Results Of Stress Relaxation Tests Research Articles

Friction-based connectors with wooden dowels for timber shear walls: Shake table and time-dependent test

Prestressing in wood connection, due to frictional forces, improves mechanical properties of the connection. This paper presents novel friction-based connectors, and corresponding tests at the connection and system levels. The friction-base connector is composed of a softwood or hardwood dowels inserted into the wood-based material and tightened to generate frictional force. Long-term performance was evaluated by stress relaxation tests and time-dependent connection tests. To evaluate dynamic performance of shear walls with friction-based connectors, shake table tests were conducted. The shake table tests showed that the restoring force characteristics of the wood friction shear walls were bilinear reflecting the behaviors of the friction-based connectors. The strength degradation of the wood friction shear walls due to cyclic input was small that more seismic inputs could be applied before any significant damage to the wood friction shear wall. Results of stress relaxation tests, for periods of 1 to 3 years, showed that the stresses were maintained at 70–90% under uncontrolled environment. On the other hand, the results of time-dependent connection tests, for 1 to 6 years, showed high initial clamping stresses caused large performance loss, but high performance could be maintained when tightened below the allowable stress for sustained loading.

A multifunctional lactic acid based plasticizer used for plasticizing PVC and PLA: Endowing PLA elastic restore capability

A novel multifunctional plasticizer, cyclohexyl triethylene glycol oligo-lactate (CTLE) was designed by a two-step esterification reaction using green non-toxic biomass lactic acid as raw material. Its structure was characterized by 1H NMR, FT-IR and TOF-MS. Thermogravimetric analysis (TGA), tensile test and leaching test were used to evaluate the plasticizer effect of PVC or PLA blends. Compared with commercial plasticizers, CTLE showed more outstanding thermal stability, flexibility and migration stability. Under the same condition with the addition of 50 phr plasticizer, the CTLE plasticized PVC film had the lowest glass transition temperature (TgCTLE-50 = 24.6 °C) and the highest initial decomposition temperature (TiCTLE-50 = 265.4 °C). In PLA blends, only the CTLE plasticized PLA film could recover after fracture, and the stress relaxation test results showed that the stress value of PLA-CTLE-20 drops sharply to a non-zero value, showing elastic characteristics. We estimate that the excellent plasticity of CTLE is due to its molecular structure. Among them, a large number of polar groups form intermolecular forces with PVC chain segments to make the binding tight, and the ring structure increases the free volume between PVC chain segments.

Effect of Microstructure on the Mechanical Response of Hydrogen-Charged Pure Iron

In this paper, we investigate how two different microstructures in pure iron affect the dislocation mobility in hydrogen-charged and non-charged samples by conducting stress-relaxation tests. The effective activation volume of the pure iron for both types of microstructures (cold-rolled and annealed samples) has been determined for both H-charged and uncharged material. Information about the dislocation structures formed during stress relaxation is provided by conducting TEM analysis. We employ a self-consistent kinetic Monte-Carlo (SCkMC) model of the ½ [111] screw dislocation in Fe to investigate how hydrogen affects the mobility and behavior of the dominant mobile dislocation in Fe at different stresses and H concentrations. The results from our simulations show the following: (i) at low stresses the deviation from the primary slip plane in the presence of H is lower than the deviation in the uncharged Fe. The deviation angle decreases with increasing H concentration; (ii) at higher shear stresses, the higher probability for kink-pair formation in the secondary (110) planes in the presence of H, leads to an enhanced deviation from the primary slip plane, which increases with increasing H concentration. We use the results of stress-relaxation tests and SCkMC simulations to propose an explanation for the formation of dislocation cell structures in pure and hydrogen charged Fe in the cold-rolled and annealed samples.

