Subject stress-strain plot

The apparent Young s modulus E, is, by definition, the ratio oAe-x) (slope of the stress-strain plot). Hence, for an off-axis test specimen subjected to uniaxial stress in the x-direction, we obtain the following equation for the modulus ... 

J7 In a tensile test on a plastic, the material is subjected to a constant strain rate of 10 s. If this material may have its behaviour modelled by a Maxwell element with the elastic component f = 20 GN/m and the viscous element t) = 1000 GNs/m, then derive an expression for the stress in the material at any instant. Plot the stress-strain curve which would be predicted by this equation for strains up to 0.1% and calculate the initial tangent modulus and 0.1% secant modulus from this graph. 

A further complication of creep is that it is nonlinear in strain, just as the stress-strain relationship is nonlinear for plastics. Since plastics are typically subjected to large deformations during their life, it is unfortunately essential to characterize this phenomenon. This phenomenon is best seen in the classic isochronous stress-strain curve which plots stress-strain relationships at several times, decades apart. These curves are invaluable for design and product performance evaluation. 

The results of typical creep experiments are shown in Fig. 4.1. We discuss here specimens subjected to shear related effects are, of course, observed in tension. If a constant shear stress Oi is applied to a viscoelastic specimen (4.N.1) the strain is observed to be time dependent (see Fig. 4.1 (a)). Suppose the specimen to be allowed to recover and a larger constant stress 02 applied. The time dependence of the strain is shown in Fig. 4.1(b). Now, if the strains at a particular time say r, after the application of the stress, are plotted against the stress it then is observed that these strains y(t ) are linear in the stress (see Fig. 

The stress—relaxation process is governed by a number of different molecular motions. To resolve them, the thermally stimulated creep (TSCr) method was developed, which consists of the following steps. (/) The specimen is subjected to a given stress at a temperature T for a time /, both chosen to allow complete orientation of the mobile units that one wishes to consider. (2) The temperature is then lowered to Tq T, where any molecular motion is completely hindered then the stress is removed. (3) The specimen is subsequendy heated at a controlled rate. The mobile units reorient according to the available relaxation modes. The strain, its time derivative, and the temperature are recorded versus time. By mnning a series of experiments at different orientation temperatures and plotting the time derivative of the strain rate observed on heating versus the temperature, various relaxational processes are revealed as peaks (243). 

Creep rupture. Creep-rupture data are obtained in the same way as creep data except that higher stresses are used and the time is measured to failure (Figs. 2-28 and 29). The strains are sometimes recorded, but this is not necessary for creep rupture. The results are generally plotted as the log stress versus log time to failure (110). In creep-rupture tests it is the material s behavior just prior to the rupture that is of primary interest. In these tests a number of samples are subjected to different levels of constant stress, with the time to failure being determined for each stress level. General technical literature and product data sheets seldom provide a complete description of a material s behavior prior to rupture. It should include the development of any crazing and stress whitening, its strain-time... 

These equations are plotted in Figure 7-2. They are formally identical to the compliance response of a Voigt element in series with a spring when the entire model is subjected to a sinusoidal stress. The complex dielectric constant is thus the analogue of the complex compliance, with the electric field playing the role of stress and the electric displacement 4 7rcr playing the role of strain. 

In the case of HIC, hydrogen diffuses into the lattice at all strain rates tested and no ductility is observed on the reduction in the area vs. strain rate plot. At the critical strain rate, in the case of SCC, the film formation cannot inhibit the ductile mpture [20]. Threshold stresses can also be determined by SSRT tests. The test is performed by using a number of samples that are subjected to different ranges of stress at a given strain rate. A plot of stress range vs. average crack velocity could be used to evaluate the threshold stress [20]. 

These conditions then represent subjecting a polymer to an oscillatory (cosine) strain input. The stress output is also oscillatory, but is out of phase with the strain input. To visualize, see the input and output results shown in Fig. 5.14 at a single frequency. The total input and total output are plotted, as well as the in phase and out of phase portions of the stress output. 

A typical creep tesF consists of subjecting a specimen to a constant load or stress while maintaining the temperature constant deformation or strain is measured and plotted as a function of elapsed time. Most tests are the constant-load type, which yield informa-... 

---