Tensile Stress Relaxation Research Articles

Uniaxial Tensile Testing Device for Measuring Mechanical Properties of Biological Tissue with Stress-Relaxation Test under a Confocal Microscope

Biological soft tissue is a non-linear and viscoelastic material and its mechanical properties can greatly affect quality of life. Many external mechanical factors can alter the tissue, for example the tissue of talipes equinovarus congenitus, also known as clubfoot, which is the most frequent congenital deformity affecting lower extremities with pathological changes of connective tissue. In clubfoot, the presence of disc-like mass of fibrous tissue, resembling intervertebral disc tissue, is described to be between the medial malleolus and the medial side of the navicular bone. The clubfoot tissue is often referred to be stiffer or rigid by clinicians, or it is referred to as contracted and less contracted tissue, however relevant evidence about mechanical properties is missing. Therefore, the description "disc-like" is informing only about relative mechanical properties of clubfoot tissue. We aim to prepare methodical approach to quantify mechanical properties of biological tissue with uniaxial tensile stress-relaxation test, in order to help clinicians and scientist to identify precisely the mechanical properties of normal and pathological tissue and their structural behaviour during mechanical testing. In this study, we test and tune the uniaxial tensile stress-relaxation test on biological tissue with high content of connective tissue such as collagen. The model tissue is porcine pericardium. The tissue has clear collagen fibres aligning parallel to the force applied. Modulus of elasticity measured here is comparable to other studies.

Size and stress dependences in the tensile stress relaxation of thin copper wires at room temperature

The tensile stress relaxation behavior of thin copper wires with diameters of 20, 35 and 50 μm is experimentally investigated at room temperature. It is revealed that the stress relaxation can occur at low starting stress below the yield strength at room temperature. Significant size and stress dependences in the stress relaxation are observed. That is at a given stress, a smaller-diameter sample exhibits a much faster stress relaxation. At a given diameter, the stress relaxation is accelerated with increasing the starting stress. The value of activation volume increases with increasing the wire diameter, while it increases with decreasing the starting stress. The strain rate elevates as the wire diameter decreases or the starting stress increases. A bilinear relation between the logarithmic strain rate and the logarithmic stress during the relaxation is observed only for the wire sample with a diameter of 20 μm. The intersection of dislocations with grain boundaries is considered to be the main deformation mechanism during the stress relaxation. A rapider exhaustion of mobile dislocations is observed in the wire with smaller diameter in the primary of relaxation. It is attributed to annihilation of dislocations easily escaping from the surface due to larger fraction volume of the near-surface zone. After the primary regime, the higher possibility of dislocations being pinned or immobilized by grain boundaries in a larger-diameter sample is accounted for the size dependence.

Thermo-Mechanical Properties of Glass Fiber Reinforced Shape Memory Polyurethane for Orthodontic Application.

Glass fiber reinforced shape memory polyurethane (GFRSMPU) has great potential to be an alternative kind of material for orthodontic archwires for overcoming the disadvantages of metal wires in terms of esthetic and allergy and deficiency of pure shape memory polyurethane (SMPU) wires in mechanical properties. The objective of this study was to investigate the thermo-mechanical properties and shape recovery functions of GFRSMPU and evaluate the feasibility of using this composite for orthodontic archwires. GFRSMPU were made from short cut glass fibers and SMPU by mixing extrusion. Scanning electron microscope (SEM) and differential scanning calorimetry (DSC) were performed to investigate the distribution of glass fibers in the mixture and glass transition temperature (Tg). Then the thermo-mechanical properties, including tensile modulus, flexural modulus and stress relaxation effects, were measured. Furthermore, shape recovery functions of GFRSMPU characterized by the shape recovery ratio and force were investigated through shape recovery tests, typodont models and finite element analysis (FEA). SEM images indicated that an excellent dispersity of glass fibers was obtained after double-extrusion. DSC experiments showed Tg was not enormously affected with the existence of glass fibers, but the mechanical properties of GFRSMPU were greatly improved. Shape recovery tests showed reduction of shape recovery ratio of the GFRSMPU material with the addition of glass fibers, but dentition aligning time was reduced by 50% in the simulation performed on identical typodont models with GFRSMPU archwires filled with 30 wt.% glass fibers. The FEA results illustrated that the reacting forces of GFRSMPU archwires with 30 wt.% glass fiber was increased by 96.36% compared with pure SMPU archwires. The mechanical properties of GFRSMPU can be considerably improved by adding glass fibers, and the shape memory function would be well preserved too. Enhanced SMPU owns a good application prospect in orthodontics for dentation aligning on the preliminary stage, as well as other medical fields.

