Constant Cyclic Stress Research Articles

Effect of Constant Cyclic Stress Coupling on the Fatigue Behavior of 304LN Stainless Steel.

The stress state of the primary circuit main pipeline is very complex mixed constant stress and periodic cyclic stress during the operation of nuclear power plants, which often affects the failure behavior of the stainless steel (SS) pipe. The fatigue behavior of 304LN SS under mixed constant stress and cyclic stress was investigated, and it was found that the coupling effect of constant stress and cyclic stress has a significant collaborative acceleration on the fatigue behavior. The research results showed that the cracks in the 304LN SS did not propagate even after 78 days under both constant load conditions of 5 KN and 7.5 KN, while the crack growth rate (CGR) of the specimen increased by about an order of magnitude when coupled with a cyclic fatigue load. The fatigue life of 304LN SS was the longest under 200 °C high-temperature water, and its life was roughly the same under 250 °C and 300 °C water. In a 300 °C high-temperature water environment, the fatigue life under the coupling of constant stress and cyclic fatigue stress is significantly lower than that under symmetric cyclic stress alone. There is a synergistic acceleration effect on the fatigue life of 304LN SS when constant stress is coupled with periodic cyclic stress, which is attributed to the combined effect of a corrosion environment and mechanical factors.

Surface integrity and fatigue performance of Ti6Al4V alloy peened by sinking bead abrasive jet

Exploring a green, low-cost peening process to improve the surface integrity of titanium alloys, increasing fatigue life, and breaking through the limitations in large-scale applications is an urgent need at present. The study presents a novel peening method using sinking beads as the shot, combined with the low dust pollution and water film cushioning advantages of waterjet peening. The impact of the process on the surface integrity of titanium alloys is meticulously discussed to elucidate the microstructural evolution. Constant stress fatigue tests are performed to evaluate the effectiveness of the proposed method. It was found that the jet pressure and feed rate determined the energy density and the number of peening for sinking bead abrasives, which were the key factors affecting the surface material properties of the peening specimen. By optimizing the process parameters, the maximum surface residual compressive stress can reach 875.5 MPa, the microhardness can attain 481.7 HV, and the surface roughness can be reduced to 1.594 μm. The surface of the peened specimen is covered with impact craters, exhibiting a plastic deformation layer with evident grain refinement and lattice distortion. The ductility of the peened specimen decreased, with axial deformation reducing from 0.278 mm to 0.260 mm under constant cyclic stress. Due to the increase in microhardness and the introduction of residual compressive stresses, the fatigue cycles of the specimen were increased from 20692 to 33615, improving the fatigue life by 62.45%.

A numerical approach for simulation of stress-controlled cyclic shear loading in a soil column considering partial drainage

Liquefaction strength curves (LSCs) are commonly generated based on undrained stress-controlled cyclic testing to define the liquefaction potential of sands. However, these LSCs do not consider the effect of partial drainage on liquefaction potential. This is mainly because there are very limited experimental techniques to run stress-controlled tests for partially drained sands. This manuscript proposes a procedure to numerically simulate a saturated sand column with a free drainage boundary at the top (or top and bottom), accelerated horizontally at the bottom with constant acceleration cycles of duration designed to generate constant cyclic shear stress histories in the column. This is done for columns subjected to low and high overburden pressures. The paper starts with numerical simulations of a typical undrained cyclic direct simple shear, CDSS tests by applying a stepped velocity wave to a single soil element, an already established approach. This approach only works for a single element in undrained condition which is fixed at the base. Then, the proposed procedure is implemented by applying stepped acceleration time histories at the base of soil columns having one or two drainage boundaries. In these stepped acceleration runs, the durations of the acceleration cycles are controlled to achieve a partially drained stress-controlled CDSS kind-of-loading at specific elevations within the soil columns. The technique is used to show the effect of partial drainage on sand liquefaction behavior at both low and high overburden pressures. The results show that the effect of drainage is much more significant at higher overburden. The existence of both top and bottom drainage boundaries resulted in less liquefaction vulnerability, as compared to having only one drainage boundary at the top of the sand column. The results show that liquefiable sand layers in the field may be less prone to liquefaction under high overburden pressure than predicted by the current state-of-practice, which relies mainly on undrained small-scale cyclic tests.

