Stress-controlled Tests Research Articles

Plastic strain-controlled short crack growth and fatigue life

Constant plastic strain-controlled and constant stress-controlled tests were performed on smooth and lightly notched specimens machined from a massive forging of 42CrMo4 steel. Comparison of the fatigue life curves plotted as function of the plastic strain amplitude and stress amplitude shows a decisive role of plastic strain amplitude. Crack initiation and the kinetics of short crack growth were studied in constant plastic strain amplitude loading and the relation between the crack growth coefficient and plastic strain amplitude was established. This is equivalent to the Coffin–Manson law and shows that the Coffin–Manson law can be interpreted in terms of short crack growth.

Residual Stress Relaxation and Cyclic Deformation Behavior of Deep Rolled AlMg4.5Mn (AA5083) at Elevated Temperatures

The cyclic deformation behavior of deep rolled and polished aluminium wrought alloy AlMg4,5Mn in the temperature range 20-300°C has been investigated. Results of quasistatic tension and compression tests of untreated specimens in the temperature range 20-300°C are presented. To characterize the fatigue behavior for stress-controlled tests as a function of test temperature, s-n curves, cyclic deformations curves and mean strains as a function of number of cycles are given. The residual stress- and work hardening states near the surface of deep rolled aluminium alloy AlMg4.5Mn before and after fatigue tests were investigated by X-ray diffraction methods. The investigated AlMn4.5Mn aluminium alloy shows cyclic hardening until fracture at all stress amplitudes in stress-controlled fatigue tests at 25-150°C. With increasing temperature the deformation behavior shifts from cyclic hardening to cyclic softening. Below a certain stress amplitude at a given temperature deep rolling led to a reduction of the plastic strain amplitude as compared to the untreated state through cyclically stable near-surface work hardening as indicated by stable FWHM-values. This reduction in plastic strain amplitude is associated with enhanced fatigue lives. The effectiveness of deep rolling is governed by the cyclic and thermal stability of nearsurface work hardening rather than macroscopic compressive residual stresses. Since nearsurface work hardening is known to retard crack initiation, deep rolling is also effective in temperature- and stress ranges where macroscopic compressive residual stresses have relaxed almost completely, but where near-surface work hardening prevails. Above certain stress amplitudes and temperatures, deep rolling has no beneficial effect on the fatigue behavior of AlMg4.5Mn. This is a consequence of instable near-surface microstructures, especially instable near-surface work hardening.

Microstructure and cyclic deformation behaviour of the magnesium die-cast alloy AZ91D hp

Abstract In this investigation, stress-controlled load increase and constant amplitude tests have been carried out on the magnesium die-cast alloy AZ91D hp. The plastic strain amplitude and the temperature change measured in load increase tests allowed an estimation of the endurance limit with one single specimen. Under constant amplitude loading, stress amplitudes above the endurance limit lead to pronounced cyclic hardening after initial cyclic softening. Compared to monotonic loading, cyclic hardening was observed. The plastic deformation predominantly occurs in the α-phase. In contrast, the β-phase fails in a very brittle manner. Light and scanning electron microscopic investigations show the different crack propagation behaviour of the α- and β-phases.

Influence of Lüders band formation on the cyclic creep behaviour of a low-carbon steel for piping applications

Abstract The paper presents a study on the influence of formation of Luders band on the cyclic creep or ratcheting behaviour in low carbon steel bound for pipeline applications. The Luders band formation has been characterized by carrying out stress-controlled monotonic tensile tests in terms of the stress required for it to propagate and the amount of strain accumulated during its propagation. Low-cycle fatigue at different strain ranges and cyclic stress-controlled tests at different stress ratios and stress rate combinations have been carried out. It has been observed that the hysteresis behaviour in conditions favouring Luders band formation leads to a significant enhancement in ratcheting and cyclic softening in the low-cycle fatigue tests. The evolution of the hysteresis loop behaviour has been characterized by the Bauschinger energy parameter. The accelerated strain softening has been attributed faster proliferation of dislocation sources and to greater dislocation annihilation rates as a consequen...