A Core Damage Constitutive Model for the Time-Dependent Creep and Relaxation Behavior of Coal

The creep and stress relaxation behaviors of coal are common in coal mining. The unified constitutive model is suitable to describe and predict both the creep and relaxation evolution characteristics of rocks. The generalized Kelvin model is the core element for traditional and improved component models to reflect both the nonlinear creep and relaxation. In this paper, an improved core damage model, which could both reflect the creep and stress relaxation in relation to the damage evolution, was established based on a comparison of the traditional and improved component models, and the responding constitutive equations (creep and stress relaxation equation) at constant stress/strain were deduced. Then, the core damage model was validated to the uniaxial compressive multistage creep and stress relaxation test results of coal, showing that the model curves had great accordance with the experimental data. Moreover, the model comparisons on accuracy, parameter meaning, and popularization among the core damage model, hardening-damage model, and the fractional derivative model were further discussed. The results showed that the parameters in the core damage model had clear and brief physical significances. The core damage model was also popularized to depict the time-dependent behaviors of other rocks, showing great accuracy.

Analysis of reclaimed asphalt blended binders using linear and nonlinear viscoelasticity frameworks

This paper presents a comprehensive analysis of blended asphalt binders extracted from mixtures that contain various amounts of reclaimed asphalt pavements (RAP). The analysis is performed within a thermodynamically consistent non-linear viscoelastic (NVE) modeling framework, which has the advantage of accounting for the contributions of each of the constituents (i.e. virgin binder and RAP binder) to the response of the blended binder. In order to calibrate and validate the NVE model, the RAP blended binders were subjected to different testing protocols: frequency sweep, multiple stress creep and recovery, repeated creep and recovery with multiple stress levels, random creep and recovery, and stress relaxation. For the binders used in this study, linear viscoelasticity (LVE) was suitable to model the frequency sweep and multiple stress creep and recovery data, but it did not capture the repeated creep and recovery with multiple stress levels results that involved higher stress and strain levels. The NVE model, on the other hand, was successful in describing the repeated creep and recovery with multiple stress levels results. The validation was achieved by comparing the NVE model predictions with the random creep and recovery and stress relaxation test results. Finally, the paper recommends a new method, based on the parameters of the NVE model, to evaluate the rutting resistance of blended asphalt binders.

Non-linearly viscoelastic constitutive equation of cancellous bone tissue and identification of material constants

In the paper a new constitutive equation for human cancellous bone tissue was formulated. A strain energy function was postulate, which describes the elastic response of the tissue to the load. The nonlinear viscoelasticity was also taken into account in the constitutive model. The material constants in the constitutive equation were identified on the basis of the results of stress relaxation tests and monotonic compression tests using the curve fitting method. The Levenberg-Marquardt algorithm of the least square method was used here. On the basis of the relaxation tests, the values of relaxation times were identified, while the elastic and viscoelastic constants were determined on the basis of monotonic compression tests. The tests were performed at room temperature. On the cuboidal samples of cancellous bone obtained from femoral heads during the surgeries of hip joint prosthesis implantation.

Effect of crosslinking density on adhesion performance and flexibility properties of acrylic pressure sensitive adhesives for flexible display applications

The use of acrylic pressure sensitive adhesives (PSAs) in flexible displays involves their attachment to each layered device. Due to the high industrial demand of flexible displays, acrylic PSAs must necessarily exhibit high flexibility. In this study, the effect of the crosslinking density of acrylic PSAs on their adhesion and flexibility properties has been investigated by incorporating a diisocyanate crosslinking agent into the PSA structure. As the content of the crosslinking agent increased, the measured peel strength and tack of the synthesized PSAs decreased, while the maximum value of the lap shear stress increased. In addition, the maximum stress and shear strain determined for the crosslinked PSA specimens decreased with an increase in the crosslinking agent concentration (although, the stress measured at low strain values was initially increasing until the crosslinking agent content reached 1 phr). The results of stress relaxation testing showed a stress increase at specified strain levels with an increase in the crosslinking agent content up to 0.5 phr, while the results of creep testing revealed that the measured strain was inversely proportional to the degree of elastic recovery. The obtained data indicate that crosslinking generally improves the PSA properties related to their use in flexible display applications; however, very high crosslinking densities produce mostly a negative effect on the PSA flexibility and adhesion characteristics.

