Constant Stress Amplitude Research Articles

The Static and Cyclic Deformation Behaviours Mg-Y<sub>2</sub>O<sub>3</sub> Nanocomposites

Mg-Y2O3 nanocomposites were synthesized by disintegrated melt deposition method (DMD). Tensile tests and constant stress amplitude fatigue tests were carried out on Mg nanocomposites with 0.5, 1.0 and 2.0 volume percentages of reinforcements. Yield strengths of the nanocomposites were found to increase with higher volume fraction of Y2O3. Both unreinforced Mg and the Mg nanocomposites were found to strain harden upon cycling. The hardening behaviour was more intense in the nanocomposites.

Fatigue Response of Hybrid Magnesium/Carbon-Fiber/Peek Nanocomposite Laminates at Elevated Temperature

High performance Mg based hybrid five-layered composite laminates were fabricated by means of sandwiching the AZ31 Mg foils with the Carbon/PEEK prepregs, in which the nanoparticles SiO2 were uniformly spreaded at the interfaces, through hot press. The basic mechanical properties of the cross-ply hybrid laminates were obtained by tensile tests, and they were found in good agreement with the prediction by the rule of mixture. Then, the response due to constant stress amplitude tension-tension (T-T) cyclic loading at elevated temperature up to 10 6 cycles was investigated.

Effects of Al 3Sc particle size and precipitate-free zones on fatigue behavior and dislocation structure of an aged Al–Mg–Sc alloy

Al–Mg–Sc alloy polycrystals have been cyclically deformed at room temperature under constant stress amplitudes. Cyclic softening after initial hardening is found in specimens with small particles of 4 nm, but no cyclic softening takes place in specimens with large particles of 11 nm. Transmission electron microscopy observations reveal that dislocations are uniformly distributed under all applied stress amplitudes in the specimens containing large particles, whereas clearly developed slip bands are formed in the cyclically softened specimens bearing small particles. Two-step aging causes a decrease in the width of precipitate-free zones in the large-particle specimen, resulting in the improvement of fatigue life.

A physically based cumulative damage formalism

A general cumulative damage methodology is derived from the basic relation specifying crack growth rate (increment) as a power law function of the stress intensity factor. The crack is allowed to grow up to the point at which it becomes unstable, thereby determining the lifetime of the material under the prescribed stress program. The formalism applies for the case of creep to failure under variable stress history as well as for cyclic fatigue to failure under variable stress amplitude history. The formulation is calibrated by the creep rupture lifetimes at constant stress or the fatigue cycle lifetimes at constant stress amplitude. No empirical (non-physical) parameters are involved in the basic formulation; everything is specified in terms of experimentally determined quantities. Several examples are given showing the inadequacy of linear cumulative damage while the present nonlinear damage accumulation method overcomes these deficiencies. The present method is extended to admit probabilistic conditions.

Fatigue strength of small-notched specimens under variable amplitude loading within the fatigue limit diagram

Although the fatigue limit diagram is defined in principle for constant stress amplitude, it is often considered that fatigue failure would not occur, even in varying loading, if applied stresses were kept within the fatigue limit diagram. However, it was shown in the case of small-notched specimens that fatigue failure occurred in some special cases of variable amplitude loading, even when all stress amplitudes were kept within the fatigue limit diagram. The cause of this phenomenon was examined using two-step stress and repeated two-step stress patterns in which the first step stress was chosen to be equal to the fatigue limit with zero mean stress and a mean stress was superposed on the second step stress. A non-propagating crack was formed by the first step stress. This crack functioned as a pre-crack for the second step stress with high mean stress. Consequently, fatigue failure occurred even when all stress amplitudes were kept within the fatigue limit diagram. It was an unexpected fracture caused by the interference effect of a non-propagating crack and a mean stress change.

On the fatigue-critical amplitude of random-amplitude stress

On the fatigue-critical amplitude of random-amplitude stress Uniaxial non-zero mean stress of constant circular frequency in the high-cycle fatigue regime is considered. It is assumed that equation of the S-N curve and modified Soderberg equation are applicable. For constant-amplitude stress, the fatigue-critical stress amplitude is defined as that which leads to failure during the required design life. For random-amplitude stress, expected values of the fatigue-critical stress amplitude and total fatigue damage accumulated during the required design life are estimated. It is found that the probability of fatigue failure is equal to the probability of exceedance of the fatigue-critical stress amplitude. As an example, for stationary random stress the equivalent random-amplitude stress and probability of fatigue failure are determined.

