Machinery's Handbook, 31st Edition
562 Plastics Applications and Properties Even at low stresses, they are sensitive to rate of loading and to temperature, as shown generically in Fig. 1 . Curves A and B identify the stress-strain behavior of brittle materials that will tend to fail suddenly under increasing strain. By comparison, curves C through E show how increasing temperatures both increase ductility and extend the strain to failure curve, albeit with decreased stiffness.
A
Increasing Temperature
B
Increasing Strain Rate
C
D
E
Strain
Fig. 1. Generic Tensile-Test Graphs for Plastics, Showing Typical Range of Behavior from Brittle to Highly Ductile with Increasing Temperatures
Structural analysis during design of components uses two independent constants, Young’s modulus ( E ) and Poisson’s ratio ( n ), but these two constants are sufficient only for elastic, isotropic materials that respond linearly to loads (when load is proportional to deformation). Designers often use the same values for these constants everywhere in the structure, which is correct only if the structure is homogeneous. Assumptions of linear elasticity, isotropy, and homogeneity are reasonable for many analyses and provide a good starting point, but use of these assumptions can lead to signif - icant design errors with plastics, particularly with glass-reinforced and liquid crystalline polymers, which are highly anisotropic. In this discussion, plastics are assumed to be lin- early elastic, homogeneous, and isotropic to allow a simpler presentation of mechanical properties in line with the data provided in plastics manufacturers’ marketing data sheets. The standard equations of structural analysis (bending, torsion, pressure in a pipe, etc.) also require these assumptions. Creep: When plastics are subjected to sustained high loads, they creep . That is, they tend to slowly deform (flow), exhibiting the viscous side of their nature. If the load is removed, the stressed object will usually not return completely to its original shape and dimensions. Creep is more rapid at higher temperatures, even at temperatures expected in normal service. Laboratory stress-strain curves obtained according to ASTM D638 at moderate strain rates and 23 ° C (73.4°F) or other temperatures will resemble those in Fig. 2 . Most plastics have no true proportional limits, instead exhibiting curved depen - dence of stress versus strain even at strains less than 1 percent. Tangent moduli from D638 therefore tend to be subjective and inaccurate when used for predicting deforma - tion. The isometric diagram of Fig. 2 shows the dependence of stress in Styron 475, a rigid modified polystyrene, at various strains and times. Note the logarithmic time scale (0.01 h = 36 s, 10,000 h = 1.14 yr). The curve on the left panel, for 0.01 h, is the stress- strain graph one might obtain at one of the slower testing rates of ASTM D638. The stress ordinate at the left edge, front, i.e., 600 psi (4.1 MPa) for a strain of 0.l5 percent, corresponds to a short-time modulus for this resin of 400 kpsi (2.8 MPa), in agreement with values reported elsewhere. When this small strain was maintained for 1000 hours (6 weeks), the stress had decayed along the first curve (left rear to right front) to about 400 psi (2.8 MPa), two-thirds of its initial value.
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