Suitability of material models in finite element simulation of stress relaxation for titanium sheets

Titanium alloys are important aerospace materials due to their low density, high strength to weight ratio and excellent corrosion resistance, but it is difficult to form them under room temperature due to poor formability and large springback, which is often formed by hot forming. And stress relaxation behaviour of material plays an important role in hot forming. In this paper, different material models are compared in simulating stress relaxation of Ti-6.5Al-2Zr-1Mo-1V sheet, and validated by experiments. A series of uniaxial tensile stress relaxation tests are performed on the Ti-6.5Al-2Zr-1Mo-1V alloy sheet over the temperature ranging from 773K to 973K, and the strain 10%. According to the result of stress relaxation tests, Time-Hardening model and Hyperbolic-Sine model are calibrated and validated. It is shown that for stress relaxation process, from 0 to 900 seconds, the maximum stress simulation error (6MPa) of Time-Hardening model is lower than Hyperbolic-Sine model (25MPa), but from 1800 to 3600 seconds, the maximum stress simulation error (15MPa) of Time-Hardening model is larger than Hyperbolic-Sine model (5MPa). In addition, it can be concluded that the stress simulation error will have little effect on shape accuracy of formed parts during the simulation.

Large, long range tensile forces drive convergence during Xenopus blastopore closure and body axis elongation.

Indirect evidence suggests that blastopore closure during gastrulation of anamniotes, including amphibians such as Xenopus laevis, depends on circumblastoporal convergence forces generated by the marginal zone (MZ), but direct evidence is lacking. We show that explanted MZs generate tensile convergence forces up to 1.5 μN during gastrulation and over 4 μN thereafter. These forces are generated by convergent thickening (CT) until the midgastrula and increasingly by convergent extension (CE) thereafter. Explants from ventralized embryos, which lack tissues expressing CE but close their blastopores, produce up to 2 μN of tensile force, showing that CT alone generates forces sufficient to close the blastopore. Uniaxial tensile stress relaxation assays show stiffening of mesodermal and ectodermal tissues around the onset of neurulation, potentially enhancing long-range transmission of convergence forces. These results illuminate the mechanobiology of early vertebrate morphogenic mechanisms, aid interpretation of phenotypes, and give insight into the evolution of blastopore closure mechanisms.

Investigating the static and dynamic tensile mechanical behaviour of polymer‐bonded explosives

AbstractThe tensile properties of a polymer‐bonded explosive (PBX) were systematically studied by using quasi‐static and dynamic experiments. A non‐linear constitutive relation was developed to describe the tensile behaviour of the PBX. The tensile properties of the PBX under different strain rates and temperatures were measured in quasi‐static tests. The tensile behaviour of the PBX was found to exhibit high strain rate and strong temperature dependence, attributable to the large fraction of the polymer binder. To obtain the rational dynamic tensile results, a modified split Hopkinson tensile bar (SHTB) setup was designed such that the specimens were in dynamic stress equilibrium and deformed homogeneously at nearly constant strain rates. To characterise the viscoelastic behaviour, the master modulus curve was derived from the tensile stress relaxation tests at different temperatures. The non‐linear constitutive model was implemented in ABAQUS to predict the tensile behaviour of the PBX. The computational results were found to be in good agreement with the experimental results.

