Machinery's Handbook, 31st Edition
Plastics Applications and Properties 561 extruding molten resin onto chilled rolls, the films are usually stronger in the “stretch” direction, weaker in the lateral or cross direction. Laminates made of laid-up reinforcing fibers or woven cloths can be nearly biaxially isotropic in the laminate plane, but their interlaminar shear is low because no fibers cross through the plane. Extremely anisotro - pic are unidirectional laminates in which all the fibers are parallel, as in golf club shafts reinforced with graphite fibers. Wood (see page 382) is a natural anisotropic material, strong and stiff in the grain (growth) direction, less so in the perpendicular and tangential directions. Commercially important fibers, both natural and synthetic, are also highly anisotropic with their greatest strength and moduli aligned with length. As the degree of anisotropy increases, the number of physical constants (moduli) required to define a material’s stress-strain behavior also increases, up to a maximum of 21. Absence of such complete information on complex laminates and the inadequacy of simple analysis methods point to the need for extensive end-use testing of plastics parts before approval of critical application requirements. Computer modeling using finite ele ment analysis (FEA) techniques can help in the successful implementation of these mate rials. Detailed properties data are required for reliable and accurate modeling. In addition to mechanical properties, isotropy and anisotropy can affect manufactured plastic part dimensions. Anisotropic shrinkage caused by patterns of flow during process - ing is important in molding crystalline and fiber-reinforced plastics, for which different shrinkage values are experienced in the flow and cross-flow directions. These values are of most concern to the tool designer and molder; however, the existence and severity of anisotropic shrinkage must be considered when a material is chosen for a part having tight tolerances. Coefficient of Friction: This property comes into play in plastics gears and bearings and other applications where low dynamic (kinetic) friction is desired between moving sur faces in contact. When a weighted block of a plastic is dragged at a steady velocity along a horizontal slab of the same or different material and the contact surfaces are smooth, the coefficient of friction is defined as the ratio of the horizontal drag force to the total weight of the block, called the normal force. In general, the observed coefficients will be different for different pairs of materials, so a test report must identify both materials. While some coefficients were (long ago) determined in this basic manner, ASTM D3028 now prescribes a sophisticated machine (“frictionometer”) for measuring the property. Coefficients of friction change with temperature. Materials such as 66-nylon, acetal resin, and polytetrafluoroethylene containing molybdenum disulfide, which have low self- coefficients, typically enjoy long operating life in unlubricated gears and bearings. Significance of Elasticity, Homogeneity, and Isotropy in Plastics: In the section The Basics of Stress and Strain in Plastics on page 563, formulas for stress and strain in a wide variety of structural geometries are based on the assumptions that 1) the materi - als are perfectly isotropic and elastic, obeying Hooke’s law in tension, compression, and shear; and 2) the only mechanical properties needed are the elastic modulus E , the shear modulus G, and, occasionally, Poisson’s ratio n . See pages 564 and 565. For discussion purposes, consider the structural material low-carbon steel, which obeys these assumptions at temperatures below dull red heat. For this metal, E = 30,000 kpsi (206.8 GPa), G = 11,400 kpsi (78.6 GPa), and Poisson’s ratio n = 0.32. The elastic limit is about 40 kpsi (276 MPa), which indicates that its elongation in tension at that limit is only 0.13 percent. Its stress-strain graph up to the elastic limit is practically a straight line. If it is loaded up to half its elastic limit and maintained at that load for a couple of decades, when the loading is removed, the 0.065 percent elongation will be instantly and com - pletely recovered. The rate and duration of loading in normal applications do not affect the steel’s apparent properties. Plastics, in contrast, are viscoelastic materials. They are complex aggregates of many different elastic and fluid elements and therefore display properties between those of crys talline metals and very viscous fluids. Their stress-strain graphs tend not to be linear.
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online