Stress-controlled Fatigue Tests Research Articles

Tensile and fatigue behaviour of as-forged AZ31B extrusion

Tensile and stress controlled fatigue tests were performed to investigate the influence of forging at a temperature of 400°C at different rates, on the performance of extruded AZ31B magnesium alloy. The obtained microstructural analysis showed that the extruded AZ31B magnesium alloy possesses a bimodal grain structure with strong basal texture. In contrast, the forged samples showed refined grains and a weaker texture. During tensile testing, a maximum yield and ultimate tensile strength of about 163 MPa and 268 MPa were obtained for the forged samples showing an increase of 102% and 7%, respectively from the as-extruded material. At the same time, a significant improvement of fatigue life was also observed for the sample forged at the rate of 100 mm/min. The fractographic analysis of the fracture surfaces showed that ductile type fractures occurred in both as-extruded and forged samples. However, more dimples and plastic deformation were identified in the fracture surfaces of the forged specimens. It is believed that forging improved the fatigue life by a combination of grain refinement and texture modification resulting in improved yield and ductility. KEYWORDS. AZ31B; Forging; Fatigue Characterization; Fracture; Texture.

Ratcheting fatigue behavior of Zircaloy-2 at room temperature

Nuclear core components of zirconium alloys experience asymmetric stress or strain cycling during service which leads to plastic strain accumulation and drastic reduction in fatigue life as well as dimensional instability of the component. Variables like loading rate, mean stress, and stress amplitude affect the influence of asymmetric loading. In the present investigation asymmetric stress controlled fatigue tests were conducted with mean stress from 80 to 150 MPa, stress amplitude from 270 to 340 MPa and stress rate from 30 to 750 MPa/s to study the process of plastic strain accumulation and its effect on fatigue life of Zircaloy-2 at room temperature. It was observed that with increase in mean stress and stress amplitude accumulation of ratcheting strain was increased and fatigue life was reduced. However, increase in stress rate led to improvement in fatigue life due to less accumulation of ratcheting strain.

Experimental study and modeling of fatigue life prediction of plain weave carbon/polymer composite under constant amplitude loading

For fiber-reinforced composites, the anisotropy and the tension-compression asymmetry have very important influence on fatigue performance. The quasi-static and fatigue mechanical behavior of plain weave laminates of carbon/polymer composites were experimentally investigated in this paper. The quasi-static and stress-controlled fatigue tests were carried out on the servo-hydraulic material testing machine. Fracture failure surfaces were observed in micro-mechanism with scanning electron microscopy. A phenomenological and nonlinear constant life diagram (CLD) model was developed based on quasi-static strength and S–N curve data-sets of different stress ratios. The relationship between six parameters of the proposed model and the failure cycles was studied. The fatigue experimental results showed that the fatigue failure type changed from tensile mode to compressive mode at nearby stress ratio R = −0.2. A satisfactory agreement between the predicted value of cycle life and experimental data was observed. The results indicated that the fatigue performance was adequately described by the Basquin S–N formulation and proposed CLD.

Influence of loading direction on the anisotropic fatigue properties of rolled magnesium alloy

The influence of loading direction on the fatigue behavior of rolled AZ31 alloy was investigated by conducting fully reversed stress-controlled fatigue tests along the rolling direction and normal to the rolling plane. Alternating twinning and detwinning behavior during initial cycling was found to cause asymmetric hysteresis loops, resulting in a compressive strain in the rolling direction and a tensile strain normal to the rolling plane. A transition in the dominant deformation mechanism from twinning–detwinning to slip occurs at around five cycles under both conditions due to cyclic hardening, thus making their loops symmetric. The lower twinning stress in tension along the normal direction leads to an increase in fatigue damage by plastic strain, resulting in a lower fatigue resistance than along the rolling direction.

Effects of Lüders-like bands on NiTi fatigue behaviors

Although the formation of Lüders bands in NiTi polycrystalline structures (strips/tubes) has been well reported and is believed to have considerable influence on the material's fatigue behaviors, there is no direct experimental evidence showing the relation between the band formation and the material's fatigue failure. For each NiTi polycrystalline strip in this study, a strain-controlled isothermal test was firstly conducted to check the pseudoelastic properties and then a stress-controlled fatigue test was performed with synchronized optical observation on the evolution (cyclic nucleation/annihilation) of Lüders bands in the strip. It is found that the fatigue failure crack always occurs within the band or at the band front, because the band formation is an indicator of the weak zones of the samples and the cyclic band nucleation/annihilation accelerates microstructure degradation in those zones. Under the same loading scheme of the fatigue tests, the samples with bands have much shorter fatigue life than the samples without bands. The experiments provide the bases for future theoretical study on the interaction between the martensitic phase transformation and the plasticity/defects and give some hints for developing a proper fatigue criterion for the softening material in engineering applications.

