Threshold Strain Research Articles

Mechanosensitive Junction Remodeling Promotes Robust Epithelial Morphogenesis

Morphogenesis of epithelial tissues requires tight spatiotemporal coordination of cell shape changes. In vivo, many tissue-scale shape changes are driven by pulsatile contractions of intercellular junctions, which are rectified to produce irreversible deformations. The functional role of this pulsatory ratchet and its mechanistic basis remain unknown. Here we combine theory and biophysical experiments to show that mechanosensitive tension remodeling of epithelial cell junctions promotes robust epithelial shape changes via ratcheting. Using optogenetic control of actomyosin contractility, we find that epithelial junctions show elastic behavior under low contractile stress, returning to their original lengths after contraction, but undergo irreversible deformation under higher magnitudes of contractile stress. Existing vertex-based models for the epithelium are unable to capture these results, with cell junctions displaying purely elastic or fluid-like behaviors, depending on the choice of model parameters. To describe the experimental results, we propose a modified vertex model with two essential ingredients for junction mechanics: thresholded tension remodeling and continuous strain relaxation. First, junctions must overcome a critical strain threshold to trigger tension remodeling, resulting in irreversible junction length changes. Second, there is a continuous relaxation of junctional strain that removes mechanical memory from the system. This enables pulsatile contractions to further remodel cell shape via mechanical ratcheting. Taken together, the combination of mechanosensitive tension remodeling and junctional strain relaxation provides a robust mechanism for large-scale morphogenesis.

Pressure-conductive rubber sensor based on liquid-metal-PDMS composite

We present a pressure-conductive rubber sensor using a liquid-metal-polydimethylsiloxane (PDMS) composite suitable for incorporation onto surfaces with a complex curvature such as the human body. The composite is synthesized by physical mixing of Galinstan and PDMS based on magnetic stirring. This composite is conductive only when a mechanical pressure exceeding the threshold value or strain is applied; the pristine state of the composite is not conductive. The threshold value can be controlled by adjusting the mixing ratio of liquid metal and PDMS. This material is mechanically robust, allowing it to operate reliably under various elastic deformations such as pressing, stretching, and bending without structural failure and performance degradation. Moreover, a fabricated sensor array can detect the distribution of the applied pressure in plane. As a feasibility study, we demonstrate a pressure-conductive rubber sensor for detecting finger movements and bio-signals such as blood pressure and respiration rate. Our results reveal that our rubber sensor is practical as a wearable sensor because of its mechanical robustness and electrical reliability.

Experimental results on the influence of gas on the mechanical response of peats

Direct observation of gas in peat layers, generated by slow degradation in anoxic conditions, raised concern in the Netherlands about its potential impact on the geotechnical response of dykes founded on peat. To address this issue, an experimental investigation was initiated, aimed at quantifying the main consequences of the presence of gas on the mechanical response of peats. The results of a series of triaxial tests on natural peat samples flushed with carbonated water are presented and discussed. Controlled amounts of gas were exsolved by undrained isotropic unloading, and the samples were sheared under undrained conditions. During gas exsolution, the samples suffered volumetric expansion, at a rate which is ruled by the relative compressibility of the fluid and the soil skeleton. The gas in the pore fluid dominates the stress–strain response upon undrained shearing, causing lower excess pore pressure compared to fully saturated samples. The experimental results suggest that local fabric changes occur during gas exsolution. However, for the amounts of gas investigated, these fabric changes seem to be almost reversible upon compression. Although the ultimate shear strength is hardly affected by gas, the reduction in the mobilised shear strength at given axial strain thresholds is dramatic, compared to fully saturated samples. The study suggests that the presence of gas must be cautiously accounted for at low stresses, when a reference stiffness is chosen for serviceability limit states, and when operative shear strength definitions, based on mobilised strength for given strain thresholds, are chosen in the assessment of geotechnical structures on peats.

