Effects Of Stress Amplitude Research Articles

Experimental Research on the Low-Cycle Fatigue Crack Growth Rate for a Stiffened Plate of EH36 Steel for Use in Ship Structures

This paper presents a straightforward approach for determining the low-cycle fatigue (LCF) crack propagation rate in stiffened plate structures containing cracks. The method relies on both the crack tip opening displacement (CTOD) and the accumulative plastic strain, offering valuable insights for ship structure design and assessing LCF strength. Meanwhile, the LCF crack growth tests for the EH36 steel were conducted on stiffened plates with single-side cracks and central cracks under different loading conditions. The effects of stress amplitude, stress ratio, and stiffener position on the crack growth behavior were examined. Fitting and verifying analyses of the test data were employed to investigate the relationship between CTOD and the crack growth rate of EH36 steel under LCF conditions. The results showed that the proposed CTOD-based prediction method can accurately characterize the LCF crack growth behavior for stiffened plate of EH36 steel for use in ship structures.

Effects of Stress Amplitude and Cold Expansion Strengthening on the Fatigue Property of TC4 Titanium Alloy

Effects of Stress Amplitude and Cold Expansion Strengthening on the Fatigue Property of TC4 Titanium Alloy

Mechanism of ratcheting deformation of HSLA steel: Effects of internal stresses and dislocation density

The objective of this study is to elucidate the mechanism of ratcheting deformation of HSLA steel in relation to the evolution of internal stresses i.e. effective stress (σef) and backstress (σb) associated with the generation of dislocations during asymmetric cyclic loading. Effective and back stresses are calculated from the stress-strain hysteresis loops and dislocation density is estimated from the X-ray diffraction technique. The results exhibit an increase in the accumulation of ratcheting strain associated with a sharp reduction in fatigue life by the increment in stresses. A continuous cyclic softening until the final failure is observed for all the ratcheting tests. It is noticed that the effect of stress amplitude is more prominent than the mean stress for controlling cyclic softening, which is evidenced by the evolution of effective stress and backstress in association with dislocation density. Additionally, transmission electron microscopic images are used to correlate strain conditions and substructural patterns.

Effect of stress amplitude on damage and fracture characteristics of austenitic heat‐resistant steel Sanicro 25 at 973 K

AbstractHigh‐temperature damage characteristics related to the deformation behavior of Sanicro 25 alloy were investigated under stress‐controlled cyclic loading. The results show that the strain curves exhibited creep‐like strain characteristics. The increase in mean stress increased the strain rate and weakened the life. Comparatively, the increase in stress amplitude only accelerated the tertiary‐regime strain rate, leading to a decrease in life and fracture strain. Microcrack initiation at grain boundaries and primary Z phases at different stress amplitudes can be evidenced by the observation of micro‐voids and high geometrically necessary dislocation densities around them. Subsequently, the increase in stress amplitude can significantly affect crack propagation. Dislocation accumulation at the transgranular crack tip caused by cyclic stress can assist the propagation of the transgranular crack at higher stress amplitude ( ). Conversely, transgranular cracks are easily suppressed and stop propagating at grain boundaries under constant loading ( ). The main reason was that the stress concentration of the transgranular crack tip at grain boundaries was dissipated through discontinuous dynamic recrystallization.

Effect of stress amplitude on mechanical and acoustic emission of sandstone under constant–cyclic loading

Effect of stress amplitude on mechanical and acoustic emission of sandstone under constant–cyclic loading

The effect of stress amplitude on ratcheting strain development in an extruded AZ31B magnesium alloy under different mean stresses

The ratcheting behavior in an extruded AZ31B magnesium alloy was investigated under uniaxial stress-controlled cyclic loading at room temperature. The results showed that basal <a> slip, prismatic <a> slip and {10–12} extension twinning/detwinning are the deformation mechanisms accommodating cyclic strain. The weight of the slips or the twinning/detwinning that relates to stress amplitude affects the valley strain and ratcheting strain. The increase of the weight of the dislocation slips results in an increment of strain hardening rate. When the stress amplitude is 105 MPa, the hardening rate is higher than that at the amplitudes of 45 MPa and 75 MPa, which leads to a ratcheting strain development deviating from the regular pattern.

