Material properties tensile yield stress

Figure 14.8 shows stress-strain curves for polycarbonate at 77 K obtained in tension and in uniaxial compression (12), where it can be seen that the yield stress differs in these two tests. In general, for polymers the compressive yield stress is higher than the tensile yield stress, as Figure 14.8 shows for polycarbonate. Also, yield stress increases significantly with hydrostatic pressure on polymers, though the Tresca and von Mises criteria predict that the yield stress measured in uniaxial tension is the same as that measured in compression. The differences observed between the behavior of polymers in uniaxial compression and in uniaxial tension are due to the fact that these materials are mostly van der Waals solids. Therefore it is not surprising that their mechanical properties are subject to hydrostatic pressure effects. It is possible to modify the yield criteria described in the previous section to take into account the pressure dependence. Thus, Xy in Eq. (14.10) can be expressed as a function of hydrostatic pressure P as... 

Figure 5.2 shows the linear relationship found between the microhardness and the yield stress for the two series of blends A /B and A2/B. However, as we will discuss later, the slope of this plot differs from that found for other PE samples (Baltd Calleja, 1985). The simultaneous increase of both H and tensile yield stress Yt with increasing content of the HOPE component results in a linear correlation between these two mechanical properties with a ratio H/Yf 2 2 (Fig. 5.2). This ratio is significantly smaller than that previously found for PE (H 3Tf) (see Section 4.7.3). The smaller H/Yt ratio found for these materials can be related to the relatively high deformation speed (50 mm min ) used in a macroscopic measurement, which is about 40 times higher than that used in a microhardness test (see Balta Calleja et al., 1995).

Sumita M, Tsukumo Y, Miyasaka K et al (1983) Tensile yield stress of polypropylene composites filled with ultrafine particles. J Mater Sci 18 1758—1764 Thellen C, Orroth C, Froio D et al (2005) Influence of montmorillonite layered silicate on plasticized poly(l-lactide) blown films. Polymer 46 1716-11727 Uyama H, Kuwabara M, Tsujimoto T et al (2003) Green nanocomposites from renewable resources plant oil-clay hybrid materials. Chem Mater 15 2492-2494 Wang SF, Shen L, Zhang WD et al (2005a) Preparation and mechanical properties of chitosan/ carhon nanotuhes composites. Biomactomolecules 6 3067—3072... 

Mechanical properties such as tensile properties (modulus, yield stress and elongation-to-break), room temperature and 0 °C impact, CLTE, weld strength and flexural modulus were measured for the TPO materials in this study. The mechanical properties of the two TPO formulations were measured using protocol outlined in appropriate ASTM standards. 

Physical characterization of polymers is a common activity that research and development technologists at the Dow Chemical Company perform. A material property evaluation that is critical for most polymer systems is a tensile test. Many instruments such as an Instron test frame can perform a tensile test and, by using specialized software, can acquire and process data. Use of an extensometer eliminates calibration errors and allows the console to display strain and deformation in engineering units. Some common results from a tensile test are modulus, percent elongation, stress at break, and strain at yield. These data are then used to better understand the capabilities of the polymer system and in what end-use applications it may be used. 

The fracture resistance of a material depends on all of the properties which have been discussed including tensile strength, yield stress, elastic modulus, flexural strength, and impact resistance, all of which depend, in part, on fillers. Fillers, consequently, are important determinants of fracture resis-Only those phenomena which are related... 

Polymer blends must provide a variety of performance parameters. Usually it is a set of performance criteria that determines if the material can be used or not. For specific application more weight can be given to one or another material property. The most important properties of polymer blends are mechanical. Two type of tests have been used the low rate of deformation — tensile, compressive or bending and the high speed impact. Immiscibility affects primarily the maximum elongation at break, and the yield stress. 

Tests for tensile properties are described in ASTM D882 tests for flexural properties are described in ASTM D790 and ASTM D6272. Table 3.4 indicates how the softness or toughness of any material relates to the values of elastic modulus, yield stress, strength, and elongation. 

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