Amount Of Stress Relaxation Research Articles

Effects of dwell time on the isothermal and thermomechanical fatigue behavior of 316LN stainless steel

Isothermal fatigue (IF) and thermomechanical fatigue (TMF) tests with and without tensile dwell time at various strain amplitudes were conducted on 316LN stainless steel. The results showed that the inelastic relaxation strain rate is more appropriate to characterize creep damage than the amount of stress relaxation. The introduction of tensile strain hold imparted a more deleterious effect on the TMF than the IF performance based on a statistical analysis of the number of secondary cracks and their size distribution, which was primarily attributed to the more severe oxidation-assisted cracking at the interface between the oxide scale and substrate. Most importantly, the lowest fatigue life occurred in the TMF test with tensile dwell time at the low strain amplitude of 0.4% rather than in the creep-fatigue test at the isothermal maximum temperature, indicating that it was not always conservative to predict the life of engineering components based on the IF data.

Mechanical Properties of Graphene Foam and Graphene Foam - Tissue Composites.

Graphene foam (GF), a 3-dimensional derivative of graphene, has received much attention recently for applications in tissue engineering due to its unique mechanical, electrical, and thermal properties. Although GF is an appealing material for cartilage tissue engineering, the mechanical properties of GF - tissue composites under dynamic compressive loads have not yet been reported. The objective of this study was to measure the elastic and viscoelastic properties of GF and GF-tissue composites under unconfined compression when quasi-static and dynamic loads are applied at strain magnitudes below 20%. The mechanical tests demonstrate a 46% increase in the elastic modulus and a 29% increase in the equilibrium modulus after 28-days of cell culture as compared to GF soaked in tissue culture medium for 24h. There was no significant difference in the amount of stress relaxation, however, the phase shift demonstrated a significant increase between pure GF and GF that had been soaked in tissue culture medium for 24h. Furthermore, we have shown that ATDC5 chondrocyte progenitor cells are viable on graphene foam and have identified the cellular contribution to the mechanical strength and viscoelastic properties of GF - tissue composites, with important implications for cartilage tissue engineering.

Glycosaminoglycans contribute to extracellular matrix fiber recruitment and arterial wall mechanics.

Elastic and collagen fibers are well known to be the major load-bearing extracellular matrix (ECM) components of the arterial wall. Studies of the structural components and mechanics of arterial ECM generally focus on elastin and collagen fibers, and glycosaminoglycans (GAGs) are often neglected. Although GAGs represent only a small component of the vessel wall ECM, they are considerably important because of their diverse functionality and their role in pathological processes. The goal of this study was to study the mechanical and structural contributions of GAGs to the arterial wall. Biaxial tensile testing was paired with multiphoton microscopic imaging of elastic and collagen fibers in order to establish the structure-function relationships of porcine thoracic aorta before and after enzymatic GAG removal. Removal of GAGs results in an earlier transition point of the nonlinear stress-strain curves [Formula: see text]. However, stiffness was not significantly different after GAG removal treatment, indicating earlier but not absolute stiffening. Multiphoton microscopy showed that when GAGs are removed, the adventitial collagen fibers are straighter, and both elastin and collagen fibers are recruited at lower levels of strain, in agreement with the mechanical change. The amount of stress relaxation also decreased in GAG-depleted arteries [Formula: see text]. These findings suggest that the interaction between GAGs and other ECM constituents plays an important role in the mechanics of the arterial wall, and GAGs should be considered in addition to elastic and collagen fibers when studying arterial function.

Low cycle fatigue of 2.25Cr1Mo steel with tensile and compressed hold loading at elevated temperature

A series of uniaxial strain-controlled fatigue and creep-fatigue tests of the bainitic 2.25Cr1Mo steel forging were performed at 455°C in air. Three different hold periods (30s, 120s, 300s) were employed at maximum tensile strain and compressive strain under fully reversed strain cycling. Both tensile and compressive holds significantly reduce the fatigue life. Fatigue life with tensile hold is shorter than that with compressive hold. A close relationship is found between the reduction of fatigue life and the amount of stress relaxation. Microstructural examination by scanning electron microscope reveals that strain hold introduces more crack sources, which can be probably ascribed to the intensified oxidation and the peeling-off of oxide layers. A modified plastic strain energy approach considering stress relaxation effect is proposed to predict the creep-fatigue life, and the predicted lives are in superior agreement with the experimental results.