Deformation Behavior of Sandstones during Multilevel Cyclic Loading and Unloading under Varying Lower Stress Limits: Effect of Stress Amplitude and Underlying Mechanism

Deformation Behavior of Sandstones during Multilevel Cyclic Loading and Unloading under Varying Lower Stress Limits: Effect of Stress Amplitude and Underlying Mechanism

Investigation on the Effect of Asymmetric Cyclic Loading on Ratcheting Deformation and Bulk Texture Development in HSLA Steel

Ratcheting deformation in association with bulk texture evolution in HSLA steel is studied in this investigation. The cyclic deformation behaviors and damage modes of the specimens are obtained at room temperature, employing asymmetric stress-controlled tests under various combinations of mean stress and stress amplitudes. Bulk texture measurements of the specimens are done adopting an X-ray diffraction analysis. The results exhibit an increase in strain accumulation from 0.23 to 30.61% associated with a sharp reduction in ratcheting life from 3320 to 880 cycles by the increment in mean stress/stress amplitude. A continuous cyclic softening until the final failure is observed for all the ratcheting tests. It is noticed that variation of mean stress is more detrimental than stress amplitude in controlling strain accumulation while the effect of stress amplitude is more pronounced than mean stress for the reduction of ratcheting life. The fracture surface morphology indicates typical striation formation in the crack propagation region along with overload failure. The bulk texture studies indicate that there is a transition between ‘ϒ’ fiber and ‘ζ’ fiber during all loading conditions. The texture intensity is increased from 5.71 to 11.68 with progressive ratcheting strain accumulation. Here also, influence of stress amplitude is more significant than the mean stress.

Friction stir welds with enhanced fatigue strength and life post laser peening

This study aims to quantify the influence of laser peening on the structural integrity of a FSWelded aluminium fuselage joint. Fatigue tests with different stress amplitudes were performed for samples in as-machined and friction stir welded conditions to characterise the loss of fatigue strength and the effect of stress amplitude on the degradation of fatigue life. Optimisation of laser parameters was carried out to achieve an appropriate through-thickness stress profile. The optimised parameters were used to peen tensile coupon specimens and analyse the fatigue life recovery. The residual stress fields were quantified using the central incremental hole drilling method on peened and unpeened samples. It was found that both the microstructure and superposition of residual stresses from the welding process and peening treatment play a crucial role in the fatigue behaviour of the FSWelded joint. Using the parameters and layout described in this article, laser peening was able to increase the fatigue life of as-welded samples by a factor of 2.4 for the lower end of the load spectrum tested. This represents a recovery of 65% when compared to the pristine aluminium reference samples.

The Effect of Stress Amplitude on Ratcheting Strain Development in an Extruded Az31b Magnesium Alloy Under Different Mean Stresses

The Effect of Stress Amplitude on Ratcheting Strain Development in an Extruded Az31b Magnesium Alloy Under Different Mean Stresses

Investigation of uniaxial ratcheting fatigue behaviours and fracture mechanism of GH742 superalloy at 923 K

In this study, the ratcheting fatigue behaviours of GH742 superalloy at 923 K were investigated through a series of symmetric and asymmetric stress-controlled tests. The effects of stress amplitude and mean stress on the fatigue behaviours were analysed through scanning electron microscopy, transmission electron microscopy and electron backscatter diffraction. The results highlight that the fatigue life significantly decreases with increasing mean stress or stress amplitude. Tensile ratcheting strain occurs and increases with the increasing number of cycles under both symmetric and asymmetric cyclic loading. The alloy initially exhibits a cyclic hardening response, followed by cyclic softening until fracture failure under symmetric and asymmetric cyclic loading. The increase in mean stress promotes the cyclic softening in the tensile strain direction to a certain extent. The fracture mode does not change with increasing mean stress. Multiple fatigue cracks initiate from the surface inclusions, crystallographic facets and slip bands, and propagate in mixed transgranular and intergranular modes. The modified Basquin equation, which considers the mean stress and stress amplitude, can accurately predict the fatigue life, and all the fatigue data are located within the 1.5X scatter band.

