Showing papers on "Stress relaxation published in 2007"

Fatigue resistance of aligned carbon nanotube arrays under cyclic compression.

Abstract: Structural components subject to cyclic stress can succumb to fatigue, causing them to fail at stress levels much lower than if they were under static mechanical loading1 However, despite extensive research into the mechanical properties of carbon nanotube structures2,3,4,5,6,7,8,9 for more than a decade, data on the fatigue behaviour of such devices have never been reported We show that under repeated high compressive strains, long, vertically aligned multiwalled nanotubes exhibit viscoelastic behaviour similar to that observed in soft-tissue membranes10,11 Under compressive cyclic loading, the mechanical response of the nanotube arrays shows preconditioning, characteristic viscoelasticity-induced hysteresis, nonlinear elasticity and stress relaxation, and large deformations Furthermore, no fatigue failure is observed at high strain amplitudes up to half a million cycles This combination of soft-tissue-like behaviour and outstanding fatigue resistance suggests that properly engineered nanotube structures could mimic artificial tissues, and that their good electrical conductivity could lead to their use as compliant electrical contacts in a variety of applications

The Universality of NBTI Relaxation and its Implications for Modeling and Characterization

Abstract: As of date many NBTI models have been published which aim to successfully capture the essential physics. As such, these models have mostly focused on the stress phase. The relaxation phase, on the other hand, has not received as much attention, possibly because of the contradictory results published so far. Particularly noteworthy are the very long relaxation tails of almost logarithmic nature, which cannot be successfully described by the reaction-diffusion model. The authors argue that understanding the nature of the relaxation phase could hold the key to unraveling the underlying NBTI mechanism. In particular, the authors stipulate that the relaxation phase follows a universal relaxation law, demonstrate the valuable consequences resulting therefrom, and use this universality to classify presently available NBTI models.

Self-Assembly and Stress Relaxation in Acrylic Triblock Copolymer Gels

Abstract: The structure and relaxation behavior of thermoreversible gels made with poly(methyl methacrylate)−poly(n-butyl acrylate)−poly(methyl methacrylate) [PMMA−PnBA−PMMA] triblock copolymers in 2-ethylhexanol, a midblock selective solvent, were studied by small-angle X-ray scattering (SAXS) and rheology. Effects of endblock length, endblock fraction, and gel concentration on the gel properties were investigated. A dramatic decrease in SAXS intensity was observed over a 20 °C interval where the gel transitions smoothly from elastic to viscous behavior. SAXS patterns were fit with a Percus−Yevick disordered hard-sphere model from which aggregation number and average domain spacing were calculated. Aggregation number increases with increasing gel concentration and endblock length. Increasing the endblock length from 9K to 25K increases the relaxation time of a gel with a polymer volume fraction of 0.15 by a factor of 10^6. For a given triblock endblock fraction and molecular weight, the micelle aggregation number is strongly correlated to the gel relaxation time. Arrhenius behavior with an effective activation energy of ~550 kJ/mol was observed for all triblocks and concentrations. This very high effective energy barrier describes gels relaxation behavior over a 40 °C temperature range, where the relaxation times vary by a factor of 10^(10).

Creep and creep fracture of polycrystalline copper

Abstract: Normal creep curves are recorded over extended stress ranges at 686–823 K for fine-grain copper. Analyses of the curve shape variations, together with the results of stress change experiments, do not support the view that a transition from dislocation to diffusional creep mechanisms occurs with decreasing stress. Instead, the observed behaviour patterns suggest that dislocation processes are dominant at all stress levels. However, strain accumulation within the grains becomes progressively less important as deformation is increasingly confined to the grain boundary zones when the stress is reduced below the yield stress at the creep temperature. New approaches are then introduced for rationalization of creep rate and creep life measurements, which account for the data trends taken as evidence for major mechanism changes when the creep properties are described using power law relationships.

The relation between collagen fibril kinematics and mechanical properties in the mitral valve anterior leaflet.

