High Stress Relaxation Research Articles

Effect of Fibre Orientation and Hostile Solutions on Stress Relaxation of Glass/Polyamide Composites.

Polyamide creates high-performance composite materials, which are replacing the traditional epoxy composites in several applications. In this context, exposure to hostile environments is expected. On the other hand, due to the viscoelastic nature of the matrix, these composite materials are prone to stress relaxation. Therefore, the stress relaxation behaviour of glass/polyamide 6 composites was studied considering different fibre directions, as well as exposure to NaOH and HCl solutions. Stress relaxation tests on the bending mode were carried out, and the stress recorded during the loading time (7200 s). All tests were characterized by a stress decrease over time, but laminates with higher fibre angles were more prone to stress relaxation. However, exposure to hostile solutions promoted more significant decreases, where the highest stress relaxation was achieved for alkaline environments with values that were three times higher for laminates with fibres at 0° and around one and half times higher for 45° fibre alignments when compared with the control samples. Finally, the Kohlrausch–Williams–Watts (KWW) model showed that it can be used to predict stress relaxation time, due to the accuracy that was obtained between the experimental and theoretical results.

Relationship between physicochemical characteristics of Korean wheat flour and quality attributes of steamed bread

The purpose of this study is to identify major factors affecting the manufacture and quality of steamed bread, consumed in Southeast Asia including China, Japan, and Korea. Hence, flours of 11 Korean wheat cultivars were used to evaluate quality attributes of two different styles of steamed bread, Korean style steamed bread (KSSB) and northern-style Chinese steamed bread (NSCSB). KSSB prepared more ingredients and higher optimum water absorption of dough than NSCSB because Korean consumers prefer white and glossy surface and soft crumb. KSSB showed lower height, larger diameter and volume of steamed bread, higher stress relaxation, and softer texture of crumb than NSCSB. The correlation between flour characteristics and quality of steamed bread was different in KSSB and NSCSB. About 90% of variability in the height and volume of KSSB could be predicted from protein content, mixing tolerance of Mixograph, average particle size of flour, final viscosity and solvent retention capacity. Protein content and quality parameters also could explain the variation of steamed bread height in NSCSB. Korean wheat carrying Glu-A3c allele produced higher volume of steamed bread (704.7 mL) than Glu-A3d allele (645.8 mL) in KSSB, although there was no significant difference in volume of NSCSB by glutenin compositions. Glu-D1d and Glu-A3c alleles had softer texture of crumb than Glu-D1f and Glu-A3d alleles in KSSB, Glu-B3i allele also showed lower hardness of crumb than their counterpart allele in NSCSB. Hard wheat showed higher height and volume of steamed bread, and lower stress relaxation and hardness of crumb than soft wheat in KSSB.

The role of geometry on the viscoelastic materials properties: tensile and stress relaxation of the yarn

The time-dependent mechanical behavior of textiles has particular importance. One of such behaviors is stress relaxation. When strain is applied constantly, there is a decreased stress with time in viscoelastic materials, which is called stress relaxation. The aim of this study is to investigate the effect of knot’s geometry (surgeon, square and eight) and the number of knots on the tensile and stress relaxation properties of the polyester yarn. Significant differences were observed for the tensile characteristics of the yarn in the presence of the knot. Generally, the knotted yarns demonstrated lower tensile stress and strain at failure. Moreover, the results revealed that the stress relaxation behavior of the yarn is affected by the number of knots and their geometry. The yarn without knot exhibited the highest stress relaxation percent while the yarn with the surgeon’s knot displayed the least stress relaxation percent. On the other hand, increasing the number of knots led to a decrease in the percentage of yarn stress relaxation.

Effect of Millisecond Annealing Temperature of Ni1-<italic>x</italic>Pt<italic>x</italic> Si Formation on Leakage Current Characteristics of Static Random- Access Memory Cells

The importance of optimizing the millisecond annealing (MSA) temperature for Pt-doped NiSi ( ${\mathrm {Ni}}_{{1}-{x}}$ Pt x Si) contact formation was highlighted by implementing and characterizing the ${\mathrm {Ni}}_{{1}-{x}}$ Pt x Si films in the static random-access memory (SRAM) cells fabricated with a 28-nm design rule. MSA at 750 °C–900 °C effectively reduced the junction leakage current compared to that with the conventional rapid thermal annealing, because of its efficient suppression of Ni diffusion in the junction region. Moreover, with an increase in the MSA temperature, the beneficial effects of maintaining a low junction leakage, such as interface flattening and grain size increase, were observed. However, the increase in the MSA temperature deteriorated the statistical distribution of the standby leakage current of the SRAM cells by producing more encroachment and spiking defects; this was attributed to the build-up of a high initial film stress and large stress relaxation during the subsequent back-end processes.

