Crystal orientation tensile testing

A single crystal of alnminum is oriented for a tensile test snch that its slip plane normal makes an angle of 28.1° with the tensile axis. Three possible slip directions make angles of 51.2°, 36.0°, and 40.6° with the same tensile axis. Which of these three slip directions is most favored ... 

If a part or several parts in a body have been strained permanently beyond the limit of plasticity and the external forces and moments are removed, the material in the overstrained region and around it will, in general, be subjected to inherent stresses which are then called residual stresses (Nadai 1963). This description is exact from the macroscopic viewpoint however, from the microscopic viewpoint, the residual stress in each grain may be different, even in a tensile test that induces perfectly uniform deformation from a macroscopic viewpoint as shown in Fig. 1. In such a case, residual stress can be understood as a stress mainly due to the different state of stress existing in the variously oriented crystals before unloading (Johnson and MeUor 1962). 

Table 3.5. Crystal orientation of PP and its composites before and after tensile tests at 25°C characterized by relative WAXD intensity /(110)//(040) [62]...

Uniaxial tensile tests of poly (ethylene terephthalate) (PET)/montmorillonite(MMT) nanocomposites were preformed over a temperature range of 85°C-105°C and stretch rate of 7.5mm/s-12.5mm/s. The stress-strain curves consisted of three regions the hnear visoelasticity, the rubbery plateau and the strain hardening. The effects of temperature and stretch rate on stress-strain behavior were discussed. The results of differential scanning calorimetry (DSC) measurements indicated that the stretch lead the increase of the crystallinity degree of specimens. The wide angle X-ray diffraction (WAXD) measurements revealed that the more perfect crystal structures were obtained with the increase of temperature and oriented along the stretch direction. 

It is well known that the mechanical properties of P(3HB-co-8%-3HV) films markedly deteriorate to stifhiess and brittleness by a process of secondary crystallization. The cold-drawn and annealed films of P(3HB-co-8%-3HV) were stored for 6 months at room temperature to study the time dependent change of the mechanical properties, and the stress-strain test was performed. The tensile strength and elongation to break of cold-drawn and annealed films remained unchanged for 6 months as summarized in Table 1. It is of importance to note that the mechanical properties of the cold-drawn and annealed film did not deteriorate during 6 months. It is concluded that a highly oriented and crystallized P(3HB) film keeps superior mechanical properties for long periods. 

Any amorphous polymer at a temperature above 7 that is stretched undergoes some orientation of chain segments. In this oriented state, crystallization may occur, which will increase the effective number of cross-links (see Section 3.4). If crystallization does not occur, the behavior of the sample to rupture is similar at large and small deformations. Figure 10.8 shows that the stress-strain curves for some elastic fibers correlate well (give a linear plot) with Equation 9.93 up to the rupture point (about 600% elongation). The most often used mechanical test is one that measures stress at a constant strain rate. The ultimate tensile stress and strain at rupture vary with temperature and rate of strain. 

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