Abstract: We have recently demonstrated that the mitral valve anterior leaflet (MVAL) exhibited minimal hysteresis, no strain rate sensitivity, stress relaxation but not creep (Grashow et al., 2006, Ann Biomed Eng., 34(2), pp. 315-325; Grashow et al., 2006, Ann Biomed. Eng., 34(10), pp. 1509-1518). However, the underlying structural basis for this unique quasi-elastic mechanical behavior is presently unknown. As collagen is the major structural component of the MVAL, we investigated the relation between collagen fibril kinematics (rotation and stretch) and tissue-level mechanical properties in the MVAL under biaxial loading using small angle X-ray scattering. A novel device was developed and utilized to perform simultaneous measurements of tissue level forces and strain under a planar biaxial loading state. Collagen fibril D-period strain (epsilonD) and the fibrillar angular distribution were measured under equibiaxial tension, creep, and stress relaxation to a peak tension of 90 N/m. Results indicated that, under equibiaxial tension, collagen fibril straining did not initiate until the end of the nonlinear region of the tissue-level stress-strain curve. At higher tissue tension levels, epsilonD increased linearly with increasing tension. Changes in the angular distribution of the collagen fibrils mainly occurred in the tissue toe region. Using epsilonD, the tangent modulus of collagen fibrils was estimated to be 95.5+/-25.5 MPa, which was approximately 27 times higher than the tissue tensile tangent modulus of 3.58+/-1.83 MPa. In creep tests performed at 90 N/m equibiaxial tension for 60 min, both tissue strain and epsilonD remained constant with no observable changes over the test length. In contrast, in stress relaxation tests performed for 90 min epsilonD was found to rapidly decrease in the first 10 min followed by a slower decay rate for the remainder of the test. Using a single exponential model, the time constant for the reduction in collagen fibril strain was 8.3 min, which was smaller than the tissue-level stress relaxation time constants of 22.0 and 16.9 min in the circumferential and radial directions, respectively. Moreover, there was no change in the fibril angular distribution under both creep and stress relaxation over the test period. Our results suggest that (1) the MVAL collagen fibrils do not exhibit intrinsic viscoelastic behavior, (2) tissue relaxation results from the removal of stress from the fibrils, possibly by a slipping mechanism modulated by noncollagenous components (e.g. proteoglycans), and (3) the lack of creep but the occurrence of stress relaxation suggests a "load-locking" behavior under maintained loading conditions. These unique mechanical characteristics are likely necessary for normal valvular function.

Indentometric analysis of in vivo skin and comparison with artificial skin models.

Abstract: Background/purpose: The mechanical properties of the skin have been previously analyzed by a number of different techniques including torsional analysis, cutometery, gas-bearing electrodynamometry, etc. The objective of this work is to present quantitative analysis of skin rheology by a technique termed indentometry. Methods: The instrument used was a texture analyzer, which is a mechanical tensiometer simulating the process of touch. The experiments were carried out on human subjects as well as on artificial skin models. They included indentometry tests performed by using spherical probes with various geometrical dimensions as well as stress relaxation and creep experiments. The experimental data were interpreted by using the Hertz theory of contact mechanics and by calculation of fundamental parameters such as the modulus of elasticity. Results: The calculated Young's modulae for skin models ranged from 5.5 × 104 to 17.7 × 104 N/m2, while the corresponding values for forearm and facial skin of ten panelists were found to be in the range of 0.7 × 104 –3.3 × 104 N/m2. In addition, stress relaxation and creep experiments were conducted, which permitted the assessment of the viscoelastic properties of skin. The results of these measurements were interpreted within the framework of the Kelvin–Voigt model of delayed elasticity leading to the calculation of viscosities and relaxation times. Indentomeric data, obtained by varying the diameter of the indentor and the indentation depth, are also discussed. Conclusion: The indentometric analysis for both in vivo skin and artificial skin models could be interpreted by using the Hertz theory of contact mechanics. The loading and unloading indentometric curves could be used to assess the viscoelasticity of the investigated materials while creep and stress relaxation processes were analyzed quantitatively by the Kelvin–Voigt model with one relaxation time.

Interpenetrating networks of elastomers exhibiting 300% electrically-induced area strain

Abstract: Mechanical prestrain is generally required for most electroelastomers to obtain high electromechanical strain and high elastic energy density. However, prestrain can cause several serious problems, including a large performance gap between the active materials and packaged actuators, instability at interfaces between the elastomer and prestrain-supporting structure, and stress relaxation. Difunctional and trifunctional liquid additives were introduced into 400% biaxially prestrained acrylic films and subsequently cured to form the second elastomeric network. The goal of this research was to determine the effect of different functional additives and concentrations on the microstructure, the mechanical properties, and the actuation of composite films. In the as-obtained interpenetrating polymer networks (IPNs), the additive network can effectively support the prestrain of the acrylic films and as a result, eliminate the external prestrain-supporting structure. However, the large amount of additive used to completely preserve prestrain was found to make the films too stiff, causing damage to IPN composite films. Furthermore, the interpenetrating network formed from a trifunctional monomer is more effective than that formed from a difunctional monomer in supporting the high tension of the VHB network. This high efficiency trifunctional additive leads to the enhancement of the breakdown field, due to less damage on the microstructure. The IPN composite films without external prestrain exhibit electrically-induced strains up to 300% in area, comparable to those of VHB 4910 films under high prestrain conditions.