Microstructural, Mechanical, and Physicochemical Behaviours of Alkali Pre-treated Oil Palm Stalk Fibres

The effect of alkali pre-treatment (sodium hydroxide, NaOH) on the microstructural, mechanical, and chemical composition of oil palm stalk fibres (OPSF) is reported for future bioconversion processes. The OPSF was pre-treated with various concentrations of NaOH (5, 10, 20, 30, and 40% w/v). Scanning electron microscopy analysis revealed that 5% w/v alkali concentration caused complete removal of silica bodies and waxy layers, whereas pronounced degradation of the fibres occurred at 40% w/v NaOH concentration. Mechanical test results showed that the maximum elastic modulus of untreated OPSF was 2.5 GPa and the modulus was not sensitive to alkali concentration. Permanent set (plastic strain) and viscoelastic behaviours of OPSF were observed from the loading-unloading and stress relaxation test results, respectively. Agreement was observed between the Prony series viscoelastic model and test results, which provided further evidence of the viscoelastic behaviour of OPSF.

Biomechanical Properties of Superficial Musculoaponeurotic System Tissue With vs Without Reinforcement With Poly-4-hydroxybutyric Acid Absorbable Mesh

IMPORTANCE It is unknown whether poly-4-hydroxybutyric acid (P4HB)-reinforced superficial musculoaponeurotic system tissue (SMAS) plication techniques will support SMAS imbrication and plication and potentially improve outcomes in rhytidectomy. OBJECTIVES To evaluate the biomechanical properties (tissue breaking strength, suture tearing force, and stress relaxation) of the SMAS with vs without reinforcement with P4HB absorbable mesh and to correlate these results with potential clinical applications. DESIGN, SETTING, AND SAMPLES In a cadaver study at an academic setting, 12 fresh frozen cadaver heads were used. Rhytidectomy incisions were made, and the SMAS was harvested and prepared for strength and stress testing using an Instron device. MAIN OUTCOMES AND MEASURES Tissue breaking strength and suture tearing force were analyzed. Stress relaxation test results were also assessed. The results of the SMAS samples alone were compared with those reinforced with P4HB absorbable mesh. RESULTS Overall, there were significant differences noted in tissue breaking strength and suture tearing force between the 2 groups. When the SMAS was reinforced with absorbable mesh, significant improvements were observed in tissue breaking strength (P < .001) and suture tearing force (P < .003). In addition, less variability was demonstrated in the maximum tensile load-bearing quality of the SMAS when the repair was reinforced with P4HB. CONCLUSIONS AND RELEVANCE Reinforcement with P4HB absorbable mesh improves tissue breaking strength and suture tearing force in cadaveric SMAS. It also reduces the variability in load vs displacement seen among samples tested. These data suggest that P4HB-reinforced SMAS imbrication would support higher loads and provide more consistent, long-lasting SMAS support among patients undergoing rhytidectomy. Further studies are needed to correlate these data with clinical outcomes in rhytidectomy.

Coupling of Crystal Plasticity Finite Element and Phase Field Methods for the Prediction of SRX Kinetics after Hot Working