Freezing does not alter multiscale tendon mechanics and damage mechanisms in tension.

It is common in biomechanics to use previously frozen tissues, where it is assumed that the freeze-thaw process does not cause consequential mechanical or structural changes. We have recently quantified multiscale tendon mechanics and damage mechanisms using previously frozen tissue, where damage was defined as an irreversible change in the microstructure that alters the macroscopic mechanical parameters. Because freezing has been shown to alter tendon microstructures, the objective of this study was to determine if freezing alters tendon multiscale mechanics and damage mechanisms. Multiscale testing using a protocol that was designed to evaluate tendon damage (tensile stress-relaxation followed by unloaded recovery) was performed on fresh and previously frozen rat tail tendon fascicles. At both the fascicle and fibril levels, there was no difference between the fresh and frozen groups for any of the parameters, suggesting that there is no effect of freezing on tendon mechanics. After unloading, the microscale fibril strain fully recovered, and interfibrillar sliding only partially recovered, suggesting that the tendon damage is localized to the interfibrillar structures and that mechanisms of damage are the same in both fresh and previously frozen tendons.

Influences of Processing and Fatigue Cycling on Residual Stresses in a NiCrY-Coated Powder Metallurgy Disk Superalloy

Oxidation and corrosion can attack superalloy disk surfaces exposed to increasing operating temperatures in some turbine engine environments. Any potential protective coatings must also be resistant to harmful fatigue cracking during service. The objective of this study was to investigate how residual stresses evolve in one such coating. Fatigue specimens of a powder metallurgy-processed disk superalloy were coated with a NiCrY coating, shot peened, and then subjected to fatigue in air at room and high temperatures. The effects of this processing and fatigue cycling on axial residual stresses and other aspects of the coating were assessed. While shot peening did induce beneficial compressive residual stresses in the coating and substrate, these stresses relaxed in the coating with subsequent heating. Several cast alloys having compositions near the coating were subjected to thermal expansion and tensile stress relaxation tests to help explain this response of residual stresses in the coating. For the coated fatigue specimens, this response contributed to earlier cracking of the coating than for the uncoated surface during long intervals of cycling at 760 °C. Yet, substantial compressive residual stresses still remained in the substrate adjacent to the coating, which were sufficient to suppress fatigue cracking there. The coating continued to protect the substrate from hot corrosion pitting, even after fatigue cracks initiated in the coating.

RF Performance of <italic>In Situ</italic> SiN<italic>x</italic>Gate Dielectric AlGaN/GaN MISHEMT on 6-in Silicon-on-Insulator Substrate

A high power-added efficiency and low dynamic on-resistance ( $R_{ \mathrm{\scriptscriptstyle ON}})$ AlGaN/GaN metal–insulator–semiconductor high-electron mobility transistor (MISHEMT) with in situ SiN x insulator design was demonstrated on 150-mm silicon-on-insulator (SOI) substrate. Compared to traditional high-resistivity Si substrate, SiN x /AlGaN/GaN MISHEMT grown on the SOI 5- $\mu \text{m}$ -thick Si active layer performed better tensile stress relaxation and surface flatness. Based on Hall measurement results, electron mobility and the sheet charge density on SOI substrate were improved simultaneously owing to a lower-defect density which is also proofed by pulse measurement and low-frequency noise measurement. Due to the low-feedback substrate capacitance, the bandwidth, and the linearity were also improved simultaneously by SOI substrate design.

