Short-term Stress Relaxation Tests Research Articles

Influence of stabilization and short-term relaxation to compression generated by stocking welt

This research was carried out to investigate the influence of yarns and knitting parameters, stabilization, and relaxation processes on compression properties of knitted fabrics, used for stocking welt or other compression products. It is well-known that there is a high correlation between the mechanical properties of the fabric and its generated pressure. Also, it is proved that generated compression values decrease during the time they are being worn. In this research, it was found that course and wale density of knitted fabric, as well as tensile force values, increased after the stabilization process. Moreover, increases of linear density of core yarn (PU) of elastomeric yarn do not have direct linear influence to its generated tensile force. The results of short-term stress relaxation tests (300 sec) showed that the major alteration of the generated compression values appears during the first 100 sec period of relaxation process.

Model and prediction of stress relaxation of polyurethane fiber

In this study, the effect of small strain (less than 10%) on hydrogen bond (H-bond) and crystallinity of dry-spun polyurethane fiber was investigated with fourier transform infrared spectroscopy and x-ray diffractometer, respectively. The results showed that the H-bond of hard segments hardly broke and its degree of crystallinity scarcely varied below strain of 10%. The fiber stress relaxation behavior at 25 °C under small strain was researched using dynamic mechanical analyzer. The stress relaxation modulus constitutive equation was obtained by transforming the non-linear relationship between stress and time into the linear relationship between stress and strain. The stress relaxation modulus master curve at 25 °C was established in terms of short-term stress relaxation tests at elevated temperatures (35 °C, 45 °C, 65 °C and 75 °C) according to time-temperature superposition principle (TTS) to predict long-term behavior within 353 year.

The role of delayed elasticity and stress relaxation in Sn-Bi-Cu lead-free solders solidified under permanent magnet stirring

This paper develops methods to explore the influence of permanent magnet stirring (PMS) on solidification microstructure, thermal behavior and creep parameters of Sn-20Bi-0.4Cu alloys predicted from the primary creep stage at 25 °C. A series of short-term stress relaxation test (SRT) and strain relaxation & recovery test (SRRT) are conducted to assess the viscoelastic behavior of tested samples. A three-components consisting of elastic strain εe, delayed elastic strain εd and viscous strain εv are developed and characterized from SRRT for anticipated creep-type constitutive equation. Significant improvement of delayed elastic strain, about 46% of the elastic strain, has been detected after applying PMS. The relative contribution of dislocation mechanism with n values of 5.4–7.7 is discussed in terms of imposed strain and the role of PMS on promoting the columnar-to-equiaxed transition (CET) of lamellar β-Sn dendrites. Moreover, n value is found to increase with increasing the ratio of εd/εv and decrease with the amount of εv. The PMS also significantly decreased the undercooling, pasty range, onset temperature, while maintaining the peak temperature at the same level. These effects have good prospects for lower temperature processed electronic interconnections.

Assessment of room-temperature short-term stress relaxation and strain relaxation with recovery in Sn-Bi lead-free solders solidified under rotating magnetic field

In this paper, short-term stress relaxation test (SRT) as well as strain relaxation and recovery test (SRRT) were performed to assessment the minimum creep rate of Sn-Bi solder from the primary creep stage at 25 °C. Both techniques allow to determination the effect of rotating magnetic field (RMF) during solidification of Sn-Bi solder on the viscous stress exponents nv and other parameters, and have been established to be appropriate for assessment the initial microstructure dependent flow properties of the alloy solder. Additionally, the viscous strain rate in irreversible viscous flow is developed during primary creep and a power-law relationship was created from experimental data to predict the creep deformation mechanism. It was found that the Sn-Bi solder solidified under RMF had the higher resistance to stress and strain relaxation than that solidified without RMF. The stress exponent values of 9.4–6.3 for dislocation creep were achieved depending on the initial imposed strain, and indicating the role of RMF on refining β-Sn dendrites that affect the density of mobile dislocations. Significant decrease in undercooling (from 13.4 to 5.6 °C) was attained after applying RMF, which has good prospects for industrial application.

Uniaxial creep and stress relaxation behavior of modified 9Cr-1Mo steel

An attempt has been made to establish a correlation between the creep strains rates predicted from a stress relaxation test with that directly obtained from a creep test results on a commercial grade of steel (P91) at three different temperatures. Tests were performed on a set of specimens made from the same plate to exclude the effect of material variability on the test data. These were used to explore if short term stress relaxation test (SRT) performed on the same grade of steel could predict its creep rupture strength. The results suggest that the magnitude of activation energy and stress exponent obtained from SRT is significantly lower than those obtained from the creep test data. This is primarily due to the microstructures that evolved during the two variants of tests which had difference in thermal exposures, and leads to the conclusion that material parameter set obtained from SRT cannot be used to predict creep rate of the steel at any given stress and temperature. It needs an additional conversion factor for the prediction of minimum creep rate.