Modeling fatigue failure in soft tissue using a visco-hyperelastic model with discontinuous damage

Soft tissue is susceptible to injury from single high-magnitude static loads and from repetitive low-magnitude fatigue loads. While many constitutive formulations have been developed and validated to model static failure in soft tissue, a modeling framework is not well-established for fatigue failure. Here we determined the feasibility of using a visco-hyperelastic damage model with discontinuous damage (strain energy-based damage criterion) to simulate low- and high-cycle fatigue failure in soft fibrous tissue. Cyclic creep data from six uniaxial tensile fatigue experiments of human medial meniscus were used to calibrate the specimen-specific material parameters. The model was able to successfully simulate all three characteristic stages of cyclic creep, and predict the number of cycles until tissue rupture. Mathematically, damage propagated under constant cyclic stress due to time-dependent viscoelastic increases in tensile stretch that in turn increased strain energy. Our results implicate solid viscoelasticity as a fundamental regulator of fatigue failure in soft tissue, where tissue with slow stress relaxation times will be more resistant to fatigue injury. In a validation study, the visco-hyperelastic damage model was able to simulate characteristic stress-strain curves of pull to failure experiments (static failure) when using material parameters curve fit to the fatigue experiments. For the first time, we've shown that a visco-hyperelastic discontinuous damage framework can model cyclic creep and predict material rupture in soft tissue, and may enable the reliable simulation of both fatigue and static failure behavior from a single constitutive formulation.

Insight into Heart-Tailored Architectures of Hydrogel to Restore Cardiac Functions after Myocardial Infarction.

With permanent heart muscle injury or death, myocardial infarction (MI) is complicated by inflammatory, proliferation and remodeling phases from both the early ischemic period and subsequent infarct expansion. Though in situ re-establishment of blood flow to the infarct zone and delays of the ventricular remodeling process are current treatment options of MI, they fail to address massive loss of viable cardiomyocytes while transplanting stem cells to regenerate heart is hindered by their poor retention in the infarct bed. Equipped with heart-specific mimicry and extracellular matrix (ECM)-like functionality on the network structure, hydrogels leveraging tissue-matching biomechanics and biocompatibility can mechanically constrain the infarct and act as localized transport of bioactive ingredients to refresh the dysfunctional heart under the constant cyclic stress. Given diverse characteristics of hydrogel including conductivity, anisotropy, adhesiveness, biodegradability, self-healing and mechanical properties driving local cardiac repair, we aim to investigate and conclude the dynamic balance between ordered architectures of hydrogels and the post-MI pathological milieu. Additionally, our review summarizes advantages of heart-tailored architectures of hydrogels in cardiac repair following MI. Finally, we propose challenges and prospects in clinical translation of hydrogels to draw theoretical guidance on cardiac repair and regeneration after MI.

Fatigue Life Scatter Modeling and Failure Probability Prediction of Multisite Damage Structural Part under Variable Amplitude Load History

An “index stress” (a constant amplitude cyclic stress) is introduced to characterize the scatter of the fatigue life under a variable amplitude load history. It is defined as a fatigue damage weighted average of the amplitudes of the individual stress cycles in a variable amplitude load history. The fatigue test results show that the scatter of fatigue life under variable amplitude load histories can be well characterized by such defined index stress. For probabilistic fatigue life prediction and failure probability estimation, a multiple-site damage structural part is taken as a series system, and the total probability theorem is applied to reflect the effect of load history uncertainty on failure probability.

Axial strain accumulation projection model for sand in cyclic loading

Strain accumulation analysis is important to understand the deformation mechanism of soil structure subjected to cyclic loading. An empirical model is a superior choice for the description of strain accumulation. However, the ambiguity and sensitivity for the physical meaning of the parameters in the empirical model hindered the understanding of the strain accumulation mechanism. Compared with conventional empirical models by directly fitting experimental data, this study proposed a new approach to formulate an empirical model of strain accumulation using the intrinsic relationship between the reloading process in cyclic loading and the monotonic loading process. All axial strain accumulation increments of silica powder during cyclic loading with constant cyclic stress amplitude were projected onto the corresponding monotonic loading stress-strain curve, and the evolution of strain accumulation was described by the projection modulus, which was regarded as an intrinsic parameter of the projection stress-strain curve. An empirical formulation for describing the connection between projection modulus and stress levels was proposed. In addition, the prediction model of total axial strain in cyclic loading was presented by combining the accurate representation of elastic and plastic moduli during primary loading. Finally, discussions on the intrinsic relation mentioned above were enhanced by presenting more experimental evidence. This paper provided novel insight into the mechanism of strain accumulation in cyclic triaxial conditions by linking strain accumulation with a unified stress-strain relation. However, limited by the triaxial stress conditions in this study, it is recommended to extend this projection method to other cyclic load tests.