Fatigue Life Prediction of CFRP Laminates under Variable Stress Amplitude and Frequency

This paper is concerned with the fatigue life prediction of CFRP laminates under variable cyclic loading using the linear cumulative damage (LCD) rule with statistical approach. Three-point bending fatigue tests for plain-woven CFRP laminates were carried out undervarious cyclic loading with constant and variable stress amplitude and frequency. As results, the applicability of LCD rule to the flexural fatigue life was confirmed for this CFRP laminates.

Whole-Field Visualization and Evaluation of Fatigue Damage Using Laser Slit Beam

In this study, we investigated a method to evaluate fatigue damage of steels in the whole view field without contact using laser. In high cycle fatigue of mild steels, slipbands are firstly produced on material surface and the slipband density increases with progress of fatigue damage. In our previous work, we used a laser spot beam illuminating the fatigue damaged area of a specimen surface and diffusion pattern of the reflected light from the specimen surface changes due to surface change. We are able to evaluate fatigue damage based on diffusion pattern change due to surface property change caused by slipbands at an observing point. But it is important to find first the position where fatigue damage starts. In this study, we tried three types of optical setups to detect fatigue damaged area in the whole view field of the observing area with a CCD camera and evaluate fatigue damage. The diffusion pattern change accompanied by the loading cycles was observed under constant stress amplitude with the three optical setups. The results showed that there is a possibility of detecting fatigue damaged area and also evaluating fatigue damage by the optical setup using slit beam. Observation in the whole view field is possible scanning the slit beam on the specimen surface.

OS5-1-6 The effect of film thickness on crystal rotation behavior with fatigue crack propagation in copper films

Using a fatigue testing method by which fatigue cracks can be initiated and propagated in a film adhered to cover an elliptical through-hole in a base plate subjected to push-pull cyclic loads, annealed copper films with the thickness of 100μm and those reduced the thickness from the 100μm to 50μm by an electro-polishing were fatigued under a constant stress amplitude with a stress ratio of zero. The crystal rotation behavior with the fatigue crack propagation was investigated by measuring the crystal orientation around the fatigue crack initiated from the notch root before and after fatigue testing, using EBSD (Electron Back-scatter Diffraction) method. Then, the change of crystal orientation with fatigue testing was evaluated quantitatively from the misorientation between the crystal orientation matrix on the same point obtained before and after fatigue testing. As a result, the angle of the crystal rotation obtained from the region showing the high fatigue crack propagation rate was larger than that obtained from the region showing the low fatigue crack propagation rate for the film with the thickness of 100μm, while the fatigue crack propagated faster in the film with the thickness of 50μm than that with the thickness of 100μm regardless of the small crystal rotation angles with the fatigue testing for the film with the thickness of 50μm.

Development of Smart Stress Memory Sensor Using AE Kaiser Effect

Reliability of structures is an important task to ensure the ease and safety of our life, and further development of non-destructive evaluation for structures such as bridges and tunnels is required. Some fatigue sensors that consist of sacrificed specimen have been developed to evaluate the fatigue damage of structures such as fatigue cyclic number and residual lifetime. However, these fatigue sensors can be used only when the applied stress amplitude is known. We tried to develop a new smart stress memory patch that measured both maximum stress and number of fatigue cycles simultaneously using Kaiser effect of Acoustic Emission (AE) and crack length. In this study, the characteristics of the smart patch was evaluated. Pure copper was used for this sensor because its good corrosion resistance, stable crack propagation and detectability of AE near yield point. Fatigue test was performed under the constant stress amplitude to evaluate the crack propagation behavior using the relationship between stress intensity factor and crack propagation rate. The obtained curve between crack length and number of fatigue cycles by these crack propagation behavior was in good agreement with experimental results. AE measurement after some fatigue tests was performed and AE was detected at the applied fatigue stress. These results demonstrated that number of fatigue cycles and the maximum stress could be measured by this fatigue sensor.

