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This investigation focuses on the axial-torsional loading interaction of an epoxy polymer, Epon 826/Epi-Cure Curing Agent 9551. Thin-walled tubular specimens were subjected to combined constant tensile (or shear) stress and cyclic shear (or tension) loading schemes. Pure tensile creep and shear creep tests were also performed to compare the creep deformation to that with superimposed cyclic shear or cyclic tension. Test data clearly showed that cyclic shear (or cyclic tension) have a readily discernible effect on the tensile (or shear) creep deformation. Similarly, a superimposed constant tensile (or shear) load affects the hysteresis responses in cyclic shear (or cyclic tension). A nonlinear constitutive model developed by the authors was used to simulate the observed normal-shear stress interaction. Due to the inclusion of an effective stress parameter in its nonlinear function, this model was able to account for the normal-shear coupling effect. However, the incorporation of a general loading/unloading rule led to inaccurate simulation of the observed oscillatory creep response. A modification of the general rule was proposed and better predictions on both the cyclic and the creep responses could be Obtained. POLYM. ENG. SCI., 50:919-928, 2010. [C] 2009 Society of Plastics Engineers, INTRODUCTION Epoxy polymers are commonly used as matrices in advanced polymeric composites. This market dominance has stimulating research activities on experimental characterization and theoretical modeling of epoxy resins (see, e.g., [...]

The experimental results presented in Part I of this study were used to evaluate the predictive capabilities of two viscoelastic constitutive models. One of the models, developed by Xia and Ellyin, is in a differential form. The other, which is a modified Schapery model by Lai and Bakker, is in an integral form. The results of the comparison indicate that the Xia-Ellyin constitutive model simulated the experimental observations well. This was attributed to the existence of a general rule that delineates the loading and unloading parts of the cyclic response. The modified Schapery model was able to predict the general trends of the deformation behavior; however, it was unable to correctly simulate the unloading behavior. This difference became more pronounced when the applied cyclic stress/strain was high. At high applied loads, the material response became more nonlinear. POLYM. ENG. SCI., 45:103-113, 2005. [c] 2004 Society of Plastics Engineers, INTRODUCTION Polymers and polymeric composites in engineering service are usually subjected to multiaxial stress states with complex loading histories, and in many cases, the loading is of a cyclic nature. [...]

A series of uniaxial cyclic tests were carried out on solid cylindrical specimens of an epoxy resin, Epon 826/Epi-Cure Curing Agent 9551. The focus of the study was to investigate time-dependent viscoelastic behavior of this thermosetting polymer material under cyclic loading and to develop a constitutive model with the capabilities to simulate the observed deformation response. The tests include stress-controlled or strain-controlled cyclic loading with/without mean stress or mean strain at various amplitudes and loading rates. It was found that the cyclic stress-strain response of this material is amplitude-dependent and rate-dependent, and the response to axial tension is different from that in compression. The stress-strain loops exhibit more pronounced nonlinearity with high amplitudes or low loading rates. For stress-controlled cyclic loading with mean stress, ratcheting strain is accumulated, which is of viscoelastic nature, and this is confirmed by its full recovery after load removal. For strain-controlled cyclic loading with mean strain, the mean stress relaxation occurs, which contributes to the observed longer life in comparison to the stress-controlled cyclic loading with mean stress. Polym. Eng. Sci. 44:2240-2246, 2004. © 2004 Society of Plastics Engineers., INTRODUCTION Solid polymers are widely used in load-bearing applications, both in the homogeneous state or as a matrix phase in fiber-reinforced polymeric composites. Polymers and plastics are typical materials with [...]

The mechanical behavior of an epoxy resin (Epon 826) was studied by performing a series of tests on thin-walled tubular specimens. These tests deal with different aspects of the mechanical behavior of this epoxy resin. The deformation behavior, such as viscoelastic behavior, hydrostatic stress effect, multiaxial behavior and loading path effect, was investigated. It was found that the Epon 826 epoxy resin is a highly nonlinear viscoelastic material. The effect of hydrostatic pressure on the deformation behavior of this epoxy is not significant. However, it shows different tensile and compressive deformation behavior. The loading path was found to have an observable effect on the deformation response of this epoxy, especially in the high stress/strain range., INTRODUCTION Polymer resins are widely used as adhesives or matrices in fiber-reinforced composites, with applications in a variety of industries, e.g., automotive, aerospace, oil and gas, and marine industries. Among [...]

Based on the experimental data presented in Part I, a nonlinear viscoelastic constitutive model, in differential form, is presented here. A distinctive feature of this model is the inclusion of a criterion to delineate loading and unloading in multiaxial stress states, and different moduli for loading and unloading behaviors. In addition, the model contains only five material constants and one modulus function, which can be calibrated in accordance with a well-defined procedure. A comparison with the experimental data shows that the current differential model is capable of predicting the nonlinear viscoelastic behavior of the epoxy polymer qualitatively and quantitatively, including both the loading and unloading behavior. The predictions of an integral form of constitutive model are also included for comparative purposes., 1. INTRODUCTION The results presented in the companion paper, Part I (1), indicated that the tested epoxy had a similar behavior as other thermoset polymers, such as highly nonlinear viscoelasticity [...]

The axial-torsional interaction of an epoxy resin was investigated by subjecting thin-walled tubular specimens to combined normal and shear stress components. It is shown that a superimposed normal stress (tensile or compressive) or hydrostatic pressure will influence shear creep behavior. Similarly, a superposed shear stress affects the normal stress response of the resin. The axial-torsional stress interaction is also observed in transient stress responses under different strain paths, and in the creep deformation with non-proportional stress histories. Linear viscoelastic constitutive models are unable to predict the aforementioned behaviors. Two typical nonlinear viscoelastic constitutive models are examined with respect to their capabilities to predict the observed response. It is shown that the predictions of these two models agree only qualitatively but not quantitatively with the experimental results., INTRODUCTION Constitutive modeling of the deformation behavior of solid polymers has attracted considerable interest recently, due to the use of polymeric materials in primary structures. To ensure reliable and safe [...]