Effect Of Stress Ratio Research Articles

Effect of Stress Ratio on Fatigue Crack Propagation Behavior of Single Crystal Superalloy

In order to study the impact of different stress ratio on the fatigue crack growth behavior of DD6 single crystal superalloy, the fatigue crack growth rate test of different stress ratio (R = 0.05, 0.2, 0.5, 0.6) at 530 °C was conducted, the influence of stress ratio on crack growth rate was studied, and the growth of crack extension rate was corrected by normalization method. The results show that the section includes the fatigue fracture section perpendicular to the load and an angle to the load. The flat section is the main section representing the crack extension. The oblique section is formed with the change of the crack extension load state, and its occurrence is related to the dispersion of (da/dN)−Δk data, reflecting the uniqueness of single crystal. The Paris formula better characterized the crack expansion behavior of DD6 alloy, and the stress is more sensitive than the crack growth rate in the early stable growth phase. Crack growth rate normalized characterization equation constructed under different stress ratios are predicted to be within the 1.3-fold dispersion band.

The Effect of Stress Ratio on Fatigue Cracks Growth Rate in Aluminum Alloy

The financial and societal consequences of the sudden failure of various mechanical structures and components can occasionally be severe. As a result, there has been extensive research done on fatigue failure in both the scientific and industrial fields. The impact of stress ratio adjustments on the rates of fatigue fracture propagation for an aluminum alloy has been investigated in the current study. This aluminum alloy was extracted from a Russian MI25 helicopter's primary blade. The stress intensity factor was calculated and employed by creating an initial crack in the test specimens after a specimen of MI25 blade was cut into testing specimens with standard dimensions. A special program was written specifically for this purpose to draw a graph of the stress intensity factor range (k) with value of the propagation of fatigue cracks (da / dN) at each stress ratio (R); Propagation of fatigue cracks has been studied. Finally, the relationship between the two different stress ratios has been discovered. Between the two, a comparison has been established. The findings indicated that when the stress ratio rises, so too does the rate at which fatigue cracks form. Conversely, with a negative stress ratio, the negative increase leads to a reduction in the rate at which fatigue cracks grow. The data have been examined, and during the second stage of crack propagation, where (R = 0.0), pertinent equations that are compatible with the Paris equation have been discovered.

The Effect of Stress Ratio on Fatigue Cracks Growth Rate in Aluminum Alloy

The financial and societal consequences of the sudden failure of various mechanical structures and components can occasionally be severe. As a result, there has been extensive research done on fatigue failure in both the scientific and industrial fields. The impact of stress ratio adjustments on the rates of fatigue fracture propagation for an aluminum alloy has been investigated in the current study. This aluminum alloy was extracted from a Russian MI25 helicopter's primary blade. The stress intensity factor was calculated and employed by creating an initial crack in the test specimens after a specimen of MI25 blade was cut into testing specimens with standard dimensions. A special program was written specifically for this purpose to draw a graph of the stress intensity factor range (k) with value of the propagation of fatigue cracks (da / dN) at each stress ratio (R); Propagation of fatigue cracks has been studied. Finally, the relationship between the two different stress ratios has been discovered. Between the two, a comparison has been established. The findings indicated that when the stress ratio rises, so too does the rate at which fatigue cracks form. Conversely, with a negative stress ratio, the negative increase leads to a reduction in the rate at which fatigue cracks grow. The data have been examined, and during the second stage of crack propagation, where (R = 0.0), pertinent equations that are compatible with the Paris equation have been discovered.

Residual stress effects on fatigue crack growth in different regions of aluminum alloy 7050 friction stir weld

AbstractDifferent distribution of residual stress in each microregion of the joint had an intensely significant influence on the fatigue crack growth. The friction stir welding (FSW) finite element model was developed. The residual stress was imported to the fracture mechanical model by the submodel technique. The residual Krs (stress intensity factor from residual stress) approaches were used to predict the fatigue crack growth rate (FCGR) in different regions of aluminum alloy 7050 friction stir welded joints. The FRANC3D in conjunction with ABAQUS, the M‐integral method was used to calculate Krs, considering the effect of Krs on the effective stress ratio, the Paris–Walker formula was used to predict the FCGR in different regions of the FSW joints. The predicting results and experimental values aligned with the better, demonstrating that the established model could accurately predict the FCGR in different regions of the FSW joints.

