Certain effects of melt-shearing and cooling on the stress-strain behavior of thermoplastic elastomers

The peculiar behavior of thermoplastic elastomers in molding suggested that the processing steps (mainly shearing) to which the material had been submitted had a great influence on the morphology. Experiments were carried out on two polysstyrene-polybutadiene-polystyrene elastomers. The effects of melt-shearing and cooling were examined both by mechanical testing and low angle X-ray scattering. Melt-shearing creates a marked morphological anisotropy but a form of annealing can occur at sufficiently high temperatures.     

Mechanical behavior at low strain of microgel containing elastomers

Synthetic routes for the preparation of microgels with defined particle size, crosslinking density and surface chemistry are described. The mechanical effects caused by incorporating such systems in crosslinked rubber (NR and SBR) are described. The mechanical effects are qualitatively related to the state of clustering which is governed by the particle size and the interfacial tension (δ-parameter difference). The reinforcing effects on Young's modulus and the dynamic shear modulus are explained against a comprehensive theoretical background which assumes kinetically controlled cluster-cluster aggregation (CCA) of colloidal filler particles dispersed in entangled polymers. This leads to a control of rubber material properties by chemical means. Received: 19 December 1997/Revised version: 7 January 1998/Accepted: 7 January 1998     

A simultaneous SAXS/WAXS and stress-strain study of polyethylene deformation at high strain rates

An experimental system has been commissioned which allows the collection of time-resolved simultaneous two-dimensional small-angle X-ray scattering/wide-angle X-ray scattering (SAXS/WAXS) and stress-strain data with a temporal resolution of 40 ms during the drawing of synthetic polymers. X-ray data collection is achieved via two CCD-based area detectors which are positioned in order to maximize the amount of scattering that can be observed from a particular sample. True stress-strain measurements are obtained from video extensometer and load cell measurements. The system was used at the Daresbury SRS to study the structural changes which occur in a sample of isotropic high-density polyethylene when subjected to an overall strain rate of ≈ 3 s⁻¹. It is shown that the onset of both a partial stress-induced crystal phase change and micro-voiding in the sample can be directly correlated to the point of yield in the true stress-strain curve. © 1997 Elsevier Science Ltd.     

The effect of microstructure on the rate-dependent stress-strain behavior of poly(urethane urea) elastomers

Segmented poly(urethane urea) materials (PUUs) exhibit versatile mechanical properties and have drawn great interest due to their potential for protection against projectile impacts and blast loadings. To optimize the performance of PUUs for various high rate applications, specific features of their mechanical behavior have to be suitably tailored by altering the microstructure. Hence the micromechanisms governing the mechanical behavior must be identified, understood and leveraged. In this study, the effects of varying microstructure on the rate-dependent mechanical behavior were examined for select PUU materials. As expected, increasing the hard segment content increased the stiffness and the flow stress levels. Interestingly, it was observed that promoting phase mixing among the hard and soft segment domains of the PUU material greatly enhanced its rate-dependent stiffening and strain hardening behavior. These insights can help design PUUs for articles that manifest improved protective abilities under impact, while maintaining their flexibility during normal use. The potential applications for such materials are extensive, including face masks and goggles, which require excellent folding/un-folding capabilities, while also providing superior impact resistance.     

A new test method to obtain biaxial tensile behaviors of solid propellant at high strain rates

