Stress Relaxation Procedure Research Articles

An approach to assess the thermal aging effects on the coupling between inelasticity and network alteration in filled rubbers

The development of a constitutive representation for predicting the durability of filled rubbers upon aggressive environmental conditions is crucial for numerous technical examples but still remains largely an open issue. This paper presents a constitutive model for the prediction of thermal aging effects on the inelastic response of filled rubbers. A network alteration theory is proposed for the thermally driven network degradation by considering a collection of chemical and physical changes occurring inside the rubber-filler material system. The network decomposition considers scission-rebound mechanisms of links (between filler aggregates and between elastically active cross-linked chains) and changes in movements of the free chains superimposed into the (newly born and original) chemically linked network. The material kinetics is designed using multi-step (physical) stress relaxation experiments performed at room temperature on a sulfur-vulcanized styrene–butadiene rubber containing different amounts of carbon-black fillers, and previously submitted to an accelerated chemical stress relaxation procedure, i.e. aging at constant stretch for various temperatures and exposure times. The time–temperature equivalence principle is used to construct master curves of the alteration kinetics for both the cross-linked chains and the superimposed free chains. The amplified effect of the fillers is also considered in our network decomposition to account for the cross-linked chains/fillers interactions and the free chains/fillers interactions. The capabilities of the proposed network alteration based constitutive model to describe and predict the material response evolution in terms of stiffening, hysteresis area increase and physical relaxation increase are shown. The chemically induced “plastic-like” deformation and stress relaxation are predicted using the developed model. The results show the importance of the inelastic effects on the prediction of long-term material response due to thermally activated degradation. From an applicative point of view, the model makes possible to estimate the operational life of pre-strained rubber mechanical components with respect to loss of pressure and flexibility.

An investigation of the glycosaminoglycan contribution to biaxial mechanical behaviours of porcine atrioventricular heart valve leaflets.

The atrioventricular heart valve (AHV) leaflets have a complex microstructure composed of four distinct layers: atrialis, ventricularis, fibrosa and spongiosa. Specifically, the spongiosa layer is primarily proteoglycans and glycosaminoglycans (GAGs). Quantification of the GAGs' mechanical contribution to the overall leaflet function has been of recent focus for aortic valve leaflets, but this characterization has not been reported for the AHV leaflets. This study seeks to expand current GAG literature through novel mechanical characterizations of GAGs in AHV leaflets. For this characterization, mitral and tricuspid valve anterior leaflets (MVAL and TVAL, respectively) were: (i) tested by biaxial mechanical loading at varying loading ratios and by stress-relaxation procedures, (ii) enzymatically treated for removal of the GAGs and (iii) biaxially mechanically tested again under the same protocols as in step (i). Removal of the GAG contents from the leaflet was conducted using a 100 min enzyme treatment to achieve approximate 74.87% and 61.24% reductions of all GAGs from the MVAL and TVAL, respectively. Our main findings demonstrated that biaxial mechanical testing yielded a statistically significant difference in tissue extensibility after GAG removal and that stress-relaxation testing revealed a statistically significant smaller stress decay of the enzyme-treated tissue than untreated tissues. These novel findings illustrate the importance of GAGs in AHV leaflet behaviour, which can be employed to better inform heart valve therapeutics and computational models.

Impact and Measurement Techniques of Residual Stress in Welding of Bimetallic Material

Objectives: The welding techniques are being developed at a very fast pace with novel and efficient techniques coming up for unconventional cases such as dissimilar metal joints between plates of various types, which have various practical applications, like in nuclear power plants. Methods/Statistical Analysis: A few strategies and procedures have been proposed furthermore have been connected for measuring residual stress (σR) in a few metals. Some of such strategies are: stress relaxation procedures, X-Ray Diffraction methods, Techniques utilizing stress touchy properties and Cracking systems. In addition, endeavors have additionally been made to use ultrasonic and hardness strategies to decide remaining burdens in metals by measuring stress sensitivity properties. Finding: Failure investigations of many dissimilar joints and literature review have revealed that several failures of the dissimilar welded joints have taken place in the Heat Affected Zone [HAZ]. One of the major reasons for failure of dissimilar welded joints is the untreated left residual stresses. In the present paper a brief description of the ideology of bimetallic weld processes, their capabilities and restrictions, phenomenon and types of σR and their treatment techniques is presented. The overall impact of σR and their measurement techniques have also been discussed. Application/Improvements: There are numerous applications of dissimilar metal joints such as in automotive industry, power generation industry etc.

