Amplitude Cyclic Loading Research Articles

Effect of time–temperature, strain level and cyclic loading on the complex Poisson’s ratio of asphalt mixtures

Complex modulus and complex Poisson’s ratio are two parameters which characterise the 3D linear viscoelastic properties of asphalt mixtures. Compared to the complex modulus, the complex Poisson’s ratio is more difficult to be measured because of the great accuracy needed for the experimental device. This investigation focuses on the complex Poisson’s ratio of asphalt mixtures and the effect of loading parameters on its values. The three loading parameters considered in the paper include the time–temperature, the strain level and the number of cyclic loadings. To fulfil the objective of study, complex modulus tests, fatigue tests combined with the rest time, large amplitude cyclic loading tests and cyclic permanent deformation tests are performed. The complex modulus tests are conducted at different temperatures, frequencies and strain amplitudes. Thanks to the fatigue tests, the large amplitude cyclic loading tests and the cyclic permanent deformation tests, the change in complex Poisson’s ratio with the number of cycle, the pre-deformation strain level of specimen and the accumulated permanent deformation (up to some %) respectively is also investigated. The obtained results show that the time–temperature and the number of cyclic loading have a clear effect on the complex Poisson’s ratio of asphalt mixtures. No significant change in the complex Poisson’s ratio is observed when varying the strain cyclic loading amplitude within the small strain domain. The change in the complex Poisson’s ratio becomes more important and is observed to be proportional with the norm of the strain deviator tensor when the axial strain is accumulated or pre-loaded up to some percent.

Modeling of Short Fiber Reinforced Polymer Composites Subjected to Multi‐block Loading

Short Fiber Reinforced Composite (SFRC) structures exhibit multiple microstructures (due to material flow during the process). They are generally subjected to variable amplitude loadings. In this context, a robust model is needed to predict fatigue life as a function of microstructure. In this paper, we propose a predictive micromechanical damage-based model allowing fatigue life prediction in the case of SFRC submitted to variable amplitude cyclic loading. An experimental study was firstly performed on Sheet Molding Compound (SMC) composite involving different microstructure configurations. Specimens were subjected to stress-controlled block loading. The influence of the order of the sequences was evaluated through Low-High amplitude (L-H) and High-Low amplitude (H-L) schemes. Damage accumulation is computed at the local scale to describe the evolution of the fiber-matrix interface damage until failure. A local failure criterion based on a critical damage state allowed predicting variable amplitude fatigue life as a function of microstructure. A good correlation was found between experimental and numerical results. Once the approach was validated, it has been used to model different useful variable amplitude loading schemes to emphasize the role of the loading sequence parameters and order.

Methodology for calculating J-integral range [formula omitted] under cyclic loading

The J-integral range or the cyclic J-integral, ΔJ, is frequently utilized to deal with the fatigue crack growth of ductile materials with large scale yielding. ΔJ was originally defined as a line integral similar to the J-integral proposed by Rice. Many researchers correlated fatigue crack growth rate of ductile materials with ΔJ defined by Jmax−Jmin. Although it is theoretically shown that the latter definition of ΔJ, that is, ΔJ=Jmax−Jmin, is not equivalent to the former defined by a line integral, why is the latter definition of ΔJ utilized so frequently? This question is main concern of the present paper. To answer this question, we derive the expression of ΔJ represented by a line integral for HRR singular fields, which govern the vicinity of a crack tip under large scale yielding, then formulate the difference between the ΔJ represented by a line integral and ΔJ=Jmax−Jmin. We perform the error estimation for three-point bending specimens to clarify how accurately (Jmax−Jmin) predicts the ΔJ-value, compared with the ΔJ represented by a line integral, which is supposed to provide the exact ΔJ-value. As a result, theΔJ-value calculated from (Jmax−Jmin) is identical to the ΔJ-value calculated from a line integral in the case of the zero-tension cyclic loading conditions, and the deviation between the former and latter values is small under near zero-tension and large cyclic loading amplitude conditions. The use of the ΔJ defined by (Jmax−Jmin) should be limited to the cases where near zero-tension and large cyclic loading amplitude conditions are satisfied.