Changes in electrical resistance of carbon‐black‐filled silicone rubber composite during compression

Abstract: We studied the changes in the electrical resistance of carbon black filled silicone rubber composite, which is the sensitive element of the flexible force sensor, as a function of time during compression. The experimental results show that there is a sudden increase of the electrical resistance along with the sudden increase of the stress immediately after the compression. When the sample strain is kept constant, the electrical resistance and the stress both decay with time. The data of the stress relaxation and the resistance relaxation both can be fitted by the linear combination of two exponential functions. Based on the shell structure theory, the experimental phenomena are explained from the view that the uniaxial pressure induces the changes in the effective conductive paths. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2700–2706, 2007

Microphase separation, stress relaxation and creep behavior of polyurethane nanocomposites

Abstract: The microphase separation of polyurethane (PU) nanocomposite was studied. The result suggests that the addition of clay leads to a decrease in the size of hard domain and an increase in the degree of microphase separation. The stress relaxation and creep behavior of blank PU and PU/clay nanocomposites were investigated. The relaxation time spectrum and retardant time spectrum were derived according to the generalized Maxwell model and Voigt model with a Tikhonov regularization method. The characteristic relaxation time was identified with the corresponding relaxation process. At a small strain, the relaxation was mainly attributed to uncoiling/disentangling of soft segment chain network in the soft phase, with a single characteristic relaxation time in the range of 5~100s. The increase in the hard segment content leads to a decrease in the relaxation time, and the addition of clay leads to an increase in the relaxation time. At large strains, the multi-peak relaxations occurred, and they were attributed to the breakup of interconnected hard domains and pull-out of soft segment chains from hard domains, together with the disentangling of soft segment chain network in the soft phase. The creep results are in consistent with that of the stress relaxation. The relaxation and creep behavior were related to microphase separation of polyurethane. This study suggested that the relaxation spectrum H(i´) can be used to examine the complicated relaxation processes for a multi-phase and multi-component polymer system.

Mechanical Characterization of Brain Tissue in High-Rate Compression

Abstract: Mechanical properties of brain tissue in high strain region are indispensable for the analysis of brain damage during traffic accidents. However, accurate data on the mechanical behavior of brain tissue under impact loading condition are sparse. In this study, mechanical properties of porcine brain tissues were characterized in their cylindrical samples cored out from their surface. The samples were compressed in their axial direction at strain rates ranging from 1 to 50 s-1. Stress relaxation test was also conducted following rapid compression with a rise time of ∼30 ms to different strain levels (20-70%). Brain tissue exhibited stiffer responses under higher impact rates: initial elastic modulus was 5.7±1.6, 11.9±3.3, 23.8±10.5 kPa (mean±SD) for strain rate of 1, 10, 50 s-1, respectively. We found that stress relaxation K(t,e) could be analysed in time and strain domains separately. The relaxation response could be expressed as the product of two mutually independent functions of time and strain as: K(t,e)=G(t)σe(e), where σe(e) is an elastic response, i.e., the peak stress in response to a step input of strain e, and G(t) is a reduced relaxation function: G(t)=0.642e-t/0.0207+0.142e-t/0.482+0.216e-t/18.9, i.e., the time-dependent stress response normalized by the peak stress. The reduced relaxation function obtained here will serve as a useful tool to predict mechanical behavior of brain tissue in compression with strain rate greater than 10 s-1.

Fluorescence studies of confinement in polymer films and nanocomposites: Glass transition temperature, plasticizer effects, and sensitivity to stress relaxation and local polarity

Abstract: Confinement effects in polystyrene and poly(methyl methacrylate) films and nanocomposites are studied by fluorescence. The ability to employ an intensive measurable, the excited-state fluorescence lifetime, in defining the glass transition temperature, Tg, of polymers is demonstrated and compared to the use of an extensive measurable, fluorescence intensity. In addition, intrinsic fluorescence from the phenyl groups in polystyrene is used to determine the Tg-confinement effect in films as thin as ~15 nm. The decrease in Tg with decreasing film thickness (below ∼60 nm) agrees well with results obtained by extrinsic pyrene fluorescence. Dye label fluorescence is used to quantify the enhancement in Tg observed with decreasing thickness (below ~90 nm) in poly(methyl methacrylate) films; addition of 2–4 wt% dioctyl phthalate plasticizer reduces or eliminates the Tg-confinement effect in films down to 20 nm thickness. Intrinsic polystyrene fluorescence, which is sensitive to local conformation, is used to quantify the time scales (some tens of minutes) associated with stress relaxation in thin and ultrathin spin-coated films at Tg + 10 K. Finally, the shape of the fluorescence spectrum of pyrene doped at trace levels in polystyrene films and polystyrene-silica nanocomposites is used to determine effects of confinement on microenvironment polarity.