A Study of Precipitation-Strengthened Creep-Resistant Austenitic Stainless Steel for Supercritical and Ultra-Supercritical Power Plants

Abstract The properties of precipitation-hardened austenitic stainless steels are adversely affected by the precipitation of unwanted brittle phases, like sigma (nonmagnetic intermetallic phase that is composed of iron and chromium) and Laves (Fe2Nb). These stainless steels are also susceptible to stress corrosion cracking. The induced residual stress in the material in turn reduces its creep strength. The strain beyond a certain limit affects its creep-resistant property appreciably, and hence, beyond a certain strain, these stainless steels are to be solution-treated for stress relief. Characterization studies on precipitation-strengthened creep-resistant austenitic stainless steel are presented in this article. Effects of strain and aging on microstructures are also evaluated. In order to understand the precipitation-strengthening behavior and effect of strain on Super304H (CC2328-2), the material was strained and aged at 650°C for a period ranging from 7 h to 4 weeks, and then the microstructures were studied using an optical microscope. Experiments were carried out on tubes with no strain and tubes with strain, which were aged at 650°C. The results show that beyond a threshold strain of ∼17 %, the material becomes deteriorated, and to regain its original microstructure, solid solution treatment is necessitated.

Small-strain deformation characteristics of frozen clay from static testing

Small-strain stiffness characteristics of frozen clay as measured in laboratory static tests are reported in this paper. Accurate strain measurement, which has rarely been attempted for frozen soils, was achieved with local gap sensors that function stably in a triaxial cell filled with refrigerant at freezing temperatures, assisted by an extremely high-resolution servomotor for axial loading. High-plasticity clay samples were frozen directly under confining pressure after being consolidated to different isotropic effective stresses, and loaded at three different rates and three different temperatures. The importance of avoiding ram–bush friction and a way to achieve it are discussed. The observed stress–strain relationships for strains of less than 0·01% indicated features surprisingly similar to those seen for unfrozen states, such as independence of the initial elastic modulus from the loading rate, onset of non-linearity (or yielding) at smaller strains for slower loading, while the linearity threshold strain was greatly increased at lower temperatures. These similarities are made clear through parallel tests on the same clay in unfrozen states. An interesting feature was seen for frozen samples; those consolidated to higher effective stress exhibited lower stiffness when being frozen. A simple model taking into account the volumetric ice content explains this feature.

Gain boundary character distribution optimization of Cu-16at.%Al alloy by thermomechanical process: Critical role of deformation microstructure

To explore the influence of deformation microstructures on the evolution of gain boundary character distribution (GBCD), various thermo-mechanical processes (TMPs) were carried out on the Cu-16at.%Al alloy. The deformation microstructures of all cold rolled samples and the evolution process of twin related domains (TRDs) during subsequent annealing were revealed by transmission electron microscope (TEM) observations and quasi-in situ electron back scatter diffraction (EBSD) observations, respectively. Besides, the influence of GBCD optimization on the mechanical property was also examined by uniaxially tensile tests. The results show that the deformation microstructures including stacking faults (SFs) and deformation twins (DTs) have significant effects on the evolution of TRDs and thus on the GBCD optimization. SFs are beneficial to GBCD optimization, for which the formation of annealing twins (ATs) can be induced by generating “stacking accidents” in a sequence of closely packed atomic planes at the front end of growing TRD. On the contrary, DTs hinder the growth of TRDs, thus impairing GBCD optimization. The optimal prior strain of TMP for the GBCD optimization of the Cu-16at.%Al alloy is thus suggested to be around the threshold strain for the appearance of DTs. Under the premise for the absence of obvious differences in yield strength and ultimate tensile strength, the tensile ductility of the GBCD optimized Cu-16at.%Al alloy is improved, due to an increased deformation uniformity and a higher resistance of AT boundaries to crack propagation.