Experiments and Modeling of the Attenuation of the Dynamic Elastic Modulus of Saturated Red Clay under Cyclic Loading

Due to the long-term effect of cyclic traffic loads, the accumulated deformation of red clay subgrades is increasing, strength is decreasing, and problems such as uneven settlement are occurring. In order to improve the stability and durability of red clay subgrade, this paper analyzes the effects of cyclic stress ratios and compactness on the development of the dynamic elastic modulus of saturated red clay. This is achieved through dynamic triaxial tests on red clay in the Qingyuan County, Zhejiang Province, China. The results show that the saturated red clay has an attenuation threshold cyclic stress ratio under cyclic loading. The attenuation threshold cyclic stress ratio prediction models were constructed based on the confining pressure and compactness, respectively. By introducing the concept of a relative cyclic stress ratio, the prediction model of the dynamic elastic modulus of saturated red clay (as based on cyclic vibration times and close cyclic stress ratio) is constructed. The model realizes the prediction of the dynamic elastic modulus of red clay under arbitrary cyclic vibration times via the initial cyclic stress ratio, compactness, and other parameters. It provides a theoretical basis for rationally evaluating the dynamic stability of red clay subgrade under cyclic loading.

The role of biaxial stress ratio on the mechanical behavior and deformation mechanisms in HCP α-Ti

The mechanical behavior of hexagonal close-packed (HCP) Ti alloy is closely related to the stress state. Renewing the mechanical behavior and associated mechanisms of HCP α-Ti alloy under biaxial stress states is critical for their manufacturability and serviceability. In this work, biaxial tensile tests with different stress ratios closing to actual service conditions were conducted to investigate the mechanical behavior of commercially pure Ti (CP-Ti). Then, comprehensive microstructural characterizations were performed using electron back-scattering diffraction (EBSD) and transmission electron microscopy (TEM) techniques. The effects of biaxial stress ratio on the evolution of dislocation and deformation twinning throughout plastic deformation were clarified. It was found that the activity of prismatic slip was suppressed while pyramidal slip was promoted, which was attributed to the decrease of external stress component on the prismatic planes and the increase of that on the pyramidal planes with increasing biaxiality. Moreover, the increased stress component in the c-axis direction also encouraged the occurrence of extension twinning at high biaxiality. The decrease of prismatic slip with low critical resolved shear stress (CRSS) and the increase of twinning and pyramidal slip with high CRSS contributed to an increase in deformation resistance and work hardening rate with increasing biaxiality.

Fatigue Characteristics of 7050-T7451 Aluminum Alloy Friction Stir Welding Joints and the Stress Ratio Effect.

The fatigue crack initiation and growth characteristics in 7050-T7451 aluminum alloy butt joints subjected to different stress ratios and owing to friction stir welding (FSW) were investigated using fatigue tests for stress ratios of 0.1, 0.3, and 0.5. The difference between the fatigue crack initiation in the base material (BM) and FSW joints, related to coarse secondary phases, was explored using scanning electron microscopy (SEM). Accordingly, Al23CuFe4, Al7Cu2Fe, and Al2Mg3Zn3 were the preferred joint crack initiation locations, whereas Mg2Si was the major fracture initiation point of the parent material, and cracks tended to propagate along dense, coarse secondary phases, becoming more pronounced for larger cracks. In addition, as the stress ratio increased, non-Mg2Si phase fracture initiation points appeared in the BM. Meanwhile, the quantity of non-Mg2Si phases in the joints continued to increase, and the crack initiation sites became increasingly concentrated in the TMAZ-HAZ region.

Assessment of soil parameters during post-liquefaction reconsolidation of loose sand

Saturated deposits of loose sand may suffer liquefaction under earthquake shaking. Liquefaction-induced damage occurs during the earthquake loading and may continue during the post-liquefaction reconsolidation stage in which excess pore pressures gradually dissipate. An accurate evaluation of soil parameters is of great significance for the full depiction of this process. Consolidation-related problems are typically investigated using Terzaghi's one-dimensional consolidation theory, assuming a constant permeability, one-dimensional stiffness, and consolidation coefficient throughout this process. Some studies based on physical model tests suggest that an assumption of a single and constant value of soil parameter is not appropriate to study the reconsolidation of liquefied sand. This paper presents the results of three geotechnical centrifuge experiments to contribute to the current understanding of this phenomenon. The centrifuge data is combined with oedometer and fluidization test data, producing several empirical relationships to estimate the soil parameters related to the reconsolidation of liquefied sand. A simplified method is adopted to reproduce the time histories of excess pore pressure dissipation and ground surface settlement using the estimated soil parameters as the input. It is shown that the coefficient of consolidation and one-dimensional stiffness vary as a function of effective stress or excess pore pressure ratio, markedly increasing during the reconsolidation of liquefied sand. The predictions of the proposed methodology appear to match the experimental measurements reasonably well as long as appropriate soil parameters are used.

