High-Low Loading Sequences Research Articles

Ratcheting Examination of 1045 Notched Steel Plates under Low-High and High-Low Sequences

Abstract The present study examines the ratcheting response of 1045 notched steel plates undergoing uniaxial step-loading cycles. Loading steps were applied with different Low-High and High-Low sequences. Ratcheting tests were conducted on notched samples, and average local strains were measured with the use of strain gauges mounted in the vicinity of notch roots. Measured ratcheting strains near notch roots were found as high as 10 times those of tests conducted on unnotched samples. The magnitude of applied cyclic load as well as notch diameter influenced local ratcheting strain at the vicinity of the notch root. The lower nominal load level over the first step in the Low-High sequence substantially promoted ratcheting magnitude over subsequent loading steps with higher load magnitudes. In the High-Low loading sequence, the previous load step with higher magnitude retarded the local accumulated plastic strain of the subsequent step with a lower load level. The plastic zone at the notch root was affected over the loading steps. Microhardness tests were conducted at several positions within a small area of 1.5 by 4 mm near the notch root to map the plastic zone and its progress after each loading step. Microhardness data collected at the notch region verified an enlarged plastic zone over the Low-High loading sequence, while the plastic zone for the High-Low loading sequence was estimated about 50 % smaller in size. Cracking at the notch root and fracture surface taken within the plastic zone were examined through optical and scanning electron microscopies.

Uniaxial Ratcheting Assessment of 304 Stainless Steel Samples Undergoing Step-Loading Conditions at Room and Elevated Temperatures

AbstractThe present study evaluates ratcheting response of 304 stainless steel samples subjected to various step-loading conditions at room and elevated temperatures using the kinematic hardening rules of Ohno–Wang (O–W), AbdelKarim–Ohno (AK–O), and Ahmadzadeh–Varvani (A–V). The hardening rules were employed along with the visco-plastic flow rule to account for the time-dependent response of 304 stainless steel samples. Ratcheting over low–high–low loading sequences consistently showed a small drop in ratcheting strain over the third loading step. This is mainly due to plastic strain accumulation over the first two loading steps preventing ratcheting strain to drop significantly with a drop in the mean stress. Moreover, dynamic recovery terms in these models were further modified through the inclusion of an exponential function developed by Kang to address the dynamic strain aging phenomenon. Low ratcheting rate and shakedown shortly after a few stress cycles within loading steps as operating temperatures varied between 400 and 600 °C were attributed to dynamic strain aging phenomenon in SS304 steel alloy. Progressive ratcheting response and their stress–strain hysteresis loops were highly influenced at various operating temperatures, stress levels, and stress rates. Coefficients in the dynamic recovery term of the A–V model controlled ratcheting progress and hysteresis loops agreeable with those of experimental data over consecutive loading steps. Choices of material constants and the number of segments defined from stress–strain curve based on the O–W and AK–O models noticeably influenced the ratcheting response of steel samples. Predicted ratcheting values by means of the A–V, O–W, and AK–O models were discussed and compared with those of the experimental data.

Viscoplastic Ratcheting Response of Materials under Step-Loading Conditions at Various Cyclic Stress Levels

The present study intends to assess ratcheting response of rate-dependent SS316L, SS316L(N), and 63Sn37Pb alloys under various step-loading conditions at room temperature through use of the Ahmadzadeh-Varvani (A-V) model developed based on a viscoplastic flow rule. The choice of viscoplastic description enabled the model to control plastic strain increments over loading progress at various stress rates. Predicted ratcheting over asymmetric loading cycles and multi-step cycles was influenced by cyclic stress levels, stress rates, and sequence of loading steps. Predicted ratcheting in steel samples SS316L and SS316L(N) over the second loading step in Low-High and High-Low loading sequences was primarily influenced with the stress amplitude in the second step. Multi-step-loading sequences High-Low-High and Low-High-Low in solder samples revealed how noticeable the impact of mean stress on ratcheting is. Predicted ratcheting values by means of the A-V model closely agreed with experimental data of alloys examined in this study.