Synthesis and characterization of bio-compatible shape memory polymers with potential applications to endovascular embolization of intracranial aneurysms

Intracranial aneurysms (ICAs) are focal dilations in the brain's arteries. When left untreated, ICAs can grow to the point of rupture, accounting for 50–80% of subarachnoid hemorrhage cases. Current treatments include surgical clipping and endovascular coil embolization to block circulation into the aneurysmal space for preventing aneurysm rupture. As for endovascular embolization, patients could experience aneurysm recurrence due to an incomplete coil filling or compaction over time. The use of shape memory polymers (SMPs) in place of conventional platinum coils could provide more control and predictability for mitigating these complications. This study was focused on characterization of an aliphatic urethane-based SMP to evaluate its potential as a novel biomaterial for endovascular embolization. Twelve compositions of the SMP were synthesized and their thermomechanical properties together with the shape recovery behavior were comprehensively investigated. Our results showed that the SMPs experienced a significant decrease in storage and loss moduli as heated above their glass transition temperatures (32.3–83.2 °C), and that all SMPs were thermally stable up to 265 °C. Moreover, the SMPs exhibited both composition-dependent stress relaxation and a decrease in elastic modulus during cyclic loading. The shape recovery time was less than 11 s for all SMP compositions, which is sufficiently short for shape changing during embolization procedures. Several candidate compositions were identified, which possess a glass transition temperature above body temperature (37 °C) and below the threshold of causing tissue damage (45 °C). They also exhibit high material strength and low stress relaxation behavior, suggesting their potential applicability to endovascular embolization of ICAs.

Modeling of viscosity and complex modulus for poly (lactic acid)/poly (ethylene oxide)/carbon nanotubes nanocomposites assuming yield stress and network breaking time

In this paper, the blends and nanocomposites containing poly (lactic acid) (PLA), poly (ethylene oxide) (PEO) and carbon nanotubes (CNT) are prepared and the viscosity of samples is measured at different frequency ranges. After that, a suitable model proposed by Zhu group is applied to predict the viscosity and the viscoelastic parameters of prepared samples. Moreover, this model is developed to suggest the complex modulus as a function of zero viscosity, yield stress, relaxation time and network breaking time. The roles of different parameters in the viscosity and complex modulus are clarified to justify the suggested models. All predictions show good agreements with experimental data at all frequency ranges. The addition of CNT to blend increases the zero viscosity, yield stress, relaxation time and network breaking time. The zero viscosity directly handles the viscosity, but the role of relaxation time in the viscosity depends on the extents of other parameter. Moreover, the low values of yield stress and network breaking time reduce the viscosity, whereas only high yield stress is enough to improve the viscosity. Additionally, high zero viscosity, high yield stress and short relaxation time grow the complex modulus of polymer systems.

Hot stretch forming of titanium sheet by resistance heating

Titanium alloys are important aerospace materials due to excellent comprehensive performance, but it is difficult to form at room temperature. Electrically assisted hot stretch forming is used to form the titanium panels of aircraft. This paper studies the effect of temperature, strain, stress relaxation time and cooling rate on forming accuracy by stress relaxation test, tensile test and forming experiment. It is shown that for the influence of temperature on forming accuracy, there is no large difference among 773K, 873K, 973K. And with increase of strain, the forming accuracy becomes lower. For the influence of stress relaxation time, there is no large difference among 0min, 15mins, 30mins. And with decrease of cooling rate, the forming accuracy is improved significantly. It’s concluded that when the temperature reaches a high level, temperature and stress relaxation time have no great influence on forming accuracy. And the forming accuracy is mainly influenced by strain and cooling rate. Electrically assisted hot stretch forming is an effective method to form titanium alloy. And the part with high forming accuracy can be obtained by adopting high temperature, low strain, proper stress relaxation time and low cooling rate.