The conventional approach to model static recrystallization (SRX) kinetics is to use the well‐known Kolmogorov–Johnson–Mehl–Avrami (KJMA) equation. However, the validity of KJMA kinetics depends on whether the real recrystallization process follows the assumptions made in the derivation of the KJMA kinetics, which are a random distribution of nucleation sites, isotropic growth and a constant growth velocity. When these assumptions are violated the KJMA kinetics loose their physical meaning and turn into an empirical formula with limited predictive capability. In such cases, phase field (PF) method can be an alternative to the KJMA model, since it is based on irreversible thermodynamics. An effective strategy to apply the PF method to static recrystallization problems is coupling it to a deformation model though which the prior deformation process is simulated. For that purpose crystal plasticity finite element method (CPFEM) can be employed. In this work, an example of a weak coupling strategy was applied to a steel grade of commercial interest (X60Mn23 TWIP steel) for which KJMA equation is invalid. Deformation of a Voronoi type representative volume element was modeled using a Crystal plasticity finite element formulation after determining the material parameters of a strain hardening model from macroscopic experiments. Afterwards, the deformed structure was transferred on to a finite difference grid with a dedicated mapping scheme, which was then used as input for phase field simulation. Finally, the simulation results were compared with the results of stress relaxation tests. It was found that although the grain morphology of the simulation differed from the experimental results, the coupled simulation strategy was able to mimic the main features of the static recrystallization (SRX) kinetics.

Quasistatic and dynamic mechanical properties of Al–Si–Cu structural films in uniaxial tension

In this article, the quasistatic and dynamic tensile properties of aluminum–silicon–copper (Al–Si–Cu) alloy films are described. The films were deposited by sputtering onto thermally oxidized Si wafers, and then half of the wafers were heat treated at 623 K in nitrogen gas for 1 h. Specially developed environment-controlled uniaxial tensile test equipment was used to carry out the quasistatic tensile test, stress relaxation test, and cyclic loading test at temperatures ranging from room temperature (RT) to 573 K in high vacuum, and the influence of annealing on the mechanical characteristics was investigated. The Young’s modulus did not show annealing dependency. The mean value was 65 GPa at RT, and gradually decreased with increasing test temperature. The yield stresses of nonannealed and annealed films were 168.5 and 129.6 MPa, respectively, which also decreased with temperature rise. In stress relaxation test results, creep exponents in respective films were obtained from curve fitting using the Norton law, which indicated that creep deformation was restricted by annealing. The cyclic loading test was performed under stress- and displacement-amplitude-constant modes. The stress-amplitude-constant mode test provided creep deformation acceleration to failure, whereas the displacement-amplitude-constant mode test showed a gradual drop of stress amplitude. The stress-amplitude change was compared with stress relaxation curves; consequently, the creep deformation was dominant to the degradation of Al–Si–Cu films subjected to cycling loading with constant displacement amplitude.

A Rationale for SRX Regression Model of Hot-Deformed Austenite Using an Orthogonal Taguchi L8 Matrix Steels

In recent past, a linear regression model to predict the activation energy (Qrex) and kinetics of static recrystallisation for hot-deformed austenite was developed based on stress relaxation test results of over 40 different carbon steels. The model is able to predict satisfactorily the static recrystallisation (SRX) kinetics of common carbon steel grades (including microalloyed steels) and also several special steel grades. In this study, the main effects of seven alloying elements, viz., C, Mn, Cr, Ni, Mo, Nb and V, on the activation energy of recrystallisation were further examined by using eight experimental steels based on an orthogonal Taguchi L8 matrix. All steels contained constant additions of B, Ti and Si. Even though originally intended for studies on phase transformation characteristics and hardenability under direct quenching conditions, the L8 matrix steels were suitably employed for further validation of the SRX regression model. In addition, the SRX characteristics and kinetics of a set of new steel compositions including C-Mn, C-Mn-Nb and C-Mn-Nb-Ti types were examined in the light of model equations, which further confirm the suitability of the regression model.