Fatigue behaviour of rail steel—a comparison between strain and stress controlled loading

Strain and stress-controlled fatigue testing of rail material UIC grade 900A has been performed. The tests were carried out in uniaxial, constant amplitude push–pull loading on polished test bars. Strain-controlled experiments were performed at 0.4, 0.6 and 1.0% total strain amplitude. Two identical tests at each strain level were made and the results show very little spread: <1.5% difference in stress amplitude during the fatigue life and <5% difference in number of cycles to failure. The stress-controlled tests were selected to match the strain-controlled experiments in terms of stabilised stress levels. The number of cycles to failure in the two types of tests is very similar, indicating little influence of initial overloads experienced in strain control.

Fatigue of deep rolled AlMg4.5Mn (AA5083) in the temperature range 20–300°C

Abstract The cyclic deformation behavior of deep rolled and polished aluminium wrought alloy AlMg4.5Mn has been investigated in the temperature range from 20 to 300 °C. Results of quasistatic tension and compression tests of untreated specimens are presented. To characterize the fatigue behavior for stress-controlled tests as a function of test temperature, stress-life-curves, cyclic deformations curves and mean strains as a function of number of cycles are given. Residual stresses and work hardening effects near the surface of the deep rolled condition before and after fatigue tests were investigated by X-ray diffraction methods.

On the dislocation mechanisms of dynamic strain ageing in fatigued plain carbon steels

Abstract The fatigue life of plain ferritic carbon steels fatigued in stress-controlled (plastic-strain-controlled) tests exhibits a maximum (minimum) at a temperature of approx. 300°C as a result of dynamic strain ageing (DSA). In the regime of DSA, an enhanced cyclic hardening related to an increased dislocation density is observed, and the dislocation arrangement changes from a dislocation cell structure (at intermediate stress amplitudes) at room temperature to a dense edge di-/multipolar bundle/wall/vein structure in the temperature range of DSA and back to a dislocation cell/subgrain structure at higher temperatures. Related effects occur also in α-iron containing some carbon when fatigued at room temperature at a very low strain rate. This behaviour is considered to be typical of body-centred cubic materials. In the present study, the dislocation mechanisms responsible for this behaviour are discussed in detail, and a dislocation glide model is developed which can explain both the enhanced dislocat...

Undrained strength behaviour of a cemented marine clay under monotonic and cyclic loading

The paper presents the results of a series of monotonic and cyclic triaxial shear tests carried out to study the influence of the strain effect and load cycles on the undrained shear strength of a cemented marine clay from the East coast of India. The undrained shear strength of Indian coastal marine clay has been established from a detailed shear testing carried out in three phases. Undisturbed soil samples taken out from the seabed were used in the test. In the first part, a comprehensive monotonic shear testing has been carried out under both stress-controlled and strain rate-controlled conditions. In the second phase on identical soil specimen, undrained cyclic shear tests were carried out at various cyclic stress ratios (CSR) and these stress levels are chosen in such a way so that no failure occurred during testing. In the final phase post cyclic monotonic shear testing was conducted to qualitatively evaluate the damage caused by cyclic loading. The monotonic shear test results bring out the influence of cementation that can be detected by the stress-controlled test. The cyclic stress results are analysed in terms of CSR. Further, the results are correlated in terms of stress path.

On a rate-type thermomechanical model for shape memory bars. Stress-controlled problems

In this paper we consider an explicit one-dimensional rate-type thermo-viscoelastic model that is capable of describing some aspects of the response of a shape memory alloy. This approach takes into account the dependence of the load-deformation curves on temperature and the heat exchange across the lateral surface of the bar. We study here the hysteretic macroscopic response predicted by this model in the case of stress-controlled problems and relaxation tests.