Preliminary Study on Effect of Chemical Composition Alteration on Elastic Recovery and Stress Recovery of Nitrile Gloves

Nitrile gloves are widely used in the medical and automobile field due to its superiority in hypo-allergic component and chemical resistance over natural latex gloves. However, poor elastic recovery of nitrile glove to compressive force also creates an aesthetic issue for customers with high levels of wrinkling after removing from glove box. This paper demonstrates the preliminary study on the varies chemical composition such as crosslinking agents, sulphur and zinc oxide, the accelerator agent added during curing process, and the rubber filler Titanium Dioxide, on the elastic recovery and stress relaxation in nitrile gloves manufacturing. These chemical were studied at different concentration level comparing the high and low level versus the normal production range. Due to the inconsistency in the analysis technique on the surface imaging, the elastic recovery result was unable to be quantified and was not conclusive at this point. The cross linking agents, sulphur and zinc oxide, and the accelerator agent, played a significant role in the mechanical strength of the gloves. Increment of these chemicals result in higher tensile strength, but a reduction in the elasticity of the materials in which causes a lesser elongation at break percentage for the gloves. Both cross-linkers demonstrate different behaviour where higher sulphur content, provide higher stress relaxation (SR%) yet zinc oxide shows otherwise.

Influence of biofilm lubricity on shear-induced transmission of staphylococcal biofilms from stainless steel to silicone rubber.

SummaryIn real‐life situations, bacteria are often transmitted from biofilms growing on donor surfaces to receiver ones. Bacterial transmission is more complex than adhesion, involving bacterial detachment from donor and subsequent adhesion to receiver surfaces. Here, we describe a new device to study shear‐induced bacterial transmission from a (stainless steel) pipe to a (silicone rubber) tube and compare transmission of EPS‐producing and non‐EPS‐producing staphylococci. Transmission of an entire biofilm from the donor to the receiver tube did not occur, indicative of cohesive failure in the biofilm rather than of adhesive failure at the donor‐biofilm interface. Biofilm was gradually transmitted over an increasing length of receiver tube, occurring mostly to the first 50 cm of the receiver tube. Under high‐shearing velocity, transmission of non‐EPS‐producing bacteria to the second half decreased non‐linearly, likely due to rapid thinning of the lowly lubricious biofilm. Oppositely, transmission of EPS‐producing strains to the second tube half was not affected by higher shearing velocity due to the high lubricity and stress relaxation of the EPS‐rich biofilms, ensuring continued contact with the receiver. The non‐linear decrease of ongoing bacterial transmission under high‐shearing velocity is new and of relevance in for instance, high‐speed food slicers and food packaging.

Movable Cross-Linked Polymeric Materials from Bulk Polymerization of Reactive Polyrotaxane Cross-Linker with Acrylate Monomers

Topological cross-linked polymers attract much attention from their unique mechanical properties derived from their cross-linking structure. Here, we have fabricated a polymeric material (Acryl-AcO-PRx) incorporating topological cross-links into various universal acrylate polymers by using a modified polyrotaxane as a polymerizable cross-linking agent. Acryl-AcO-PRx obtained by a simple photocuring technique under bulk conditions showed a distinctly higher extensibility compared with the chemical cross-linked polymers. In addition, Acryl-AcO-PRx materials showed high stress relaxation and deformation hysteresis. These results indicate that the topologically cross-linked structure composed of polyrotaxane plays an important role in mechanical properties even in bulk state.

KOBELCO CAC

Abstract Kobelco Alloy CAC 5 has higher stress-relaxation strength. The alloy is nominally Cu-0.8Ni-1.2Sn-0.07P. CAC 5 has the good strength of C52100 and C26000. This datasheet provides information on composition, physical properties, hardness, elasticity, and tensile properties. It also includes information on forming. Filing Code: Cu-858. Producer or source: Kobe Steel Ltd, Aluminum and Copper Business.

Comparison of creep strength and intrinsic ductility for serviced and reheat treated T91 steel based on stress relaxation testing

High precision stress relaxation tests (SRT) at four temperatures were conducted on T91 (9%Cr) steel after extended boiler service, and also after re-heat treatment. Relative differences in creep strength, measured over five decades in strain rate were dependent on test temperature. Using an established correlation between strain rate sensitivity and elongation at failure, intrinsic ductility values as a function of stress and test temperature were determined. The general trend of a minimum in ductility in terms of stress or strain rate was consistent with long term creep rupture data on T91, and with literature data on alloy steels. However, the precision and repeatability of the SRT analysis contrasted with the appreciable scatter and heat to heat variation in traditional testing. It is argued that the current creep strength evaluation based on the nearly constant state measurement from the SRT test is superior to the measurement of stress dependence of minimum creep rate in traditional creep rupture testing. The complexity of primary creep in laboratory testing, which may not be significant at operating stresses where loading strains may be fully recoverable (anelastic), does not apply to the SRT. Since very low strain rates are achieved in a one day test, the procedures for setting of design allowables and design analysis based on the SRT data should not be significantly different from current practice. This technique offers accelerated alloy development and optimisation for creep strength and also ductility, and hence resistance to notch sensitivity.