Stress relaxation behaviour in IN718 nickel based superalloy during ageing heat treatments

Designing microstructure of components made from Inconel 718 nickel based superalloy (IN718) with tailored mechanical properties for high temperature applications, require sequential thermo-mechanical processing. This often includes straining and annealing at solution annealing temperature (i.e. ≈ 980°C) followed by water quenching and subsequent aging heat treatments at lower temperatures. In addition to the microstructure development (i.e. precipitation) the aging heat treatment partially relieve the residual stresses generated at previous stages of forging and water quenching, however the stress field will not be completely relaxed.In this study, a series of experiments were conducted on round tensile specimens made from IN718 bar to investigate tensile stress relaxation behaviours at elevated temperatures used for aging heat treatments. The stress relaxation curves obtained can be described by a hyperbolic function with a non-zero asymptotic stress (σ∞), which seems to be proportional to the initially applied stress (σ0) for a fixed temperature. This behaviour is investigated at temperatures between 620°C and 770°C that is a temperature range used in industry to perform the aging heat treatments to obtain microstructures with tailored mechanical properties. It has been shown that the σ∞/ σ0 ratio has decreased rapidly with increasing temperature at this range. The relaxation behaviour has been assessed numerically and an empirical relationship has been defined for each temperature that can be used for modelling purposes.

Mechanical properties of cerium oxide-modified vulcanised natural rubber at elevated temperature

ABSTRACTCerium oxide (CeO2) was used as the reinforcement filler to improve the mechanical properties of rubbers. Rubber components are often used at high temperature, so the mechanical properties of CeO2-modified vulcanised natural rubbers (NR/CeO2) were investigated at elevated temperatures. The tensile properties, stress relaxation and creep recovery response were studied from 30°C to 150°C. NR/CeO2 demonstrated higher crosslink density and elastic modulus. The degradation of elastic modulus could be alleviated in the NR/CeO2 at elevated temperatures. Moreover, a remarkable enhancement in stress relaxation resistance and creep resistance was achieved in the NR/CeO2 at elevated temperatures. The improved stress relaxation resistance and creep resistance were attributed to the enhanced interaction and bonding force of rubber chains caused by CeO2 filler. CeO2 was an effective reinforcement filler to improve the mechanical properties of NR at elevated temperatures.

P.097 Tensile properties of Polyvinyl Alcohol Cryogel (PVA-C) formulations and generation of a tissue mimicking artificial lumbar intervertebral disc prototype

Background: Current lumbar intervertebral disc prostheses provide suboptimal symptom relief with little natural load-cushioning. PVA-C is a promising biocompatible material, however previous studies from our lab show that it does not have adequate elastic modulus to mimic the annulus fibrosus. Here we present a prototype of an artificial lumbar intervertebral disc. Methods: The tensile properties of pure (5-35% PVA-C) and particle-reinforced (15% PVA-C with 5% of either Sephadex or hydroxyapatite) composite PVA-C formulations were evaluated. Simple tension and tensile stress relaxation tests were performed. Woven Teflon mesh was embedded in PVA-C and tested under compression. Endplate pull-out tests were performed. Results: Tensile testing showed that all PVA-C formulations behaved linearly for physiologic levels of strain (<20%). Tensile elastic modulus is an order of magnitude lower than the annulus fibrosus. Teflon has similar elastic modulus as collagen and compression of the hybrid Teflon-PVA-C construct revealed good biomechanical mimicry with elastic modulus of 20-25MPa at 20% deformation, similar to human data. Bonding between PVA-C and porous titanium endplate is excellent. Conclusions: A fiber-reinforced PVA-C impregnated composite adequately mimics the annulus fibrosus. Our prototype of a tissue mimicking artificial intervertebral disc utilizes a woven Teflon fiber with 20% PVA-C (+Hydroxyapatite) annulus and 5% pure PVA-C nucleus bonded to porous titanium foam endplates.

