Applied Strain Level Research Articles

Longitudinal Fatigue Response of a Metal Matrix Composite Under Strain Controlled Mode at Elevated Temperature

The fatigue characteristics of a unidirectional titanium-based metal matrix composite (MMC) (SCS-6/Ti-15-3) were investigated at elevated temperature (427°C). A hybrid strain controlled loading mode was employed for this purpose. This hybrid control mode did not allow the specimen to experience compressive stress in order to prevent any possible buckling effects. To fully understand the fatigue behavior of the MMC under this control mode, fatigue tests, microscopic evaluation, and micromechanical analysis were performed. Based on this combination of activities, the damage and deformation mechanisms were systematically identified. It was found that the fatigue behavior was initially dominated by creep deformation of the matrix. This was accompanied by plastic deformation in those specimens that were subjected to a maximum strain level of 0.55% or higher. Depending on the maximum strain level, the specimen failure was a result of either fiber fracture or matrix cracking. Using the combined approach involving experiments, microscopy, and analysis, the interrelationships among applied strain levels, fatigue life, damage mechanisms, and macroscopic response were established to characterize the longitudinal fatigue characteristics of the tested MMC.

Bending and torsional fatigue of nylon 66 monofilaments

AbstractUsing newly developed test equipment, the fatigue behavior of nylon 66 monofilaments was studied under two loading conditions, pure bending or simple torsion. For each mode, results are expressed in terms of the measured decay in stiffness with numbers of cycles over a range of maximum applied strain levels. Fatigue lifetimes are presented in S–N format where the log number of cycles of fatigue for a 40% decay in stiffness (N) is plotted as a function of applied strain (S). The failure mechanism for these fibers in each fatigue mode reflects the morphology of semicrystalline‐oriented synthetic fibers. In torsion, many longitudinal cracks form around the perimeter of a fiber as the result of cleavage of the relatively weak interfibrillar bonds in nylon 66. In bending, cracks form within kink band boundaries and grow at an oblique angle to the fiber axis. © 1992 John Wiley & Sons, Inc.

Approximate techniques of J estimation applicable to part-through surface cracks

This investigation concerns the accuracy with which three different J estimation schemes can predict the variation of applied Jwith applied strain for semi-elliptical surface cracks. The three estimation schemes considered were a weight function technique proposed by Bhandari, a modification of the EPRI estimation scheme proposed by Ainsworth, and Turner's Engineering- J design curve. Each of these techniques is simple enough to employ in design, the most arduous calculation required being that needed to determine the linear elastic stress intensity factor. Estimation scheme accuracy was assessed by comparing J predictions to finite-element results for three small semi-elliptical surface cracks in a moderately hardening steel loaded in either pure tension or pure bending. The results obtained indicate that, for applied strains up to three times the yield strain, both the weight function and the modified EPRI schemes under estimate applied J by between 23 and 83% depending on the applied strain level. Conversely, Turner's Engineering- ssJ design curve provides accurate or conservative (i.e. over) estimates of applied J in both tension and bending provided total crack size is less than 3% of the total cross sectional area and maximum crack depth is less than 25% of the plate thickness. Application to larger and deeper cracks loaded in tension is not recommended as the design curve does not conservatively account for net section yielding in these situations. The design curve can still be applied in bending to cracks of size up to 7.4% of the total cross sectional area. However, the degree of conservatism inherent in this application may be considered excessive in certain situations.