DP 鋼の繰返し硬化-軟化挙動に及ぼす微細析出物の効果

The precipitation hardening with fine carbides of nm size has already been applied to various advanced high-strength steels. In the present study, the role of fine TiC precipitates in the ferrite phase on cyclic deformation behavior was investigated for a Ti-added dual-phase steel with a strength of 780 MPa. In particular, the cyclic hardening and softening behavior characterized by the changes in cyclic plastic strain during stress-controlled fatigue testing was discussed from the viewpoints of the evolution of dislocation substructures. The fatigue life of the precipitation-hardened steel with 2 to 5-nm TiC precipitates is remarkably long as compared with that of a Si-added solution-hardened steel with a comparable ultimate tensile strength. For a specimen which eventually fractures before 2×106 cycles at least, the cyclic plastic strain gradually decreases, i.e., the cyclic hardening takes place, at the beginning of cyclic loading. After a certain number of cycles, the cyclic softening starts, i.e., the cyclic plastic strain reaches a minimum and, then, continuously increases, up to fracture. It was confirmed that the cyclic hardening is accompanied by an increase in dislocation density, while cyclic softening is caused by the dislocation rearrangement. The commencement of cyclic softening is remarkably delayed in the steel with fine TiC precipitates. The strong pinning effect by the precipitates, probably due to the Orowan mechanism, not due to the cutting mechanism, was observed by transmission electron microscopy (TEM). Therefore, it is considered that the long fatigue life of the precipitation-hardened steel is led by the fine TiC precipitates which play a role of the obstacles against mobile dislocations.

Ratcheting Strain Accumulation Due to Asymmetric Cyclic Loading of Zircaloy-2 at Room Temperature

Asymmetric stress cycling leads to accumulation of plastic strain which is called as ratcheting strain. The problem is generally associated with nuclear fuel cladding materials used in nuclear power plants and in pressurized pipe lines. In the present investigation asymmetric stress controlled fatigue tests were conducted with three different parameters namely, mean stress, stress amplitude and stress rate (keeping two parameter constant and varying third parameter) to see the plastic strain accumulation and its effect on fatigue life of Zircaloy-2 at room temperature. The tests were conducted with mean stress varying from 80 to 150 MPa, stress amplitude varying from 270 to 340 MPa and stress rate varying from 30 to 750 MPa/s respectively. The experimental outcomes show that with increase in mean stress and stress amplitude, the ratcheting strain accumulation increases with reduction in fatigue life. However, increase in stress rate leads to improvement in fatigue life of the material due to small ratcheting strain accumulation. Cyclic softening is observed during initial cycles for lower values of mean stress and stress amplitude and cyclic hardening is predominant for higher values of mean stress and stress amplitude. However, cyclic hardening is observed with increase in stress rate.

Crack mode and life of Ti-6Al-4V under multiaxial low cycle fatigue

This paper studies multiaxial low cycle fatigue crack mode and failure life of Ti-6Al-4V. Stress controlled fatigue tests were carried out using a hollow cylinder specimen under multiaxial loadings of ?=0, 0.4, 0.5 and 1 of which stress ratio R=0 at room temperature. ? is a principal stress ratio and is defined as ?=sigmaII/sigmaI, where sigmaI and sigmaII are principal stresses of which absolute values take the largest and middle ones, respectively. Here, the test at ?=0 is a uniaxial loading test and that at ?=1 an equi-biaxial loading test. A testing machine employed is a newly developed multiaxial fatigue testing machine which can apply push-pull and reversed torsion loadings with inner pressure onto the hollow cylinder specimen. Based on the obtained results, this study discusses evaluation of the biaxial low cycle fatigue life and crack mode. Failure life is reduced with increasing ? induced by cyclic ratcheting. The crack mode is affected by the surface condition of cut-machining and the failure life depends on the crack mode in the multiaxial loading largely.

Growth behavior of fatigue cracks in ultrafine grained Cu smooth specimens with a small hole

In order to study the growth mechanism of fatigue cracks in ultrafine grained copper, stresscontrolled fatigue tests of round-bar specimens with a small blind hole as a crack starter were conducted. The hole was drilled on the surface where an intersection between the shear plane of the final ECAP processing and the specimen surface makes an angle of 45° or 90° with respect to the loading axis. At a low stress ( ? a = 90 MPa), the direction of crack paths was nearly perpendicular to the loading direction regardless of the location of the hole. Profile of crack face was examined, showing the aspect ratio (b/a) of b/a = 0.82. At a high stress ( ? a = 240 MPa), although the growth directions inclined 45° and 90° to the loading-axis were observed depending on the location of the drilling hole, crack faces in these cracks were extended along one set of maximum shear stress planes, corresponding to the final ECAP shear plane. The value of aspect ratios was b/a = 0.38 and 1.10 for the cracks with 45° and 90° inclined path directions, respectively. The role of deformation mode at the crack tip areas on crack growth behavior were discussed in terms of the mixed-mode stress intensity factor. The crack path formation at high stress amplitudes was affected by the in-plane shear-mode deformation at the crack tip.