Effect of fines on liquefaction resistance of sand

To evaluate the effect of non-plastic fines on liquefaction resistance of Babolsar sand, a series of undrained static and cyclic triaxial tests alongside a series of numerical analysis were carried out. The cyclic triaxial tests were conducted in stress-controlled conditions with 1 Hz frequency at 0.35 constant cyclic stress ratio for 50 kPa and 0.25 for 100 and 200 kPa confining pressures. The samples contained 0%, 10%, 20%, 30% and 40% of fine grains. The numerical analysis was performed by a finite difference method, and Finn’s constitutive model was applied to investigate the liquefaction resistance of the mixtures. The tests’ results showed that the number of cycles leading to liquefaction of a sand–silt mixture decreases after increasing fine-grained percentage. Poorer performance in compression and better performance in tension was observed in this situation. The test outcomes also showed that dominant behavior of the mixtures changes from sand to silt at 20% fines content. It was observed from the test results and the numerical analysis that in low percentages of silt, the behavior of the sand–silt mixture is similar to those of the clean sand sample. But by increasing silt, the mixture’s behavior becomes more dependent on contacts between fine and granular particles. The concept presented by Thevanayagam was used to check the contacts between fine and coarse particles. It was also observed that the Finn constitutive model is in good consistency with the test results as long as the behavior of the sand is dominant in the mixture.

The Reliability of a Component Under Several Distributed Cyclic Numbers at the Corresponding Constant Cyclic Stress Levels

The probabilistic stress-number of cycles curve (P-S-N curve) approach is widely accepted for describing the fatigue strengths of materials. It is also a widely accepted fatigue theory for determining the reliability of a component under fatigue loadings. However, it is an unsolved issue in the P-S-N curve approach that the calculation of reliability of a component under several distributed cyclic numbers at the corresponding constant cyclic stress levels. Based on the commonly accepted concept of the equivalent fatigue damage, this paper proposes a new method to determine the reliability of the component under several distributed cyclic numbers at the corresponding constant cyclic stress levels. Four examples including two validation examples will be provided to demonstrate how to implement the proposed method for reliability calculation under such fatigue cyclic loading spectrum. The relative errors in validation examples are very small. So, the proposed method can be used to evaluate the reliability of a component under several distributed cyclic number at different stress levels.

Effect of frequency of cyclic loading on liquefaction and dynamic properties of saturated sand

The effect of frequency of cyclic loading on liquefaction and post-liquefaction resistance of original Ahmedabad sand has been presented in this technical paper. Saturated and consolidated specimens prepared to a particular relative density were subjected to varied frequencies at a constant cyclic stress ratio under undrained conditions. Specimens were cyclically loaded till liquefaction. It was observed that the rate of generation of excess pore water pressure increases with increase in frequency of cyclic loading as a result the cyclic liquefaction resistance decreases with increase in the loading frequency. From the study on dynamic properties, it is found that the shear modulus decreases and damping ratio increases with increase in frequency. The frequency of cyclic loading has no influence on the shear modulus at a particular shear strain. It is also seen that the shear modulus decreases and damping ratio increases with increase in shear strain.

Behavior of Nonplastic Silty Soils under Cyclic Loading

The engineering behavior of nonplastic silts is more difficult to characterize than is the behavior of clay or sand. Especially, behavior of silty soils is important in view of the seismicity of several regions of alluvial deposits in the world, such as the United States, China, and Turkey. In several hazards substantial ground deformation, reduced bearing capacity, and liquefaction of silty soils have been attributed to excess pore pressure generation during dynamic loading. In this paper, an experimental study of the pore water pressure generation of silty soils was conducted by cyclic triaxial tests on samples of reconstituted soils by the slurry deposition method. In all tests silty samples which have different clay percentages were studied under different cyclic stress ratios. The results have showed that in soils having clay content equal to and less than 10%, the excess pore pressure ratio buildup was quicker with an increase in different cyclic stress ratios. When fine and clay content increases, excess pore water pressure decreases constant cyclic stress ratio in nonplastic silty soils. In addition, the applicability of the used criteria for the assessment of liquefaction susceptibility of fine grained soils is examined using laboratory test results.