Liquefaction of sand under monotonic and cyclic shear conditions: Impact of drained preloading history

In-situ sand deposits inevitably undergo a preloading history caused by factors such as overlying structures, adjacent construction work, or repeated sedimentation and erosion; this will in turn affect their mechanical behavior and liquefaction susceptibility during subsequent loading. An experimental program was developed to explore the potential influence of the drained preloading history on the undrained response of sand subjected to both monotonic and cyclic loadings. The results indicate that the shear strength, dilatancy, and stiffness characteristics in monotonic tests depend on the relative directions of the drained preloading and subsequent undrained shearing. Two typical types of behavior are observed and identified in the cyclic tests, i.e., cyclic mobility and residual deformation accumulation failure, depending on the stress state prior to the undrained cycling. The cyclic resistance is quantified based on a failure criterion defined in terms of the triggering of flow behavior, and it is compared to that determined using a criterion of specified strain threshold. In particular, the direction–wise (compressional or extensional) influence of the preloading history on the liquefaction resistance and pore pressure generated in sand during cyclic loading agrees well with that observed during monotonic loading. Furthermore, a correspondence between the cyclic and monotonic tests can be derived, and a unified approach is proposed to evaluate both the monotonic and cyclic responses, irrespective of the drained preloading history.

Hepatocyte growth factor enhances the clearance of a multidrug‐resistant Mycobacterium tuberculosis strain by high doses of conventional chemotherapy, preserving liver function

Tuberculosis (TB) is one of the deadliest infectious diseases in humankind history. Although, drug sensible TB is slowly decreasing, at present the rise of TB cases produced by multidrug-resistant (MDR) and extensively drug-resistant strains is a big challenge. Thus, looking for new therapeutic options against these MDR strains is mandatory. In the present work, we studied, in BALB/c mice infected with MDR strain, the therapeutic effect of supra-pharmacological doses of the conventional primary antibiotics rifampicin and isoniazid (administrated by gavage or intratracheal routes), in combination with recombinant human hepatocyte growth factor (HGF). This high dose of antibiotics administered for 3 months, overcome the resistant threshold of the MDR strain producing a significant reduction of pulmonary bacillary loads but induced liver damage, which was totally prevented by the administration of HGF. To address the long-term efficiency of this combined treatment, groups of animals after 1 month of treatment termination were immunosuppressed by glucocorticoid administration and, after 1 month, mice were euthanized, and the bacillary load was determined in lungs. In comparison with animals treated only with a high dose of antibiotics, animals that received the combined treatment showed significantly lower bacterial burdens. Thus, treatment of MDR-TB with very high doses of primary antibiotics particularly administrated by aerial route can produce a very good therapeutic effect, and its hepatic toxicity can be prevented by the administration of HGF, becoming in a new treatment modality for MDR-TB.

Rate effect on crack propagation measurement results with crack propagation gauge, digital image correlation, and visual methods

The rate-dependent measurement gaps among the crack propagation gauge (CPG), digital image correlation (DIC), and Visual methods for measuring dynamic rock crack propagation were determined using a case study. Dynamic notched semi-circular bending (NSCB) tests of a rock material, namely, red sandstone, were conducted using a modified split Hopkinson pressure bar system to form different crack propagating velocities (CPVs). From the viewpoint of the initial crack time, a normal strain threshold of 2% in the DIC was effective in obtaining a match between the CPG and DIC methods at a low CPV. However, the selected 2%–5% normal strains in the DIC were pivotal factors that could compromise the results from the CPG and Visual methods with a low CPV in terms of the peak CPV, whereas 2%–3% normal strains were recommended for cases with a high CPV. In addition, the equivalent crack-velocity-dependent strain thresholds (tiDIC equal to tiCPG and tiDIC equal to tiVisual) for the CPG, Visual, and DIC techniques displayed a linear fit with the CPV. The measurement gap between the DIC and Visual methods was narrower than the measurement gap between the DIC and CPG methods. The critical initial crack time of CPG for red sandstone needed the condition of a CPV of approximately 500 m/s. A potential explanation for these gaps was proposed in terms of the changes in the fracture process zone with CPV, and three suggestions were subsequently recommended based on the experimental results to aid in the interpretation of other experimental results.