A thermo-viscoelastic analysis of process-induced residual stress in fibre-reinforced polymer–matrix composites

Abstract: Process-induced residual stress in fibre-reinforced thermoset polymer–matrix composites was analysed using a thermo-viscoelastic micromechanical model and the finite element method. A three-dimensional unit cell with glass fibre and epoxy polymer–matrix, representing the periodic microstructure of unidirectional fibre-reinforced composites, was considered to compute cure residual stress of fibre composites induced by chemical shrinkage of the epoxy resin and thermal cooling contraction of the whole fibre and resin system. The constitutive behaviour of the epoxy matrix was described by a cure and temperature-dependent viscoelastic material model. Compared to an elasticity solution, a reduction in residual stress was predicted due to the stress-relaxation caused by the viscoelastic behaviour of the epoxy matrix. Calculated residual stress shows strong dependency on the fibre volume fraction and fibre packing. After the cure process is complete, residual stress tends to relax to a constant value. The effect of residual stress on damage and failure of the model was also studied using the maximum stress failure criterion combined with a post-failure stiffness reduction technique. Damage onset, in terms of the location and the load level, was shown to be clearly influenced by the residual stress for both normal and shear loading. Initial and final failure envelopes, predicted for biaxial normal (longitudinal and transverse) loading and combined shear (longitudinal) and normal (transverse) loading, were shown to be shifted and contracted by the inclusion of residual stress. For final failure, residual stress was seen to have little effect on the load levels for longitudinal failure but greatly affected the load levels for transverse and shear failure. Residual stress could be detrimental or beneficial depending on the state of existing residual stress and the loading conditions.

Nanotwin formation in copper thin films by stress/strain relaxation in pulse electrodeposition

Abstract: Nanotwinned copper has been shown to greatly improve the yield strength while maintaining good electrical conductivity. It has great potential to be incorporated in the very-large-scale integration of Cu interconnect technology. The influence of stress/strain on nanotwin formation is studied using first principles calculations of the total crystal binding energy. Under biaxial stress, the total energy of strained Cu can be larger than that of strain-relaxed periodic nanotwinned Cu. We propose that, during pulse electrodeposition of Cu films, highly strained Cu can undergo recrystallization and grain growth to relax stress and form strain-relaxed nanotwins.

Effect of nano-particles on the creep resistance of Mg-Sn based alloys

Abstract: A study has been made on the microstructure and high-temperature properties of die-cast Mg–Sn–Al–Si (TAS831) alloy. Microstructure of TAS831 alloy is characterized by the presence of thermally stable Mg2Sn nano-particles within the matrix. It also contains Mg2Sn and Mg2Si particles along grain boundaries. It has been shown that the morphology of matrix Mg2Sn particles varies depending on cooling rate, forming rod-type Mg2Sn particles at fast cooling rates and polygonal particles at slow cooling rates. Elevated-temperature creep tests show that the die-cast TAS831 alloy has superior creep resistance compared with commercial AZ91 alloy. Analyses of creep and load relaxation at elevated temperatures in the context of internal-variable theory indicate that the presence of thermally stable nano-particles within the matrix increases the resistance to dislocation movement, thereby improving the creep resistance.

Strain-Induced Crystallization of Crosslinked Natural Rubber As Revealed by X-ray Diffraction Using Synchrotron Radiation

Abstract: Strain-induced crystallization (SIC) of natural rubber (NR) has been extensively studied even before the advent of macromolecular physics. However, there are still some unsolved basic issues in this field. In this review article, classic studies on SIC of NR are briefly introduced, and then recent results by synchrotron X-ray diffraction studies in separate papers by different authors are categorized and interpreted on the basis of molecular models. Cyclic deformation experiments provided information on partial orientation of the network-chains, on nucleation and morphological changes of crystals and on stress field around the strain-induced crystals. On the other hand, experiments under constant strain provided information on kinetics of SIC, on stress relaxation due to SIC, and so on. The experimental results could be explained under the assumption that the SIC is dominated by strain, and that the crystals are of folded-chain type. However, in order to consistently explain the various experimental results, we have to establish a unified molecular model of the network structure.