A new test method was proposed and applied for studying the biaxial tensile behaviors of hydroxyl-terminated polybutadiene (HTPB) propellant at high strain rates. The biaxial tensile stress responses of the propellant at room temperature and at different strain rates (0.40–85.71 s^?1) were obtained through the use of biaxial tensile strip samples, a new designed aluminum apparatus and a uniaxial Instron testing machine. A high-speed camera and scanning electron microscop (SEM) were employed to observe the biaxial tensile deformation and the damage of HTPB propellant under the test conditions. The results indicated that strain rate could remarkably influence the biaxial tensile behaviors of HTPB propellant. The effect of strain rate on the characteristics of stress–strain curves, mechanical properties and fracture mechanisms was consistent with that in uniaxial tension. However, the biaxial weakening of HTPB propellant was obvious. The strain at biaxial maximum tensile stress was between 10 and 30 % lower than that at the corresponding uniaxial case. Finally, the correlations between the fracture mechanisms and the mechanical properties of HTPB propellant, stress state and the damage of HTPB propellant were discussed. The damage of the propellant under the biaxial tensile test was less serious than that under uniaxial tension at the same strain rate. In addition, continuously increasing strain rate could change the fracture mechanism of the propellant under the biaxial and uniaxial tensile tests. In this investigation, the dominating fracture mechanism of HTPB propellant changed from the dewetting and matrix tearing at lower strain rate to the particles fracture at higher strain rate.     

Effects of temperature and strain rate on the tensile behavior of unfilled and talc‐filled polypropylene. Part I: Experiments

The tensile behavior of unfilled and 40 w% talc‐filled polypropylene has been determined at four different temperatures (21.5, 50, 75 and 100°C) and three different strain rates (0.05, 0.5 and 5 min^?1). Experimental results showed that both unfilled and talc‐filled polypropylenes were sensitive to strain rate and temperature. Stressstrain curves of both materials were nonlinear even at relatively low strains. The addition of talc to polypropylene increased the elastic modulus, but the yield strength and yield strain were reduced. The temperature and strain rate sensitivities of these materials were also different. An energy‐activated, rate sensitive Eyring equation was used to predict the yield strength of both materials. It is shown that both activation volume and activation of energy increased with the addition of talc in polypropylene.     

Model for tensile fracture of concrete at high rates of loading

Mechanisms of tensile fracture of concrete are described. A model is developed for an idealized material. The amount of simultaneous cracking and the path of each crack depend on the rate of stressing. The fracture energy and the tensile strength have been determined as functions of the rate of loading. The results of earlier experiments on concrete under impact tensile loading can be explained by this model.     

Coupled strain rate and temperature effects on the tensile behavior of strain-hardening cement-based composites (SHCC) with PVA fibers

This paper reports the experimental findings on the tensile behavior of Strain-hardening cement-based composites (SHCC) subjected to elevated temperatures and different strain rates and to combinations of these parameters. Uniaxial tension tests with in-situ temperature control were performed at 22 °C, 60 °C, 100 °C and 150 °C. In addition, the effect of loading rate was investigated using the strain rates of 10^? 5 s^? 1, 3 × 10^? 4 s^? 1 and 10^? 2 s^? 1 at all four temperatures considered. It was shown that tensile strength decreases both with an increase in temperature and with a decrease in the strain rate. The strain capacity increases with decreasing strain rate at temperatures of 22 °C and 60 °C, but for the temperature of 100 °C this material property increases when the strain rate increases. At 150 °C the investigated SHCC loses its ductility and no noticeable strain rate effect can be observed. Furthermore, the residual properties of SHCC were evaluated using uniaxial tensile tests at room temperature on the specimens which were previously heated to 60 °C, 100 °C or 150 °C. The residual tests showed that the strength, strain capacity, and work-to-fracture decrease with increasing pre-treatment temperature. However, in comparison with the results of the in-situ tests with elevated in-situ temperatures, the residual tests on SHCC yielded higher tensile strength and lower ductility. These results and possible mechanisms leading to changes in mechanical performance are discussed on the basis of the observed crack patterns on the specimens' surfaces as well as the microscopic investigations of the condition of fibers on fracture surfaces.     