Failure behavior of SA508 Gr.1a LAS and SA312 TP316 SS pipes with a circumferential through-wall crack under large amplitude cyclic loads

This paper presents cyclic fracture test results using small-scale through-wall cracked pipe specimens to investigate the effects of large-amplitude cyclic loads on failure behaviour for different pipe materials. SA508 Gr.1a low-alloy steel (LAS) and SA312 TP316 stainless steel (SS) pipes were used in the experiment. The tests were conducted under quasi-static monotonic, displacement-controlled, and load-controlled cyclic loadings at room temperature (RT). Regardless of the pipe material, displacement-controlled reversed cyclic loading significantly reduced deformation-ability and fracture resistance, promoting crack growth. These effects were more pronounced in the SA312 TP316 SS than in the SA508 Gr.1a LAS specimen. The material-dependence on fracture behaviour was considerable under monotonic load, but was near-negligible under displacement-controlled reversed cyclic loading. Under load-controlled cyclic loading, the number of cycles to failure, the maximum allowable crack extension, and deformation-ability of pipe specimens decreased as the amplitude and compressive level increased. For all load-controlled cyclic loads, the SA312 TP316 SS specimen was less resistant to failure than the SA508 Gr.1a LAS specimen. Regardless of the loading type, a circumferential through-wall crack grew out of the initial crack plane for the SA508 Gr.1a LAS specimen, but propagated in a straight line from the initial crack plane for the SA312 TP316 SS specimen.

Comparison Between Monotonic, Cyclic and Post-Cyclic Pullout Behavior of a PET Geogrid Embedded in Clean Sand and Clayey Sand

A series of monotonic and multistage pullout tests were performed using a large pullout apparatus to study the monotonic, cyclic and post-cyclic pullout behavior of a uniaxial woven geogrid under different conditions. The monotonic and multistage pullout tests were performed for clean sand and sand containing 10% and 20% clay. The effect of vertical effective stress on the pullout resistance and deformation along the geogrid was investigated using monotonic testing. In the multistage pullout test, the influence of vertical effective stress, cyclic load amplitude and the frequency and the number of tensile load cycles on the post-cyclic pullout resistance and deformation during the cyclic stage was studied. The monotonic test results indicated that an increase in clay content increased the pullout resistance at the lower effective vertical stress (20 kPa), while at higher vertical stresses (40 and 60 kPa), this effect was insignificant. Moreover, when a geogrid was embedded in clean sand and 20% clayey sand, the post-cyclic peak pullout resistance was slightly lower than the values attained in monotonic tests at a vertical effective stress of 20 kPa. The reductions varied from 3.5 to 8% for clean sand and 5.5 to 7% for 20% clayey sand. An increase in the cyclic load amplitude had no effect on the post-cyclic resistance of the geogrid-embedded in clean sand, but post-cyclic resistance of geogrid embedded in 20% clayey sand decreases with increasing cyclic load amplitude.

Part 2. Review and meta-analysis of studies on modulation of longitudinal bone growth and growth plate activity: A micro-scale perspective.

Macro-scale changes in longitudinal bone growth resulting from mechanical loading were shown in Part 1 of this review to depend on load magnitude, anatomical location, and species. While no significant effect on longitudinal growth was observed by varying frequency and amplitude of cyclic loading, such variations, in addition to loading duration and species, were shown to affect the morphology, viability, and gene and protein expression within the growth plate. Intermittent compression regimens were shown to preserve or increase growth plate height while stimulating increased chondrocyte presence in the hypertrophic zone relative to persistent and static loading regimens. Gene and protein expressions related to matrix synthesis and degradation, as well as regulation of chondrocyte apoptosis were shown to exhibit magnitude-, frequency-, and duration-dependent responses to loading regimen. Chondrocyte viability was shown to be largely preserved within physiological bounds of magnitude, frequency, amplitude, and duration. Persistent static loading was shown to be associated with overall growth plate height in tension only, reducing it in compression, while affecting growth plate zone heights differently across species and encouraging mineralization relative to intermittent cyclic loading. Lateral loading of the growth plate, as well as microfluidic approaches are relatively understudied, and age, anatomical location, and species effects within these approaches are undefined. Understanding the micro-scale effects of varied loading regimes can assist in the development of growth modulation methods and device designs optimized for growth plate viability preservation or mineralization stimulation based on patient age and anatomical location.