A Recruitment Model of Quasi-Linear Power-Law Stress Adaptation in Lung Tissue

Abstract: When lung tissue is subjected to a step in strain, it exhibits a stress adaptation profile that is a power function of time. Furthermore, this power function is independent of the strain, even though the quasi-static stress–strain relationship of the tissue is highly nonlinear. Such behavior is known as quasi-linear viscoelasticity, but its mechanistic basis is unknown. We describe a model of soft tissue rheology based on the sequential recruitment of Maxwell bodies. The model is homogeneous in its elemental constitutive properties, yet predicts both power-law stress relaxation and quasi-linear viscoelasticity even when the stress–strain behavior of the model is nonlinear. The model suggests that stress relaxation in lung tissue could occur via a sequence of micro-rips that cause stresses to be passed from one local stress bearing region to another.

Impression creep behavior of lead-free Sn–5Sb solder alloy

Abstract: Creep behavior of the lead-free Sn–5Sb solder alloy in the cast and wrought conditions was investigated by impression testing. The tests were carried out under constant stress in the range of 18–135 MPa and at temperatures in the range of 298–403 K. Assuming a power law relationship between the impression rate and the punching stress, stress exponents of 2.8 and activation energies of 41.3 kJ mol −1 were determined for the wrought material over the whole stress and temperature ranges studied. For the cast condition, however, stress exponents of 5.4 and 11.4 and activation energies of 53.8 and 75.8 kJ mol −1 were obtained at low and high stresses, respectively. The n value of 2.8 and the activation energy of 41.3 kJ mol −1 , which is very close to the activation energy for grain boundary diffusion of β-Sn, together with a very fine grain size of 4.5 μm and a uniform distribution of fine SnSb particles, may suggest that grain boundary sliding is the dominant creep mechanism in the wrought condition. For the cast material with a coarse grain size of 280 μm, the low stress regime activation energy of 53.8 kJ mol −1 , which is close to that of the self diffusion of pure Sn, and a stress exponent of 5.4 suggest that the operative creep mechanism is dislocation climb. This behavior is in contrast to the high stress regime in which, the n = 11.4 and Q = 75.8 kJ mol −1 are indicative of a dislocation creep mechanism.

Interface effect on metal-insulator transition of strained vanadium dioxide ultrathin films

Abstract: The interface effects on the metal-insulator transition (MIT) of strained VO2 ultrathin films grown epitaxially on TiO2 (001) single crystal substrate were investigated. Varying the surface conditions of TiO2 substrate, such as the roughness and the surface reconstructions, produced the remarkable changes in the MIT events of VO2 thin films, including the transition temperature and the abruptness. The presence of the surface reconstructions was found to be detrimental for applying effectively strain effects due to the strain relaxation in the c axis of VO2 thin films. The abrupt MIT in strained VO2 thin films, deposited on the substrate without such detrimental surface reconstructions, was successfully maintained down to around 5nm film thickness.

Annealing of intrinsic stresses in sputtered TiN films: The role of thickness-dependent gradients of point defect density

Abstract: Morphology, structure and thermal behavior of magnetron sputtered TiN thin films with the thickness in the range 100–2900 nm are characterized. The films are thermally cycled and the relationship between film thickness, defect density and the intrinsic stress relaxation is analyzed. The results indicate that the residual stresses in the as-deposited films and the amount of stress relaxation depend decisively on the specific depth gradient of point defects originating from film evolution during growth. The compressive stresses, representing different driving forces and the amount of stress relaxation decrease, while the onset temperature of stress relaxation increases with increasing film thickness.

Non-linear mechanical response of the Red Blood Cell

Abstract: We measure the dynamical mechanical properties of human red blood cells. Single cell response is measured with optical tweezers. We investigate both the stress relaxation following a fast deformation, and the effect of varying the strain rate. We find a power law decay of the stress as a function of time, down to a plateau stress, and a power law increase of the cell's elasticity as a function of the strain rate. Interestingly, the exponents of these quantities violate the linear superposition principle, indicating a nonlinear response. We propose that this is due to breaking of a fraction of the crosslinks during the deformation process. The Soft Glassy Rheology Model accounts for the relation between the exponents we observe experimentally. This picture is consistent with recent models of bond remodeling in the red blood cell's molecular structure. Our results imply that the blood cell's mechanical behavior depends critically on the deformation process.