The effects of degree of mixing and concentration of carbon black on the tensile properties of filled elastomers

Based on the concept of entropy of mixing, a new method for determining the degree of mixing of the filler in a polymer matrix has been established. The measurement used in this method is based on a new variable, the effective volume fraction of the filler, ?′, which is a function of the mixing index, A, and the volume fraction of the filler, ?. The mixing index can be determined from the tensile modulus data of filled elastomers. Excellent agreement is obtained between the mixing index and the dispersion of the filler as determined by a morphological study of cryogenically fractured surfaces of filled elastomers. Using the new variable, the effective volume fraction of the filler, useful relationships for the tensile modulus and the ultimate stretch ratio of filled elastomer have been established. Experimental data confirm that these relationships can well describe the effects of concentration and degree of mixing on the two tensile properties of filled elastomers.     

Stress-strain behaviour of concrete and mortar at high rates of tensile loading

The Split-Hopkinson-Bar technique was used in the investigation on tensile stress-strain behaviour of concrete and mortar at high stress rates (5–30 N/mm²ms).The single loading tests showed that the impact tensile strength was higher than the static one, and that impact strains at the maximum stress were larger than static strains.The impact fatique tensile tests indicated an increase of strains in the course of fatigue loading and increasing fatique life with decreasing maximum stress level.These phenomena are discussed with the aid of fracture mechanics concepts and explanations for differences in the behaviour of concrete and mortar are suggested.     

Effects of temperature and strain rate on the tensile behavior of short fiber reinforced polyamide-6

Tensile behavior of extruded short E-glass fiber reinforced polyamide-6 composite sheet has been determined at different temperatures (21.5°C, 50°C, 75°C, 100°C) and different strain rates (0.05/min, 0.5/min, 5/min). Experimental results show that this composite is a strain rate and temperature dependent material. Both elastic modulus and tensile strength of the composite increased with strain rate and decreased with temperature. Experimental results also show that strain rate sensitivity and temperature sensitivity of this composite change at a temperature between 25°C and 50°C as a result of the glass transition of the polyamide-6 matrix. Based on the experimental stress-strain curves, a two-parameter strain rate and temperature dependent constitutive model has been established to describe the tensile behavior of short fiber reinforced polyamide-6 composite. The parameters in this model are a stress exponent n and a stress coefficient σ*. It is shown that the stress exponent n, which controls the strain rate strengthening effect and the strain hardening effect of the composite, is not only strain rate independent but also temperature independent. The stress exponent σ*, on the other hand, varies with both strain rate and temperature.     

Direct tension test and tensile strain capacity of concrete at early age

The tensile strain capacity of concrete under uniaxial tension is investigated using the direct tension test method. The adopted method of testing improves the weak bond strength between the embedded bar and concrete and reduces the stress concentration at the end of the embedded bar. The method has overcome the difficulties in centralizing and aligning the two embedded bars in the specimens. Seven mixes of concrete were designed to study the effects of age, compressive strength and mineral admixture on the tensile strain capacity. The investigation shows that the tensile strain capacity of concrete is a relatively independent parameter. The average tensile strains at failure and at 90% failure load are 120 and 100 μ, respectively. The corresponding characteristic tensile strain values at failure and at 90% failure load are 86 and 78 μ, respectively.     

Binary and ternary particulate composites. II. Tensile behavior over a wide range of strain rates

Tensile properties of both a binary material, i.e., polystyrene (PS) reinforced by 15 vol % of glass beads, and ternary composites, i.e., showing either a maleated styrene/ethylene-co-butylene/styrene copolymer or a styrene-co-methacrylic acid copolymer (SAMA) adduct at the PS/glass-beads interface, are analyzed at room temperature and over a wide range of strain rates. Because the poor quality of the adhesion at the PS/glass-beads interface, the fracture toughness of these binary composites is strongly reduced, whatever the strain rate. The presence of the rubbery interlayer does not change the deformation mechanisms of the composite and the work to break is not significantly enhanced. This results from the poor compatibility between PS and the rubbery interphase leading to the debonding of coated glass beads. The good adhesion quality at the interfaces between phases in the ternary composite showing the SAMA adduct, i.e., SAMA/glass-beads and PS/SAMA interfaces, hinders the decohesion phenomenon. This results in an improvement in both the transfer load and the maximum strength. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1041–1046, 1997     