Interfacial properties of CFRP sheets and concrete subjected to variable amplitude fatigue loading

AbstractIn order to better understand the interfacial fatigue behavior of concrete and fiber‐reinforced polymer (FRP), bending fatigue tests were conducted on plain concrete beams strengthened with carbon FRP (CFRP) sheets under variable amplitude cyclic loading which deteriorates structural performance in actual projects more dramatically than constant amplitude loading. The test parameters were four types of loading modes including static loading, constant amplitude cyclic loading, alternating amplitude cyclic loading and incremental amplitude cyclic loading. Miner damage rule, modified Miner damage rule and Corten‐Dolan cumulative damage rule were used to predict the fatigue life of specimens. The relationship between loads and deflections, the relationship between strains and cycle times and the crack propagation were analyzed. The results have shown that the fatigue life of specimens under variable amplitude cyclic loading, especially alternating one, is shorter than that of specimens under constant one, and the modified Miner damage rule is more suitable to predict fatigue life. The crack lengths on the lateral faces of beams recorded by digital image correlation (DIC) method developed more slowly than those at the bottom sides recorded by strain gauges. A proposed damage model describing the relationship between the crack lengths and fatigue life for specimens under alternating amplitude fatigue loading is consistent with the experimental results well.

Shear strength behaviour of coral gravelly sand subjected to monotonic and cyclic loading

The paper presents an experimental study on the strength behaviour of a coral gravelly sand from Vietnam subjected to monotonic and cyclic loading. A series of direct shear tests were carried out to investigate the shear strength behaviour and the factors affecting the shear strength of the sand such as relative density, cyclic load, amplitude of the cyclic load and loading rate. The study results indicate that the shear strength parameters of the coral gravelly sand include not only internal friction angle but also apparent cohesion. These parameters vary with the relative density, cyclic load, the amplitude of the cyclic load and loading rate. The shear strength increases with the increase of the relative density. The shear strength increases after subjecting to cyclic loading. The amplitude of the cyclic load affects the shear strength of coral gravelly sand, the shear strength increases as the amplitude of the cyclic load increases. The loading rate has insignificantly effect on the shear strength of the coral gravelly sand.

A comparison of damage tolerance behaviour of two different nickel base super alloys under a turbine standard spectrum loads

In this investigation, the damage tolerance behavior of two different aero-engine materials i.e., GTM718 and GTM720 nickel base super alloys subjected to a turbine standard spectrum loads was experimentally determined and compared. Compact tension (CT) specimens as per ASTM test standard specifications were cut and prepared from the forged discs of these materials. Fatigue crack growth (FCG) tests were conducted using CT specimens in a 100 kN servo-hydraulic universal test machine at ambient temperature and lab air atmosphere. Constant amplitude cyclic fatigue loads were applied on CT specimen to initiate a pre-crack of about 2.0 mm in length from the notch root. Further, the fatigue crack was propagated under standard cold-TURBISTAN spectrum load sequence. The crack length was measured by compliance method as well as by using optical traveling microscope at regular intervals of completion of load blocks. Loading waveform employed during the test was triangular with a frequency of 2 Hz i.e., four reversals per second. The average fatigue crack growth life under turbine standard spectrum loads observed was observed to be lower in GTM718 when compared to GTM 720 material. The measured crack growth life was about 48 blocks and 38 blocks in GTM 720 and GTM 718 respectively. It was clear from these experiments that GTM720 is more damage tolerant than GTM718 material.

Experimental study of the damage evolution and radon exhalation characteristics of quasi-uranium ore under constant amplitude cyclic loading and unloading

Cyclic loading damage in uranium mining is often accompanied by changes in radon exhalation characteristics. Understanding the relationship between rock damage and radon exhalation of uranium ore is therefore important for predicting rock instability and improving monitoring methods to ensure the safety of uranium mines and other underground projects. In this study, we performed constant amplitude cyclic loading and unloading tests on quasi-uranium ore to measure the degree of damage and used the closed chamber method to measure the accumulated radon concentration. The results show a range of damage values between 0.089 and 0.315. The sample with the highest damage value also showed the highest radon exhalation rate of 0.0411 ± 0.00384 Bq m− 2 s− 1. We use the dissipated energy as a damage variable and find a positive correlation between damage and radon exhalation rate. The damage value was fitted with radon exhalation rates, yielding correlation coefficients of 0.97, and shows an inverted S-shaped trend. The results provide a basis for monitoring rock stability using radon exhalation in future uranium mining operations.