Quasi Static and Fatigue Properties of Long Carbon Fiber Reinforced Polyamide

In this experimental work, the quasi static and fatigue properties of a 40 wt.% long carbon fiber reinforced partially aromatic polyamide (Grivory GCL-4H) were investigated. For this purpose, microstructural parameter variations in the form of different thicknesses and different removal directions from injection-molded plates were evaluated. Mechanical properties decreased by increasing misalignment away from the melt flow direction. By changing the specimen thickness, no change in the general fiber distribution pattern transversal and normal to the axis of melt flow was observed. It has shown that with increasing specimen thickness the quasi static properties along the melt flow direction decreased and vice versa resulting in superior properties normal to the melt flow axis. At around 5 mm, an intersection suggests quasi-isotropic behavior. In addition, the fatigue strength of the material was significantly higher in the flow direction than normal to the flow direction. No change in fatigue life was observed while changing specimen thickness. The Basquin equation seems to describe the effect of stress amplitude on the fatigue strength of this composite. Scanning electron microscopy was used to investigate fracture surfaces of tested specimens. Results show that mechanical properties and morphological structures depend highly on fiber orientation.

Stress analysis and lifetime prediction for Ti–6Al–4V welding joint under fatigue loading

A welded joint has a complex gradient microstructure. The effects of local deformation in the fusion zone, heat-affected zone (HAZ), and other areas, on the macroscopic fatigue properties of an inhomogeneous welded joint, are not fully understood. Thus, it is necessary to correlate the microstructural features and their related local deformation mechanisms with macroscopic fatigue properties. In this study, cyclic stress–strain curves and lifetime model parameters of materials in different HAZs were determined by hardness distribution and base–metal material properties. A user element subroutine was developed in the elastic–plastic finite element analysis to obtain the local stress and strain in the joint. ‘Danger points’ were selected to predict fatigue lifetime with the Manson–Coffin damage equation. The prediction results agreed closely with experimental observations within the low-cycle fatigue lifetime regime. Highlights A new LCF life analysis method, for coated specimens, was developed based on the fracture mechanics. Life prediction method for coated specimens was proposed and verified by LCF tests. Coating-induced lifetime degradation was analysed quantitatively at different temperatures. Effects of stress amplitude on the surface crack initiation were investigated.

Study on Mechanical Properties and Damage Evolution of High‐Porosity Concrete under Cyclic Loading and Unloading

Understanding the mechanical properties and energy response of high‐porosity concrete under the cyclic loading and unloading is the foundation of road construction in sponge city. In this study, the concrete with the porosity of 15% was taken as the research object, and the cyclic loading and unloading tests on the high‐porosity concrete were performed under the stress amplitude of 25 MPa, 30 MPa, and 35 MPa in the elastic stage. The effects of stress amplitude and cycle number on the mechanical characteristics and damage evolution law of concrete were obtained. The experimental results show the following. (1) With the increase of cycle number, the loading and unloading elastic modulus of concrete under different stress amplitudes first increases and then decreases; the greater the stress amplitude, the faster the growth and deceleration of the loading and unloading elastic modulus. (2) With the increase of the cycle number, the peak strain and residual plastic deformation increase. (3) The greater the stress amplitude, the higher the damage of concrete; with the increasing number of cyclic loading and unloading, the damage of concrete is enhanced gradually. When the damage variable value is 1, the relationship between the cycle number and the initial stress amplitude satisfies a negative exponential function.