Surface versus internal fatigue crack initiation in steel: Influence of mean stress

Stress-controlled fatigue tests were run at different R ratios (=σmin/σmax) up to at most 3million cycles on a 2.5%Cr–1%Mo steel (ASTM A182 F22) used in riser tubes connectors for offshore oil drilling. The fatigue lives, as well as the slope of the S–N curves were found to decrease with increasing R and the endurance limit to follow Gerber’s parabola. Surface crack initiation without any defect involved, was most often observed for R=−1, −0.5 and 0, while an R ratio of 0.25 triggered crack initiation from either surface or internal pores or chemically inhomogeneous areas, leading, in the latter case, to fish-eye patterns for relatively low numbers of cycles. A further increase in R ratio to 0.5 promoted only defect-initiated surface cracks, while no fatigue fracture was observed within 10million cycles above R=0.6. These transitions in crack initiation mode are discussed based on X-ray diffraction analyses of residual stresses, elastic–plastic F.E. computations on a unit cell model containing a pore and some fracture mechanics analyses, with a particular attention to environmental effects.

A two-parameter, heat energy-based approach to analyse the mean stress influence on axial fatigue behaviour of plain steel specimens

Recently, fatigue data generated from fully reversed stress- and strain-controlled tests on plain and notched stainless steel specimens were rationalised in a single scatter band by using the specific heat energy per cycle as fatigue damage index. In this paper, the energy approach is extended to analyse the mean stress influence on the axial fatigue behaviour of un-notched bars made of cold drawn AISI 304L stainless steel or hot rolled quenched and tempered C45 steel. In view of this, stress controlled fatigue tests at different load ratios R were carried out. A new two-parameter, energy-based approach is defined to account for the R-ratio effects, which combines the specific heat loss and the thermoelastic temperature corresponding to the maximum stress of the load cycle. Such parameters can be readily evaluated at a point of a specimen or a component undergoing a fatigue test by means of temperature measurements, while controlling or monitoring the thermal boundary conditions of the tests is unnecessary. The new two-parameter approach was able to rationalise the stress ratio effect observed experimentally.

Effect of mean stress on multiaxial ratcheting life: A simplified life prediction model based on average equivalent stress amplitude

This article proposes a model to predict uniaxial and multiaxial ratcheting life by addressing the three primary parameters that influence failure life: fatigue damage, ratcheting damage and the multiaxial loading path. These three factors are addressed in the present model by (a) the stress amplitude for fatigue damage, (b) mean stress-dependent Goodman equation for ratcheting damage and (c) an inherent weight factor based on average equivalent stress to account for the multiaxial loading. The proposed model requires only two material constants which can be easily determined from uniaxial symmetric stress-controlled fatigue tests. Experimental ratcheting life data collected from the literature for 1025 and 42CrMo steel under multiaxial proportional and nonproportional constant amplitude loading ratcheting with triangular sinusoidal and trapezoidal waveform (i.e. linear, rhombic, circular, elliptical and square stress paths) have shown good agreement with the proposed model.

Some recent results on the fatigue strength of notched specimens made of 40CrMoV13.9 steel at room and high temperature

The work summarizes a large bulk of experimental data from specimens made of 40CrMoV13.9 steel. The first part of the paper deals with multiaxial fatigue strength of notched round bars tested under combined tension and torsion loading, both in-phase and out-of-phase. The results from multi-axial tests are discussed together with those obtained under pure tension and pure torsion loading from notched specimens with the same geometry. The second part of the paper summarizes data from uniaxial-tension stress-controlled fatigue tests on specimens made of the same steel. Tests are performed varying temperature, from room temperature up to 650°C. Altogether more than 180 new fatigue data are summarised in the present work, corresponding to more than two-years of testing programme. All fatigue data are presented first in terms of nominal stress amplitudes referred to the net area and then re-analysed in terms of the mean value of the strain energy density evaluated over a given, crescent shape volume embracing the stress concentration region. For the specific steel, the radius of the control volume is found to be independent of the loading mode.