Experimental Study on Acoustic Emission and Resistivity Response of Sandstone under Constant Amplitude Cyclic Loading

A lot of engineering practice shows that, during the construction and operation of rock mass projects, in addition to the static load, the rock is more subjected to cyclic load. Constant amplitude cyclic load is one of the simpler cyclic loading methods. The damage of rocks gradually accumulates under the action of periodic cyclic load, and finally, fatigue damage occurs, which affects the stability of the project. Therefore, it is necessary to study the mechanical properties of the rock under the action of constant amplitude cyclic load. Acoustic emission (AE) parameters and resistivity are both sensitive to the damage process of the rock and can well describe the damage evolution law of the rock. Under the action of constant amplitude cyclic loading, different loading conditions, different state specimens, and so on, factors will make a big difference to the AE signal and resistivity, such as the difference in stress amplitude, loading rate, and saturation. Therefore, in this paper, the typical sandstone in the Chongqing area is taken as the research object, the AE characteristics and resistivity characteristics of sandstone under different test conditions such as different stress amplitude, different loading frequency, and different saturation are tested, and the evolution of AE is found. It also has a three‐stage rule: in the initial stage, the deformation is faster and the AE signal is strong; in the constant velocity stage, the deformation develops slowly and the AE signal is also stable; in the acceleration stage, the deformation develops sharply and the AE signal also becomes more intense. With the increase of cyclic loading stress amplitude, or the decrease of cyclic loading frequency, or the decrease of rock sample saturation, the acoustic emission signal generated in each cycle will be stronger. The resistivity as a whole shows a rapid decline in the loading phase and a rapid rebound in the unloading phase. The changes in resistivity at different stress amplitudes show differences in the first few cycles; the overall change trend of resistivity at different frequencies is consistent, but the rate of change increases with decreasing frequency, and the variation trend of resistivity is very different under different saturation.

Residual Stress Relaxation of DRWDs in OSDs under Constant/Variable Amplitude Cyclic Loading

An orthotropic steel deck (OSD) has a complicated structure, and its fatigue life is mainly determined by various welding details. Fatigue assessment of deck-to-rib welding details (DRWDs) under long-term train loads is an important concern for engineers. Properly assessing the initial residual stress and the mechanism of stress relaxation in DRWDs under long-term external loading is a prerequisite for predicting the fatigue damage and service life of OSDs. In this paper, a finite element analysis method is proposed to calculate the residual stress relaxation in DRWDs of OSDs under constant/variable amplitude cyclic loading. First, experiments on full-size OSD specimens were carried out using the hole drilling strain-gauge method, and the multi-axial distribution characteristics of residual stress on the sub-surface of the deck were obtained. On this basis, a refined residual stress analysis model of DRWDs using thermal-structural sequence coupling analysis and life and death unit technology is established, and the accuracy of the model is verified by the test data. Second, a coupling stress analysis model that considers the welding residual stress and mechanical stress using cyclic plastic constitutive model is established. The combined influence of number of cycles, stress amplitude, and stress ratio on multi-axial residual stress relaxation effect under constant/variable amplitude cyclic loading is investigated. Finally, a release formula of welding residual stress relaxation coefficient is proposed based on the external loading stress amplitude, stress ratio, and material yield stress. The results show that (1) with the increase in the number of loading cycles, the stress decreases until it is stabilized, while the global distribution of welding residual stress remains unchanged. Most of the welding residual stress release (about 95%) occurs in the first cycle; (2) the residual stress relaxation decreases with the increase in stress amplitude and increases linearly with the stress ratio; (3) the residual stress release is controlled by the maximum amplitude stress in the variable amplitude cyclic loading. After the residual stress is released, the stress will not continue to be released if the DRWDs have the same or smaller amplitude loading.

Fatigue performance of additive manufactured metals under variable amplitude service loading conditions including multiaxial stresses and notch effects: Experiments and modelling

Additive manufacturing technology has gained significant attention in recent years. However, due to lack of sufficient understanding of their fatigue behavior under service loading conditions, design of critical load carrying parts using this technique is still at early stages. These conditions include multiaxial stress states, notches and stress concentrations, and variable amplitude load cycles. Some of the distinguishing features of AM metals as compared to the conventionally fabricated metals include defects, surface roughness, anisotropy and build orientation effects, and residual stresses due to the fast solidification during the fabrication process. These factors resulting from AM processes may significantly affect the fatigue performance of AM parts under multiaxial variable amplitude loading. In this work, unnotched and notched fatigue behavior of two commonly used AM metals with different surface roughness conditions were studied under variable amplitude multiaxial loads. Both crack initiation approach using critical plane-based model and fracture mechanics approach using crack growth from the rough surface or surface defects at the notch were used to estimate the fatigue lives. Fatigue estimations are then compared to the experimental results.