Influence of Nanoreinforced Particles (Al2O3) on Fatigue Life and Strength of Aluminium Based Metal Matrix Composite

Abstract
 In this investigation, Al2O3 nano material of 50nm particles size were added to the 6061 Al aluminium alloy by using the stir casting technique to fabricate the nanocomposite of 10wt% Al2O3. The experimental results observed that the addition of 10wt% Al2O3 improved the fatigue life and strength of constant and cumulative fatigue. Comparison between the S-N curves behaviour of metal matrix (AA6061) and the nanocomposite 10wt% Al2O3 has been made. The comparison revealed that 12.8% enhancement in fatigue strength at 107cycles due to 10wt% nano reinforcement. Also cumulative fatigue life of 10wt% nanocomposite was found to be increased by 33.37% and 39.58% for low-high and high-low loading sequences, respectively, compared to the metal-matrix cumulative life.
 Keywords: Al2O3 nanoparticles, AA6061/10wt%, constant and cumulative fatigue, MMCs.

Ratcheting prediction of Al 6061/SiCP composite samples under asymmetric stress cycles by means of the Ahmadzadeh–Varvani hardening rule

The present study intends to examine ratcheting response of SiCP particle-reinforced Al 6061 matrix (Al 6061/SiCP) composite samples over asymmetric load cycles based on the kinematic hardening rule of Ahmadzadeh–Varvani. The Ahmadzadeh–Varvani hardening rule offered a simple framework by taking into account of both material and stress level dependent coefficients to predict the quasi-shakedown of ratcheting of composite samples with various volume fractions under single- and multi-step loading conditions. The coefficients in the Ahmadzadeh–Varvani rule were estimated by means of mathematical expressions involving material properties, mean stress, and stress amplitude for any given stress levels. Ratcheting strain progressively increased as composite materials experienced low–high loading sequences when stress level increased over steps of loading histories. Ratcheting strain curves of low-high and high-low loading histories were successfully predicted. The predicted ratcheting curves with high–low loading sequences have shown change in ratcheting direction consistent with the experimental data.

Loading history effect on fatigue crack growth of extruded AZ31B magnesium alloy

An experimental investigation was conducted on the fatigue crack growth (FCG) behavior of an extruded AZ31B magnesium (Mg) alloy in ambient air. Compact tension (CT) specimens oriented in three different directions with respect to the extrusion direction were employed in the study. The influences of overloading and two-step high-low sequence loading on FCG were investigated in detail. Single tensile overloads with three overload ratios (1.5, 1.75, and 2.0) were applied during otherwise constant amplitude loading. A single overload retarded the crack growth rate in all the three specimen orientations. The crack growth rate decreased immediately to a minimum value right after the application of a tensile overload and increased quickly to a stable value expected at constant amplitude loading. No visible torn fracture was observed at the crack tip right after overloading but the crack tip was clearly blunted. Results from the two-step high-low sequence loading reveal that FCG retardation occurred at the beginning of the lower amplitude step when the maximum load was lowered in the second loading step, which is similar to the case of a single overload. Walker’s model can correlate well the crack growth experiments with different R-ratios. Wheeler’s model can reasonably predict the influence of overload and high-low loading sequence on the FCG in the AZ31B Mg alloy.

An experimental investigation of fatigue crack growth of stainless steel 304L

A series of fatigue crack growth experiments were conducted using round compact tension specimens of AISI 304L stainless steel under Mode I loading. The influences of the R-ratio (the ratio of the minimum load to the maximum applied load in a cycle), notch size, the tensile and compressive overloads, and the loading sequence on crack growth were studied. The results show that the material displays sensitivity to the R-ratio. The application of a tensile overload results in a short period of acceleration in the crack growth rate followed by a significant retardation in the crack growth rate. A compressive overload (underload) produces a short period of acceleration in crack growth and the magnitude of such an acceleration depends on the value of the loading amplitude of the constant-amplitude loading. Results from the two-step high-low loading sequence reveal a period of crack growth retardation at the beginning of the lower amplitude step, an effect similar to that of a single overload. Two existing crack growth models which are based on the stress intensity factor concept are evaluated using the experimental results. A two-parameter crack driving force approach together with a modified Wheeler’s model is found to correlate well the crack growth experiments.