Rheological characterization of low-calcium fly ash suspensions in alkaline silicate colloidal solutions for geopolymer concrete production

The rheological behavior of low-calcium fly ash suspensions in activating solutions of colloidal silica and alkali hydroxide is investigated. The study is aimed at relating the rheological behavior of alkali-silicate activated low-calcium fly ash (AAF) suspensions with Portland cement paste suspensions. The yield stress and the viscosity of the AAF suspensions increase with the alkalinity of the colloidal solution, which is due to changes in the surface charges on fly ash particles with the change in the ionic medium. Increasing the alkalinity results in a less negative zeta potential of fly ash in the alkaline solution of colloidal silica and an increase in the yield srtess of the AAF suspension. The thixotropic behavior in AAF suspensions is associated with structure breakdown to a finely dispersed suspension of particles produced by shearing. Energy measurements indicate a very slow change in the internal particle structure with age at room temperature. A comparison of AAF suspensions is presented with suspensions of Portland cement in water, which are proportioned for similar physical flow characteristics and yield stress. AAF suspensions have a larger solid fraction than the cement suspensions in water of comparable yield stress. The zeta potential of cement particles in water is less negative when compared to fly ash in alkaline-silicate solutions. The AAF suspensions of comparable yield stress exhibit a significantly higher viscosity than the cement suspensions in water. Cement paste and AAF suspensions exhibit a rate dependent yield response. Cement paste suspensions exhibit a threshold strain rate for minimum yield stress. In AAF suspensions there is a continuous decrease in the yield stress at lower strain rates. The thixotropic behavior in cement paste is influenced by chemical ageing which produces a rapid recovery of yield stress. In comparison, there is very little ageing at room temperature in the AAF suspensions and a very slow recovery of yield stress after shearing.

Dilatancy-triggering surface for advanced constitutive modelling of sand

This paper presents, for sand, a formulation for a surface in stress space defining the onset of dilatancy based on the volumetric threshold strain concept, the expression for the small-strain shear modulus in terms of soil state and an expression for secant shear modulus degradation with increasing shear strain. The size of the surface depends on both density (void ratio) and mean effective stress. The use of the proposed surface is demonstrated for Toyoura sand. The paper also presents a multiaxial formulation for the surface that can include kinematic hardening. The proposed surface can be useful in constitutive modelling, especially to capture seismic or cyclic loading conditions.

Stability analysis of strain-softening slopes based on the gravity increase method

With traditional slope stability analysis methods, it is difficult to accurately describe the progressive failure process and dynamic variation law of slope stability. The strain-softening constitutive model was therefore used to simulate the progressive failure process of a strain-softening slope based on the gravity increase method (GIM), with the displacement interface employed to determine the sliding surface. A sensitive analysis of the characteristic parameters within the softening stage was then conducted. The results are as follows: There are similar space-time evolution laws of residual strength factor and shear strain increment, with failure starting from the slope toe and extending gradually. The sliding surface of strain-softening slopes is located between that of the slope with peak strength and the sliding surface of the slope with residual strength. The stability coefficient shows an exponential growth trend with the increase of residual cohesion and residual plastic shear strain threshold, with a positive linear correlation between the residual friction angle and stability factor. The residual friction angle is the most sensitive factor in slope stability, followed by the residualcohesion, with the residual plastic shear strain threshold being the least sensitive.

On the onset of deformation twinning in the CrFeMnCoNi high-entropy alloy using a novel tensile specimen geometry

Deformation-induced nanoscale twinning is one of the mechanisms responsible for the excellent combination of strength and fracture toughness of the single-phase, face-centered cubic CrMnFeCoNi (Cantor) alloy, especially at cryogenic temperatures. Here, we use a novel, modified dogbone geometry that permits the sampling of varying stress and strain regions within a single tensile specimen to characterize the onset of twinning in CrMnFeCoNi at 293 K, 198 K and 77 K. Electron backscatter diffraction (EBSD) and backscattered electron (BSE) imaging revealed the presence of deformation nano-twins in regions of the samples that had experienced plastic strains of ∼25% at 293 K, ∼16% at 198 K, and ∼8% at 77 K, which are similar to the threshold strains described by Laplanche et al. (Acta Mater. 118, 2016, 152–163). From these strains we estimate that the critical tensile stress for the onset of twinning in this alloy is on the order of 750 MPa.