INFLUENCE OF MICROSTRUCTURE ON SHORT FATIGUE CRACK GROWTH

AbstractThe influence of microstructure and processing method on the initiation and growth of fatigue cracks in an aluminum‐magnesium‐silicon alloy has been studied with extruded and squeeze‐cast material cycled in strain control. Using the replication technique, initiation and early growth of short cracks was found to be dependent upon the applied strain level. At low strains decohesion at the particle/matrix interface occurred whereas at high strain levels both particle cracking and decohesion were observed. The micromechanisms of subsequent short crack growth were also influenced by strain amplitude. In the case of the squeeze‐cast material cycled at low total strain amplitudes subsequent growth took place from particle to particle along the soft precipitate‐free interdendritic regions, in the direction of maximum shear. Notable deflection occurred at triple points. Short cracks in the extruded material cycled at all total strain amplitudes were found to initiate by debonding of particles in banded zones. Early crack growth at low strain amplitudes over the first 20μ along the surface was significantly impeded by subgrains and some second phase particles, whose effect was much greater than for the squeeze‐cast material due to a finer microstructure (the average subgrain size was 6 μ) and absence of soft interdendritic regions. The fracture surface was also rougher which presented a greater degree of resistance to propagation. In both types of material, Stage I crack growth extended to a depth of about 250‐350 μ. During this portion of life short fatigue crack growth behaviour was observed. Long crack behaviour coincided with the crack switching to Stage II growth. Short crack propagation behaviour and the crack depth at which Stage I was complete could be expressed using a crack growth model based on the strength of slip bands emanating from the crack tip and the strength of barriers which were in the immediate area.

Bending and Torsional Fatigue of Nylon 66 Monofilaments

AbstractUsing newly developed test equipment, the fatigue behavior of nylon 66 monofilaments was studied under two loading conditions, pure bending or simple torsion. In each case, the deformation was applied in balanced cycles of positive and negative strains. For each mode, results are expressed in terms of the measured decay in stiffness with numbers of cycles over a range of maximum applied strain levels. Fatigue lifetimes are presented in S-N format where the log number of cycles of fatigue for a 40% decay in stiffness (N) is plotted as a function of applied strain (S). The failure mechanism for these fibers in each fatigue mode reflects the morphology of semicrystalline oriented synthetic fibers. In torsion, many longitudinal cracks formed around the perimeter of the fiber as the result of cleavage of the relatively weak interfibrillar bonds in the nylon 66 fibers. In bending, cracks grew across the fiber at an oblique angle to the fiber axis along kink band boundaries.

On the Mechanics of Microcracking in Fiber Composite Laminates Under Combined Stress

The formation of matrix microcracks in fiber composite laminates has been widely observed. The present paper addresses the mechanics of this microcracking by means of a shear lag model in conjunction with a Weibull failure criterion for further cracking. This shear lag model considers both shear loading as well as cross-fiber tension in the cracked ply. The reduction in inplane shear and tensile modulus of the cracked ply is computed directly from the model. The increase in applied load required to produce new cracks is computed and shows general agreement with previous experimental results. A general approximate relationship between the applied strain levels and the modulus reduction is presented.

CYCLIC MECHANICAL BEHAVIOR AND MICROSTRUCTURE OF A 12Cr‐Mo‐V MARTENSITIC STAINLESS STEEL

Abstract— The low cycle fatigue properties of a 12Cr‐IMo‐V martensitic stainless steel are investigated at room temperature and correlated with its microstructure. The mechanical test results show that the behavior of this steel depends on the applied strain level. The SEM and TEM studies bring out the role of ferrite islands embedded in a tempered martensite lath matrix. Plastic deformation is essentially accommodated by ferrite; this results in extrusion formation on the surface specimen and a cellular dislocation structure. When the strain level is high, it requires a greater contribution of the tempered martensite, ferrite alone being unable to accomplish all the plastic deformation.

Influence of slow strain rate tensile deformation on creep–fatigue endurance of 20Cr–25Ni–Nb stainless steel at 593°C

Tests have been conducted on a 20Cr–25Ni–Nb stabilized stainless steel at 593°C during which a creep component was introduced into the tensile half of a fully plastic fatigue cycle. This was achieved either by the introduction of hold period at the maximum tensile strain of a rapid fatigue cycle or by the reversal of slow strain rate tensile deformation by rapid compressive straining. These cycles result in a significant reduction in fatigue endurance from that obtained during rapid fatigue cycling tests. The present results indicate that the most damaging cycle is one in which slow strain rate tensile deformation is reversed by rapid compressive straining. Here an order of magnitude reduction in life from that in cycles with equal rates of tensile and compressive straining was observed over a wide range of applied strain levels. Detailed metallographic examination of failed specimens has indicated that these life reductions occur because failure takes place by the interaction between a surface nucleated fatigue crack and internal grain boundary creep cavitation. A theoretical model based on this damage mechanism is presented which describes the experimental data obtained from the different cycle shapes.