Stress drop effects in time dependent behavior of quartz sand

Time effects in granular materials relate to crushing of grains, which is a time dependent process. Depending on the proximity of the load applied to a grain relative to the instantaneous failure load of the grain, the time increases as the load is decreased below the failure load. A study of grain crushing associated with stress drop-creep and stress drop-stress relaxation experiments on dense Virginia Beach sand consisting mainly of quartz grains is presented. In these experiments the triaxial specimens are isotropically consolidated to 8000 kPa, sheared at a constant rate up to a given deviator stress followed by either creep or stress relaxation over a given time period. After dismantling the triaxial specimens, the grain size distributions are determined and their changes, as expressed through Hardin's relative breakage factor, are related to the amount of energy input. Results of the stress drop-creep tests and the stress drop-stress relaxation tests show that creep deformations and the amount of stress relaxation are considerably reduced with increasing amounts of stress drop. Depending on the amount of stress drop, a delay is detected before creep deformation or stress relaxation is initiated. Multiple stress drop-creep experiments are also performed. Structuration effects, which describe changes in yield surface location during creep or stress relaxation and are due to change in grain configuration and interlocking with time, are not observed in the specimens of Virginia Beach sand.

Creep–fatigue interaction behavior of 316LN austenitic stainless steel with varying nitrogen content

Creep–fatigue interaction (CFI) behavior of 316LN stainless steel with varying nitrogen content (0.07, 0.14 and 0.22wt.%) is investigated at temperature of 873K under strain-controlled fatigue tests with a tensile-hold period of 1, 13.3 and 30min. Dynamic strain aging, thermal-recovery and/or dislocation–precipitate interactions are found to strongly control CFI deformation. At all the nitrogen contents in 316LN SS, the amount of stress relaxation and the associated relaxation strain rate decreased with increase in hold duration. Irrespective of the hold period, wavy-slip dislocation structures (cells, tangles, walls) are noticed at 0.07wt.% N, whereas dislocation structures such as planar slip bands impinging on grain boundary are noticed, in addition, at nitrogen contents above 0.07wt.%. Nitrogen content played a dual-role in influencing the CFI life. At 1-min hold where the failure is by transgranular plus intergranular fracture, increase in nitrogen from 0.07 to 0.14wt.% caused an improvement in CFI life. On the other hand, CFI life decreased with increasing nitrogen content at 13.3 and 30-min hold durations that induced significant intergranular damage. Intergranular damage is evidenced in the form of triple point cracks and interconnected networks of grain boundary decohesion.

Swelling and de-swelling of gels under external elastic deformation

We show that the equilibrium Poisson ratio of electrically neutral gels depends on their shear modulus. When immersed in a good solvent, gels increase their volume on imposed external deformation, but stiffer gels swell less and exhibit a larger Poisson ratio, closer to 0.5, while the gels with a higher solvent content (and correspondingly lower shear modulus) approach a Poisson ratio of 0.25. We monitor the full process of stress and shape relaxation after an instantaneous deformation by using the technique of digital image correlation (DIC), and show that the amount of stress relaxation in uniaxially strained gels is proportional to the shear modulus of the free swollen state and a change in effective strain. Experiments were conducted on polyacrylamide (PAAm) gels in a custom built setup to give the Poisson ratio to high accuracy and time resolution, as well as verification of homogeneous deformation in equilibrium. In addition to water, hydrophilic gels were stretched in three poor solvents: silicone oil, mineral oil, and in air. All three exhibited water loss on imposed deformation and a resulting increase in stress, with mineral oil presenting the smallest change due to its lower permeability to water. Mineral oil and silicone oil are of particular interest as they are often used in mechanical testing to prevent solvent loss.

Characterization of azobenzene polymer networks using in situ solid state NMR and temperature dependent photostriction

Azobenzene liquid crystal polymer networks (azo-LCNs) undergo a complex light-driven molecular conformation change of the azobenzene chromophore which imparts a macroscopic shape change within a glassy polymer network. To better understand molecular conformational changes which underlie macroscopic polymer deformation, we have collected solid-state nuclear magnetic resonance (NMR) data on 19F fluorinated side-chain azo-LCNs using an in situ visible light (450–458 nm) LED light source. We illustrate measurable changes in 19F NMR lineshapes under light irradiation, indicating that conformational changes can be probed by NMR. The measured effects of light on NMR spectra are also found to be reversible upon removal of the light source. We further show that sample heating does not affect azobenzene isomerization through analysis of temperature-dependent magic-angle-spinning NMR lineshapes. These results illustrate a narrowing of the lineshapes, but no change in the NMR peak positions, indicating that heating from 30 and 60 ° C affects molecular dynamics but does not change the azobenzene conformation. In addition to NMR data, benchtop photomechanical uni-axial measurements are taken over a temperature range from 23 to 60 ° C. Samples with the fluorinated side-chain azo-LCNs are compared to samples composed only of non-fluorinated main-chain azo-LCN composition. Similar stress relaxation was observed in both compositions under high pre-stretch. The amount of stress relaxation was found to depend on the pre-stretch, the ambient temperature, and the polarization of light.