Previbration Signature on Dynamic Properties of Dry Sand

Abstract The shear modulus and damping ratio of soils at low to medium strains are essential input parameters in dynamic response analysis of soils. As soil elements in the field always have a history of being subjected to vibrations due to earthquakes, construction works, and vehicle motions, etc., the previbration effect on the dynamic properties of soils needs to be addressed appropriately. This study presents a series of tests performed by a newly developed resonant column system that enables the assessment of the dynamic properties of soils that have only undergone dozens of vibration cycles to investigate the impact of the vibration history on the dynamic properties of dry sand. Both the shear modulus and damping ratio of sand under different confining pressures (p′) are measured after the application of a previbration with a wide range of vibration amplitudes (γpre) and number of cycles (N). The results show that an elastic threshold strain exists for the dynamic properties of sand, beyond which the effect of the vibration history can be identified from the shear modulus reduction and damping ratio curves. The results also confirm that the effect of the vibration history on dry sand depends on the test variables of γpre, N, and p′. In addition, a new interpretation method is proposed to quantify the combined effects of γpre and N on the signature of the vibration history, which could potentially be useful for interpreting the dynamic properties of the in situ soils that may experience previbration.

Large number tracking of depth and mechanics dependent calcium signaling in chondrocytes of articular cartilage

Purpose: Moderate loading of cartilage stimulates chondrocytes to synthetize matrix proteins, but excessive loading can lead to apoptosis. It is well known that macroscale strains affect cell health, but the mechanism behind how tissue level strain causes cellular and subcellular biological response is poorly understood. Previous work done by our group showed impact-induced tissue strain resulting in mitochondrial depolarization in chondrocytes, with correlation between strain and chondrocyte death, demonstrating the importance of microscale mechanics driving chondrocyte fate after impact. The correlation between strain and mitochondrial dysfunction indicates the mitochondria’s role in mechanosensing. However, the mechanotransduction and cell signaling pathway driving this process has not been fully characterized. In traumatic brain injury, Ca2+ signaling is known to produce mitochondrial swelling, while in other tissues Ca2+ is buffered by mitochondria. However, if too much Ca2+ is taken up, the mitochondrial permeability transition pore will open, leading to depolarization and cell death. In brain injury, this process can be averted if the mitochondrial swelling is addressed immediately. This indicates an opportunity to introduce mitoprotective therapies in a similar vein in articular cartilage. To better understand chondrocyte fate and investigate the process of mitochondrial depolarization after strain, an understanding of the Ca2+ signaling during mechanical stimulation is required. We predict that chondrocytes in different regions of cartilage will show different Ca2+ signal responses, in relation to the depth dependent compressive and shear properties of the tissue. In order to test this prediction, we compare changes in Ca2+ signaling under various magnitudes of compressive and shear strain, and compare the signaling response with previous work that determined impact-induced strain thresholds for mitochondrial dysfunction and chondrocyte death. Methods: Explants of articular cartilage were sterilely dissected from the femoral condyles of neonatal calves. The explants were cultured in a solution of DMEM and Antibiotic-Antimycotic for 24 hours. After removal from culture, the explants were vertically bisected into hemicylinders. They were stained for Ca2+ ions in 5μM Fluo-8 AM for one hour at 37° C, and rinsed in pure DMEM three times for ten minutes each to remove excess stain. The bisected samples were affixed to a custom-built Tissue Deformation Imaging Stage (TDIS) that can apply compressive and shear strain while directly imaging the tissue on a Zeiss LSM 510 inverted confocal microscope [Fig. 1]. Each sample was subjected to compression and shear. Recordings were made for 1000s while uncompressed, immediately following compression, post-relaxation, and immediately following shear. From these videos we were able to extract cell locations as a function of time through a centroid tracking algorithm [Fig. 2]. We extracted location and intensity information for all cells framed within a 636× 636μm frame (∼1000 cells per video), and identified cells showing Ca2+ transients. Local strain was calculated using a 2D digital image correlation program and the intensity curve of each cell was matched with the corresponding local strain. Results: Cell tracking showed different families of curves in Ca2+ signal intensity for cells located in the superficial (0 to 200μm depth) and middle (> 200μm depth) regions [Fig. 3], as well as multiple different families of cell signals present for uncompressed, compressed, and sheared samples [Fig. 4]. Increased proportion of cells showing Ca2+signaling was observed at greater compressive strain and in comparison with unstrained samples, indicating a threshold at which abnormal Ca2+ signaling occurs. Conclusions: The development of a technique to track a large number of cells in a non-equilibrium state is an improvement upon previous methods. This method allows for the study of transient signals despite changes in location and brightness, allows for large-scale tracking of cells for population statistics and provides the basis to directly study the relationship between Ca2+ signaling and mitochondrial response. The observation of differences in cell Ca2+ signaling between the surface and middle zones before any tissue strain was applied reinforces the idea that the superficial layer has a protective role for articular cartilage. This is in agreement with previous work showing that removal of the superficial layer allowed strain to penetrate deeper into the tissue. The identification of strain thresholds for increased Ca2+ signaling aligns with previous work showing strain thresholds for mitochondrial depolarization and cell death, further promoting the role that Ca2+ has in the cellular response pathway to strain.View Large Image Figure ViewerDownload Hi-res image Download (PPT)View Large Image Figure ViewerDownload Hi-res image Download (PPT)View Large Image Figure ViewerDownload Hi-res image Download (PPT)