Fracture Mechanics Analysis for Short Fatigue Cracks

AbstractElastic and elastic-plastic fracture mechanics solutions are modified to predict the growth of short fatigue cracks by introducing an effective crack length, which is equal to the actual crack length increased by an amount l 0, into the solutions for intensity factors and the J integral method of analysis. Since the threshold stress approaches the fatigue limit of the material as the crack length tends to zero the value of l 0 can be obtained from the threshold stress intensity factor and the fatigue limit. Crack growth results for short cracks, in both elastic and plastic strain fields of smooth specimens, when interpreted in terms of the modified solutions, show excellent agreement with elastic long crack data. The accuracy of the term l 0 in predicting crack growth rates for short cracks is found to be independent of the applied strain level. It varies linearly with grain size for low carbon steels and can be considered at the surface as a measure of the reduced flow resistance of surface grain...

Fatigue Crack Propagation of Short Cracks

Previous studies have shown that both threshold stress intensity factors and fatigue crack growth rates are dependent on crack size. The average growth rates for very short cracks considerably exceed those given by conventional stress intensity-crack growth laws fitted to long crack data. Elastic and elastic plastic fracture mechanics solutions are modified to predict this behavior of short cracks by introducing an effective crack length l0 into the solutions for intensity factors and the J integral method of analysis. The threshold stress at a very short crack length approaches the fatigue limit of the material, and therefore the value of l0 can be obtained once the threshold stress intensity factor and the fatigue limit are known. The accuracy of the term l0 in predicting crack growth rates for short cracks is found to be independent of the applied strain level. It varies linearly with the grain size of the material and can be considered at the surface as a measure of the reduced flow resistance of surface grains due to their lack of constraint.

Complex Modulus of a Cohesive Soil from Stress Relaxation Response using the One-Sided Fourier Transform

The storage and dissipation resistance to deformation of a cohesive soil as represented by the complex modulus is obtained from stress relaxation response using the one‐sided Fourier transform. The soil response is nonlinear. An approximate method of evaluating the transform with digital computer methods is based on the use of straight line segments to form a piecewise continuous representation of the relaxation response. This allows comparison of static and dynamic response characteristics as functions of applied strain level and moisture content through the transformation from the time domain to the frequency domain. For each strain level, large and small time plateau regions of complex modulus are connected with a transition region showing an increase in modulus with increase in frequency. Complex modulus decreases with increased strain level and increased moisture content.

Complex Modulus of a Cohesive Soil from Stress Relaxation Response using the One-Sided Fourier Transform

The storage and dissipation resistance to deformation of a cohesive soil as represented by the complex modulus is obtained from stress relaxation response using the one-sided Fourier transform. The soil response is nonlinear. An approximate method of evaluating the transform with digital computer methods is based on the use of straight line segments to form a piecewise continuous representation of the relaxation response. This allows comparison of static and dynamic response characteristics as functions of applied strain level and moisture content through the transformation from the time domain to the frequency domain. For each strain level, large and small time plateau regions of complex modulus are connected with a transition region showing an increase in modulus with increase in frequency. Complex modulus decreases with increased strain level and increased moisture content.

Energy Dissipation of a Cohesive Soil by the Fourier Transformation of Stress Relaxation Response

The energy dissipation of a cohesive soil is obtained by the Fourier transformation of stress relaxation test data. Energy dissipation is expressed in terms of the loss tangent. The soil response is nonlinear. Because of the difficulty in obtaining a mathematical expression to fit the relaxation data adequately over the entire time scale of interest, an approximate method of evaluating the transform is used. The relaxation response functions are approximated by straight line segments to form a piecewise continuous relation. By using a large number of straight line segments, the error between the actual and approximate transformation can be made negligible. The segment slopes with corresponding values of time are used in a digital computer program to evaluate the transform. The results clearly indicate a decrease of loss tangent with increased applied strain level. For constant strain level, the variation of loss tangent as a function of moisture content is not clearly defined. The loss tangent vs. logarithm of time relation gives a characteristic single peak bell-shaped distribution for each strain level.