A STUDY OF MICROSTRUCTURE AND TENSILE PROPERTIES OF PROTON BEAM IRRADIATED Al–Mg–Si ALLOY

In the present paper, the microstructure and tensile properties of Al – Mg – Si alloy irradiated with 2 MeV protons for 5, 10, 20, and 40 mins at 300 K are investigated. The microstructure of irradiated specimens, observed using metallurgical microscope, shows that aspirates are emitted out from the surface of irradiated specimens, which take the form of clusters and/or precipitates with increase of exposure time. The tensile behavior of irradiated specimens was investigated using universal testing machine and compared with that of unirradiated one. The yield stress, ultimate tensile stress and % elongations remain unchanged after 5 and 10 mins of irradiation. However with an increase of irradiation time up to 20 mins, an increase in yield stress, ultimate tensile stress and decrease in % elongation was observed. With further increase of exposure time to 40 mins, the increase in yield stress and decrease in plasticity became more prominent. The results of stress relaxation tests performed during the tensile deformation have also been presented. The XRD analysis reveals that the microstructural changes controlling the tensile properties in irradiated samples appear to be physical rather than chemical.

Creep Behaviour of Closed Cell Aluminium Foams from Stress Relaxation Tests

Creep behaviour of closed cell aluminium foams and transitions from power law to power law breakdown (PLB) creep regimes are investigated from results of stress relaxation tests (SRT) carried out on Alporas foams with densities between 0.20 to 0.32 g/cm3. Tests were carried out at temperatures between 200°C and 300°C and stress relaxation was measured from the collapse stress under compression of the foams. Under similar foam density, temperature and stress conditions, the values of strain rate calculated from SRT tests were of the same order of magnitude than those previously reported in the literature from conventional constant load creep tests. Under stress values close to the collapse stress, the creep mechanism seems to follow a PLB regime, with values of the power law exponent n=10-17, much higher than those corresponding to the power law creep in the solid material (n=4.4-6.5) and with activation energy values close to Q = 150 KJ/mol. Having in mind the limitations of available creep models for closed cell foams and the need for additional experimental results, the use of SRT testing would offer advantages for the assessment of the high temperature behaviour of aluminium foams, due to its lower testing times and reduced experimental effort.

Isothermal physical aging characterization of Polyether-ether-ketone (PEEK) and Polyphenylene sulfide (PPS) films by creep and stress relaxation

This paper considers the experimental characterization of isothermal physical aging of PEEK and PPS films using a dynamic mechanical analyzer. Using the short-term test method established by Struik, momentary creep and stress relaxation curves were measured at several temperatures within 15–35°C below the glass transition temperature (Tg) at various aging times. Stress and strain levels were such that the materials remained in the linear viscoelastic regime. These curves were then shifted together to determine momentary master curves and shift rates using the PHYAGE program. In order to validate the obtained isothermal physical aging behavior, the results of creep and stress relaxation testing were compared and shown to be consistent with one another using appropriate interconversion of the viscoelastic material functions. Time–temperature superposition of the master curves was also performed. The temperature shift factors and aging shift rates for both PEEK and PPS were consistent for both creep and stress relaxation test results.

Stress-relaxation tests in the work-hardening regime of tungsten single crystals below 300 K

The influence of work hardening on the results of stress-relaxation tests was studied for highly pure tungsten single crystals isothermally deformed at four temperatures of 274, 241, 131, and 78 K. A method accounting for strong work hardening on the determination of the strain-rate sensitivity from stress-relaxation tests is introduced by establishing special diagrams of SR tests denoted as YX diagrams.

Effects of initial load levels and crosshead speed levels on viscoelastic properties of biodegradable loose fill extrudates

The viscoelastic properties of biodegradable loose fill extrudates can provide information at the structural level of the biodegradable cushioning materials used. The objective of this study was to test two rheological models of linear viscoelastic theory (generalized Maxwell with three units in parallel, and Burgers) using the results of stress relaxation tests in biodegradable loose fill extrudates. The experiments were run at 105 °C with screw speed 500 rpm and die diameter 3.92 mm. The extrudates were then submitted to tests of stress relaxation in an equipment of mechanical measurements, Instron Universal Testing Machine (Model 4464). The deformation, imposed to the extrudate, was 1%, guaranteeing the permanency in the domain of the linear viscoelasticity. The models were fitted to experimental data (stress × time) by nonlinear regression. The generalized Maxwell model with three units in parallel and the Burgers model represented the relaxation curves of stress satisfactorily. The viscoelastic properties varied in a way that they were less dependent on the initial stress levels.

