Polymers yield strength values

Let us now see whether materials really show this strength. The bar-chart (Fig. 9.2) shows values of Oy/E for materials. The heavy broken line at the top is drawn at the level it/E = 1/15. Glasses, and some ceramics, lie close to this line - they exhibit their ideal strength, and we could not expect them to be stronger than this. Most polymers, too, lie near the line - although they have low yield strengths, these are low because the moduli are low. 

On comparison of the yield strengths and elastic moduli of amorphous polymers well below their glass transition temperature it is observed that the differences between polymers are quite small. Yield strengths are of the order of 8000 Ibf/in (55 MPa) and tension modulus values are of the order of 500 000 Ibf/in (3450 MPa). In the molecular weight range in which these materials are used differences in molecular weight have little effect. 

In the case of commercial crystalline polymers wider differences are to be noted. Many polyethylenes have a yield strength below 20001bf/in (14 MPa) whilst the nylons may have a value of 12 000 Ibf/in (83 MPa). In these polymers the intermolecular attraction, the molecular weight and the type and amount of crystalline structure all influence the mechanical properties. 

For many polymers and metals the yield point is not clearly defined. In these cases the offset yield strength is used. To obtain this value, a line parallel to the linear portion of the curve is drawn such that... 

Application to Adhesional Wear. For adhesional wear, as opposed to adhesion itself, the property or process that is of direct interest is the rupture of interfacial crazes and the transfer of polymeric material, rather than the force or energy requirement. It is clear that the inequalities, (lb), (3b) and (4b) are the criteria that predict the occurrence of polymer wear. In terms of molecular properties, small values of the yield strength, and a rapid decrease in Oy with temperature, should lead to the occurrence of polymer wear, and to the transfer of polymer to the solid. Large values of Oy and small values of dOy/dT will be conducive to the absence of wear. 

It is important to remark that the mechanical properties of polymers are very sensitive to temperature. Hence, a material such as PS with a Tg values of approximately 100°C, at anibient temperature is at nearly 80% of the Tg value on an absolute temperature scale. Changing the Tg by 65 K has a large effect on this ratio, and the result is a potentially large reduction in the polymer mechanical properties, such as yield strength, modulus, and creep resistance. Dielectric response is similarly affected by proximity to the glass temperature. 

The graph depicted in Fig. 8.3 illustrates this phenomenon. The upper curve indicates that the value at O F is 14,000 Ib/in. At 72°F, it has dropped to around 12,000 Ih/in. By the time it reaches 140°F, the tensile yield strength is approximately 7000 Ih/in. This data is for nylon, a polymer particularly affected by moisture. The lower curve illustrates the effect of 2.5% moisture. In the range of temperatures between 30 and 100°F, the tensile yield strength appears to be about 20% lower for the moist material. Note that the curves begin to nm together beyond 150° F as most of the water has been driven off by that point. 

Other estimates of the ultimate shear strength of amorphous polymers have been made by a number of authors and generally all fall within a factor of 2 of each other (38,77,78). Stachurski (79) has expressed doubt as to the validity of the concept of an intrinsic shear strength based on the value of the shear modulus, G, for an amorphous solid. He questions which modulus is the correct value to use— the initial small strain value or the value at higher strain (the yield point or the ultimate extension). Further, the temperature and strain-rate dependence of both the yield strength and modulus (however defined) suggests that perhaps the ratio of yield strength to modulus is not a true intrinsic material property. We remark however that the temperature and strain-rate dependence of both the yield stress and the shear modulus are often similar. 

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