In situ multi-level analysis of viscoelastic deformation mechanisms in tendon collagen

Tendon is a hydrated multi-level fibre composite, in which time-dependent behaviour is well established. Studies indicate significant stress relaxation, considered important for optimising tissue stiffness. However, whilst this behaviour is well documented, the mechanisms associated with the response are largely unknown. This study investigates the sub-structural mechanisms occurring during stress relaxation at both the macro (fibre) and nano (fibril) levels of the tendon hierarchy. Stress relaxation followed a two-stage exponential behaviour, during which structural changes were visible at the fibre and fibril levels. Fibril relaxation and fibre sliding showed a double exponential response, while fibre sliding was clearly the largest contributor to relaxation. The amount of stress relaxation and sub-structural reorganisation increased with increasing load increments, but fibre sliding was consistently the largest contributor to stress relaxation. A simple model of tendon viscoelasticity at the fibril and fibre levels has been developed, capturing this behaviour by serially coupling a Voigt element (collagen fibril), with two Maxwell elements (non-collagenous matrix between fibrils and fibres). This multi-level analysis provides a first step towards understanding how sub-structural interactions contribute to viscoelastic behaviour. It indicates that nano- and micro-scale shearing are significant dissipative mechanisms, and the kinetics of relaxation follows a two-stage exponential decay, well fitted by serially coupled viscoelastic elements.

Influence of residual stresses from shot peening on fretting fatigue crack growth

ABSTRACTOne method to improve fretting fatigue life is to shot peen the contact surfaces. Experimental fretting life results from specimens in a Titanium alloy with and without shot peened surfaces were evaluated numerically. The residual stresses were measured at different depths below the fretting scar and compared to the corresponding residual stress profile of an unfretted surface. Thus, the amount of stress relaxation during fretting tests was estimated. Elastic–plastic finite element computations showed that stress relaxation was locally more significant than that captured in the measurements. Three different numerical fatigue crack growth models were compared. The best agreement between experimental and numerical fatigue lives for both peened and unpeened specimens was achieved with a parametric fatigue growth procedure that took into consideration the growth behaviour along the whole front of a semi‐elliptical surface crack. Furthermore, the improved fretting fatigue life from shot peening was explained by slower crack growth rates in the shallow surface layer with compressive residual stresses from shot peening. The successful life analyses hinged on three important issues: an accurate residual stress profile, a sufficiently small start crack and a valid crack growth model.

Fretting Fatigue and Stress Relaxation Behaviors of Shot-Peened Ti-6Al-4V

Abstract Relaxation behavior of residual stress and its effect on fretting fatigue response of shot-peened titanium alloy, Ti-6Al-4V were investigated at room and elevated temperatures under constant amplitude cycling condition using two contact configurations, cylinder on flat and flat on flat. Measurements by X-ray diffraction method before and after tests showed that residual compressive stress relaxed during fretting fatigue. Fretting fatigue life depended on the amount of stress relaxation as well as on the applied stress range. Elevated temperature induced more residual stress relaxation relative to that at room temperature, which, in turn, resulted in a reduction of fretting fatigue life. There was no effect of contact geometry on fretting fatigue life on the basis of the applied cyclic stress on the specimen, which did not account for any effects from contact stresses, even though less residual stress relaxation occurred with flat pad. A critical plane based fatigue crack initiation model, modified shear stress range parameter (MSSR), was computed from finite element analysis for tests incorporating various levels of stress relaxation. It showed that not only crack initiation but also crack propagation should be considered to characterize fretting fatigue behavior of shot-peened specimens.

Creep and Stress Relaxation of Polyurethane-Shape Memory Polymer Foam.

The compressive deformation properties of polyurethane-shape memory polymer foam depending on time were investigated experimentally. The results obtained are summarized as follows. (1) The compressive deformation of the foam initiates at a certain local part and progresses from that part. During the unloading process, the shape recovery starts from the part which is deformed at the end of the loading process. (2) Creep deformation is large if the initial deformation is small. Both creep strain and creep recovery strain are large at temperatures below the glass transition temperature Tg. (3) Relaxed stress after a certain time is proportional to initial stress. The ratio of stress relaxation is large below Tg but is small above Tg. (4) The amount of stress relaxation is large if strain rate is high during the loading process. (5) If large strain is applied, followed by keeping the stress constant above Tg, irrecoverable strain appears. If large strain is kept constant above Tg, stress almost relaxes and the strain is fixed.