Microstructural evolution during low cycle fatigue of a 3003 aluminum alloy

A polycrystalline 3003 aluminum alloy was cyclically deformed at room temperature under constant stress amplitudes in the range of 40–68 MPa. The mechanical responses of the stress-controlled tests were compared with those of the strain-controlled tests conducted earlier. Similar to the strain-controlled tests, cyclic softening after initial hardening was found at medium strain amplitudes in the stress-controlled tests. Secondary hardening occurred in the later stage of fatigue after softening. Microstructural analysis using transmission electron microscopy (TEM), revealed evolutions of dislocation structures at each stress amplitude. A labyrinth structure, which is composed of over two sets of dislocation-dipole walls with spacing of about l μm, was typically formed during the softening stage. Veins, i.e. tangled edge dislocation dipoles, were formed during the monotonic hardening stage at both lower strain and stress amplitudes. On the other hand, cell structures were dominant at the final stage in the high stress amplitude tests. The evolutions of dislocation structures in stress-controlled tests are well correlatable with those in strain-controlled tests.

Analysis of certain factors affecting the unstable behaviour of saturated loose sand

The factors affecting the onset of unstable behaviour in saturated loose sand loaded under drained stress-controlled conditions are not as well known as those involved in its undrained behaviour, which has been analysed in detail. This paper is intended as a first step towards a better understanding of the onset of instability in drained stress-controlled tests and reports on the analysis of certain factors affecting the unstable behaviour of saturated loose sand samples subjected to drained stress-controlled triaxial compression tests. The samples are loaded continuously at a constant loading rate and the results are compared with those obtained by other authors who discontinuously applied a load using finite increments. A special experimental apparatus was designed so that the axial load could be applied in various proportions by a dead weight and by a pneumatic cylinder in a triaxial cell with no friction on the loading ram. The paper focuses first on the influence of the load application method (i.e. by dead weights and by a pneumatic cylinder), which proves important, since the sample always collapses at a more or less constantly low mobilized friction angle when loaded with dead loads, whereas it may have unstable strain jumps starting from a high value of the mobilised friction angle, but may even be stable up to the steady state of deformation, when the axial load is applied by a pneumatic cylinder. Then the influence of the loading rate and loading path is examined and found to be negligible, at least in a first approximation. On the other hand, the effects of preshear and initial density are shown to have a major role in the onset of collapse: both the preshear and the increase in density lead to an increase in the resistance of the loose sand to collapse under drained stress-controlled conditions. Copyright © 2000 John Wiley & Sons, Ltd.

Notch-Strengthening Phenomenon Under Creep-Fatigue Loading Conditions

A study of the notch and frequency effects on fatigue life at high temperature is carried out using notched and unnotched plate specimens of SS 304 under stress-controlled testing conditions. Analysis of the σ-Nf results obtained at 600°C under fatigue and creep-fatigue conditions allowed the generalization of the σ-Nf-Kt relation proposed in an earlier study. Examinations of the experimental data with hold-time testing suggested that in these conditions, the frequency effect should be incorporated in the relationship. Results obtained from the modified relation are in agreement with the experimental data, within a factor of two. Finite element analysis was carried out to determine the state of stresses and strains at the notch root by simulating four creep-fatigue cycles. The computed results indicated that, under zero-to-tension cyclic loading with controlled nominal stress, the maximum local stress at the notch root relaxes; this results in a minimum local stress in compression, and as a consequence, the mean local stress is significantly reduced. The stress relaxation as well as the creep strain accumulation were found to occur only in the vicinity of the notch (within 0.75 mm). The numerical results concerning the local stress relaxation and the time-dependent strain accumulation are used to explain the notch-strengthening effect on life observed in the present study. [S0094-9930(00)00401-7]

The effect of matrix microstructure on cyclic response and fatigue behavior of particle- reinforced 2219 aluminum: Part I. room temperature behavior