A micromechanical crack bridging model considering the slip-softening interface with the residual shear stress for SFRCC

Characterizing the bond properties of the steel fiber/matrix interface plays an important role in describing the crack bridging effect in steel fiber-reinforced cementitious composites (SFRCC). In this paper, a new interface law considering the residual shear stress is proposed to interpret the slip-softening effects of the steel fiber/matrix interface. Accordingly, a micromechanical crack bridging model for SFRCC is formulated by combining the single steel fiber pullout model with the image analysis-based fiber orientation distribution. Comparisons with the experimental data and the existing models show that the proposed model can well predict SFRCC’s tensile softening behavior. Meanwhile, the decrease rate of the bridging stress, the composite fracture energy, the peak bridging stress and the corresponding peak crack opening displacement (COD) are employed to quantitatively evaluate the crack bridging curves. Finally, the effects of the residual shear stress ratio, the interfacial softening coefficient and the fiber orientation distribution on the crack bridging relation are discussed with the proposed framework.

High cycle and very high cycle fatigue of TC17 titanium alloy: Stress ratio effect and fatigue strength modeling

The fracture surface of TC17 alloy tends to exhibit facet characteristic at R > 0, but it presents rough area morphology at R = −1 for the internal crack initiation and early growth region. The former is attributed to basal slip or cleavage of α grains. The later is due to the cracks caused by the nanograin formation and the cracks formed irrespective of nanograins. The present TC17 alloy and various alloys in literature indicate that Walker’s equation [(1-R)/2]a is more accurate than Goodman’s equation and Smith-Watson-Topper’s equation [(1-R)/2]1/2 for the effect of stress ratio on high cycle and very high cycle fatigue strength.

Effects of frequency and CSR on dynamic properties of marine clay in drained condition

This paper presents an experimental investigation of the effects of frequency and cyclic stress ratio (CSR) on soil dynamic properties in terms of three parameters (k, ε p, and S) obtained from hysteresis loops under cyclic loading in drained conditions. A series of dynamic loading tests were carried out on Lianyungang marine clay samples with a sinusoidal wave at various values of frequency and CSR. Results showed that both frequency and CSR had significant effects on the three parameters. A rapid decrease in dynamic stiffness (related to k) and then a slow decrease with the increasing number of cycles characterized the overall dynamic response of soil under dynamic loading with various frequencies and CSRs, while the residual plastic strain (related to ε p) and the energy dissipation capacity (related to S) increased rapidly at a relatively lower number of cycles. The higher the frequency, the greater is the k at a given time. The higher the CSR, the lower is the k at a certain number of cycles. The higher the frequency, the lower is the ε p or S at a given time. The higher the CSR, the greater is the ε p or S at a certain number of cycles.

Stress ratio effect on crack growth behavior of dissimilar friction stir welded butt joint between AA6061 and AA7075 alloys

Many engineering components are subjected to random nature of loading during their life period. In this work, the fatigue crack growth in friction stir welded dissimilar AA6061 and AA7075 was investigated under different stress ratio. AA6061 was kept on the advancing side due to having a higher solidus temperature and softer than AA7075 for smooth material flow. The microstructure characterization, tensile and crack growth behavior was studied. It was observed that the joint material exhibited lower strength and ductility as compared to the base material. About 41%, 34%, and 66% reduction in tensile strength, yield strength, and percentage of elongation as compared to AA6061 (base material) were observed. About 27% higher hardness in the HAZ of retreating side than the advancing side was noticed. The crack deflection and tortuosity features were observed in the fatigue fractured specimens. In this paper, a comparative study using different crack growth models has been studied and a simple crack growth rate equation has been proposed considering the crack closure effect.