Transition-behaviours in fatigue-driven delamination of GFRP laminates following step changes in block amplitude loading

Fatigue damage accumulation in laminated fiber reinforced polymer composites is highly sensitive to interactions of load events occurring in variable amplitude loading. This work aims to experimentally characterise fatigue-driven delamination growth due to step changes in block amplitude loading. Double cantilever beam glass/epoxy specimens are subjected to two-level block loading and constant amplitude cyclic loading under mode I crack opening. Crack propagation is monitored using a recently developed highly accurate digital image-based method for automated tracking of delamination fronts in translucent materials (Bak and Lindgaard, 2020). The results prove a significant difference in the crack growth rate following from a step change in block amplitude loading in comparison to the crack growth rate under constant amplitude cyclic loading at the same value of the strain energy release rate. Special emphasis is placed on the transition crack growth phenomenon, which is strongly influenced by the cyclic load history. Any of the load amplitude transitions under investigation, cause a characteristic and non-negligible transition-behaviour, which is currently ignored in state-of-the-art prediction models for fatigue-driven delamination growth. Supplementary tests are conducted to discuss the role of bridging fibres in the crack wake during load amplitude transitions.

The impact of internal stratifications on the performance of oil and gas pipes

The work investigated pipes of the main gas pipeline with a service life of more than 45 years. Initially, this gas pipeline was designed for the technological purposes. It was intended to remove associated gases generated during the purification of methane from acidic impurities, but subsequently, as production was modernized, it was transferred to purified natural gas transport. Nevertheless, the presence of corrosive components in the transported gas during the initial period of operation of the gas pipeline led to the appearance of structural defects in the form of delamination in the pipe wall. The purpose of the study was to evaluate the performance of defective pipes based on a set of laboratory and bench tests. During the construction of the pipeline, imported longitudinal welded pipes made from steel of strength class X53 and X56 were used. The study of the chemical composition and mechanical properties of pipe steel did not reveal significant deviations of these characteristics from normalized values. It was noted that the content of non-metallic inclusions was high, but acceptable for pipes Ø 1020 mm. Based on an analysis of the modes of gas transport through the gas pipeline over the past 10 years, a model of its operation has been formed, which includes alternately repeating combinations of amplitude and pulsation loading cycles. To carry out life tests, a research stand was constructed containing pipe fragments with extended zones of internal stratifications. Test results showed that delamination up to 1.5 m2 in size and/or up-to 3.5 m in length, as an independent type of defects, do not significantly affect the performance of the pipes.

Combined peridynamics and kinetic theory of fracture for fatigue failure of composites under constant and variable amplitude loading

This study presents a fatigue model for constant and variable amplitude cyclic loading by combining kinetic theory of fracture (KTF) with peridynamics (PD). The KTF is applicable to constant and variable amplitude loading, and cumulative damage is inherent in the assumptions. Also, it is extremely suitable for implementation in the PD model of laminates. The standard stress versus cycles to failure data for unnotched laminates provides the determination of necessary parameters for the KTF. Using the test data generated by the Air Force Research Laboratory under the Tech Scout Project, the fidelity of this model is established by simulating three different composite laminates with an open-hole under cyclic loads for stiffness reduction and failure progression. The PD predictions agree with the measured reduction in stiffness as a function of number of load cycles. Also, the progressive damage predictions capture the general characteristics of the experimentally observed damage patterns.

Determination of the Non-Proportional Cyclic Hardening Coefficient Sensitive to the Loading Amplitude

The research on determination of sensitivity of metal materials to non-proportional cyclic loading and the accompanying effects is analyzed. The indirect methods for determining the coefficient of non-proportional cyclic hardening of materials based on simple experiments are considered. A comparative analysis of methods for predicting non-proportional cyclic hardening for out-of-phase with a 90°displacement of biaxial cyclic loading with strain control is performed. It is shown that methods that use only the parameters of the stress–strain curve of materials are less effective, since they do not take into account the effect of cyclic loading amplitude on non-proportional hardening, which is quite significant for metals and alloys with face-centered cubic microstructure. Analysis of experimental data obtained at cyclic non-proportional deformation of stainless and carbon steels, aluminum and titanium alloys showed that the coefficient of non-proportional cyclic hardening varies depending on the loading amplitude. For most of the studied materials with face-centered cubic structure, there is a tendency to increase this hardening coefficient with increasing amplitude of cyclic loading and to its stability or slight decrease for metals with volume-centered cubic structure. Based on the behavior of the vast majority of materials, a logarithmic functional dependence is chosen to take into account the effect of the strain amplitude on the non-proportional cyclic hardening. A modification of the key parameter for determining mentioned hardening is proposed, taking into account both the characteristics of the stress–strain curve and the dimensionless value of the cyclic strain amplitude. It is shown that its use allows one to obtain a better correlation of experimental data with the approximation dependence proposed earlier by the author. The results of determining the coefficients of non-proportional cyclic hardening due to the improved approach meet the requirements of engineering use.