Intrinsic ductility for structural materials as a function of stress and temperature

In recent years, a method has been developed to measure current creep strength at nearly constant structure in a high precision stress relaxation test (SRT), covering at least five decades in creep rate in a one day test. Results have been reported on a wide range of metallic alloys, polymers, composites and ceramics. In the present paper it is shown that these same data can be used to determine a measure of intrinsic ductility over the same range of stress, using results on a low alloy Cr–Mo–V steel. This is based on an experimental and theoretical correlation between elongation at failure and strain rate sensitivity, m. This refined SRT test can now be used to evaluate both the intrinsic creep strength and the intrinsic ductility as a function of stress in a single short-time test. The test can detect embrittling phenomena at very low creep rates as a function of temperature. This measure of ductility may be used directly in engineering design and remaining life assessment.

Recent developments in small punch testing: Applications at elevated temperatures

The paper is focused on summarizing the high temperature tensile, creep and stress relaxation small punch testing (SPT) of ferritic-martensitic steel P92. The relation of SPT results with conventional uniaxial tests results is presented. Two heats of P92 steel with different heat treatment are compared in SPT creep. The finite element modeling of SPT creep setup is compared with experimental SPT results. The stress relaxation SPT is discussed and values of initial stress and residual stress were evaluated from the test. Very good agreement of the stress relaxation SPT, stress relaxation uniaxial test and numerical modeling is shown by their comparison. An overview of the ongoing round robin on creep SPT is briefly outlined.

Time, stress, and temperature-dependent deformation in nanostructured copper: Stress relaxation tests and simulations

In the present work, stress relaxation tests, high-resolution transmission electron microscopy (HRTEM), and molecular dynamics (MD) simulations were conducted on coarse-grained (cg), nanograined (ng), and nanotwinned (nt) copper at temperatures of 22 °C (RT), 30 °C, 40 °C, 50 °C, and 75 °C. The comprehensive investigations provide sufficient information for the building-up of a formula to describe the time, stress, and temperature-dependent deformation and clarify the relationship among the strain rate sensitivity parameter, stress exponent, and activation volume. The typically experimental curves of logarithmic plastic strain rate versus stress exhibited a three staged relaxation process from a linear high stress relaxation region to a subsequent nonlinear stress relaxation region and finally to a linear low stress relaxation region, which only showed-up at the test temperatures higher than 22 °C, 22 °C, and 30 °C, respectively, in the tested cg-, ng-, and nt-Cu specimens. The values of stress exponent, stress-independent activation energy, and activation volume were determined from the experimental data in the two linear regions. The determined activation parameters, HRTEM images, and MD simulations consistently suggest that dislocation-mediated plastic deformation is predominant in all tested cg-, ng-, and nt-Cu specimens in the initial linear high stress relaxation region at the five relaxation temperatures, whereas in the linear low stress relaxation region, the grain boundary (GB) diffusion-associated deformation is dominant in the ng- and cg-Cu specimens, while twin boundary (TB) migration, i.e., twinning and detwinning with parallel partial dislocations, governs the time, stress, and temperature-dependent deformation in the nt-Cu specimens.

High Temperature Strength and Stress Relaxation Behavior of Dilute Binary Mg Alloys

Monotonic compression and stress relaxation tests were carried out on specimens of 6 cast binary alloys with (at. pct) 2.5 Al, 0.6 Sn, 2.2 Zn, 0.9 Nd, 0.8 Gd and 1.3 Y, and of a similarly cast AZ91D alloy for reference. The solute concentration of the binary alloys was kept deliberately low to limit precipitation hardening effects during the testing, done in the solution heat treated and quenched condition. Compression testing was carried out at 298 K, 373 K and 453 K (25 °C, 100 °C and 180 °C) for all of the alloys and at 493 K and 523 K (220 °C and 250 °C) for the Nd-, Gd- and Y- containing ones. Stress relaxation was done at 453 K (180 °C) at either a predetermined strain (0.05) or stress (150 MPa). The Mg-Al and the AZ91 alloys softened considerably above 373 K (100 °C). The rest of the alloys exhibited increasing linear strain hardening in compression and reduced stress relaxation, in the order Sn, Zn, Nd, Gd and Y, an indication of a progressively stable dislocation substructure, hence of an increasingly extended athermal regime in the strength-temperature relationship. The overall strain hardening behavior matches that of commercial alloys involving the same solutes at comparable or higher concentrations, and can be accounted for through the respective tendency of the solute atoms to develop short range order. This tendency is lowest for the near-random solid solution introduced by Al, and highest for Nd, Gd and Y, in agreement with their respective phase diagrams. The implications for creep resistant alloy selection and design are discussed.