The low-cycle and high-cycle fatigue behavior and cyclic response of naturally aged and overaged 2219/TiC/15p and unreinforced 2219 Al were investigated using plastic strain-controlled and stress-controlled testing. In addition, the influence of grain size on the particle-reinforced materials was examined. In both reinforced and unreinforced materials, the naturally aged conditions were cyclically unstable, exhibiting an initial hardening behavior followed by an extended region of cyclic stability and ultimately a softening region. The overaged reinforced material was cyclically stable for the plastic strains examined, while the overaged unreinforced material exhibited cyclic hardening at plastic strains greater than 2.5 × 10−4. Decreasing grain size of particle-reinforced materials modestly increased the cyclic flow stress of both naturally aged and overaged materials. Reinforced and unreinforced materials exhibited similar fatigue life behaviors; however, the reinforced and unreinforced naturally aged materials had superior fatigue lives in comparison to the overaged materials. Grain size had no effect on the fatigue life behavior of the particle-reinforced materials. The fatigue lives were strongly influenced by the presence of clusters of TiC particles and exogenous Al3Ti intermetallics.

Improvements to a Stress-Path Triaxial Cell

Abstract Stress-path triaxial cells are presently used all over the world. Accurate use of this type of apparatus allows an investigation into the mechanical behavior of soils with regard to a wide range of stress and strain. A correct evaluation of small-strain stiffness requires an accurate definition of the compliance curve of the apparatus and the possibility of performing tests in stress-controlled conditions. However, in the standard stress-path cell, compliance curves are often nonlinear and nonrepeatable. Moreover, in most standard stress-path apparatus, the pressure supply is limited to 1000 kPa, which makes it impossible to carry out stress-controlled tests at high confining pressures. The observation of these limitations prompted adjustments in the design of the cell, described in the present paper. With the revised design, it is possible to apply a deviatoric stress up to 1500 kPa under stress-controlled conditions with a cell pressure of 1000 kPa using a 1000-kPa pressure supply. The stiffness of the cell was also significantly increased, up to ten times the stiffness of the original design.

Zyklisches Kriechen und Mittelspannungsrelaxation bei Zungschwellbeanspruchung von normalisiertem und vergütem 42 CrMo 4

Stress and total strain controlled fatigue tests selecting R = 0 and R E = 0, respectively, were performed with normalized as well as quenched and tempered 42 CrMo 4 steel specimens. The following results were obtained: - Under cyclic stress control quenched and tempered specimens worksoften whereas normalized specimens workharden. However, at larger stress amplitudes normalized specimens, after an initial period of workhardening, also reveal worksoftening up to cyclic fracture. - In total strain-controlled experiments quenched and tempered specimens show cyclic softening in contrast to normalized specimens which only reveal small changes of plastic strain amplitudes around quasi-saturation states. - In stress-controlled tests both material states show features of cyclic creep. The cyclic creep resistance of the quenched and tempered specimens is markedly higher than that of the normalized ones, - In total strain-controlled experiments the mean stresses of normalized specimens relax nearly totally in the lifetime range 10 4 ≤ N ≤ 5 . 10 5 , whereas quenched and tempered specimens reveal only a small mean stress relaxation in the same range of lifetime. - The different stability of the mean stresses of the material states investigated leads in total strain-controlled tests for both states to a unique Wohler-curve for N ≤ 3 . 10 4

A Multiple Purpose Soil Testing Apparatus

Abstract A relatively compact soil testing system that is capable of performing monotonic/cyclic triaxial and torsional simple shear tests has been developed. A high-torque direct-drive, servo/stepper motor is used to provide the driving force. Under the servo mode, the motor could be controlled to provide a constant torque, which is used in stress-controlled tests. Strain-controlled tests are conducted using the stepper control mode. To perform triaxial tests, the rotational motion of the motor is transformed into linear motion using a preloaded ball screw shaft. A preloaded ball spline shaft directly attached to the motor is used to perform torsional simple shear tests. Because there are no gears in the driving system, there is no backlash during reversals of loading directions. The motor is equipped with an encoder and an indexer. The extremely rigid driving system and high resolution of the encoder enable the linear and rotational displacement to be monitored through the encoder/indexer system. Test control and data logging are fully automated using a personal computer. This paper provides details of the apparatus. A set of strain-controlled test data are presented to demonstrate capabilities of the new apparatus.