Analytical modeling of residual stress formation in workpiece material due to cutting

Residual stress on surface layer of workpiece has significant effects on service life of parts, including fatigue strength and corrosion resistance. Metal cutting process is widely used on manufacturing parts, which will cause severe mechanical and thermal stress on the material and then result in the formation of residual stress on workpiece surface layer. A thorough analysis of residual stress formation due to cutting is essential to create parts with desired residual stress distribution. After analyzing stress field in workpiece based on previous work, the authors in this paper observe that stress of a point during cutting is time dependent, and then proposed that initial condition is mandatory in order to determine initial stress state in stress relaxation procedure. Then the authors propose a criterion to determine initial condition based on the calibration of surface residual stress. Using this criterion and calibration method, the residual stress in different depth of machined surface can be compute using the proposed analytical residual stress model, and the computed results are consistent with experiment results. Moreover, the computed results show that the residual stress component in cutting speed direction is higher than that in axial direction, which is consistent with the general conclusion in the existing literatures. The proposed concept of initial condition and its calibration method in stress relaxation promote the understanding of physical mechanism of residual stress formation in cutting, and can be extended to the situations where the evolution of flow stress in workpiece during cutting is similar with that of in orthogonal cutting.

Multivariate analysis of the effect of drying method and particle size of flour on the instrumental texture characteristics of paste made from two varieties of taro ( Colocasia esculenta L. Schott) flour

The present study aimed at determining the effect of variety (red and yellow varieties), drying technique (solar and electric drying) of precooked taro and flour particle size (500, 1000 and 1500 μm) on the texture of achu (taro based paste). The flours were reconstituted into achu and their texture evaluated using an Instron machine under the stress-relaxation procedure. The results show a significant effect of variety on the force of adhesion (FOA), with the yellow variety exhibiting the higher value compared to the red variety. Achu electric dried was observed to exhibit lower force of relaxation (FOR) and rate of relaxation (ROR) and a higher viscoelasticity index (VEI). An increase in particle size was found to induce significant ( p < 0.05) decrease in FOA, hardness and FOR, while no consistent variations were observed on VEI and ROR. Principal component analysis showed that a more than 50% similarity between achu sun dried particle size of 500 and 1000 μm and that traditionally made.

Stress relaxation and medium-range order in diamond-like amorphous carbon films

ABSTRACTIn this paper, we have studied the relationship between medium-range order structures in and stress relaxation in amorphous diamond-like carbon films with fluctuation microscopy. Our preliminary results show strong correlation between stress relaxation and medium-range order. Our previous results showed that annealing films that had been through stress relaxation procedures caused great increase of medium-ranger order. In this paper, we have found that the increase of medium-range order in films that have been annealed before going through stress relaxation through removal of substrates is less pronounced than that of films that have been annealed after removal of substrates. We will discuss interpretations and implications of these results.

Experimental and numerical determination of texture evolution during deep drawing tests

This paper deals with the study of texture evolution during deep drawing tests. The aim of these tests is to obtain the whole range of deformation paths encountered in real industrial processes. Nakazima tests are carried out in order to obtain deformation states ranging from simple tension to equibiaxial stretching. Shear tests are also achieved in a simple shear setup developed for this work. The experimental strain fields are determined accurately with an optical measurement system based on the correlation of digital images. Texture measurements were carried out for the undeformed material and after each test. The texture evolution is compared to the texture obtained from an elastic–plastic rate-independent polycrystalline material model recently developed. Boundary conditions applied in the model reproduce the in-plane stretching of the material with an out-of-plane stress relaxation procedure in order to reach plane stress conditions.

Creep Behavior of FRP-Reinforced Wood Members

In this study, the writers aim to provide a basic understanding of the creep behavior of wood members reinforced with fiber-reinforced plastic (FRP) materials epoxy-bonded to the tension faces. An analytical method is presented first for the deformation of cross sections based on a Burger model combined with a mechanosorptive element for wood, Findley's model for composite materials, and a stress-relaxation procedure to yield the cross-section stresses and strains as a function of time, for both constant and variable temperature and humidity conditions. Parametric studies assessing the effect of the type and area fraction of composite material on the creep response of FRP-reinforced wood members are also presented, for both constant and variable environment. Finally, the analytical model is employed to predict the time-dependent deflections of wood beams reinforced with carbon fiber-reinforced plastic (CFRP) laminates of different thicknesses, and the results of a short experimental program involving FRP-...