High Temperature Deformation Mechanism in Hierarchical and Single Precipitate Strengthened Ferritic Alloys by In Situ Neutron Diffraction Studies

The ferritic Fe-Cr-Ni-Al-Ti alloys strengthened by hierarchical-Ni2TiAl/NiAl or single-Ni2TiAl precipitates have been developed and received great attentions due to their superior creep resistance, as compared to conventional ferritic steels. Although the significant improvement of the creep resistance is achieved in the hierarchical-precipitate-strengthened ferritic alloy, the in-depth understanding of its high-temperature deformation mechanisms is essential to further optimize the microstructure and mechanical properties, and advance the development of the creep resistant materials. In the present study, in-situ neutron diffraction has been used to investigate the evolution of elastic strain of constitutive phases and their interactions, such as load-transfer/load-relaxation behavior between the precipitate and matrix, during tensile deformation and stress relaxation at 973 K, which provide the key features in understanding the governing deformation mechanisms. Crystal-plasticity finite-element simulations were employed to qualitatively compare the experimental evolution of the elastic strain during tensile deformation at 973 K. It was found that the coherent elastic strain field in the matrix, created by the lattice misfit between the matrix and precipitate phases for the hierarchical-precipitate-strengthened ferritic alloy, is effective in reducing the diffusional relaxation along the interface between the precipitate and matrix phases, which leads to the strong load-transfer capability from the matrix to precipitate.

白板紙の罫線曲げにおける動的解放挙動への曲げ保持時間の影響と非線形緩和特性の解析

Mechanical experiments have been performed to investigate the dynamic creasing characteristics of coated paperboard mainly used in the food packaging industry. The correlation between the time-dependent relaxation of bending resistance at a specified folded position and the dynamic release behavior of creased paperboard was experimentally investigated for a time period of 0.1-1000s. The transition of in-plane tensile stress relaxation and that of bending moment relaxation were detected in the time period of 10-30s in a specified paperboard, when using a constant deformation rate for creating the initially loaded state. We varied the stopping time of the folded state of a creased part, and analyzed the time-dependent released behavior (unfolded deformation) using the improved relaxation model, which is based on the power rule of relaxation time. The results obtained with this model show a good fit to the experimental results, and the proposed release time of the unfolding process is found to be almost independent of the stopping time.

Improvement of hardness and compression set properties of EPDM seals with alternating multilayered structure for PEM fuel cells

In Proton Electrolyte Membrane (PEM) fuel cells, seals are used to keep intimate contact between all components as well as to keep the reactant gases and liquid within their respective regions. Suitable seal materials should be chosen and designed to sustain the long term operation. In this paper, EPDM seal materials with soft and hard alternating multilayered structure are fabricated through the multilayered co-extrusion system. The structure and morphology of multilayered and blended EPDM vulcanizates were observed by Scanning electron microscopy (SEM). The results of tensile, hardness, compression set and compression stress relaxation test show that multilayered vulcanizates have lower tensile strength and compression set but higher hardness than blended one. The hardness of multilayered vulcanizates increases with the increase of the layer number, leading to higher sealing force and better dimensional stability. The compression set of multilayered vulcanizates decreases with the increase of the layer number, indicating better sealing capacity and longer working life. The permeability test shows that multilayered materials have nearly same gas permeability property as blended one. The results of aging test reveal that the mechanical properties of EPDM seal materials decrease with time, but multilayered vulcanizates still have lower compression set than blended one after aging.

A Constitutive Model for Soft Materials Incorporating Viscoelasticity and Mullins Effect

Soft materials including elastomers and gels are widely used in applications of energy absorption, soft robotics, bioengineering, and medical instruments. For many soft materials subject to loading and unloading cycles, the stress required on reloading is often less than that on the initial loading, known as Mullins effect. Meanwhile, soft materials usually exhibit rate-dependent viscous behavior. Both effects were recently reported on a new kind of synthesized tough gel, with capability of large deformation, high strength, and extremely high toughness. In this work, we develop a coupled viscoelastic and Mullins-effect model to characterize the deformation behavior of the tough gel. We modify one of the elastic components in Zener model to be a damageable spring to incorporate the Mullins effect and model the viscous effect to behave as a Newtonian fluid. We synthesized the tough gel described in the literature (Sun et al., Nature 2012) and conducted uniaxial tensile tests and stress relaxation tests. We also investigated the two effects on three other soft materials, polyacrylate elastomer, Nitrile-Butadiene Rubber, and polyurethane. We find that our presented model is so robust that it can characterize all the four materials, with modulus ranging from a few tens of kilopascal to megapascal. The theory and experiment for all tested materials agree very well.