On the effect of stress amplitude on fracture and energy evolution of pre-flawed granite under uniaxial increasing-amplitude fatigue loads

Rock mass containing discontinues is often subjected to fatigue loads in many mining and civil engineering. The stress amplitude (SA) as an important factor impacts rock fracture evolution during fatigue loading condition. Existing effects to investigate the impacts of SA on rock fatigue behaviors were mainly on constant SA loading condition, the influences of incremental stress amplitude (ISA) for rock subjected to cyclic uniaxial increasing-amplitude conditions is not well understood. In this work, uniaxial increasing-amplitude fatigue loads and post-test 3D CT scanning experiments were conducted on granite containing two pre-existing flaws. Our experimental results reveal the impact of four applied ISA (i.e., 5, 10, 20 and 30 MPa) on rock fracture evolution, energy conversion and crack network pattern at rock bridge segment. The damage accumulation and volumetric deformation increase with the increase of applied ISA. The accelerated damage evolution stage is different for sample with different ISA, and it starts to appear the earliest for a sample subjected to ISA of 30 MPa. Energy conversion analysis reveals that the dissipated energy used to drive the crack propagation is relatively large for a sample with higher ISA. Post-test 3D CT visualization reveals a most striking finding that the crack network pattern becomes complicated as the applied ISA increases. It is suggested that crack coalescence between the flaws is easy to be formed for rock subjected to low ISA, rock bridge segment fracturing is stress amplitude dependent. The testing results are expected to improve the understanding of the influence of stress amplitude on the fatigue behaviors of pre-flawed rock when rock mass is subjected to increasing-amplitude stress disturbance.

Influence of stress amplitude on the dynamic characteristics of phyllite samples under triaxial multi-stage cyclic loading

The generation of a rockslide in an earthquake event is strongly dependent on the dynamic characteristics of the rock materials constituting the rock slope. In this study, phyllite samples were extracted from the Donghekou landslide area of Qingchuan County. The landslide occurred during the 2008 Wenchuan earthquake and the samples were made to undergo multi-stage cyclic loading and unloading tests under triaxial compression conditions. The confining pressure during the tests was 10 MPa, the loading frequency was 2 Hz, the loading waveform was sine wave, and 60 loading cycles were carried out at each level of stress. From the dynamic axial stress–strain curves, the effect of stress amplitude on the hysteresis loop area, the dynamic elastic modulus, the dynamic damping ratio, and the damping coefficient of the phyllite samples was analyzed. The results are as follows. (1) The strain–stress hysteresis loop area experienced a rapid increase as the stress amplitude exceeded a certain value, indicating that the rock samples have entered an accelerating crack propagation stage under high levels of cyclic loading. (2) As the stress amplitude increases, the dynamic elastic modulus, the dynamic damping ratio, and the damping coefficient of the rock sample followed a similar two-phase evolution law. There was a rapid reduction in the three parameters relative to the initial value in Phase I. Moreover, there was a minor change in Phase II. (3) The effect of the number of loading cycles number on dynamic elastic modulus and damping parameters was much more obvious under the lower levels of cyclic loading than under the higher levels.

Fatigue Life Degradation Modeling of SnAgCu Solder Joints After Aging

The reliability of electronic assemblies is exceedingly reliant on the life of solder joints. Many factors may influence the fatigue life and mechanical properties of solder joints. Aging is one of the main factors that adversely affect the reliability of solder joints. In this article, the mechanical reliability of individual SnAgCu solder joints is studied at different aging conditions. The experiments include aging then cycling solder joints in stress-controlled shear fatigue at different stress amplitudes (16, 20, and 24 MPa) until complete failure. The aging conditions include three levels of aging temperatures (50 °C, 100 °C, and 150 °C) and four levels of aging time (2, 10, 100, and 1000 h). The effect of stress amplitude, aging time, and aging temperature on the fatigue life is investigated. The evolutions in the hysteresis loop, including the inelastic work dissipation, are explored by studying the stress–strain diagrams. The results showed that the fatigue life was affected by the stress amplitude, where increasing the stress by a factor of 2 leads to life reduction by a factor of 22. It was also found that increasing the aging time leads to nonlinear life reduction, where the first few hours of aging had the largest effect. Increasing the aging temperatures aggregates the aging time effect and leads to less fatigue resistance. Overall, increasing the aging time, temperature, and/or stress amplitudes leads to an increase in the work dissipation per cycle, which creates more damage and leads to less number of cycles to failure. The effect of these factors was fit into mathematical equations. Then, a general reliability model as a function of those parameters was developed.