Ratcheting induced cyclic softening behaviour of 42CrMo4 steel

Ratcheting is an important field of fatigue deformation which happens under stress controlled cyclic loading of materials. The aim of this investigation is to study the uniaxial ratcheting behavior of 42CrMo4 steel in annealed condition, under various applied stresses. In view of this, stress controlled fatigue tests were carried out at room temperature up to 200 cycles using a servo-hydraulic universal testing machine. The results indicate that accumulation of ratcheting strain increases monotonically with increasing maximum applied stress however; the rate of strain accumulation attains a saturation plateau after few cycles. The investigated steel shows cyclic softening behaviour under the applied stress conditions. The nature of strain accumulation and cyclic softening has been discussed in terms of dislocation distribution and plastic damage incurred in the material.

Stress-Controlled Fatigue Testing of E-Glass Epoxy Composite: Monitoring of Micro-Damage

The paper deals with progressive and fatigue damage of long fiber E-glass epoxy composite, its residual stiffness degradation and corresponding transverse matrix crack density induced by load-controlled tension. Constant-amplitude fatigue tests in repeated tension of plain [±60]S; [±30]S; [0]8 and [0/902/±45/90]S samples were performed. Sudden onset of transverse matrix cracking and consequent gradual increase of its density has been observed in off-axis plies. The crack density increases with increasing number of cycles or load. Consequently, residual stiffness of the laminate decreases. It has been concluded that progressive/fatigue damage of the laminate is not a continuous homogenous process but the series of discrete sudden events emerging at ply level.

Fatigue Life of Welded High-strength Steels under Gaussian Loads

Within the scope of the investigation of welded high-strength steels for application in crane structures, a Gaussian-like test spectrum is derived from an analysis of recorded load time histories. In addition to stress-controlled fatigue tests under constant amplitude loading, the test spectrum is used for the experimental investigation of MAG-welded butt joints and tubular sample components under variable amplitude loading. A linear damage accumulation using Palmgren-Miner-Elementary is conservative for a damage sum of D = 0.5. Application of the theoretical damage sum Dth = 1 results in a closer approximation of the Gaßner-curve. For further improvement of this approximation, a rotation of the calculated Gaßner-curve, i.e. a variable damage sum, is suggested for both butt joints and sample components.

Extension of linear elastic strain energy density approach to high temperature fatigue and a synthesis of Cu-Be alloy experimental tests

Article history: Received October 6, 2014 Accepted 28 January 2015 Available online 29 January 2015 The present paper summarizes the results from uniaxial-tension stress-controlled fatigue tests performed at different temperatures up to 650°C on Cu-Be specimens. Two geometries are considered: hourglass shaped and plates weakened by a central hole (Cu-Be alloy). The motivation of the present work is that, at the best of authors’ knowledge, only a limited number of papers on these alloys under high-temperature fatigue are available in the literature and no results deal with notched components. The Cu-Be specimens fatigue data are re-analyzed in terms of the mean value of the Strain Energy Density (SED) averaged over a control volume. Thanks to the SED approach it is possible to summarize in a single scatter-band all the fatigue data, independently of the specimen geometry. © 2015 Growing Science Ltd. All rights reserved.

Experimental investigation on HCF strength affected by predamage from LCF of a near alpha titanium alloy

Abstract

Influence of Heat Treatment on Ratcheting Fatigue Behavior and Post Ratcheting Tensile Properties of Commercial Aluminum

The objective of this work is the study of ratcheting fatigue behavior and post fatigue tensile properties of the commercial aluminum via heat treatment like annealing and normalizing. In view of this, stress controlled fatigue tests carried out at room temperature up to 150 cycles under different combinations of mean stress and stress amplitude using a servo-hydraulic testing machine. It is found that in both annealed and normalized conditions, accumulation of ratcheting strain increases with increasing stress parameters. Strain accumulation in normalized condition is less due to increased dislocation density during asymmetric cyclic loading. In normalized, the saturation in strain accumulation was also achieved earlier as compared to annealed due to attainment of stable dislocation configuration. It has been noticed that the Post fatigue tensile strength enhances while the ductility decreases with increasing stress parameters because of cyclic hardening. Fractographic studies have demonstrated that dimple size decreases with increasing strain accumulation.

High Temperature Fatigue Tests of a Cu-Be Alloy and Synthesis in Terms of Linear Elastic Strain Energy Density

The present paper summarises the results from uniaxial-tension stress-controlled fatigue tests performed at different temperatures up to 650°C on Cu-Be specimens. Two geometries are considered: hourglass shaped and plates weakened by a central hole (Cu-Be alloy). The motivation of the present work is that, at the best of authors’ knowledge, only a limited number of papers on these alloys under high-temperature fatigue are available in the literature and no results deal with notched components.The Cu-Be specimens fatigue data are re-analyzed in terms of the mean value of the Strain Energy Density (SED) averaged over a control volume. Thanks to the SED approach it is possible to summarise in a single scatter-band all the fatigue data, independently of the specimen geometry.