Time‐Dependent Behavior of RC Members Strengthened with FRP Laminates

This study attempts to develop a fundamental understanding of the time-dependent behavior of reinforced-concrete beams strengthened with fiber reinforced plastic laminates. The paper describes an analytical procedure for the deformation of cross sections based on the age-adjusted effective modulus method for concrete, Findley's (1960) model for composite materials, and a stress relaxation procedure to yield the cross sectional stresses and strains as a function of time. Also presented are parametric studies asesssing the effect of the type and area fraction of composite material, and the area fraction of steel reinforcement on the long term response of strengthened elements. The analytical model is used to predict the deflections of concrete beams strengthened with FRP laminates of different thicknesses.

Longitudinal modulus and hydraulic permeability of poly(methacrylic acid) gels: effects of charge density and solvent content

The equilibrium longitudinal modulus, hydraulic permeability, and fixed charge density of poly(methacrylic acid) gels were measured over a wide range of pH, ionic strength, and uniaxial strain (solvent content). Mechanical measurements were performed using a compression stress relaxation procedure which was analyzed in the context of a poroelastic theory of gel swelling kinetics. The longitudinal modulus varied with ionization state and solvent content in a manner that suggests the importance of ionized charge density as a critical determinant of gel stiffness and swelling. The modulus decreased while the hydraulic permeability increased substantially with increasing gel solvent content

Creep and shrinkage analysis of composite systems under axial load and biaxial bending

The instantaneous and creep and shrinkage behaviour of composite steel-concrete sections subjected to axial load and biaxial bending is examined analytically using the age-adjusted effective modulus method and a stress relaxation procedure. Any cross-section that can be discretized in rectangular elements can be analysed using the approach described in this study. Material non-linearities (concrete cracking and crushing as well as steel yielding) are included in the formulation by empolying an interative numerical procedure. The analytical method was implemented in a computer program and case studies demonstrate that composite columns under complex loads experience a considerable redistribution of stresses and strains, causing the degree of cracking and the secondary load effects (P-delta) to increase.

Origins and measurements of internal stress in plastics

The term ‘internal stress’ is given various meanings in the literature and these are listed and discussed. The origin of deformation-induced internal stress is considered, and the development of residual (moulding) stresses in a number of polymer processing operations is described. The deformation-induced internal stress can be investigated by stress relaxation procedures, and has been found to be sensitive to the state of ageing or annealing, although no quantitative interpretation has been developed to date. The layer removal process for determining residual stress distribution is described in some detail and examples of applications are presented, including a full biaxial treatment. The results of this technique are quite revealing and some of the practical implications are indicated.

Some measurements of internal stress and activation volume in acrylic polymers

Stress relaxation tests conducted on poly(methyl methacrylate) (PMMA) in various forms have been analysed using the procedure of Kubat and Rigdahl. The values ofσi, the internal stress parameter, obtained from this analysis are subject to large errors and do not provide useful comparators, nor do they correlate with residual stress measurements made by the layer removal method. The index,n, changes with test temperature in a systematic manner and reveals marked differences in relaxation behaviour between the different classes and specimens investigated. A new method for estimating activation volume from the stress relaxation procedure has been tested and found to give values that are much higher for injection-moulded bars than for “Perspex” sheets as-received or hot-drawn.

Measurement of internal stresses in chemically cross-linked high-density polyethylene

Residual stresses in injection-moulded high-density polyethylene bars have been assessed by the stress-relaxation procedure of Kubat and Rigdahl, and also by the layer removal technique. Stresses were found to be tensile in the interior and compressive near to the surface. The introduction of cross-links through moisture-activated alkoxysilyl groups grafted onto the polyethylene molecules was found to increase the value of the internal stress parameter derived from the relaxation experiments and also to produce higher stress values by the layer removal analysis. Some of the relaxation curves have been analysed in addition by the technique due to Li and the significance of the internal stress parameter, σi obtained by the two different relaxation analyses is discussed.