Mechanical behavior of two hamstring graft constructs for reconstruction of the anterior cruciate ligament.

We compared the mechanical behavior of two common hamstring graft constructs that are frequently used for reconstruction of the anterior cruciate ligament-Graft A: quadrupled semitendinosus tendon fixed with titanium button/polyester tape and suture/screw post, and Graft B: a double semitendinosus and double gracilis tendon fixed with a cross pin and two screws over washers. The experimental protocol used to evaluate each graft construct included stress relaxation (with and without preconditioning), cyclic loading, and a tensile load-to-failure test. The amount of stress relaxation without preconditioning was 60.6% for Graft A and 53.8% for Graft B. With preconditioning, it significantly decreased (p < 0.05) to 48.7 and 42.3%, respectively. Elongation of the graft construct in response to 100 cycles of loading (20-150 N) was 1.8 and 0.6% of the original length for Grafts A and B, respectively. However, after a series of five cyclic loading tests, the residual permanent elongation for each construct was 3.8 +/- 1.2 and 0.3 +/- 0.2 mm, a significant difference (p < 0.05) between the two graft constructs. Further analysis found more than 90% of the permanent elongation in the proximal and distal regions of Graft A, which consisted of polyester tape tied to a titanium button (proximal) and sutures tied around a screw post (distal). The tensile load-to-failure tests also revealed significant differences (p < 0.05) between the two graft constructs. Linear stiffness was 32 +/- 1 and 119 +/- 19 Nmm and ultimate load was 415 +/- 36 and 658 +/- 128 N for Grafts A and B, respectively. For Graft A, the polyester tape consistently failed; for Graft B, slippage or tearing from the washers was the mode of failure. We conclude that a quadruple-hamstring graft fixed over a cross pin proximally and with metal washers distally (Graft B) has less permanent elongation in response to cyclic loading and has structural properties superior to those of a graft construct that includes suture and tape material (Graft A). The large permanent elongation following repetitive loading of a graft construct with tape and suture material during the early postoperative period is of concern.

Design Enhancements for Automotive Integrated Shell Structures

Recent attempt to enhance the safety against collision reshaped the simple shell structures into the integrated complex shell structures. Moreover, due to various regulations continuously tightened for environment protection, weight reduction of automobiles becomes an increasingly important issue. Auto parts lightening is mainly accomplished by more reasonable design, adoption of lighter materials and miniaturization of the auto bodies. Focusing on the locally enhanced design approach among the above three ways, we here attempt to develop a patching optimization method, and also to determine the thicknesses of an integrated shell structure, both bringing a specified amount of stress relaxation. We first select a cross member as a patching optimization model. Based on the finite element stress calculations, we relieve the stress of cross member by patching in two ways-nonuniform thickness patching and optimized uniform thickness patching, the latter of which is more effective in a practical point of view for the preset amount of stress relaxation. Selecting a box type subframe as another finite element analysis model, we then determine the thickness of each part by axiomatic design approach for a preset amount of stress relaxation. The patching methodology and the axiomatic approach adopted in this work can be applied to the other complex shell structures such as center member and lower control arm.

The effect of creep on the residual stress in vapour-deposited thin films

The residual stress state of coatings can be critically important in dictating their performance. When the residual stress in the coating is compressive the substrate is put into tension and, as the temperature increases, it can reduce this tensile stress by creep processes, which in turn will reduce the stress in the coating. A model has been developed to characterize this process by predicting how stress is generated and relaxed during coating deposition or scale growth. The output from the model has been validated by experimental measurements on vapour-deposited TiN on stainless steel and shows excellent agreement with experimental data measured by X-ray diffraction. The amount of stress relaxation by creep depends on the thickness of the substrate and coating, the time of exposure and temperature and, to a lesser extent, the cooling rate and the creep properties of the substrate material.

Thermal Stresses, Interfacial Reactions and Microstructures of Al/Ti and Al/TiN Thin Films Encapsulated by SiOF

AbstractVariations in stress and grain size of Ti- and TiN- capped Al thin films passivated by fluorinated silicon dioxide (SiOF) during repetitive thermal cycling are investigated. The amount of stress relaxation, elastic and plastic behavior of these thin film structures are compared. Ti and TiN cap layers strengthen the single Al film significantly while the presence of SiOF induces plastic deformation of metal layers. Less grain growth is associated with a dielectric passivated Al film. The penetration of fluorine into Al upon annealing can be reduced by a TiN barrier layer.