Investigation on the uniaxial ratchetting and fatigue failure behaviors of silicone seal adhesive bonded butt-joints under tensile and torsional cyclic loading

In order to investigate the uniaxial ratchetting behaviors and fatigue failure of silicone seal adhesive bonded hollow cylinder butt-joints under uniaxial tensile and torsional cyclic loading, the effects of stress amplitude, mean stress, cycle time and loading history on the ratchetting strain, ratchetting strain rate and fatigue life were discussed respectively. The experimental results show that ratchetting strain appeared in both tensile and torsional cyclic loading. The ratchetting strain of tensile cycle evolved in three stages, while that of torsion cycle evolved in two stages. Furthermore, the effect of uniaxial tensile cycle on ratchetting strain is greater than that of uniaxial torsion cycle, and the fatigue life of torsion cycle is higher than that of tensile cycle. The stress amplitude, mean stress and cycle time have influence on the ratchetting strain, ratchetting strain rate and fatigue life of the adhesively bonded butt-joints. The ratchetting strain and ratchetting strain rate increase, and the fatigue life decreases with the increase of stress amplitude and mean stress, or the decrease of cycle time. In addition, it is also found that silicone seal adhesive bonded butt-joint has a strong memory of loading history for the stress amplitude. Prior stress cycle with higher stress amplitude restrains the ratchetting strain in subsequent cycle with lower stress amplitude.

Uniaxial ratchetting and low-cycle fatigue failure behaviors of adhesively bonded butt-joints under cyclic tension deformation

Ratcheting and low-cycle fatigue failure behaviors of the adhesively bonded hollow cylindrical butt-joints has been experimentally investigated. A series of uniaxial cyclic tension experiments were carried out under stress-controlled mode. The effects of stress amplitude, mean stress and cycle time on the uniaxial ratcheting response, fatigue damage variable evolution and fatigue life of the adhesively bonded butt-joints were analyzed. The results show that the ratcheting strain, ratcheting strain rate and fatigue damage variable all increase with the increase of stress amplitude and mean stress. The shorter cycle time results in the increase of fatigue damage variable and the degradation of the stiffness of the adhesive material. It is also found that the increase of stress amplitude and mean stress can reduce the low-cycle fatigue life. Meanwhile, the fatigue life increases with the increase of cycle time for the adhesively bonded butt-joints.

Experimental study on the whole‐life heterogeneous ratchetting and ratchetting‐fatigue interaction of SUS301L stainless steel butt‐welded joint

AbstractUniaxial fatigue tests of butt‐welded joint, made from SUS301L stainless steel, were carried out under asymmetric stress‐controlled cyclic loading conditions in this work. The effects of stress amplitude and mean stress on the whole‐life heterogeneous ratchetting and fatigue life of the butt‐welded joint were investigated, respectively, for the specified subzones. The experimental observations show that the whole‐life inhomogeneous ratchetting strain concentrating in a specific fusion zone (denoted as the FZ‐1 subzone) of the welded joint becomes more significant as the stress level increases; the fatigue failure also occurs in the FZ‐1 subzone, and the fatigue life depends on both the applied mean stress and stress amplitude and is determined by the combination of ratchetting damage and fatigue one in the localized FZ‐1 subzone.