An environmental instrument for measuring creep and stress relaxation in polymers

The construction and operation of an instrument for measuring tensile stress relaxation and creep, particularly of polymers, is described. The instrument is comparatively inexpensive to build and enables measurements to be carried out in vacuo or in a controlled atmosphere of gas or vapor. The design is based on principles used for some earlier stress relaxometers modified to enable characterization of samples having a very wide range of moduli either as stress relaxation or, additionally, as creep measurements. The instrument can therefore be used to evaluate material properties of hard plastics or of soft rubbers when exposed to selected environments.     

An improved method for numerical solution of distillation processes

A new method for the numerical solution of a stagewise process is presented. The method consists of total linearization of all the equations in the distillation process, using the method of quasi-linearization. The resulting equations form a block-band matrix which can be readily solved by a method developed by the author. The method used for obtaining convergence is presented. The method has been tested for both binary and multi-component systems. The method has been compared with the Amundson-Pontinen method for a five component mixture. The method gives more rapid convergence. Numerical experiments indicate that the vapor and liquid flow rates are more stringent requirements than the bubble point.     

Tensile creep and recovery in ultra-high modulus linear polyethylenes

The tensile creep and recovery of oriented linear polyethylene (LPE) monofilaments have been studied for a range of samples of different structure. Starting with a comparison of samples of different draw ratio prepared from one grade of polymer, the measurements were extended to examine the effects of molecular weight. Although the viscoelastic behaviour is markedly non-linear it was found valuable to model the creep and recovery at each level of stress by a simple linear solid representation. This representation enabled a clear distinction to be made between recoverable and irrecoverable creep, both of which are affected by draw ratio and molecular weight. When the irrecoverable creep was examined further in terms of an activated Eyring process, a clear distinction between the two molecular weight grades could be made. In particular, the high molecular weight grade displayed a critical stress below which irrecoverable creep fell to a negligible level. This finding could be of considerable importance with regard to the application of ultra-high modulus LPE fibres in reinforcement.     

Improved models of viscosity and relaxation modulus for epoxy resin during cure

In this work, epoxy resin viscosity was calculated by using reference viscosity and “viscosity‐cure” shift factor which was established to research the dependence of viscosity on curing degree. The predicted results were compared with experimental data and the agreements between them were better than those in reference. Generalized Maxwell model was used to characterize the viscoelastic behavior of epoxy resin and the relaxation modulus was modeled by Prony series. A simple model of stress relaxation times was derived as functions of viscosity and stiffness. The stress relaxation times at different curing degrees were calculated after acquiring the stress relaxation times at the reference curing degree. The relaxation modulus predicted by this method agreed well with experimental data. The above results showed that these models could well be used to predict the viscosity and relaxation modulus of thermosetting resin during cure. POLYM. ENG. SCI., 56:617–621, 2016. © 2016 Society of Plastics Engineers     

Crazing limit of polymers in creep and stress relaxation

The appearance of the first visible damage in polymers in the form of crazes or microcracks may be assumed to be a sign of failure. Experimental investigations have shown that in uniaxial creep and stress relaxation experiments, under isothermal conditions, a certain time between the quasi-spontaneous loading and visible crazing is needed. This ‘incubation time’ is very strongly dependent on the magnitude of the quasi-spontaneously applied stress or induced strain. Based on the Reiner-Weissenberg theory of strength, simple relations allowing the prediction of crazing are developed. The agreement between theoretical computation and experiment is very good.     

Quantitative correlation of creep and stress relaxation in synthetic fibres

The quantitative correlation of creep and stress relaxation was determined. The reliability of the indicial equation and calculated deformation characteristics for describing the different conditions with a given law of deformation was confirmed.All-Russian Scientific-Research Institute of Technical Fabrics, Yaroslavl'. Institute of Textile and Light Industry, St. Petersburg. Translated from Khimicheskie Volokna, No. 6, pp. 41–42, November–December, 1992.     

On the tensile testing of high modulus polymers and the compliance correction

The application of a method to determine and correct for the influence of non-specimen extension in tensile testing is reviewed and demonstrated using two different thermotropic liquid crystalline polymers. In the tensile testing of high modulus polymer extrudate, where the amount of specimen extension is inferred from the crosshead travel, the error associated with system compliance can be significant and parameters such as modulus and elongation will be in error. The need to apply the correction depends on the magnitude of the product of the sample modulus and cross-sectional area, divided by the test gage length, relative to the system compliance value. Its application is not necessarily restricted to high modulus materials, and can be extended to samples of larger cross-sectional area and lower modulus. Guidance to assist in the choice of a suitable gage length to avoid compliance correction and a method to quantify the error contribution is presented.     