Biaxial Ratcheting Under Strain or Stress-Controlled Axial Cycling With Constant Hoop Stress

Strain or stress-controlled tension-compression cyclic tests were conducted on pressurized thin-walled tubular specimens of 304 stainless steel. In strain-controlled mode ratcheting strains in hoop direction, and under stress-controlled mode ratcheting in both hoop and axial directions, were observed. To predict the observed ratcheting behavior, an additional evolution rule for the stress memory surface has been introduced in the constitutive model developed recently by the authors. Qualitative and quantitative comparisons with the test results indicate a fairly good agreement in predicting ratcheting deformations.

Effect of Laminate Orientation on the Thermomechanical Fatigue Behavior of a Titanium Matrix Composite

Thermomechanical fatigue (TMF) tests were conducted on the silicon carbide fiber and titanium alloy matrix composite, SCS-6/TIMETAL®21S. Three different laminate orientations were considered: [0]4, [0/90]s, and [0/±45/90]s. Both in-phase and out-of-phase stress-controlled tests were conducted under a temperature cycle of 150° to 650°C and a stress ratio of 0.1. The fatigue lives for these different orientations can be consolidated within a factor of 3 by normalizing the maximum applied stress (Smax) by the ultimate tensile strength at the Smax temperature of the TMF cycle. For all laminate orientations, the maximum and minimum strain increase during in-phase cycling, whereas only maximum strain increases during out-of-phase cycling. Damage accumulation under inphase cycling is attributed to a combination of the increasing stress carried by the [0] fibers due to cyclic matrix stress relaxation and the gradual breakage of [0] fibers. The damage during out-of-phase cycling is attributed to matrix cracks which initiate at the surface with the aid of the environment and/or initiate internally at the transverse fibers in [0/90]s and [0/±45/90]s composites. A simple model to compute cyclic strain accumulation based on the proposed mechanisms successfully captures the experimentally observed behavior. TMF life is shown to be sensitive to fiber volume fraction under inphase cycling, but not under out-of-phase cycling. TMF conditions are shown to be more severe than isothermal fatigue.

Effect of loading ramp on cyclic stress-strain response and cyclic creep behaviour

The cyclic stress-strain response and cyclic creep behaviour of polycrystalline copper were shown to depend not only on the length but also on the mode of the start-up ramp in stress-controlled tests at zero mean stress. In tests with a non-zero mean stress, these phenomena are additionally affected by the magnitude of the mean stress. There is a strong correlation between the cyclic creep strain and the saturated plastic strain amplitude. In the studied region of small amplitude cycling, the explanation lies in the response of the one-phase matrix dislocation structure.

The Influence of Matrix Microstructure and TiC Reinforcement on the Cyclic Response and Fatigue Behavior of 2219 Al

The low-cycle and high-cycle fatigue behavior and cyclic response of naturally aged and artificially aged 2219/TiC/15p and unreinforced 2219 Al were investigated utilizing plastic strain-controlled and stress-controlled testing. The cyclic response of both the reinforced and un-reinforced materials was similar for all plastic strain amplitudes tested except that the saturation stress level for the composite was always greater than that of the unreinforced material. The cyclic response of the naturally aged materials exhibited cyclic hardening and, in some cases, cyclic softening, while the cyclic response for the artificially aged materials showed no evidence of either cyclic hardening or softening. The higher ductility of the unreinforced material made it more resistant to fatigue failure at high strains, and thus, at a given plastic strain, it had longer fatigue life. It should be noted that the tensile ductilities of the 2219/TiC/15p were significantly higher than those previously reported for 2XXX-series composites. During stress-controlled test-ing at stresses below 220 MPa, the presence of TiC particles lead to an improvement in fatigue life. Above 220 MPa, no influence of TiC reinforcement on fatigue life could be detected. In both the composite and unreinforced materials, the low-cycle and high-cycle fatigue lives were found to be virtually independent of matrix microstructure.