High-amplitude Stress Research Articles

Effect of the hot forming with synchronous quenching process on high cycle fatigue properties of the 2A97 Al-Li alloy

The 2A97 Al-Li alloys have been used in aircraft to achieve lightweight, its high cycle fatigue performance is one of the important factors that determines aircraft safety. The 2A97-T8 Al-Li alloy is obtained through the 2A97-T3 Al-Li alloy undergoes the hot forming with synchronous quenching process. The 2A97-T3 and 2A97-T8 Al-Li alloys are tested for high cycle fatigue under different stress amplitudes. The fatigue crack propagation path of the 2A97-T3 Al-Li alloy is not only along the direction of maximum shear stress at 45° to the stress direction but also along the direction perpendicular to the stress axis. The fatigue crack propagation path of the 2A97-T8 Al-Li alloy propagates along the direction perpendicular to the stress axis. Under high amplitude stress, the characteristics of quasi-cleavage fracture are found in the fracture morphology of 2A97-T3 Al-Li alloy. However, the characteristics of cleavage fracture are found on the fracture morphology of 2A97-T3 Al-Li alloy under low amplitude stress. Under high amplitude stress, the quasi-cleavage characteristics and the dimples are found on the fracture morphology of 2A97-T8 Al-Li alloy. Under low amplitude stress, the small voids and the dimples are found on the fracture morphology of 2A97-T8 Al-Li alloy. The precipitated phases of the 2A97-T3 Al-Li alloy are multitudinous, fine, and dispersed δ’ phases and a few T1 phases. The precipitates of the 2A97-T8 Al-Li alloy are large, fine, and acicular T1 phases and a few δ’ phases. The hot forming with synchronous quenching process significantly improves the high cycle fatigue properties of the 2A97 Al-Li alloy. In addition, the fatigue cycles, the fatigue life rise and fall plots, the S-N curves, and the fatigue crack growth behaviors of the 2A97-T3 and 2A97-T8 Al-Li alloys are obtained.

Pt-Al bond coat dependence on the high-cycle fatigue rupture and deformation mechanisms of a fourth-generation single crystal superalloy at various temperatures

The impact of a Pt-Al bond coat on the high-cycle fatigue (HCF) behaviour of fourth-generation single crystal superalloys at 760℃ and 900℃ was investigated. The Pt-Al bond coat was found to be almost negligible under low stresses at 760℃; however, the bond coat effect was detrimental at 900℃ or under high stresses at 760℃. At 760 °C with increasing high-amplitude stress, the tip cracks preferentially nucleated from defects within coat and propagated inwards, accelerating the fracture. The fatigue life of the coated alloys was considerably decreased at 900 °C, which was attributed to the damage accumulated in the bond coating via oxidation, crack-induced oxide cracking and interfacial microstructure degradation. An Elevated temperature led to the crack-induced oxides shifting from a small fan-shape to a large umbrella-shape, increasing the density of slip bands and quantity of secondary cracks in the substrate. Furthermore, more Ru diffused outwards at 900℃ than that at 760℃, which caused the disappearance of L-C dislocation locks and tertiary γ′ phases and aggravated TCP phases precipitation in the substrate at 900℃. Ultimately, to assess the degradation of the HCF life, an empirical life prediction method was developed, and the calculations results well matched the test results.

The Attenuation Ability of Saturated Joints Filled with Granular Materials under High-Amplitude Stress Wave Loading

Existing experimental evidence shows that the propagation of explosive waves in the free fields of soils is remarkably affected by the degree of saturation. In the surrounding rocks of underground protective structures, the underground water is normally unavoidable, which is supposed to reduce the isolation efficiency of a passive antiblast barrier. To investigate the effect of water saturation on the stress wave attenuation ability of infilled joints, impact tests were carried out on artificial joints filled with dry and saturated granular materials using a split Hopkinson pressure bar (SHPB). The test results revealed that under the same conditions, the stress and energy transmission coefficients of the waves crossing saturated sand-filled joints were about 3.16–4.13 times and 9.75–11.4 times those of joints filled with dry sand, respectively. The dynamic stress-strain relationship of the filling layer during the impact process and the crushing index of the infill were analyzed. The results showed that the compressibility and the granular crushing index of the dry sand were much greater than that of the saturated sand, and the dynamic stress-strain relationship of the dry sand exhibited three-stage nonlinear characteristics. The experimental results quantitatively uncovered the serious adverse effect of water on the wave absorption properties and markedly diminished the potential of the filled joints as a wave elimination barrier, which should be a matter of great concern in the design of underground protective structures.

