Machinery's Handbook, 31st Edition
560 Plastics Applications and Properties designed to operate in their material’s elastic ranges, and plastic deformation may render a part useless for its designed function, plastic deformation can be used to prevent cata - strophic failure with a fail-safe design. A few plastics have such compliant plasticity that they can be cold-formed by metalworking methods—rolled, stretched, or even forged into final products. This property in metals is called ductility and also applies to plastics. Toughness: This property governs the ability of a material to absorb mechanical energy without cracking or breaking. Often it depends on the rate at which the specimen is de- formed. In a standard tensile test (ASTM D638), the strain rate is fairly low—from 0.15 to 15 (in/in)/min. In a tensile-impact test (ASTM D1822) the specimen is abruptly stretched to failure in milliseconds at a strain rate of about 20,000 (in/in)/min. The tensile tester of D638 generates a graph of stress (psi) versus strain (in/in). The area under this curve is the energy in ft-lb f /in 3 absorbed by the gage volume as it is stretched and finally broken. The impact tester, on the other hand, directly determines how many foot-pounds of energy are needed to stretch and break the specimen. That value, too, can be converted to energy per unit of specimen volume, but the impact energy usually will be very different from the tensile energy to failure. A widely used test of toughness for plastics is the Izod impact test (ASTM D256), in which a falling pendulum strikes and breaks off the upper half of a cantilever test bar whose leading face has a 0.1 in. (2.54 mm) deep, V-shaped notch across it just above the clamping point. This test (the oldest in ASTM’s Plastics list, first published in 1926) indicates not only toughness but also the notch-sensitivity of the plastic. The test result is expressed in ft-lb f /in of notch width. Izod-test values for 1 / 8 -inch (3.2 mm) wide specimens of neat thermoplastics range from 0.4 for crystal polystyrene or wood-flour-filled phenolics to 14 for polycarbonate and phenolics with a high content of glass fiber. Particulate fillers may increase or decrease Izod impact strength; fibrous reinforcements usually increase it. Thus, applying tabulated toughness values to plastics part design is not a simple task. Brittleness: Lack of toughness, characterized in plastics by Izod impact test values below 1.0 ft-lb f /in (53 J/m) or by abrupt fracture with little or no yield in tensile tests, is referred to as brittleness. Notch Sensitivity: This measurement indicates the ease with which a crack progresses through a material from an existing notch, crack, or sharp corner. Hardness: The ability of a material to resist deformation by surface indentation or abra- sion, hardness is often related to modulus, strength, and wear resistance. Several ASTM test methods are in use for plastics, the Rockwell hardness test (ASTM D785) being a popular one. One of three hardened steel balls is pressed into the test specimen, first with a low force, then a much higher one, while a micrometer indicates the increase in depth of impression. Five scales are available for plastics: R, L, M, E, and K, in order of increasing hardness, requiring progressively smaller balls and stronger forces. The scale must be identified in the report of Rockwell hardness numbers. When an article is expected to suffer abrasion in service, the abrasion loss of candidate plastics may be experimentally determined with an abrasion machine (ASTM D1242). Homogeneous plastics have the same composition throughout a part down to the microstructural level, but not to the molecular level as in simple chemical compounds. A 3 ⁄ 16 inch (5 mm) BB-sized pellet of a neat resin contains many long-chain molecules of various chain lengths. However, even compounds containing several additives, pigments, and powdery fillers behave in molded articles as if they were homogeneous. Heterogeneous (inhomogeneous) plastics vary in composition and structure. A com pound reinforced with centimeter-long, randomly oriented, chopped glass fibers will not look homogeneous under 50 3 magnification, yet it may be safely treated as such in the design of parts that are larger than the test samples whose properties have been measured. Isotropy and Anisotropy: These properties are related to homogeneous and heteroge neous types of plastics. In isotropic materials, measured properties are equal in all direc tions. Cast metals and unfilled, amorphous plastics tend to be isotropic. Processing effects such as flow orientation that do not change homogeneity can create substantial direc - tional differences in properties. This effect is called anisotropy . In plastic films made by
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