Surface Alteration of Planar P-Type Gallium Nitride to Porous Structure Using 50 Hz Alternating Current-Assisted Photo-Electrochemical Etching Route

Porous p-type GaN films were obtained after photo-electrochemical (PEC) etching in a mixed sulfuric acid and methanol solution under ultraviolet illumination for 90 min with the assistance of 50 Hz alternating current (AC) current (40, 60, 80, and 100 mA). Porous film obtained by etching at 60 mA has demonstrated the highest photoluminescence intensity and bandgap (3.42 eV) value, owing to the generation of the highest amount of point defects and the lowest total dislocation density, which have respectively served as the radiative and non-radiative defects. In addition, the highest root-mean-square surface roughness was obtained for this film, which could be corroborated through the detection of A1(TO) phonon mode that indicated an increased scattering effect off the sidewalls of the pores as well as the acquisition of a high in-plane biaxial stress relaxation. A simple mechanism on AC-assisted PEC etching of the films was also proposed and described.

耐応力緩和特性に優れる車載コネクタ用 70Cu/30Zn ベース合金「C44710」の開発

耐応力緩和特性に優れる車載コネクタ用 70Cu/30Zn ベース合金「C44710」の開発

Intergranular damage during stress relaxation in AISI 316L-type austenitic stainless steels: Effect of carbon, nitrogen and phosphorus contents

This work concerns a study of the mechanisms responsible for intergranular cracking during high temperature stress relaxation in AISI 316L-type austenitic stainless steels. This phenomenon, also known as reheat cracking, is typically present in heat affected zones of massive welded parts used in the energy industry. Here, five steel grades with different C, N and P contents were considered to assess the effects of chemical composition on the three main mechanisms potentially responsible for reheat cracking, namely intergranular M23C6 carbide precipitation, stress relaxation phenomena, and intergranular P segregation. After testing the five AISI 316L-type grades under reheat cracking conditions by a pre-compressed CT-like specimen technique, different degrees of intergranular damage were observed in the specimens by optical microscopy and synchrotron X-ray tomography. Detailed grain boundary analyses by SEM and TEM in the five different steel grades showed the main mechanism responsible for reheat cracking to be the nucleation of microcavities at intergranular M23C6 carbides in high residual stress regions. The addition of P was found to increase the number of cavitated GBs but not to be the dominant mechanism responsible for intergranular damage. A comparison between elasto-plastic finite element predictions of the residual stresses in the CT-specimens and the results of microstructural investigations revealed no intergranular damage in regions where the initial maximum principal stresses in the specimens were below 740 ± 30 MPa.

Effects of soy-based oils on the tensile behavior of EPDM rubber

The effects of soy-based oil on the constitutive behavior of EPDM rubber have been investigated. Soy polyols with various hydroxyl numbers as well as unmodified soybean oil have each been incorporated in the EPDM rubber as a partial replacement for petroleum oil. Tensile tests have been performed at low and high rates, and hysteresis and stress relaxation tests have been performed at low rates. It has been found from the tensile tests that the rubber with soy polyol of low hydroxyl number can provide very similar constitutive characteristics as the control. On the other hand, the constitutive characteristics are significantly changed by using soy polyol of high hydroxyl number or unmodified soybean oil. For example, the rubber with unmodified soybean oil elongates substantially up to stretch ratios of 13, which is approximately 100% greater than that of the control. It has also been found from the tensile and hysteresis tests that the soy-based rubbers generally have better energy storing and dissipation characteristics than the control, such as storing more energy while maintaining lower peak stress, for an impact application. The test results indicate the potential for application based tuning of soy rubber properties by selecting an appropriate soy oil/EPDM blend or a polyol/EPDM blend with optimized functionality.