3D Textural Analysis of Fatigue Fracture Surfaces

Three CT specimens from stainless steel AISI 304L were subjected to constant amplitude cyclic loadings with various asymmetries. Crack growth was recorded in detail. Fracture surfaces were documented by 3D maps in about 110 locations in the crack growth direction. 3D maps and their local gradients were represented by 2D wavelet decompositions in 10 levels resulting in 60 textural features. Statistical models expressing crack growth rate as a function of textural features were optimized. Training and testing approach, a high ratio of overfitting, and testing of significance of components ensured model's robustness. Quality of results is documented by graphs confronting model outputs with real data known from experiment. Results are acceptable in all cases.

Alternating Shear Orientation During Cyclic Loading Facilitates Yielding in Amorphous Materials

The influence of alternating shear orientation and strain amplitude of cyclic loading on yielding in amorphous solids is investigated using molecular dynamics simulations. The model glass is represented via a binary mixture that was rapidly cooled well below the glass transition temperature and then subjected to oscillatory shear deformation. It was shown that periodic loading at strain amplitudes above the critical value first induces structural relaxation via irreversible displacements of clusters of atoms during a number of transient cycles, followed by an increase in potential energy due to the formation of a system-spanning shear band. Upon approaching the critical strain amplitude from above, the number of transient cycles required to reach the yielding transition increases. Interestingly, it was found that when the shear orientation is periodically alternated in two or three dimensions, the number of transient cycles is reduced but the critical strain amplitude remains the same as in the case of periodic shear along a single plane. After the yielding transition, the material outside the shear band continues strain-induced relaxation, except when the shear orientation is alternated in three dimensions and the glass is deformed along the shear band with the imposed strain amplitude every third cycle.

Effect of Interspersed Hold at Peak Load on the High-Temperature Fatigue Crack Growth Rate Response of SS 316 L(N) Stainless Steel

Abstract Low-carbon, nitrogen-enhanced austenitic stainless steels (316 L(N)) are often used in the construction of power plant components, such as heat transport systems that operate at elevated temperatures. Some of these components may be subjected to a combination of creep and fatigue loads during the high-temperature operating conditions, and the synergistic action leads to accelerated failure life. The present study aims to assess the effect of interspersed hold at peak load on the fatigue crack growth rate in SS 316 L(N) at test temperatures of 823 K (550°C), 873 K (600°C), and 923 K (650°C). The fatigue hold-time experiments were conducted using repeated blocks of 1,500 cycles of constant amplitude cyclic loading at a stress ratio of 0.1 followed by a hold-time waveform at the peak load with a dwell period of 600 s. In some experiments, the dwell time was set as 900 or 1,200 s. The results indicate a marginal slowdown in the crack growth rate with dwell time testing at 550°C compared with continuous cycling at high temperatures. This could be due to increased crack tip plasticity, crack tip blunting due to hold time, and associated crack closure during subsequent cycles of fatigue loading. Crack length estimated by unloading compliance technique prior to and after hold time suggested a crack length increase after hold period for the first unloading cycle, but upon subsequent cycling, the crack growth rate appeared to slow down. It is presumed that there is a competition between crack closure due to enhanced plastic zone sizes (due to creep) and crack extension due to creep during hold time. It is noted that SS 310 L(N) is a creep-resistant alloy and exhibits dynamic strain aging at the test temperatures. The scanning electron microscopy fractographs in general indicated evidence of conjoint damage due to fatigue (through striations) and creep. The presence of both transgranular and intergranular modes of fracture could be seen. Formation of the triple-point crack as a result of grain boundary sliding was observed from the electron fractography in addition to void nucleation along the grain boundaries, indicating the damage accumulation during the creep.