A cyclic plasticity model for single crystals

The Armstrong–Frederick type kinematic hardening rule was invoked to capture the Bauschinger effect of the cyclic plastic deformation of a single crystal. The yield criterion and flow rule were built on individual slip systems. Material memory was introduced to describe strain range dependent cyclic hardening. The experimental results of copper single crystals were used to evaluate the cyclic plasticity model. It was found that the model was able to accurately describe the cyclic plastic deformation and properly reflect the dislocation substructure evolution. The well-known three distinctive regimes in the cyclic stress–strain curve of the copper single crystals oriented for single slip can be reproduced by using the model. The model can predict the enhanced hardening for crystals oriented for multislip, showing the model's ability to describe anisotropic cyclic plasticity. For a given loading history, the model was able to capture not only the saturated stress–strain response but also the detailed transient stress–strain evolution. The model was used to predict the cyclic plasticity under a high–low loading sequence. Both the stress–strain responses and the microstructural evolution can be appropriately described through the slip system activation.

On the description of transient cyclic hardening behaviour of mild steel CK 15

A series of strain-controlled experiments have been performed on thin-walled tubes made of mild steel CK 15. The experimental program includes uniaxial standard tests such as cyclic straining between fixed strain limits until cyclic stress amplitude saturation, as well as non-standard tests that include prestrain and cyclic mean stress relaxation. In addition, a tension-torsion test program has been designed. Nonradial strain paths of circular, rectangular, and triangular patterns have been examined, and significant extra hardening compared to the uniaxial case occurs. Specifically, the triangular pattern causes peak stresses exceeding the equivalent uniaxial stress amplitude by more than 50%. Furthermore, the v. Mises kinematic and isotropic parts of hardening are quantified by a reliable procedure involving nonlinear regression with the help of which the yield point is determined. On the basis of the experimental knowledge, a plasticity model has been developed that contains a new feature. This is represented by an isotropic memory surface that either expands or shrinks during plastic loading and thus allows for simulating hardening and softening due to high-low loading sequences.

Characterization of small-crack growth in 12% CrMoV steel under high-temperature, low-cycle fatigue conditions: Earthman, J.C. Mater. Sci. Eng. A Feb. 1991 A132, 89–95

A series of fatigue crack growth experiments were conducted using round compact tension specimens of AISI 304L stainless steel under Mode I loading. The influences of the R-ratio (the ratio of the minimum load to the maximum applied load in a cycle), notch size, the tensile and compressive overloads, and the loading sequence on crack growth were studied. The results show that the material displays sensitivity to the R-ratio. The application of a tensile overload results in a short period of acceleration in the crack growth rate followed by a significant retardation in the crack growth rate. A compressive overload (underload) produces a short period of acceleration in crack growth and the magnitude of such an acceleration depends on the value of the loading amplitude of the constant-amplitude loading. Results from the two-step high-low loading sequence reveal a period of crack growth retardation at the beginning of the lower amplitude step, an effect similar to that of a single overload. Two existing crack growth models which are based on the stress intensity factor concept are evaluated using the experimental results. A two-parameter crack driving force approach together with a modified Wheeler’s model is found to correlate well the crack growth experiments.

Modelling of fatigue crack growth following overloads

In variable-amplitude loading there are interaction effects between the loading history and the crack propagation rate. The most important of these effects is the retardation in the crack propagation, which may raise the life of the cracked structureconsiderably. The main objective of this research is to analyse and quantify the retardation of crack propagation in a thin plate of the high-resistance aluminium alloy 2024-T3, comparing the results obtained from the mathematical models proposed to account for the retardation effect. The specimens were tested under high-low loading sequences, for different crack sizes and overload ratios. A simplified model was developed, based on crack closure theory, that could represent the crack behaviour during retardation very well.

A mathematical model of fatigue crack propagation under variable amplitude loading

A mathematical model of fatigue crack propagation based on the crack closure concept is proposed. It allows prediction of fatigue crack growth under complex loading sequences on the basis of data obtained under constant amplitude and simple Low-High, High-Low loading sequences. The model explains the influence of single and multiple positive overloads and the interaction of positive and negative overloads. An algorithm for cycle-by-cycle calculation of crack growth is proposed.