Growth and structure evaluation of strain-relaxed Ge1- xSnx buffer layers grown on various types of substrates

Abstract: We have performed growth and structure evaluation of strain-relaxed Ge1−xSnx buffer layers grown on Si(0 0 1), virtual Ge(0 0 1) and bulk Ge(0 0 1) substrates. In the case of Si(0 0 1), amorphous Ge1−xSnx phases are partially formed as well as many threading dislocations in Ge0.98Sn0.02 layers. Employing virtual Ge substrates to reduce the lattice mismatch at the interface leads to epitaxial Ge0.978Sn0.022 layers with a flat surface. Most of threading dislocations in the Ge0.978Sn0.022 layer comes from pre-existing ones in the virtual Ge substrate and propagates laterally, leaving misfit segments at the Ge0.978Sn0.022/virtual Ge interface, after post-deposition annealing (PDA). This simultaneously results in the reduction of threading dislocation density and the promotion of strain relaxation. In the case of bulk Ge(0 0 1), although low threading dislocation density can be achieved, less than 106 cm−2, the film exhibits surface undulation and a lesser degree of strain relaxation even after PDA.

Stress-corrosion constitutive laws as a possible mechanism of intermediate-term and short-term seismic quiescence

Abstract: SUMMARY Three types of seismic quiescence are recognized in the earthquake cycle. Post-seismic quiescence occurs due to the stress drop caused by a previous major earthquake, and results in long-term seismic gaps; infermediate-term quiescence occurs due to a small stress relaxation in the volume around the next mainshock area; and short-term quiescence during foreshock sequences occurs due to slipweakening or dilatancy hardening concentrated on the nucleation point. In this paper a single quantitative model for intermediate-term and short-term quiescence is developed from (a) observation of subcritical crack growth due to stress corrosion, and (b) a general model-for subcritical damage development where a fractal population of fractures results, irrespective of the underlying mechanism. In the former the stress intensity K of a single dominant macrocrack is the appropriate constitutive variable, while the latter more general formulation relies on a mean potential strain energy release rate (G) proportional to (a) the square of the applied effective stress and (b) the mean fracture length. Stress corrosion provides an important concrete example as well as a useful analogy for interpreting the general theory. We then consider the effect of a stress decrease in the intermediate-term on seismic event rates using the approporiate constitutive laws for both variables. Simple calculations for a material of stress corrosion index n = 30 show that a 45 per cent reduction in event rates is consistent with only a 2 per cent reduction in K, and a 90 per cent reduction results from only a 7 per cent decrease in K. A similar order of magnitude of quiescence can be predicted theoretically by considering the effect of similar small changes in (G) for a fractal pupulation of faults or cracks averaged over a range of length scales. Such intermediate-term quiescence occurs in the model when K or (C) decreases because of the reduction in applied stress, during a phase of strain softening late in the earthquake cycle. Such a decrease in K or (G) moves the system temporarily further from the failure condition K = K, (or (G) = (G),) and is therefore stable. This temporary stability is consistent with the relatively long duration of intermediate-term quiescence (months or years). Observed intermediate-term seismic quiescences are relatively easily explained in the model by stable, regional decreases in stress in a volume much larger than the mainshock area, but prior to the period when concentrated accelerating crack growth in the nucleation zone dominates the strain softening. The general mechanism is compared to the Kaiser effect as a possible alternative explanation for intermediate-term seismic quiescence. The two models are not necessarily mutually exclusive, though the Kaiser effect involves local stress and strain relaxation, and predicts more abrupt changes in event rate when the stress decreases. Short-term quiescence is also a feature of the general model, and is predicted when a concomitant decrease in seismic b-value occurs, as the larger events near the nucleation zone begin to dominate the stress relaxation, thereby inhibiting fracture

Effect of temperature on the tensile properties of an as-cast aluminum alloy A319

Abstract: The tensile properties of an as-cast A319 alloy were investigated as a function of temperature. It was found that the A319-Al alloy remained inherently brittle in the temperature range of −90 °C T T > 270 °C the mode of failure shifts to being essentially ductile by the development of numerous dimples. Under these conditions the development of critical stresses at matrix/particle interfaces needed for brittle fracture no longer occurs. Apparently, at these temperatures thermally activated processes lead to significant relaxation of stress incompatibilities at particle/matrix interfaces and results in appreciable plastic deformation within the matrix.