A comparative study on uniaxial tensile property calculation models in spherical indentation tests (SITs)

The objective of this study was to verify the effectiveness and accuracy of current uniaxial tensile property calculation models in spherical indentation tests (SITs). Four representative models, the empirical, numerical, analytical, and incremental models, were selected, and experiments were performed on 14 kinds of metals. Comparisons proved that the empirical model yields similar hardening behaviors for all materials, the accuracy and stability of numerical and analytical models largely depend on specimen materials, while the incremental model may even characterize the yield plateau on a stress-strain curve. However, even the most demanding incremental model, which demands both cyclic loading and additional digital image correlation (DIC) measurements, may have more than 15% errors for austenitic stainless steels. The source of error in analytical and incremental models was investigated through an examination of the expanding cavity model (ECM) at different indentation depths and friction coefficients, which explained the reason why the analytical model may cause more errors. The significant importance of the strain threshold on tensile property evaluation in the incremental model was also discussed, and a method to determine the optimal strain threshold was suggested.

Size effects and magnetoelastic couplings: a link between Hall–Petch behaviour and coercive field in soft ferromagnetic metals

ABSTRACTSize effects regarding Hall–Petch (HP) relation are studied in this work for cobalt, nickel and Fe–3wt.%Si (FeSi), from polycrystalline to multicrystalline states. The materials show a breakdown in HP plot for thickness (t) to grain size (d) ratio less than a critical value. This appears in the beginning of plasticity for cobalt and FeSi whereas a plastic strain threshold must be overcome for nickel. Measurements of the coercive field on strained samples are able to depict such modification for low t/d ratio. Values of the coercive field in the polycrystalline domain allow an estimation of the magnetocrystalline anisotropy energy, related to the grain volume fraction concerned by reversal mechanisms for magnetic domains. Multicrystalline samples of cobalt and FeSi becomes magnetically softer at the yield stress. This is linked to a delay of the maximum intergranular stress towards higher strains for FeSi. For cobalt, non-linear elasticity and strong basal texture modify the magnetoelastic effects in coarse grain samples. For nickel, size effect on the coercive field appears after a few per cent of plastic strain as for HP relationship. A mean internal stress can be captured by magnetic measurements on polycrystals, related to the intragranular part of the kinematic stress. The softening of the magnetic properties for strained nickel multicrystals is due to a competition between the apparition of dislocation cells, which increases the coercive field by mechanisms of magnetic domain wall pinning, and surface softening of multicrystals, which tends to decrease the value of Hc.