Experimental implementation of the laser shock peening method aimed at an increase in the fatigue properties of metals

The study is aimed at the development and implementation of an experimental setup for treating metal parts with complex geometry to induce compressive residual stresses in the surface layers. Modern methods of surface treatment demonstrated the possibility of increasing the durability of parts by several times through creation of high-amplitude residual compressive stresses. We managed to form the residual compressive stresses up to a depth of 1 mm using the titanium alloy specimens. The developed installation consists of a solid-state laser with a pulse energy of up to 10 J, a six-axis robot manipulator, and a system for measuring residual stresses by hole drilling method. The processing is realized in automatic mode with the possibility of continuous change of specimens. The geometry of parts and processing features are worked out on a digital three-dimensional model of the part. A number of tests have been carried out to reveal the dependence of the values of residual stresses on the processing conditions and demonstrate the necessity of numerical analysis and preliminary modeling of the process of laser shock peening. The distribution of residual stresses was measured by hole drilling method in the specimens before and after laser shock peening under various processing conditions, and the profiles of these stresses in depth were plotted. It is shown that along with the pulse power, the value and distribution of residual stresses are significantly affected by the number of repeated passes, the overlap degree, and the technology of preliminary preparation of the specimen surface. The analysis made it possible to choose the optimal processing mode for titanium alloys providing the values of residual compressive stresses up to 1 GPa.

Numerical study on the attenuation of high-amplitude stress wave in rock bar along with rock damage evolution

The attenuation of high-amplitude stress waves in the rock and the rock failure caused by the stress waves have significance for vibration source analysis and rock engineering support design, where the interactions between the two deserve further study. In this paper, a FEM based numerical investigation on high-amplitude stress wave attenuation and rock damage evolution in one-dimensional rock bar is performed, aiming to understand the interaction between the variation of rock damage and wave attenuation. The numerical model is calibrated by experimental results. The spatial distribution of damage in rock bar is quantified by statistical windows. The effects of wave amplitude and heterogeneity index of rock on wave attenuation as well as the distribution of damage are analysed in details. The results indicate that the high-amplitude stress wave decays exponentially in the rock bar, and the attenuation of wave becomes significant as the incident wave amplitude increases. The incident wave amplitude corresponding to the lower limit of the amplitude effect is approximately 30% of the static compressive strength of rock, while that corresponding to the upper limit is the dynamic compressive strength of rock. Under the same incident wave amplitude, the attenuation of stress wave is more obvious in the rock with higher heterogeneity. The rock damage is quantified in a specific window, and its spatial distribution is described along with the movement of the window. The rock damage is concentrated in a region near the incident end. The damage development in space is consistent with that of high-amplitude stress waves and is more obvious in the models with higher heterogeneity. Findings in this work is helpful for the understanding of interaction between high-amplitude stress wave attenuation and rock damage and facilitate design and protection of rock engineering.

Stress Controls Rupture Extent and Maximum Magnitude of Induced Earthquakes

AbstractSeismic hazard forecasts of induced seismicity often require estimates of the maximum possible magnitude (Mmax). Empirical models suggest that maximum magnitudes, or expected number of earthquakes, are related to the volume of injected fluid. We perform a suite of 3D physics‐based earthquake simulations with rate‐ and state‐dependent friction, systematically varying the area of the pressurized region and the amplitude of the initial homogeneous or heterogeneous shear stress. Using the resulting catalog we explore the conditions that result in pressure‐controlled versus runaway ruptures that extend outside the pressurized zone. We find that proposed empirical scaling laws correctly predictMmaxwhen shear stresses are further from failure (≤90% of maximum shear stress) and for high amplitude stress fields. Runaway ruptures are observed for higher initial shear stresses and smoother stress fields. In these cases, runaway ruptures occur early after the onset of injection and rarely preceded by foreshock activity.