Fatigue life prediction of nanoparticle/fibrous polymeric composites based on the micromechanical and normalized stiffness degradation approaches

In this paper, a new model based on the micromechanical and normalized stiffness degradation approaches is established. It has been assumed that during the fatigue condition, only material properties of composites (fiber and matrix) were degraded and nanofillers remain intact under different states of stress. A normalized stiffness degradation model was proposed for laminated fibrous composites reinforced with nanoparticles to derive a novel model to predict the stiffness reduction. The developed model is capable of predicting the fatigue life of nanoparticle-filled fibrous composites based on the experimental data of fibrous composites without nanofillers. The new fatigue model is verified by applying it to different experimental data provided by different researchers. The obtained results by the new fatigue model are in very good agreement with the experimental data of nano-silica glass/epoxy composites under constant cyclic stress amplitude fatigue and also for silica/epoxy nanocomposites in various states of stress with negligible error.

Cyclic fatigue of polymer nanocomposites

This paper presents an experimental study on cyclic fatigue of two polymer nanocomposites in two common failure modes: mechanical failure in epoxy nanocomposites and thermal softening in polyamide (PA, nylon) 6 nanocomposites. For epoxy nanocomposites, the effects of hard (silica) and soft (rubber) nano-particles on un-notched samples under constant cyclic stress amplitude fatigue were studied. Hard particles were shown to increase but soft particles decrease the fatigue life of nanocomposites compared to unmodified epoxy. At the same stress amplitude, the extent of fatigue crack growth prior to fast fracture was largest in rubber nanocomposites and least in pure epoxy, reflecting the differences in their fracture toughness values. Ternary nanocomposites with both hard and soft (silica and rubber) particles were also investigated and their fatigue performances were compared to the binary nanocomposites. Further, the stress ( σ a ) versus life ( N r ) test data of pure epoxy and its binary and ternary nanocomposites are well described by Basquin’s law. PA6 nanocomposites exhibited fatigue failure due to thermal softening when the maximum local temperature of the specimens subjected to cyclic loading reached the glass transition temperature, T g , of the material. Critical stress ( σ a ) versus frequency ( ω) envelopes for design against thermal failure were obtained for PA6/organoclay, PA6/POE-g-MA and PA6/pristine clay. Experimental results compared favorably with theoretical predictions.

Dissimilar-dimension interference model of fatigue reliability under uncertain cyclic load

The stress-strength interference model for static strength failure is well developed, while difficulties still exist when use the concept of stress-strength interference to calculate the reliability of time-dependent failure modes such as fatigue, abrasion, corrosion, etc. There is not yet a straight forward and effective model such as stress-strength interference model even for fatigue under constant amplitude cyclic stress with uncertainty in stress amplitude. Traditional stress-strength interference model is interpreted as that to express the statistical average of the probability that strength is greater than stress, or vice versa. Thus, the same model format, which traditionally can only be used for the situation of the parameters (load and stress) of the same unit, can be applied to more general situation. In other words, the traditional model was extended to the situation of any two variables, as long as one of the variables can be expressed as a function of the other. By such a model, fatigue failure probability/reliability can be calculated with known load distribution and fatigue life distribution.

銅膜材の疲労き裂伝ぱに伴う結晶方位の変化

The fatigue crack initiation and propagation behaviors were examined for pure copper films of 100 μm thickness. The cold rolled films were annealed and adhered to cover a circular through hole in a base plate. The base plate was subjected to constant cyclic stress with a stress ratio of R = 0. In order to discuss correlation between fatigue crack propagation and change of crystal orientation, crystal orientation on the surface of the copper film was measured before and after fatigue testing. The crystallographic information of these films was analyzed using the Electron Back-scatter Diffraction (EBSD) system and the quantitative evaluation method for the crystal rotation angle and the rotation axis, the rotation direction with fatigue was developed. As a result, the crystal rotation angle near the fatigue crack is larger than that apart from the crack but that is quantitatively small around the crack propagated on the annealing twin boundary.