Creep rates of heat-affected zone of grade 91 pipe welds as determined by stress-relaxation test

Cross-weld creep testing conducted at 650 °C under 69 MPa stress has reproduced either the Type I or Type IV failure mode in Grade 91 welds, depending on the post-weld heat treatment procedures. Welds post-weld heat treated below the AC1 temperature have ruptured in the Type VI failure mode, while welds heat-treated above the AC1 temperature of the alloy have ruptured in the Type I failure mode. Heat-treatments at lower temperatures and shorter durations have produced a reduced creep rate. The accelerated short term stress-relaxation test has been conducted to obtain the input creep rates for a finite element model for the cross-weld creep testing. The model predicted secondary creep rates are in good agreement with the results from the conventional cross-weld creep tests. From the finite element model, the creep damage by cavitation is believed to start at regions where the first principal stress and stress triaxiality concentrate.

An Accelerated Method for Creep Prediction From Short Term Stress Relaxation Tests

With the development of ultrasupercritical power generation technology, creep strength of high-temperature materials should be considered for safety evaluation and engineering design. However, long-time creep testing should be conducted by traditional creep assessment methods. This paper established a high-efficient prediction method for steady creep strain rate and creep strength based on short-term relaxation tests. Equivalent stress relaxation time and equivalent stress relaxation rate were defined according to stress relaxation characteristics and the Maxwell equation. An accelerated creep prediction approach from short-term stress relaxation tests was proposed by defining the equivalent relaxation rate as the creep rate during the steady stage. Stress relaxation and creep tests using high-temperature material 1Cr10NiMoW2VNbN steel were performed to validate the proposed model. Results showed that the experimental data are in good agreement with those predicted solutions. This indicates that short-term stress relaxation tests can be used to predict long-term creep behavior conveniently and reliably, and the proposed method is suitable for creep strength design and creep life prediction of 9–12%Cr steel used in ultrasupercritical unit at 600 °C.

Prediction of creep life of X10CrMoVNbN-91 (P-91) steel through short term stress relaxation test methodology

The methodology proposed here adopts short-term stress relaxation testing (SRT) to predict creep strength/life of X10CrMoVNbN 91 (P-91) steel. SRT has been performed at three temperatures (853, 873, 898 K), and at each temperature two prestrains (0.7 and 1.2%) were applied with a hold time of 7 h. The initial data, stress versus time, were converted to stress versus inelastic strain rate by computation and compared with conventional steady state creep rate. Creep life was predicted from the SRT results using the Monkman-Grant relationship, and compared with creep data. The comparison showed the predicted life to agree extremely well with the creep-rupture life (≈30 000 h). The calculated activation energy for SRT (719 kJ mol-1) was found to be higher than the creep (415 kJ mol-1). Detailed SEM and TEM studies revealed that the degradation of microstructure in terms of coarsening of M23 C6, and martensitic lath grains, had taken place and was commensurable to long term creep exposure.

Comparison of measured and empirically predicted creep of reinforced polyphthalamide

The long-term creep properties of a commercial thermoplastic resin system were determined at test temperatures of 23°C, 100°C, and 150°C. In addition, the applicability of using short-term stress relaxation test (SRT) results to predict long-term creep behavior was investigated. Creep strain and strain rate were empirically predicted from the SRT and quantitatively compared to measured values at two applied stresses for each test temperature. The SRT method predictions correlated well with measured creep curves at temperatures below the glass transition and in the secondary creep region. Poor correlations in other regimes are a primary result of differences in the initial deformation.

Influence of ionic environment on the stress relaxation behavior of an invertebrate connective tissue

The rheological properties of an invertebrate connective tissue were measured in three different ionic environments. Short-term stress relaxation tests were conducted on sections of holothurian (Echinodermata) body wall immersed in isotonic monovalent and divalent salt solutions and deionized water. Using a reduced modulus format, the viscoelastic behavior over the experimental time scale was described by a two term Maxwell equation with empirically determined spring constants and relaxation times. In addition, equilibrium relaxation moduli ( G e ) were estimated from the empirical relationship of Chasset and Thirion (1965, in Physics of Non Crystalline Solids, ed. Prins, North Holland). The experiments indicated that both relaxation times and equilibrium moduli decreased in the presence of monovalent and divalent inorganic ions whereby the effect of the Na + was greater than that of the Ca ++. The present findings are compared with those reported for vertebrate connective tissue.