Microstructure and properties of CoCrFeNiMo0.2 high-entropy alloy enhanced by high-current pulsed electron beam

Abstract Recently, high-entropy alloys have drawn lots of attention due to their outstanding properties. In this paper, a promising new surface modification technique was acted on CoCrFeNiMo0.2 high-entropy alloy to improve its mechanical and corrosion properties. CoCrFeNiMo0.2 high-entropy alloys were synthesized via vacuum arc melting then, their surfaces were processed by high-current pulsed electron beam (HCPEB). Microstructure, microhardness, wear and corrosion resistance were studied systematically by means of X-ray diffraction (XRD), optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Before and after HCPEB irradiation, CoCrFeNiMo0.2 high-entropy alloy had a face-centered cubic (FCC) structure, and the surface of the irradiated samples revealed preferential orientation on the (111) and (200) planes. In addition, a compact and homogenization surface layer formed after irradiation. Also, high-amplitude stress caused high-density dislocations and stacking faults on the surface. After HCPEB irradiation, the properties of the samples were significantly improved, the maximum microhardness (392.9 HV) and lowest wear rate (0.92 × 10−4 mm3·N−1·m−1) was obtained after 35-pulsed irradiation. Furthermore, corrosion resistance was obviously enhanced after 25-pulsed irradiation.

Cumulative Damage Modeling of Fuel Cell Membranes

The strongly varying humid environment under which fuel cell membranes operate introduces detrimental mechanical stresses that inflict damage to the membrane1. The damage incurred by the membrane during each humidity cycle when accumulated over time may lead the membrane to fatigue failure. The residual fatigue life of the membrane is determined here by using a cumulative-damage model2. A time-temperature-humidity dependent constitutive model and a multi-physics fuel cell finite element model are first developed to characterize the material properties and estimate the coupled mechano-hygral-thermal stress field in the membrane. Using this multi-level modeling approach, and with the aid of experimentally obtained fatigue data, fail-safe regions (see Figure) are identified for the case of accelerated stress test loading. For general loads simulating fuel cell operation, the distribution of stress-reversals is calculated for the load history using rain-flow counting algorithm. The regions of membrane that are more susceptible to fatigue damage are identified. It is also found that high amplitude stress cycles with low occurrence percentage are more detrimental than high occurrence low-amplitude cycles.This work was supported by Natural Sciences and Engineering Research Council of Canada, Simon Fraser University Community Trust Endowment Fund, Canada Research Chairs, Mitacs through the Mitacs Accelerate program, and Ballard Power Systems. 1 Borup, R., et al., Scientific aspects of polymer electrolyte fuel cell durability and degradation. Chemical Reviews, 2007. 107(10): p. 3904-3951. 2 Fatemi, A. and L. Yang, Cumulative fatigue damage and life prediction theories: a survey of the state of the art for homogeneous materials. International Journal of Fatigue, 1998. 20(1): p. 9-34. Figure 1

Residual fatigue life modeling of fuel cell membranes

Fuel cell membranes operate in a dynamically varying humid environment that introduces detrimental mechanical stresses which may lead the membrane to fatigue failure. The damage caused by these stresses is investigated using a cumulative-damage model and the residual membrane fatigue life is determined. A time-temperature-humidity dependent constitutive model is first developed based on material data obtained from a range of tensile experiments conducted at different conditions. The constitutive model is then incorporated into a multi-physics fuel cell finite element model developed to characterize the material behavior and estimate the coupled hygro-thermo-mechanical stress field in the membrane during fuel cell operation. Using this modeling approach, and with the aid of experimentally obtained fatigue data, fail-safe regions are identified for a given load spectrum corresponding to accelerated stress test and simulated drive cycle conditions. For each load spectrum, the distribution of stress-reversals is calculated for the load history using rainflow counting algorithm. In addition to identifying the regions of membrane that are more susceptible to fatigue damage, it is found that high amplitude stress cycles with low occurrence percentage are more severe than high occurrence low-amplitude cycles.