Fatigue behavior of nano-grained copper prepared by ECAP

In order to study the fatigue behavior of ultra-fine grained copper, rotating bending fatigue test has been carried out. After 4 passes of ECAP (equal channel angular pressing) with Bc route, grains with about 300 nm diameter were formed. Specimens were fatigued at three constant stress amplitudes; σ a = 240, 120 and 80 MPa. Significant differences in morphological feature in fatigued surfaces between high and low cyclic deformation amplitudes were observed. To clarify the formation process of surface damage, morphological changes in surface damage caused by cyclic stresses were monitored successively by optical microscope and SEM.

High-cycle fatigue behavior of type 316 stainless steel at 288 °C including mean stress effect

Establishment of a life evaluation method for structural components is essential for the rational operation and maintenance of light water reactor plants. High-cycle fatigue induced by mechanical vibration or temperature fluctuation is still a major cause of troubles of plants. In this study, high-cycle fatigue tests for austenitic stainless steel SUS316NG were conducted at 288 °C with/without mean stress. Fatigue tests with constant stress amplitude less than a certain level ran out over 10 8 cycles, which suggested SUS316NG has a fatigue limit at 288 °C. The effect of mean stress on fatigue life was investigated through the correlation with the variation of mean strain and strain amplitude. Methods to predict fatigue life under mean stress condition was newly developed based on the concept of ductility exhaustion. The stress-based approach such as the modified Goodman constituting the basis of the current design rule predicted excessively conservative fatigue life, while the developed methods gave more accurate prediction.

Very high cycle fatigue of normalized carbon steels

Fatigue properties in the very high cycle regime of normalized carbon steel with carbon content 0.61% C (Ck60) and 0.15% C (Ck15) have been investigated with the ultrasonic fatigue testing technique at load ratio R = −1 and cycling frequency 20 kHz. Both steels show a distinct change of slope in the S– N curves at approximately 10 7 cycles. No specimen failed above 2.2 × 10 8 cycles although 25 specimens were stressed to over 10 9 cycles. Calorimetric measurements were used to evaluate the cyclic plastic strain amplitude at constant stress amplitude. Ck60 shows initial cyclic softening followed by cyclic hardening at stresses above and below the mean fatigue limit at 10 9 cycles. The lower the stress amplitude, the lower was the maximum plastic strain amplitude and the higher was the number of cycles when cyclic softening started. Cyclic hardening in Ck60 is seen to continue until 10 9 cycles. Ck15 undergoes cyclic softening mainly at stresses above the mean fatigue limit and specimens can fail at very low values (10 −6) of the cyclic plastic strain. No sub-surface crack initiation was detected.

Formation process of shear bands and protrusions in ultrafine grained copper under cyclic stresses

Abstract Ultrafine grained copper processed by equal channel angular pressing was fatigued at two constant stress amplitudes: σ a = 240 and 120 MPa (corresponding fatigue lives were N f ≑ 2 × 10 5 , and 4 × 10 6 cycles, respectively). Significant differences in the morphological features of post-fatigued surfaces between the high and low cyclic stress amplitudes were observed. To clarify the formation process of the surface damage, the morphological changes in the surface damage caused by the cyclic stresses were monitored successively by an optical microscope. Putting the optical microscopy and scanning electron microscopy observations of the post-fatigued surfaces together, the formation process of surface damage was discussed.

Rheological‐Dynamical Theory of Visco‐Elasto‐Plasticity and Fatigue: Part 2

The paper deals with the rheological‐dynamical analogy in which the three‐dimensional stress‐strain relations are defined under cyclic variation of stress for Hencky’s total strain theory. In many practical visco‐elasto‐plastic problems, like as multiaxial fatigue under loading at constant stress amplitude and constant stress ratio, the load‐carrying members are subjected to proportional loading. The classical Hencky’s theory has the advantage of mathematical convenience but its disadvantage is that the deformations predicted for the volume element are independent of the loading path. The existing formulations of the constitutive models for metals are mainly based on the Prandtl‐Reuss incremental theory of elasto‐plasticity, slip theory of plasticity or continuum damage mechanics. They have been shown capable of reproducing satisfactorily most experimental results available for metallic specimens. However, from the theoretical viewpoint little has been said about how these formulations relate to realistic predicting many different inelastic and time dependent problems of two‐ or threedimensional solids, such as fatigue, discontinuous plastic deformation etc. In this paper, fundamentally new aspect of isochronous constitutive relations for Hencky’s theory, which are dependent of the each loading path, is achieved by systematically introducing RDA concept into the continuum framework. Specific inelastic and fatigue formulation of triaxial state of stress is developed and discussed within the new theoretical tool and related to von Mises plasticity..

