field theories seem probable.

Role of dynamic strain ageing in low cycle fatigue

Low cycle fatigue (LCF) at elevated temperatures is known to be influenced by time-dependent processes like creep, oxidation and metallurgical instabilities. Another time-dependent phenomenon namely, dynamic strain ageing (DSA) has been found to exert an influence on LCF behaviour at high temperatures. Research activities carried out in the present author’s laboratory with a view to understanding the effects of DSA on LCF are highlighted in this paper. Occurrence of DSA manifests during total strain-controlled fatigue tests in the form of serrated plastic flow in stress-strain hysteresis loops, increased cyclic work hardening and reduced plastic strain range. Further, DSA causes localization of plastic flow leading to enhanced planarity of slip and widely-spaced slip bands. Impingement of slip bands on grain boundaries causes increased grain boundary decohesion, leading to reduced fatigue life. The influence of prior microstructure such as second phase particles and grain size on the effects of DSA on LCF is also discussed.

Toward A Sound Understanding of Dislocation Plasticity

A novel theory is presented for work hardening in stage II shown by single crystals of ductile face-centered cubic (fcc) metals. The theory is based on the assumption that slipbands are highly elongated ellipsoidal zones with weak interiors, oriented at small, alternating angles with the crystallographic slip planes. Consideration of the stacking of such ellipsoids, together with a condition for the nucleation of a new slipband at the center of an obstacle nearby, yields both the average angle of inclination and the shear offset in the bands, both of which remain constant throughout stage II. The theory makes several predictions concerning the internal stress distribution and the plastic behavior. Before expounding the theory, the concepts upon which it is based are discussed in the context of two much simpler problems: the work hardening of dispersionhardened metals and the cyclic work hardening which forms the basis of understanding fatigue properties. The picture confirms in a natural way that the value of σ III , the stress which marks the end of stage II hardening, may be taken to be equal to the fatigue endurance limit.

Cyclic work hardening and formation of dislocation substructures during low-cycle fatigue of tempered dual-phase steels: Mediratta, S.R., Ramaswamy, V., Singh, V. and Rao, R.Steel Res. July 1990 61, (7), 325–332

Cyclic work hardening and formation of dislocation substructures during low-cycle fatigue of tempered dual-phase steels: Mediratta, S.R., Ramaswamy, V., Singh, V. and Rao, R.Steel Res. July 1990 61, (7), 325–332

Cyclic work hardening and formation of dislocation substructures during low‐cycle fatigue of tempered dual‐phase steels

A C‐Mn‐Si dual‐phase steel, containing 33.8% lath martensite of 0.45% C content (called steel 1), was tempered at 200, 460 and 650°C and designated steels J, K and L, respectively. Sheet specimens of 3 mm thickness and 10 mm gauge length pertaining to steels I, J, K and L were subjected to tensile and low cycle fatigue testing at room temperature. The steels I, K and L show cyclic softening while the steel J exhibits a small cyclic hardening. The cyclic stress response varies significantly at higher and lower applied strain amplitudes for each of the steels which has been discussed in terms of competing processes of hardening and softening during strain cycling. The value of the cyclic strain hardening exponent decreases continuously in the order steels I>J>K>L. Varied dislocation substructures form at higher and lower strain amplitudes during cyclic deformation.

Viscoplastic behavior of type 304 stainless steel after cyclic preloading. 1st report. Experimental results.

Viscoplastic behavior of Type 304 stainless steel after cyclic prestraining and/or axial cyclic prestraining of various strain amplitudes, and of various numbers of cycles was experimentally investigated at room temperature. The difference between flow stress levels depending on strain rate considerably decreases after cyclic loading, which suggests that viscosity of metal is related to low cycle fatigue phenomena. On the other hand. the relaxation stress drop does not decrease. but increases with an increase in the flow stress level due to cyclic work hardening. Work hardening coefficient (plastic modulus) in reloading is strongly concerned to plastic work per cycle in preloading.

３１６ステンレス鋼の高温低サイクル疲労変形特性と転位構造の関係

Low cycle fatigue tests with constant plastic strain range Δep were carried out on 316 stainless steel at room and elevated temperatures (650°C) in order to investigate a relationship between macroscopic cyclic deformation and microscopic dislocation structures. Macroscopically, cyclic work hardening behaviors under constant temperature and two-step temperature change tests (a room to high temperature test and a high to room temperature test), and cyclic stress-strain curves by means of three different methods (a companion specimens, an increasing step and a multiple step methods) were obtained. Microscopically, the quantitative variation of dislocation structures was measured by transmission electron microscopy observation. The results obtained were summarized as follows, (1) The cyclic work hardening properties under two-step temperature change tests were explained from the dislocation structures at the time of temperature change and the dislocation rearrangement after the change during the fatigue process.(2) The cyclic stress-strain curves by the three different methods at 650°C were almost the same in the range of Δep>0.3%.(3) The area fraction occupied by cell Sc/S0 increased with the number of cycles N and the Δep, and the increasing tendency during the fatigue process at 650°C was very different from that at room temperature. The Sc/S0 at 650°C increased at a very early stage of fatigue and the value was remarkably larger than that at room temperature.(4) The relations between the normalized stress amplitude (Δσ/2G) and the average cell size lc, and between the (Δσ/2G) and the total dislocation density ρt were obtained as follows, (Δσ/2G)∝lc-0.5, (Δσ/2G)∝ρt0.5where G is a shear modulus.

Two stage cyclic work hardening and two slope coffin-manson relationship in dual phase steels

Etude des deux etapes du durcissement dans les aciers double phase C-Mn-Si soumis a des essais de fatigue oligocyclique. Cet effet est observe sur les droites de Coffin-Manson par une rupture de pente