Fatigue character comparison between high modulus asphalt concrete and matrix asphalt concrete

An innovative approach for characterizing the fatigue character of high modulus asphalt concrete (HMAC) and matrix asphalt concrete was presented. Strain-life fatigue equations based on uniaxial tensile test are employed to extract parameters characteristic of the asphalt concrete's resistance to fatigue. The feasible test conditions including specimen size, failure criterion, control mode, loading frequency and test temperature are proposed. In addition, the effect of the mean stress on the fatigue life is analyzed that mean stress has minor influence on fatigue life evaluation. Fatigue strain threshold, limit strain and fatigue life equation are put forward based on the fatigue test results at different strain levels. Such fatigue equation as well as tensile strain threshold and limit value can guide the development of the two asphalt mixtures, which can estimate the lifetime of load-bearing structures. From the fatigue character comparison between the two asphalt mixtures, tensile strain threshold and limit tensile strain of HMAC are smaller than those of matrix asphalt concrete, and HMAC can better resist the fatigue damage than matrix asphalt concrete.

Strain hardening behavior of Ni-carbonyl Chemical Vapor Deposited (CVD) material with bimodal grain structures: Ultrafine (UF) grains and large grains with UF/nano twins

Through Ni-carbonyl CVD process, bulk Ni material (hereafter referred as CVD Ni) with bimodal grain structures, i.e. UF grains and large grains with UF/nano twins, can be produced. In tensile test, this material demonstrated a very good combination of strength and ductility as well as a special strain hardening behavior. Upon investigating the material's hardening behavior, a critical threshold strain, εc, of 0.02 was identified for the distinct change in the strain hardening behavior. Prior to this strain level, the strain hardening rate of the CVD Ni was found always higher than that of its counterpart coarse-grained (CG) Ni; but it dropped, after this strain, to the same level as the CG material. Furthermore, through the analysis of results from Tension Unloading Reloading-in-Tension (TURT) tests, this specific strain value was also found to be significant in the change of the hysteresis loop width as well as the back stress strain hardening responsible for the material's Bauschinger behavior. Microstructural evolution examination suggests that most of the twin boundaries (TBs) initially acted as obstacles to dislocation movement providing strong dislocation back stress and hence contributing to the strong hardening behavior. From the strain level of 0.02 and onwards, however, massive detwinning was detected which is responsible for the drop in the hardening rate. Finally, dislocation cell structures, that were typically evolved from entanglements of dislocation inside CG Ni, were also found inside the detwinned large grains of CVD Ni in the high strain regime, which yielded similar strain hardening behavior as in the CG Ni material.

Influence of Strain Rate and Waveshape on Environmentally-Assisted Cracking during Low-Cycle Fatigue of a 304L Austenitic Stainless Steel in a PWR Water Environment

In this paper, the low cycle fatigue resistance of a 304L austenitic stainless steel in a simulated pressurized water reactor (PWR) primary water environment has been investigated by paying a special attention to the interplay between environmentally-assisted cracking mechanisms, strain rate, and loading waveshape. More precisely, one of the prime interests of this research work is related to the consideration of complex waveshape signals that are more representative of solicitations encountered by real components. A detailed analysis of stress-strain relation, surface damage, and crack growth provides a preliminary ranking of the severity of complex, variable strain rate signals with respect to triangular, constant strain-rate signals associated with environmental effects in air or in PWR water. Furthermore, as the fatigue lives in PWR water environment are mainly controlled by crack propagation, the crack growth rates derived from striation spacing measurement and estimated from interrupted tests have been carefully examined and analyzed using the strain intensity factor range ΔKε. It is confirmed that the most severe signal with regards to fatigue life also induces the highest crack growth enhancement. Additionally two characteristic parameters, namely a threshold strain εth* and a time T*, corresponding to the duration of the effective exposure of the open cracks to PWR environment have been introduced. It is shown that the T* parameter properly accounts for the differences in environmentally-assisted growth rates as a function of waveshape.