Prior Cyclic Loading Research Articles

Fatigue-life prediction of SiC aluminum composite using a Weibull model

The feasibility of the acoustic emission technique in predicting the residual fatigue life of 6061-T6 aluminum matrix composite reinforced with 15 vol.% SiC particulates (SiC p) is presented. Fatigue damages corresponding to 40, 60 and 80% of total fatigue life were induced at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. The acoustic emission activities were monitored during tensile tests. The number of cumulative AE events increased exponentially with the increase in strain during tensile tests. This exponential increase occurred when the material was in the plastic regime and was attributed mainly to SiC particulate/matrix interface decohesion. Cumulative events during post fatigue tensile tests reduced with a decrease in the residual fatigue life. Based on the high cycle fatigue damage accumulation model, a Weibull probability distribution model is developed to explain the post fatigue AE activity of specimens during tensile tests. Using the model, the residual fatigue life can be predicted by testing the specimen in tension and monitoring the AE events. In high cycle fatigue, it was observed that the residual tensile strengths of the material did not change significantly with prior cyclic loading damages since the high cycle fatigue life was dominated by the crack initiation phase.

Fatigue-life prediction of SiC particulate reinforced aluminum alloy 6061 matrix composite using AE stress delay concept

A study of the residual fatigue life prediction of 6061-T6 aluminum matrix composite reinforced with 15 vol % SiC particulates (SiCp) by using the acoustic emission technique and the stress delay concept has been carried out. Fatigue damages corresponding to 40, 60 and 80% of total fatigue life were stimulated at a cyclic stress amplitude. The specimens with and without fatigue damage were subjected to tensile tests. The acoustic emission activities were monitored during tensile tests. It was found that a lower stress level was required to reach a specified number of cumulative AE events for specimens fatigued to higher percentage of the fatigue life. This stress level is called stress delay. Approximately a linear relation was found between stress delay and fatigue damage. Using the procedure defined in this study, the residual fatigue life can be predicted by testing the specimen in tension and monitoring the AE events. The number of the cumulative AE events increased exponentially with the increase of strain during tensile tests. This exponential increase occurred when the material was in the plastic regime and was attributed mainly to SiC particulate/matrix interface decohesion and linkage of voids. In high cycle fatigue, it was observed that the residual tensile strengths of the material did not change with prior cyclic loading damages since the high cycle fatigue life was dominated by the crack initiation phase.

Assessment of cyclic properties of 18G2A low-alloy steel at biaxial stress state

An evolution of plastic properties of the 18G2A low-alloy steel due to cyclic predeformation in different directions of the two-dimensional stress space (σ xx ,τ xy ) is assessed on the basis of cyclic curves. The initial material properties have been experimentally evaluated by the analysis of preliminary yield surface. It was made by studying the position in stress space and typical dimensions of the yield surface. The initial yield locus has been determined using a number of specimens which were loaded up to the plastic range along different stress directions. This surface was used as the starting point for comparative studies of plastic properties evolution due to the cyclic prestraining. Cyclic predeformations were induced by monotonic loading at ambient temperature under constant and gradually decreasing amplitude. It is shown that prior cyclic loading induced a softening effect observed during subsequent monotonic loading of the steel. A new concept of the assessment of plastic properties of the cyclically prestrained material is proposed. It deals with the “cyclic yield surface” determination.

Effect of cyclic loading on the yield surface evolution of 18G2A low-alloy steel

Experimental analysis is presented of the plastic properties of 18G2A steel (notation according to Polish Standards) in the as-received state, and of the same material subjected to cyclic predeformation in different directions of the two-dimensional stress space ( σ xx, τ xy ). The analysis was made by studying the position in stress space and by determination of typical dimensions of the yield surfaces. The initial yield surface has been determined using a number of specimens which were loaded up to the plastic range along different stress directions, and this surface was used as the starting point for comparative studies of yield surfaces of the cyclic prestrained material. Cyclic predeformations were induced by loading at ambient temperature. After predeformation, yield surfaces were determined by the technique of sequential probes of the single specimen. The anisotropic yield condition due to Szczepiński was shown to model the experimental results well. Prior cyclic loading induced the softening effect observed during subsequent monotonic loading of the steel in the plastic strain range considered.

Conditioning monitoring by microstructural evaluation of cumulative fatigue damage

The objective of this work is to evaluate the damage induced below and above the fatigue limit (Δσt=360 MPa) in pressure vessel steels, such as SA508. Fatigue damage was induced in samples taken from an SA508 steel plate by various loading histories in order to examine the influence of prior cyclic loading below the fatigue limit. Cell-to-cell misorientation differences were measured by the selected area diffraction (SAD) method. Surface cracking was also studied by the replication method. Small cracks were observed after precycling both below and above the fatigue limit. It was, however, found that fatigue test bars had a longer lifetime after precycling below the fatigue limit, while precycling above the fatigue limit caused other specimens to fail even when subsequently cycled below the fatigue limit. Cell-to-cell misorientation usually increases with accumulation of fatigue damage, but it was found that the misorientations measured after precycling below the fatigue limit decreased again at the beginning of the subsequent cycling above the fatigue limit. It should be noted that the misorientation at failure was always about 4 to 5 deg, regardless of loading histories. Misorientation showed good correlation with the fatigue lifetime of the samples.

Strength Changes in Sand Following Cyclic Shear

Abstract Static torsional simple shear tests were performed on poorly graded sand, with and without prior cyclic loading, to evaluate changes in static shear strength. Specimens were prepared at three densities (loose, medium, and dense) and were subjected to one of three levels of cyclic loading corresponding to 0.25, 0.50, or 0.75 times the static stress level, causing failure with no prior cyclic loading. The test results indicate that post-cyclic shear strength increases in specimens that are contractive and decreases in specimens that are dilative during cyclic loading. Results showing the change in friction angle as a function of initial density and level of prior cyclic loading are presented. The observed behavior is consistent with a model that predicts strength changes following cyclic loading as a function of soil volume change characteristics and drainage conditions.