A numerical model for elastic modulus of concrete considering interfacial transition zone

The effect of interfacial transition zone on mechanical properties of concrete has been found to be significant, thus the interfacial transition zone should be considered in the analysis for better estimation of elastic modulus of concrete. However, it is difficult to estimate elastic modulus of concrete practically using simple models proposed so far. In this study, a numerical concrete model that adopts three-phase model and finite element with material discontinuity was proposed to analyze concrete with complex interface in three dimensions. The validity of the proposed model was verified by comparing the calculated elastic moduli of concrete with those obtained from experiments. The effect of interfacial transition zone on elastic modulus of concrete with either low or high w/c was also investigated. The analysis results suggest that careful selection of characteristics for interfacial transition zone should be made for the accurate estimation of elastic modulus of concrete.     

A novel,efficient route for the crosslinking and creep improvement of high modulus and high strength polyethylene fibres

A new route is presented for the chemical crosslinking of solution‐spun, ultra‐drawn Ultra‐High‐Molecular‐Weight Polyethylene (UHMW‐PE) fibres. UHMW‐PE fibres with a range of draw ratio's, Young's moduli and tensile strengths were impregnated with a radical initiator using supercritical carbon dioxide as a carrier. After impregnation, the drawn fibres were crosslinked with ultra‐violet light and fibres with a high gel content (> 90%) were obtained. It was found that the chemical crosslinking strongly reduces the plateau creep rate of the fibres and that the threshold stress for irreversible creep is enhanced. Simultaneously, the high Young's modulus and the high tensile strength of the drawn fibres are preserved which illustrates that the long term properties of the fibres (i. e. creep) are improved without a large sacrifice short term mechanical properties such as Young's modulus.     

Early age creep and relaxation of UHPFRC under low to high tensile stresses

This paper presents experimental results on creep and relaxation of UHPFRC at early age (3–8 days), under low to high tensile stresses (1.2, 2.7, 5.4, and 8.1 MPa—estimated as 13, 30, 60, and 90% of the tensile strength, respectively) up to the strain hardening domain. Various loading programs (single and incremental loading steps) were used to reveal non-linear viscoelasticity. For a very low load level of 1.2 MPa, an unexpected important decrease of the creep response was observed. For the 2.7 MPa load level, the viscoelastic responses obtained for creep and relaxation were in the same range. Non-ageing linear viscoelastic models were used to analyze and discuss the obtained results. Non-linear viscoelasticity was observed in several test series and possible underlying mechanisms were discussed. The hypothesis of shrinkage being the same for free and loaded specimen remains an open question for the analysis of test results.     

Generalized age-adjusted effective modulus method for creep in composite beam structures. Part I — Theory

The age-adjusted effective modulus method for approximate analysis of structures exhibiting linear aging creep law and underlying Ba?ant's theorem is generalized to a form which is more accurate for composite beam structures in flexure. A rigorous derivation of a linear algebraic relationship between the internal force histories and the deformation histories of the beam is given using the integral operator symbolics.     

An additive for increasing the strength and modulus of amine-cured epoxy resins

The reaction product of 4-hydroxyacetanilide and 1,2-epoxy-3-phenoxypropane, when added at 19 wt% to a conventional epoxy-resin-curing agent mixture, increases the tensile strength of the cured system from 82 MPa to 123 MPa and increases the shear modulus (20°C, 1 Hz) from 970 MPa to 1560 MPa. As well as showing increased strength, the tensile-test specimens also fail in a ductile fashion, i.e., the slope of the stress–strain curve is negative at failure, with appreciable localized deformation occurring during fracture. For notched samples (compact tension specimens), the fracture properties are strongly strain-rate-dependent. At low strain rates the additive-containing sample has a fracture energy (??, critical strain energy release rate) about twice that of the additive-free control, but at higher strain rates ?? falls to about 65° of the control value. The critical stress for crack propagation is also strain-rate-dependent and is about 50% higher than the control at low strain rates and about 10% less than the control at higher strain rates. Dynamic mechanical analysis and dielectric loss measurements indicate that the additive causes a decrease in the T_g and a suppression of the β-relaxation. Chemically, the additive accelerates the cure process but does not significantly alter the final extent of reaction of the epoxy resin. After curing, the additive is almost totally extractable by solvent indicating that it is not chemicaly bound to the polymer. These observations are discussed in terms of the concept of antiplasticization.     

An improved permanganic etchant for polyolefines

The technique of permanganic etching reveals lamellar detail in polyethylene and other polyolefines allowing representative melt-crystallized morphologies to be studied with the electron microscope. When etching with the original recipe is prolonged artefacts on a scale of ～ 10 μm can develop and have probably been misinterpreted as genuine features in some instances. It is emphasized that the conditions of etching should be adjusted to suit the needs of individual specimens. The morphology of artefacts is demonstrated so that they may be recognized as such should they occur. A simple modification of the etchant by incorporating orthophosphoric acid has been found which avoids formation of artefacts on polyethylene.     

对异丙基苯甲酸合成方法的改进

报道以对异丙苯为原料,经Friedel-Crafts酰化和溴仿反应制备对异丙基苯甲酸的方法。原料廉价易得,操作简单,总收率可达80％。