Effect of stress ratio on high cycle fatigue behavior of TNM‐TiAl alloy

AbstractIn order to investigate the effect of stress ratio on room temperature high‐cycle fatigue life of Ti‐43.5Al‐4Nb‐1Mo‐0.1B alloy, high‐cycle fatigue life test of TNM‐TiAl alloy at stress ratio of −1, −0.5, 0.1, 0.5, 0.8, and 1 was performed on the universal testing machine. The fracture mechanism of TNM‐TiAl alloy under different stress ratios was determined by observation and analysis of microstructure and fracture surface by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), and optical microscope (OM). The results show that fatigue life of TNM‐TiAl alloy with different stress ratios is mainly affected by the mean stress and stress amplitude. The fatigue life decreases with the increase of the stress amplitude and with the decrease of the mean stress. When the stress amplitude is constant, the higher the stress ratio is, the higher the mean stress in the loading process is, the lower the fatigue life is. The sensitivity of the stress amplitude of TNM‐TiAl alloy gradually increases with the increase of the stress ratio. The different cracking mechanism and fracture mechanisms at various stress ratio are revealed.

Effect of Stress Ratio and Evaluation of Crack Sizes on Very-High-Cycle-Fatigue Crack Propagation Life Prediction of Carburized Cr-Ni Steel

Carburized Cr-Ni steel is widely used in the manufacture of components in many fields due to excellent performance, of which the service life has been a concern. In order to investigate the high-cycle-fatigue and very-high-cycle-fatigue properties of carburized Cr-Ni gear steel, axial loading fatigue tests were performed by QBG-100 with stress ratios of −1, 0 and 0.3. The Generalized Pareto distribution was used to evaluate the inclusion size of carburized Cr-Ni gear steel. Based on the stress ratio and the evaluated crack size, a new fatigue life prediction model for carburized Cr-Ni gear steels was constructed. The results show that the S–N characteristics of carburized Cr-Ni gear steel represent the continuously descending tendency. Based on the long crack propagation threshold and the instability propagation threshold of carburized Cr-Ni gear steel, the sizes of FGA, fisheye and surface smooth area (SSA) can be evaluated, respectively. In addition, the maximum size of surface and interior inclusion of carburized Cr-Ni gear steel are 17.50 μm and 6.46 μm with a cumulative probability of 99.9%. By validating the new established model, the prediction result is acceptable according to the good consistency between the predicted life and the experimental life.

Effects of specimen size and stress ratio on fatigue crack growth after a single tensile overload

This paper investigates the effect of specimen size, stress ratio, and loading pattern on fatigue crack growth for centrally cracked low carbon steel specimens. Elastic-plastic calculations were conducted under cyclic loading based on FEM using the domain integral method. The computed fatigue lives were firstly verified against the experimental tests taken from the literature. The numerical calculations showed that thickness has no significant effect on crack growth at a given stress ratio under the constant amplitude loading pattern. While the thinner specimens revealed longer fatigue lives compared to the thicker specimens at a certain stress ratio due to the application of a single tensile overload. A remarkable reduction in fatigue lives with a clear acceleration in crack growth rates was noticed for specimens with a small ligament length-to-specimen width ratio. The retardation response due to the application of a single tensile overload depended on a particular combination of specimen size and stress ratio. The variation in the crack driving force due to the change in thickness and ligament length was investigated. The relation between the crack driving force and the local stress state ahead of the crack tip was also discussed.

Influence of temperature on the R‐effect for the fatigue crack growth of nickel‐base superalloy GH4169

AbstractThe stress ratio (R) effect, especially at elevated temperatures, is associated with the fatigue crack growth (FCG) prediction in aero‐engine hot‐end components under complex operation conditions. The FCG experiments with three R were conducted on Ni‐based superalloy GH4169 at room temperature (RT), 550°C, and 700°C. The results indicate that the R‐effect on the FCG of GH4196 is temperature‐dependent. Therefore, efforts were made to identify the R‐effect at various temperatures to describe the FCG. The concept of the crack‐closure or the two‐driving‐force was employed to quantify the R‐effect considering the temperature influence. Fractographic analyses on the fracture surface were performed to discuss the underlying mechanism responsible for the temperature influence. The study can contribute to the R‐dependent FCG modeling at various temperatures.

Characterising the stress ratio effect for fatigue crack propagation parameters of SAE 1045 steel based on magnetic flux leakage

The aim of this study was to determine the relationship between fatigue crack length, a, the number of cycles, N, the magnetic flux gradient intensity, dH/dx and the stress ratio, R. Fatigue crack growth tests were performed using a constant tensile load amplitude on SAE 1045 steel in the form of single-edge cracks. Stress ratio values of 0, 0.1, 0.2, 0.3 and 0.4 were used to study the characteristics of the magnetic flux gradient. A crack opening displacement device was used to detect the parameters of the stress intensity factor range, ΔK, fatigue crack growth rate, da/dN, and N. Meanwhile, a metal magnetic memory sensor device was used to detect the H value at the stress concentration zone when a 1 mm-increase in the crack occurred. The experimental results showed that as the fatigue cycle and crack length increased for each R, dH/dx also increased exponentially. The governing equations were derived based on the magnetic flux gradient for the fatigue crack length equation and the fatigue cycle equation, with the stress ratio. Based on a statistical analysis of the magnetic flux leakage signals, it was found that all the data were within a range based on a 90 % confidence level. Thus, the magnetic flux leakage parameters could be used to construct fatigue crack growth behaviour models for ferromagnetic materials and potentially as an alternative method for predicting the fatigue life.

