Stress relaxation

About: Stress relaxation is a research topic. Over the lifetime, 12959 publications have been published within this topic receiving 270815 citations.

Modeling Viscoelastic and Viscoplastic Behavior of High Density Polyethylene (HDPE)

Abstract: The viscoelastic and viscoplastic behaviors of high density polyethylene (HDPE) under uniaxial monotonic and cyclic loading are modeled using the modified viscoplasticity theory based on overstress (VBO). The viscoelastic modeling capabilities of the modified VBO are investigated by simulating the behavior of semicrystalline HDPE under uniaxial compression tests at different strain rates. In addition, the effects of the modification (introducing the variable "C" into an elastic strain rate equation) on VBO that has been made to construct the change in the elastic stiffness while loading and unloading are investigated. During first loading and unloading, the modification in the elastic strain rate equation improves the unloading behavior. To investigate how the variable "C" that is introduced in the elastic strain rate equation evolves during reloading, the cyclic behavior of HDPE is modeled. For a complete viscoelastic and viscoplastic behavior, the relaxation and creep behaviors of HDPE are simulated as well in addition to stress and strain rate dependency. The influences of the strain (stress) levels where the relaxation (creep) experiments are performed are investigated. The simulation results are compared with the experimental data obtained by Zhang and Moore (1997, Polym. Eng. Sci., 37, pp. 404-413). A good match between experimental and simulation results are observed.

An integrated process for modelling of precipitation hardening and springback in creep age-forming

Abstract: Creep age-forming (CAF) process has been developed and used to manufacture complex-shaped panel components in aerospace applications. CAF is based on the complex combination of stress relaxation, creep and age hardening. The aim of this paper is to introduce an integrated technique to model stress–relaxation, creep deformation, precipitate hardening and springback in a CAF process. Firstly, a new set of physically-based, unified creep-ageing constitutive equations is presented, which is based on the high temperature creep and ageing kinetics, and, is determined for a solution-treated and quenched AA7010. This new material model is then implemented in the commercial FE solver ABAQUS through a user defined subroutine. An integrated FE simulation process is introduced for the simulation of CAF and springback. In addition to the stress relaxation, creep-age precipitate growth and yield stress evolution during CAF are predicted.

Rheological Investigation of Polybutadienes Having Different Microstructures over a Large Temperature Range

Abstract: The rheological behavior of five polybutadienes having different microstructures has been characterized using dynamic mechanical, stress relaxation, and viscosity measurements in a temperature range from just below the conventional glass transition to 100 °C above it. The data covered a broad enough frequency (time) and temperature range that we were able to characterize the responses in both the glassy and terminal dispersions of the polymers and to address the question of the validity of thermorheological simplicity. Uncritical application of time-temperature superposition principles to these data resulted in reduced viscoelastic responses that cover 12-14 decades in frequency or time. Close examination of the data in the glassy and terminal dispersions shows that the temperature shift factors required to superpose the data in the two regions are, however, different. Such a deviation from thermorheological simplicity can be analyzed within the framework of the coupling model of Ngai 1-3 that relates the shapes of the dispersion to the temperature dependence of the viscoelastic spectrum. Comparison of the polybutadienes shows differences in glass transition temperature, shape of the segmental relaxation, and fragility that depend on microstructure. Increasing the content of vinyl side groups causes an increase of spectral broadening as well as an increase in fragility-two features which can be related within the coupling model.

Studies on the Transient Shear Flow Behavior of Liquid Crystalline Polymers

Abstract: An understanding of the flow behavior of liquid crystalline polymers (LCP's) is of immense practical value because of the potential to form high modulus materials from these polymers. These fluids exhibit a high degree of structure even in the quiescent state, as evidenced by their ability to transmit polarized light. In an effort to understand how the structure changes during flow, we have carried out a study on the transient shear flow properties of two thermotropic copolyesters of 60‐ and 80‐mole % para‐hydroxybenzoic acid (PHB) and polyethyleneterephthalate (PET) and a lyotropic system of poly‐p‐phenyleneterephthalamide (PPT) in 100% sulfuric acid. In one of the first theories concerned with the flow behavior of liquid crystalline fluids, which was proposed by Ericksen, the transient flow properties are all predicted to be due to changes in orientation of a director which describes the orientation of packets of rod‐like molecules. Stress growth, interrupted stress growth, and stress relaxation experiments are carried out on the three LCP's and at first sight are in qualitative agreement with the predictions of Ericksen's theory. However, wide angle X‐ray scattering analysis of quenched samples subjected to shear flow along with annealing experiments on oriented samples indicate that these materials do not orient readily in shear flow. Furthermore, orientation generated during extensional flow relaxes at a rate much faster than is indicated by the interrupted stress growth experiments. It is concluded that the stress growth response of LCP's is due to a disruption of a domain structure which exists within the fluid rather than to orientation changes of the domains of rod‐like molecules.

Percolation in a Model Transient Network: Rheology and Dynamic Light Scattering†

Abstract: Step strain experiments and dynamic light scattering measurements are perfomed to characterize the dynamic behavior of an oil-in-water droplet microemulsion into which is incorporated a telechelic polymer. At sufficient droplet and polymer concentrations, above the percolation threshold, the system is viscoelastic and its dynamic structure factor consists of two steps for the relaxation of concentration fluctuations: the fast one is dominated by the diffusion but the slower one is almost independent of the wave vector. The terminal time of the stress relaxation τR and the slow time of the dynamic structure factor τS are both presumably controlled by the residence time of a sticker in a droplet: consistently, τR and τS are of the same order, they both vanish at the percolation threshold according to power laws but with different exponents. We discuss these features in terms of deviations at the transition, from the usual mean field description of the dynamics of transient networks.