Multiple Impacts and Multiple-Compression Process in the Dynamics of Granular Chains

Abstract In this paper, we study the dynamics of one-dimensional chains composed of elastoplastic beads. Three uniform chains, which were experimentally studied in the existing literature, are taken as benchmark examples for manifesting wave propagation induced by multiple impacts between particles and by multiple-compression process in a single contact point. We perform simulations using an elastoplastic contact model developed recently for the binary contact of a sphere. Numerical results show good agreement with the experimental observations, including the profile and amplitude of the incident and reflected solitary waves, the travel time of the wave propagation, and the high-frequency oscillations residing in the high-amplitude stress wave. Our simulations also show that the multiple-compression process of the contact between particles is responsible for the oscillations residing in the pulse profile.

Low-cycle fatigue of ship hull damaged in collision

Abstract This work aims to extend the conventional post-accidental structural safety assessment by investigating the possibility of damage propagation in a damaged ship during the salvage period as a low-cycle high-stress fatigue process caused by fluctuating wave loads. Low-cycle loading can cause high-amplitude stresses at the edge of the damage opening, which consequently lead to high-stress concentrations. In the present study, a finite element model is created for an intact ship and 50 random damage scenarios for which the stress concentration factors are calculated. Random damage scenarios are generated using damage parameters obtained from historical data on ship accidents. The number of constant-amplitude wave load cycles to initiate a fatigue crack is calculated using the strain-life method defined according to Det Norske Veritas classification notes. Accumulated low-cycle fatigue damage is then estimated by Monte Carlo simulation, where individual vertical wave bending moment amplitudes are drawn as random numbers according to Weibull distribution. Parametric analysis is also performed in order to investigate the influence of cyclic material properties, sea environment and the number of towing days on the overall low-cycle fatigue damage accumulation. Realistic sea state scenarios are selected in order to investigate the influence of low-cycle fatigue damage accumulation during some specific constant sea states, enabling to determine marginal sea conditions for a safe rescue operation.

Effect of joint thickness on seismic response across a filled rock fracture

This study aims to investigate the influence of joint thickness on seismic response across a filled fracture with strong nonlinear deformability. To simulate seismic attenuation of thicker joints subject to high-amplitude stress waves, the split Hopkinson pressure bar is utilised to generate normally incident P wave and the dry quartz sand is used to simulate the filled joints. Three joint thicknesses – that is 5, 10 and 15 mm, are studied under identical incident waves. The stress–strain response of the filling materials is described by Barton–Bandis model having different loading–unloading behaviours. The initial stiffness and the maximum allowable closure of the joints changing with the joint thickness are studied. The thicker joints result in lower initial stiffness and cause lower seismic wave transmission across the fracture. The high-amplitude stress strengthens the nonlinearity of the filling materials and increases the stiffness. Besides, the seismic attenuation factor Q, derived from the energy dissipation, is lower than that computed by the transmission coefficient due to the frequency filtering.

Effect of Filling Humidity on the Propagation of High-Amplitude Stress Waves through an Artificial Joint

Abstract The purpose of this article is to further investigate the seismic response of an artificial filled joint under high-amplitude stress waves considering the effect of filling humidity, following our earlier work on dry infill. A steel split Hopkinson pressure bar system is utilized to induce high-amplitude stress waves to the filled joint. In this study, the wet infill is a mixture composed of quartz sand, kaolinite clay, and water. It is found that when the water content is relatively low, i.e., 8.25 %, the seismic response of the joint with wet infill is similar to that of the joint with dry infill, as shown in the literature. When the stress wave amplitude increases, the infill is progressively compacted and the transmission coefficient increases. However, there exists a crushing deformation stage for the infill in which many particles are crushed, and the transmission coefficient decreases as the incident wave amplitude increases. The water in the infill can reduce the friction between grains, which may lead to the decrease of the joint stiffness. As a result, the transmission coefficient is smaller than the case with dry infill under similar loading conditions. When the water content is moderate, such as 16.75 %, particles are very difficult to crush and the infill dominantly experiences compaction even when loaded by very high-amplitude stress waves. Consequently, the transmission coefficient through the wet infill always increases with the increase of the incident wave amplitude. When the infill is fully saturated (water content = 25.0 %), it can only experience approximately elastic deformation, and few particles can be crushed. In this case, the transmission coefficient is independent of the incident wave amplitude. When the infill is dry or fully saturated, the transmission coefficient is insensitive to the amplitude of the incident wave.

