Tensile stress growth experiment

Figures 7.20 and 7.21 present experimental results which may look at first view quite astonishing. Flowing polymer melts are birefringent, for obvious reasons, and the experiment compares for tensile stress growth experiments on polystyrene the evolution with time of the birefringence...

The maximum strain rate (e < Is1) for either extensional rheometer is often very slow compared with those of fabrication. Fortunately, time-temperature superposition approaches work well for SAN copolymers, and permit the elevation of the reduced strain rates kaj to those comparable to fabrication. Typical extensional rheology data for a SAN copolymer (h>an = 0.264, Mw = 7 kg/mol,Mw/Mn = 2.8) are illustrated in Figure 13.5 after time-temperature superposition to a reference temperature of 170°C [63]. The tensile stress growth coefficient rj (k, t) was measured at discrete times t during the startup of uniaxial extensional flow. Data points are marked with individual symbols (o) and terminate at the tensile break point at longest time t. Isothermal data points are connected by solid curves. Data were collected at selected k between 0.0167 and 0.0840 s-1 and at temperatures between 130 and 180 °C. Also illustrated in Figure 13.5 (dashed line) is a shear flow curve from a dynamic experiment displayed in a special format (3 versus or1) as suggested by Trouton [64]. The superposition of the low-strain rate data from two types (shear and extensional flow) of rheometers is an important validation of the reliability of both data sets. 

Figure 9.18 refers to two other standard experiments. It depicts the results of stress growth experiments, conducted again on a polyethylene melt. The figure includes both measurements probing shear and tensile properties, thus facilitating a direct comparison. Curves show the building-up of shear stress upon inception of a steady state shear flow at zero time and the development of tensile stress upon inception of a steady state extensional flow. Measurements were carried out for various values of the shear rate 7 or the Hencky rate of extension ch ... 

High temperature crack growth behavior under creep conditions was studied in SiC whisker-reinforced mullite. Four-point flexumral specimens were prepared for crack growth experiments by introducing Vickers indentation-induced cracks on the tensile surface. Samples were tested in creep in air at a nominal smface tensile stress at 150 MPa and a temperature of 1400°C. Crack growth and nucleation, and extent of creep-strain were... 

An important aspect of EIC experiments concerns the shape of the test specimens and whether they contain a crack or not at the start of an experiment. One distinguishes two broad categories precracked specimens and non-precracked specimens. Precracking is accomplished by subjecting the test specimen to a cychc stress by means of a tensile test machine in order to initiate a crack. The precracked specimen is then subjected to the chosen test conditions for the study of the rate of crack growth. In non-precracked specimens both crack initiation and crack growth occur during the actual test. 

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