Tensile stress relaxation in unsaturated granular materials

The mechanics of granular media at low liquid saturation levels remain poorly understood. Macroscopic mechanical properties are affected by microscale forces and processes, such as capillary forces, inter-particle friction, liquid flows, and particle movements. An improved understanding of these microscale mechanisms is important for a range of industrial applications and natural phenomena (e.g. landslides). This study focuses on the transient evolution of the tensile stress of unsaturated granular media under extension. Experimental results suggest that the stress state of the material evolves even after cessation of sample extension. Moreover, we observe that the packing density strongly affects the efficiency of different processes that result in tensile stress relaxation. By comparing the observed relaxation time scales with published data, we conclude that tensile stress relaxation is governed by particle rearrangement and fluid redistribution. An increased packing density inhibits particle rearrangement and only leaves fluid redistribution as the major process that governs tensile stress relaxation.

High-Temperature Low-Cycle Fatigue Behavior of MarBN at 600 °C

This paper presents the high-temperature low-cycle fatigue (HTLCF) behavior of a precipitate strengthened 9Cr martensitic steel, MarBN, designed to provide enhanced creep strength and precipitate stability at high temperature. The strain-controlled test program addresses the cyclic effects of strain-rate and strain-range at 600 °C, as well as tensile stress-relaxation response. A recently developed unified cyclic viscoplastic material model is implemented to characterize the complex cyclic and relaxation plasticity response, including cyclic softening and kinematic hardening effects. The measured response is compared to that of P91 steel, a current power plant material, and shows enhanced cyclic strength relative to P91.

Thickness-modulated metal–insulator transition of VO2 film grown on sapphire substrate by MBE

VO2 films with precisely controlled thickness on the nanoscale ranging from 15 to 60 nm were grown on single crystal sapphire substrates by molecular beam epitaxy. X-ray diffraction and atomic force microscopy measurements indicated that high quality single phase VO2 films with condense and smooth surface and free of cracks could be achieved only when the film was thicker than 30 nm. The temperature-dependent resistance measurement indicated a drastic modification of metal–insulator transition (MIT) properties which was achieved through the variation of film thickness, especially the transition magnitude and curve abruptness. The corresponding mechanism was supposed to be associated with the tensile stress relaxation effect with increasing thickness caused by thermal mismatch within VO2 films, as demonstrated by Raman spectra. Our present finding provides an effective and convenient alternative to modulate the MIT properties of VO2 films.

Influences of Specimen Size and Temperature on Viscoelastic Tensile Properties of SU-8 Photoresist Films

In this paper, the influences of specimen size and test temperature on the viscoelastic properties of SU-8 photoresist films are described. Films with the thicknesses of 1 μm and 10 μm are subjected to quasi-static uniaxial tensile tests and stress relaxation tests at temperatures ranging from 293 K to 473 K. The average glassy modulus at 293 K is 3.2 GPa, which decreases with an increase in the test temperature irrespective of specimen size. The mean fracture strain depends on film thickness as well as temperature. The fracture strain of the 1-μm thick films is approximately half of that of the 10-μm thick films at each temperature. Stress relaxation tests are conducted for constructing the master curves of the relaxation moduli. There is no apparent thickness dependence on the master curve. Above glass transition temperature, Tg, apparent activation energies for the two films are almost identical, whereas the activation energy for the thinner films is smaller than that for the thicker films below Tg. This size effect is discussed using Fourier transform infrared spectroscopy (FTIR).