Application of the fung equation to the constant strain rate behavior of plasticized polyvinyl chloride

A quasilinear viscoelastic equation originally proposed by Fung to characterize the uniaxial viscoelastic behavior of rabbit mesentary is used in this study to characterize the viscoelastic behavior of plasticized polyvinyl chloride for strains up to 24%. An experimentally determined relaxation function is used to predict the constant strain rate behavior of plasticized polyvinyl chloride. The predictive ability of the Fung equation is also compared with the well-known BKZ and Lianis theories. It was found that the Fung equation agrees quite closely with the BKZ and Lianis theories but that all three theories showed only moderate agreement with experiment.     

Effect of thermal pretreatment on the fracture behavior of polycarbonate in medium strain rate tensile tests

The fracture properties of as-received and annealed (48 h at 403 K) commericial polycarbonates (PC) were examined in tensile tests with an average strain rate of ? = 2 s^?1. Both materials were subjected to tensile tests at temperatures ranging from 223 to 423 K. The results were processed by a computer interfaced to the testing machine. The tensile strength of the annealed (pretreated) PC was superior to that of the as-received (untreated) material. The elongation at break and the fracture energy, then, decreased due to annealing at all temperatures, yet followed impact strength curves. Fracture analysis and fracture morphology revealed clear differences in the behavior of the materials. Fracture nucleation occurred commonly at several points in the pretreated specimens, whereas only one nucleation point was found in the untreated specimens. Shear yielding morphology, which indicated plastic deformation, started to appear at a lower temperature in the pretreated specimens than in the untreated ones.     

Determination of optimal reactor system configuration for polymerization reactions. I. Free-radical polymerization at constant temperature

A systematic procedure was developed for selecting the type of reactors or a reactor system configuration for polymerization reactions. Two different mechanisms were investigated, and the “best” reactor system to give the desired quality of the product was determined using a systems synthesis technique. The behavior of the system in the neighborhood of the optimal solution was explored, and the effect of variation in the rate constants and the initial concentrations of the catalyst and the monomer on the optimal reactor system was examined. Recycle streams were introduced and their effect on the system performance was investigated, and finally the applicability of the systems synthesis technique to other polymer reactor design problems was discussed.     

Effect of tensile strain,time, and temperature on gas transport in biaxially oriented polystyrene

The permeability and diffusion coefficients (P and D) for gases in a biaxially oriented polystyrene film have been found to increase when a specimen is stretched in simple tension and to decrease with time when the strain is held constant. These effects are attributed, respectively, to an increase in free volume with strain and to the continuous volume recovery (densification) at constant strain. The strain dependence, at small strains, of P and D for Ar, Kr, N₂, CO₂, and Xe at 1 atm pressure and 50°C indicates that the size distribution of free-volume elements is not distorted when a specimen is stretched. At a constant strain of 1.8 percent at 50°C, P and D for xenon decrease about 13.8 and 11.8 percent, respectively, per decade of time—two to threefold faster than for carbon dioxide. These results and those obtained with argon, whose molecular diameter is significantly smaller than that for xenon, suggest that the larger free-volume elements decrease in size faster than the smaller ones as volume recovery progresses.     

Particulate reinforcement of polyacrylate elastomers. I. Introduction and Preparation of Materials

The series of papers, of which this is the first, aims to demonstrate the importance of chemical reactions between filler particles and elastomer matrix in determining the physical properties of filled elastomers. A model system comprising small glass beads as the filler and crosslinked polyethylacrylate as the elastomer has been used. The beads themselves are inert towards the matrix during polymerisation, but the reactivity of the surfaces of the beads can be varied by suitable treatments. This paper describes techniques for fractionating the beads to give narrow particle-size distribution, for preparing unfilled and filled elastomer sheets, and for preparing composites in which an elastomer sheet is polymerised in contact with a glass plate which may or may not have been previously subjected to surface treatment.