Stress relaxation in the interface between creep particle and elastic matrix-strengthening mechanism

The effect of a power law creep particle on interface behavior between the particle and elastic matrix is investigated by stress analysis. Using the results obtained through the stress analysis, the forces due to interaction of an applied stress and stress concentration with an edge dislocation are determined. The direct interaction between the edge dislocation and the creeping particle is studied under fully relaxed stress conditions. Through the investigation the following results are derived. Stress relaxation in the interface can be caused by power law creep along or by diffusion, or a combination of both mechanisms. The degree of stress relaxation caused by diffusion can be defined in terms of the relaxation time for both boundary diffusion and volume diffusion. The amount of stress relaxation caused by the power law creep particle is characterized by the quantity α 2 which is a function of Γ 0 = 2( 1 √3 ) 1 + m × ( σ ∞ 2μ ) m( σ 0 σ ∞t m ) , where m is strain rate hardening exponent, σ ∞ is applied stress, μ is the shear modulus, σ 0 is the material constant of the power law creep particle, and t is elapsed time. The value α 2 = 1.0 corresponds to the fully relaxed condition and α 2 = −0.6 corresponds to the initial state. The time to reach a fully relaxed condition is very sensitive to the strain rate exponent, with the smaller m values leading to longer times. The stress state of complete relaxation in the elastic matrix is equivalent to the solution of a void in an elastic matrix superposed on the solution of positive surface traction on the void. This result is identical to that obtained by Srolovitz et al. [ Acta. Metall. 32, 1979 (1984)]. When the stress is completely relaxed in the particle, all stress components ( σ r , σ θ and σ rθ ) are relaxed, while in the matrix relaxations are observed only for σ r and σ θ . The critical resolved shear stress and critical stress to climb the dislocation in the neighborhood of the particle exceed the Orowan stress. Also, the particle attracts the dislocation. Therefore the strengthening of a power law creep particle in an elastic matrix is caused by the Orowan mechanism and by attraction of the dislocation.

Simplified analysis of cyclic elastoplastic creep responses in elevated temperature structures under cyclic loading

A simplified analysis of cyclic elastoplastic creep responses in structures subjected to cyclic mechanical loads with a holding time at high temperature is presented in this paper. The main feature of this method is to transform the complicated response analysis to another approximately equivalent problem—the elastoplastic analysis of a structure subjected to a cyclic loading with variant load amplitude, while the load amplitude variation in each loading branch is determined by the amount of stress relaxation during the holding time before the subsequent load reversal. Then two methods which are used in simulating cyclic elastoplastic responses and creep stress redistribution respectively as well as a load equivalence technique to deal with their interaction are combined to track the response evolution of cyclic elastoplastic creep at a critical location concerned in a structure.

Stable encapsulation structures for Permalloy films

This paper presents a study of the effects of annealing on the micromagnetic properties of encapsulated Permalloy films. The stress relaxation during annealing causes observable changes in the micro-magnetic state of the Permalloy. The amount of stress relaxation is highly sensitive to the shape of the Permalloy film and the encapsulation material. After Permalloy films with negative magnetostrictive coefficient are annealed, the easy axis of bar-shape yokes remains in the transversal direction while those of ring-shape yokes and heart-shape yokes rotate so that the easy axis is normal to the film edge. The annealing properties of bar-shape Permalloy films in six different encapsulation structures are compared. Permalloy films encapsulated entirely in hardbaked photoresist are consistently more stable. The uneven external stress is absorbed by the relatively softer hardbaked photoresist buffer layer and is re-distributed evenly to the Permalloy. A better control of the anisotropy of the Permalloy yoke can be obtained by placing a hardbaked photoresist buffer layer between the alumina overcoat and the Permalloy.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Thermal dimensional stability in fiber reinforced magnesium alloys

The thermal dimensional stability of short alumina fiber reinforced magnesium alloys (FRMg) was examined with the following results. (1) In the case of the pure magnesium matrix FRMg, the thermal stress originating in the difference of the thermal expansion coefficients between the fiber and the matrix is effectively relaxed by the plastic deformation in the matrix upon quenching. As a result of the stress relaxation, the FRMg shows excellent dimensional stability during subsequent annealing. (2) In the case of Mg-Nd alloy matrix FRMg, a high residual tensile stress is induced in the matrix by quenching, because slip deformation is highly suppressed owing to the high flow stress level of the alloyed matrix. The FRMg undergoes creep elongation during subsequent annealing. (3) The thermal dimensional stability of the FRMg depends on the thermal stress and the amount of stress relaxation induced by slip deformation in the matrix. In order to obtain superior dimensional stability, it is important to select th...