Stress Relaxation Behavior of Molten Slags

In the present work, a method to measure stress relaxation of molten slags, developed earlier by one of the authors, was applied to mould flux slags as well as synthetic slags. The molten slag was sandwiched by two Pt plates and was compressed. And stress relaxation tests (the structural relaxation of molten slags) have been conducted by compressing the molten slags as well as mould flux slags at strain rates of 10 mm/min. The results of stress relaxation tests were discussed from the viewpoint of relaxation time and structure of slags with the aid of Raman spectroscopy. The stress relaxation behavior is found to be linked to the structure of slags. The residual stress after a compression was increased with increasing of Al2O3 content and with decreasing of CaF2 content. The relaxation behavior was changed drastically with increasing of temperature for some molten slags.

Textural and Rheological Properties of Processed Cheese

Cheddar cheeses made from ultra filtered (UF) as well as vacuum condensed milks (CM) containing two protein levels (4.5 and 6.0%) were used to manufacture processed cheeses. These processed cheeses were evaluated for instrumental textural profile analysis (TPA), stress relaxation characteristics using Sintech universal testing machine, and visco elastic characteristics (Elastic modulus-G′ and viscous modulus-G′′) using a Haake Viscometer. A small amplitude oscillatory shear test was employed to assess the visco elastic characteristics. Peleg model and six elements Maxwell model were used to assess stress relaxation characteristics of the cheese. Results indicated that the instrumental TPA hardness of UF2 cheeses made from high protein UF milk was highest (149 N) as compared to cheeses made from low protein UF milk (125 N), high and low protein vacuum condensed milks (103 and 62 N, respectively), and control (53 N). The values of elastic and viscous moduli of cheeses made utilizing high protein UF milk were 3.53 and 2.32 MPa respectively, which indicated its higher viscoelastic nature. The UF2 cheese also showed higher values of modulus of elasticity during stress relaxation (21.8 and 16.2 kPa respectively for Peleg and Maxwell models) and required longer time to relax the given amount of stress as compared to rest of the cheeses. The higher values of hardness and visco-elasticity were attributed to higher protein and lower fat contents in the cheeses. The methods of instrumental texture and rheology evaluation had common outcome for hardness and visco-elasticity of cheeses. Results of stress relaxation tests were more useful to differentiate the cheese characteristics as compared to the results of instrumental TPA and dynamic rheology tests.

Mechanical relaxations in heat‐aged polycarbonate. Part II: Statistical analysis of low‐molecular weight data

AbstractThe significance of heat‐aging effects on low‐molecular‐weight polycarbonate has been studied by performing a two‐factor Analysis of Variance (ANOVA). Although this work was primarily motivated by the large experimental scatter observed in stress relaxation results for LMW 2608 (part I), the effect of heat‐aging on the characteristics of secondary transitions (γ and β1) generated by dynamic testing was also investigated. Both types of tests were performed using a dynamic mechanical analyzer. The statistical analysis verified an earlier suggestion that both the secondary transitions were insensitive to heat‐aging. In the quasi‐static stress relaxation tests, the curve‐fitted KWW parameters (τ, Eo′ β′) were evaluated using ANOVA for increasing heat‐aging time and test temperature. Two other statistical techniques were also applied to test repeatability—the power of each aging time/test temperature combination and the number of observations needed to achieve 90% repeatability. In conclusion, both τ and β′ could describe the self‐retarding nature of volume recovery although the repeatability of β′ was substantially higher. However, the unrelaxed modulus, Eo, was found to be an unreliable indicator of whether heat‐treatment had caused changes in the intrinsic structure. Overall, the study showed that the repeatability of the stress relaxation test results is generally very poor for the confidence levels tested.