Effect of high-amplitude loading on accumulated fatigue damage under variable-amplitude loading in 316 stainless steel

To investigate whether high-amplitude stress and cycle ratios affect cumulative fatigue damage below the fatigue limit in 316 stainless steel, fatigue tests were carried out under incorporating high- (fixed at 277 MPa) and low-amplitude stress in which high stress level was set higher than previous applied (fixed at 260 MPa) and low stresses were below the fatigue limit with varying cycle ratios. Cumulative fatigue damage values for various cycle ratios calculated via previously proposed approach are inaccurate, it indicates that both high-amplitude stress and cycle ratios affect cumulative fatigue damage below the fatigue limit. In additional, saturation cycles of low-amplitude stress in one test block which can contribute to fatigue damage effectively is less than previous value. Thus, a rational evaluative approach for predicting cumulative fatigue damage below the fatigue limit has been developed on the basis of incorporating high- and low-amplitude stress tests outcomes, whose accuracy has been proved by repeated four-step and random-amplitude stress tests.

Relationships between along-fault heterogeneous normal stress and fault slip patterns during the seismic cycle: Insights from a strike-slip fault laboratory model

We use a strike-slip fault analog model to study experimentally the role played by along-fault non-uniform and asymmetric applied normal stress on both coseismic slip and long-term fault behavior. Our model is based on a visco-elasto-plastic multi-layered rheology that allows to produce several hundreds of scaled analog microquakes and associated seismic cycles. Uniform or heterogeneous applied normal stress along the fault plane is imposed and maintained constant during the whole experiment durations. Our results suggest that coseismic slip patterns are strongly controlled by spatial normal stress variations and subsequent accumulated shear stress along fault strike. Major microquakes occur preferentially in zones of major shear stress asperities. Coseismic slip distributions exhibit a pattern similar to the along-fault applied normal stress distribution. The occurrence of isolated low to moderate microquakes where residual stresses persist around secondary stress asperities, indicates that stress conditions along the fault also control the whole variability of fault slip events. Moreover, when fault slip stability conditions are modulated by normal stress distribution, our experiments suggest that the along-fault stress heterogeneity influences the seismic cycle regularity and, consequently, long-term fault slip behavior. Uniform applied normal stress favors irregular seismic cycles and the occurrence of earthquakes clustering, whereas non-uniform normal stress with a single high amplitude stress asperity generates strong characteristic microquake events with stable return periods. Together our results strengthen the assumption that coseismic slip distribution and earthquake variability along an active fault may provide relevant information on long term tectonic stress and could thus improve seismic hazard assessment.

Numerical Simulation on Seismic Response of the Filled Joint under High Amplitude Stress Waves Using Finite-Discrete Element Method (FDEM).

This paper numerically investigates the seismic response of the filled joint under high amplitude stress waves using the combined finite-discrete element method (FDEM). A thin layer of independent polygonal particles are used to simulate the joint fillings. Each particle is meshed using the Delaunay triangulation scheme and can be crushed when the load exceeds its strength. The propagation of the 1D longitude wave through a single filled joint is studied, considering the influences of the joint thickness and the characteristics of the incident wave, such as the amplitude and frequency. The results show that the filled particles under high amplitude stress waves mainly experience three deformation stages: (i) initial compaction stage; (ii) crushing stage; and (iii) crushing and compaction stage. In the initial compaction stage and crushing and compaction stage, compaction dominates the mechanical behavior of the joint, and the particle area distribution curve varies little. In these stages, the transmission coefficient increases with the increase of the amplitude, i.e., peak particle velocity (PPV), of the incident wave. On the other hand, in the crushing stage, particle crushing plays the dominant role. The particle size distribution curve changes abruptly with the PPV due to the fragments created by the crushing process. This process consumes part of wave energy and reduces the stiffness of the filled joint. The transmission coefficient decreases with increasing PPV in this stage because of the increased amount of energy consumed by crushing. Moreover, with the increase of the frequency of the incident wave, the transmission coefficient decreases and fewer particles can be crushed. Under the same incident wave, the transmission coefficient decreases when the filled thickness increases and the filled particles become more difficult to be crushed.

