STRESS RELAXATION IN SOLIDS

A cooperative model for stress relaxation is described. It is based on a two-level system where stimulated emission of phonons may occur and was formulated in order to circumvent some of the problems encountered with the theory of stress-dependent thermal activation. The cooperative model also provides some insight into the well-documented universal similarity in stress relaxation behaviour among different materials. The dynamic-mechanical response of the cooperative model is also analyzed in some detail. The experimental stress relaxation behaviour of composites based on high density polyethylene is described and it is suggested that the internal stress concept can be a valuable tool when trying to quantify the effect of fillers and surface treatments of fillers on the long-term mechanical properties of polymeric composites

Influence of mold design on the mechanical properties of high-pressure injection molded polyethylene

High-pressure injection molding (nominal pressure 500 MPa) is known to substantially improve the mechanical properties of high-density polyethylene of a high molecular weight (HMWPE). This work shows that if the mold is equipped with an exit cavity, the tensile modulus and strength of HMWPE-bars molded is further improved at high pressure levels. The maximum values of the stiffness and strength (thin bars, 1 mm) obtained with the exit chamber is about 12 GPa and 260 MPa, respectively. The improvement due to the exit cavity is of the order of 30 percent for the tensile strength for thicknesses lower than 4mm, while the modulus increases about 1 to 1.5 GPa for bars with thicknesses between 1 and 6 mm. The orientation of the melt during the filling of the mold was also found to have an influence on the mechanical properties of the HMWPE bars

A new engineering approach to predict the hydrostatic strength of uPVC pipes (CD-rom)

Extruded unplasticised Poly(Vinyl Chloride) (uPVC) pipes are certified using pressurised pipe tests. During these tests the pipes are subjected to a certain temperature and internal pressure, while the time-to-failure, the time at which the internal pressure drops due to rupture or fracture, is measured. These tests are time consuming and are therefore costly. To circumvent these costs a model-based approach is proposed where the time-to-failure is predicted. The input parameters for this approach can be determined using short term measurements. The approach uses the observation that the timeto- failure kinetics of uPVC pipes subjected to an internal pressure is independent of the type of failure mode (ductile, semi-ductile or brittle). This supports our statement that the underlying mechanism that initiates failure is similar for these types of failure. Local deformation of the material up to a critical value of the anelastic strain is believed to determine the start of failure of the material. This critical strain appears to be constant for the testing conditions used during this study. A pressure modified Eyring expression is employed to calculate the strain rate resulting from the applied stress at a certain temperature. The time-to-failure follows from the calculated strain rate and the critical strain of the material. This approach has been verified against literature data and shown to hold quantitatively. Furthermore, the model seems to hold for different processing conditions