Detection Method of Fatigue Damage in Carbon Steel Using Laser Ultrasonics

Ultrasonic attenuation was measured by the laser ultrasonic technique among the carbon steel specimens which were subjected to five stages of fatigue damage by constant cyclic tensile stress amplitude. Irradiation of a Q-switched laser was used for ultrasonic generation, and a laser interferometer which consists of a frequency-doubled continuous wave laser and a Fabry-Perot etalon was used for detection of ultrasonic vibration on the specimen surface. From the comparison of the broadband ultrasonic waveforms among five stages of fatigue damage, the attenuation coefficient of the longitudinal wave by the intrinsic material attenuation increased progressively with increasing the number of fatigue cycle. Ultrasonic attenuation at 2.5, 5.0, 7.5 and 10.0MHz were also calculated by Fourier transformation in order to compare the decay of the longitudinal wave among these frequency components. From the results, ultrasonic attenuation increased as the number of fatigue cycle increased, in both broadband and narrowband frequency components. Therefore, the measurement of broadband and narrowband ultrasonic wave obtained by the laser ultrasonic technique would be effective as the detection method of fatigue damage in nuclear power plant components.

Detection Method of Fatigue Damage in Carbon Steel Using Laser Ultrasonics.

Ultrasonic attenuation was measured by the laser ultrasonic technique among the carbon steel specimens which were subjected to five stages of fatigue damage by constant cyclic tensile stress amplitude. Irradiation of a Q-switched laser was used for ultrasonic generation, and a laser interferometer which consists of a frequency-doubled continuous wave laser and a Fabry-Perot etalon was used for detection of ultrasonic vibration on the specimen surface. From the comparison of the broadband ultrasonic waveforms among five stages of fatigue damage, the attenuation coefficient of the longitudinal wave by the intrinsic material attenuation increased progressively with increasing the number of fatigue cycle. Ultrasonic attenuation at 2.5, 5.0, 7.5 and 10.0MHz were also calculated by Fourier transformation in order to compare the decay of the longitudinal wave among these frequency components. From the results, ultrasonic attenuation increased as the number of fatigue cycle increased, in both broadband and narrowband frequency components. Therefore, the measurement of broadband and narrowband ultrasonic wave obtained by the laser ultrasonic technique would be effective as the detection method of fatigue damage in nuclear power plant components.

Strain and strain rate behaviour of a low carbon 18Cr-12Ni stainless steel under conditions of creep low cycle fatigue interaction

Results of an investigation of the application of a low frequency “rectangular” cyclic stress, during both the initial and advanced stages of primary creep, on the time dependent strain and strain rate behaviour of a low carbon 18Cr-12Ni (304L) stainless steel are presented. The strain rates immediately before, as well as immediately after, any stress decrement or increment were measured accurately. It is shown that at any given relative amplitude of applied stress Δσ/σ and superimposed applied stress period Δt c , these strain rates depend linearly on time. The relations between these strain rates and the relative amplitude of stress cycling, Δσ/σ, are also linear at any constant cyclic loading stress period Δt c . This finding makes it possible to estimate the contribution of the net strain owing to superimposed cyclic loading to the total plastic strain. Further, it is shown that the creep strain rate after the cyclic loading superposition lasting ΣΔt c = 36 ks, is only slightly affected by this superimposed loading. An interpretation of strain and strain rate behaviour under conditions of cyclic loading superposition is based on the processes of creep strain strengthening and creep recovery, though the backward straining after any stress reduction must be taken into account as well. All the above-mentioned processes depend not only on temperature and the mean applied stress, but also on the cyclic loading history.

On crystallographic crack transfer across interfaces in four types of aluminum bicrystal

Crack growth behavior across grain boundaries (GBS) in four types of aluminum bicrystal, including[121] tilt symmetric,[121] tilt pseudosymmetric, [012]-[111], and[121] 180° tilt bicrystals, was studied. Cracks generally decelerate at Gbs in these bicrystals under a constant cyclic load. It was also found that the variation in growth rate at a GB depends on the relative orientation of the two grains on opposite sides of the GB, the GB microstructure, and the applied stress. Two modes of crack transfer across GBs were identified: at a GB with a pair of nearly parallel slip systems, located respectively, on its two sides, the crack transfers smoothly by following the pair of slip bands; at other GBs the crack splits, branches into up to three secondary slip bands, and decelerates dramatically at a constant cyclic stress. Interface-resistance to fatigue crack transfer across a GB was rationalized according to dislocation movement across it.

Cyclic deformation of 100μm thin polycrystalline copper foils

Several earlier investigations considered the cyclic deformation behavior of thick specimens under constant cyclic stress or strain control. The question which remains to be solved is whether experimental results obtained in this way can be applied directly to the problems of stress concentrations in thin specimens. In order to observe the continuous evolution of the dislocation structure, many specimens were examined at different cyclic stresses and at various numbers of cycles by interrupting the fatique tests and examining the microstructure. The observation considered the fatigue damage changes.