3548 純アルミニウムと6061アルミニウム合金の疲労き裂進展挙動の比較(S12-3 軽金属合金,S12 先進性金属材料の疲労特性と組織)

The fatigue crack propagation with unstable behavior of local slip deformation in A1050 and A6061 aluminum under the cyclic tensile loading are investigated. Delay and non-propagation mechanisms of fatigue crack under constant stress amplitude are studied in detail. Results of investigation shown that fatigue crack propagate by mode I in A6061 under the cyclic tensile loading. However, modes of crack propagation in A1050 became mixed mode, which combined with mode I and II being dependent on the testing conditions. Under the some experimental conditions, Delay of crack propagation phenomenon was observed in A1050. Difference of fatigue crack propagation are discussed from the viewpoint of unstable behavior of local multiple-slip deformation.

超微細粒銅の高サイクル疲労における表面損傷

In order to study the formation process of surface damage of ultrafine grain copper, rotating bending fatigue tests of smooth specimens have been carried out. After a total of 8 passes of ECA pressing with Bc route, grains with about 250 - 300nm diameter were formed. Specimens were fatigued at three constant stress amplitudes; σα = 240, 120 and 80MPa (corresponding fatigue lives were Nƒ ≈ 2 × 105, 4 × 106 and 5 × 107cycles, respectively). Significant differences in morphological feature in post-fatigued surfaces between high and low cyclic stress amplitudes were observed. To clarify the formation process of surface damage, morphological changes in surface damage caused by cyclic stresses were monitored successively by an optical microscope. Putting those OM observations and SEM observations of post-fatigued surfaces together, the formation processes of surface damage were discussed. In addition to those experiments, SEM observation of fracture surface and measurement of surface hardness during fatigue tests have been made.

W03I Development of stress memory patch(International Workshop on "New Frontiers of Smart Materials and Structural Systems")

A new sensing method called "smart stress memory patch", which could estimate the maximum stress, the stress amplitude and the fatigue cyclic number simultaneously using Kaiser effect of Acoustic Emission (AE) and crack length of this patch, was developed. In this study, the crack growth characteristic of this patch was evaluated. Pure copper was used for this patch because its good corrosion resistance, stable crack propagation and so on. Two kinds of samples which were rolled and electrodeposited copper were prepared to investigate the effect of microstructure on crack growth behavior. Fatigue test was performed under constant stress amplitude to evaluate the crack growth behavior using the relationship between stress intensity factor range and crack propagation rate. The scattering in fatigue crack growth was also investigated to obtain the relationship between crack length and the fatigue cyclic number including two-sided 95% confidence interval. The effect of thickness and grain size on the scattering was discussed. Finally, good crack growth behavior was obtained and the fatigue cyclic number could be estimated by this patch.

Nondestructive detection of fatigue damage in austenitic stainless steel by positron annihilation

Austenitic stainless steel specimens have been examined by positron-lifetime measurements at various stages until failure during fatigue tests at constant stress or plastic strain amplitudes. A positron-source-detector assembly has been mounted on the servohydraulic testing machines that allowed truly non-destructive positron annihilation studies without removing the specimens from the load train. Positrons were generated by a 72Se/72 As source with a maximum activity of 0.9 MBq (25 μCi). The average positron lifetime has been determined by a β+ − γ-coincidence applying a simplified data evaluation procedure. Under constant stress or plastic strain amplitudes early stages of fatigue damage could be detected. The strong increase of the average positron lifetime already during the first 10% of fatigue life could be related to the fatigue life of the specimens. Issues of lifetime prediction by positron annihilation measurements are discussed.

Temperature-Dependent Microstructures in Fatigued Ultrafine-Grained Copper Produced by Equal Channel Angular Pressing

Ultrafine-grained copper samples produced by equal channel angular pressing were cyclically deformed at different temperatures under a constant stress amplitude of 200 MPa, and the corresponding microstructures were investigated by transmission electron microscope. The increase in temperature during cycling facilitated the grain coarsening and led further to the general change of microstructures from dislocation walls to well-defined cells as well as to the occurrence of a few annealing twins in final microstructures of fatigued UFG copper at the temperature above recrystallization.