Compression-shear performance and failure criteria of seawater sea-sand engineered cementitious composites with polyethylene fibers

• Tests on the compression-shear performance of seawater sea-sand ECC were performed. • Seawater sea-sand ECC under the combined loading shows ductility failure mode and high shear strength. • Failure law is reflected by the criterion based on octahedral stress or stress invariant. • The failure criterion of seawater sea-sand ECC could be applied in future structural analysis. Seawater sea-sand engineered cementitious composites (SS-ECC) with polyethylene (PE) fibers holds wide application prospects. However, the understanding on its mechanical behaviors and failure criteria under combined compression-shear loading, which is essential for structural application, is lacking. The object of this research is to obtain the mechanical performance and establish the corresponding failure criteria of SS-ECC under combined loading. The influences of fiber content and normal stress ratio on failure modes, shear strength, peak shear displacement and shear strength components were analyzed. SS-ECC under combined loading exhibited ductile failure mode. Shear strength increased significantly with the increasing normal stress ratio, while increased slightly with higher fiber content. The effects of fiber content and normal stress ratio on the combined compression-shear behaviors were clarified. Further, the failure criteria based on octahedral stress and stress invariant were proposed for describing the failure law of SS-ECC under combined loading. This research lays the groundwork for understanding the mechanical performance of SS-ECC under combined compression-shear loading and preparing its application in structural members.

NIMS fatigue data sheet on gigacycle fatigue properties of A6061-T6 (Al-1.0Mg-0.6Si) aluminium alloy at high stress ratios

he new fatigue data sheet, No. 132, discloses gigacycle fatigue properties of the A6061-T6 aluminium alloys at high stress ratios. The fatigue tests were conducted mainly by the ultrasonic fatigue testing at 20 kHz, while conventional fatigue tests at 100 Hz were also conducted for comparison. The fatigue test results indicated that fatigue limits were obscure in the A6061-T6 alloys. Many specimens failed at over 107 cycles, developing internal fractures as well as surface fractures. On the other hand, fatigue failures at over 109 cycles were very rare, suggesting the presence of new fatigue limits in the gigacycle region. The differences in fatigue test results were negligible between the 20 kHz and 100 Hz tests, demonstrating that the 20 kHz tests were comparable to the conventional fatigue tests on this material. The fatigue strengths evaluated by the stress amplitude were decreased according to increase in the stress ratios, while the degradation of the fatigue strengths was not so serious. The fatigue strengths were higher than the modified Goodman lines, meaning that the stress ratio effects could be estimated by conventional ways.

Modelling dynamic failure of geometrical-similar concrete subjected to tension-compression loads: Effect of strain rate and lateral stress ratio

Concrete materials are widely acceptable to practical structural buildings exposed to not only uniaxial loads but also complicated stress states. Additionally, concrete structures under quasi-static loads may suffer from inevitable occasional dynamic loads. In this study, mesoscopic modelling on geometrical-similar concrete was conducted. Numerical experiments were performed under dynamic biaxial Tension-Compression loads with different strain rates (ranging from 10−5 s−1 to 1 s−1) and lateral stress ratios (ranging from −1.00 to 0). The effect of lateral stress ratio and strain rate on the mechanical damage behavior and size effect of geometrical-similar concrete was discussed. The numerical results indicate that the size effect on dynamic spindle compressive strength and dynamic lateral tensile strength are weakened with the increasing strain rate. However, the increasing lateral stress ratios bring different impacts on the size effect, namely enhancing the size effect on dynamic spindle compressive strength and weakening the size effect on dynamic lateral tensile strength. Moreover, considering the coupling effects between lateral stress ratio and strain rate, a dynamic Tension-Compression failure criterion for concrete was proposed. Finally, a static-dynamic universal size effect formula on the lateral tensile strength of concrete under dynamic Tension-Compression loading was established and verified.