Residual Stress Distribution and Microstructure Evolution of AA 6061-T6 Treated by Warm Laser Peening

The aim of this paper is to study the effects of laser peening (LP) on the residual stress distribution and microstructure evolution of AA 6061-T6 under different temperatures. A laser peening experiment was conducted on the square-shape samples by using single spot and 50% overlap shock. Three-dimensional surface morphologies of treated samples were observed. The influence of peening temperature on the distribution of compressive residual stress was analyzed. An optical microscope (OM) and a transmission electron microscope (TEM) were employed to observe the microstructure evolution of the samples before and after LP. The results indicate that, as the peening temperature increases, the micro-hardness increases first and then decreases. The LP process induces high-amplitude compressive residual stress on the surface at different temperatures even if the compressive residual stress slightly reduces with increases in temperature. The maximum compressive residual stress affected layer depth is about 0.67 mm, appearing at a temperature of 160 °C. The OM test revealed that the grain size was significantly decreased after warm laser peening (WLP) and that the average value of grain size was reduced by 50%. The TEM test shows that more dislocation tangles were produced in AA 6061-T6 after WLP; compared to the LP process, the precipitate-dislocation interaction can benefit both strength and ductility for AA 6061-T6, thus enhancing the mechanical properties of the material.

Modeling the dynamic behavior of turbine runner blades during transients using indirect measurements

Turbine start-up transients are induced by the wicket gates opening sequence and generate high amplitude stress cycles. These stress cycles have a detrimental effect leading to faster crack growth in the runner blades. Using a series of direct measurements taken on a prototype runner in order to find the optimal start-up parameters exposes both the runner and the instrumentation to a series of successive damaging transient events during the optimization process. To solve this, finding sensors strongly correlated to strain gauges and whose signals can be easily obtained to identify a model to predict the strain, instead of directly measuring it, would reduce the risk, cost and downtime associated with a measurement campaign. This paper shows that turbine shaft torsion measurements is highly correlated to the strain at a runner blade hotspot, and we demonstrate that the ARMAX model can be used to represent the dynamic system in order to minimize the strain on blades.

Propagation of high amplitude stress waves through a filled artificial joint: An experimental study

This paper investigates the propagation of high amplitude stress waves through a filled joint using a modified steel split Hopkinson pressure bar (SHPB) system. Quartz sand fillings with various thickness are placed in a steel tube and then sandwiched between the incident and transmitted bars to simulate the filled rock joints. Using SHPB, the incident stress waves with similar frequency spectrum but varying amplitude are induced to load the artificial filled joints. The particle size distributions of the fillings after tests are analyzed. It is discovered that as the amplitude of the incident wave increases, the fillings experience three stages of deformation: initial compaction, crushing and crushing and compaction. In the initial compaction stage and the crushing and compaction stage, the fillings are mainly compacted, and thus the transmission coefficient increases with the amplitude of the incident wave. However in the crushing stage, the transmission coefficient decreases with the increase of the amplitude of the incident wave. This is a result of energy consumption due to particle crushing. The observed dependence of the transmission coefficient on the wave amplitude is consistent with the particle size distribution of recovered fillings.

Nature of short, high-amplitude compressive stress pulses in a periodic dissipative laminate.

We study the evolution of high-amplitude stress pulses in periodic dissipative laminates taking into account the nonlinear constitutive equations of the components and their dissipative behavior. Aluminum-tungsten laminate was selected due to the large difference in acoustic impedances of components, the significant nonlinearity of the aluminum constitutive equation at the investigated range of stresses, and its possible practical applications. Laminates with different cell size, which controls the internal time scale, impacted by plates with different thicknesses that determine the incoming pulse duration, were investigated. It has been observed that the ratio of the duration of the incoming pulse to the internal characteristic time determines the nature of the high-amplitude dissipative propagating waves-a triangular oscillatory shock-like profile, a train of localized pulses, or a single localized pulse. These localized quasistationary waves resemble solitary waves even in the presence of dissipation: The similar pulses emerged from different initial conditions, indicating that they are inherent properties of the corresponding laminates; their characteristic length scale is determined by the scale of mesostructure, nonlinear properties of materials, and the stress amplitude; and a linear relationship exists between their speed and amplitude. They mostly recover their shapes after collision with phase shift. A theoretical description approximating the shape, length scale, and speed of these high-amplitude dissipative pulses was proposed based on the Korteweg-de Vries equation with a dispersive term determined by the mesostructure and a nonlinear term derived using Hugoniot curves of components.