Machinery's Handbook, 31st Edition
572 Time-Related Properties of Plastics log time, and the strain limit of 20 percent of the yield or ultimate strength mentioned above should not be exceeded. Impact loading describes a situation in which a load is applied extremely rapidly. Any moving body has kinetic energy, and when the motion is stopped by a collision, the energy is dissipated. Ability of a plastic part to absorb energy is determined by the shape, size, thickness, and type of material. Impact-testing methods now available do not provide designers with information that can be used analytically. The tests can be used for comparing relative notch sensitivity or relative impact resistance, and so can be useful in choosing a series of materials to be evaluated for an application or in grading materials within a series. Impact testing by the Izod and Charpy (ASTM Method D6110) methods, in which a pendulum arm is swung from a certain height to impact a notched test specimen, is the most widely used for measuring impact strength. Impact with the test specimen reduces the energy remaining in the arm, and this energy loss is recorded in ft-lb f (J). The value of such tests is that they permit comparison of the relative notch toughness of two or more materials under specific conditions. Table 2b provides the mean and range of the notched Izod impact strength (measured in J/cm) for different thermoplastic and thermoset (TS) materials. In general, materials with higher strain to failure will exhibit greater impact resistance. Thermosetting materials tend to have lower impact resistance than thermoplastics, though thermosets with flexible and unsaturated networks (e.g. rubber and polyester) can have high impact resistance. The addition of reinforcements, such as glass fiber (GF), tends to increase modulus and strength but lower strain to failure and thus reduce the impact properties of composite systems. While such Izod and Charpy impact tests provide comparative data about material performance, the tensile impact test mentioned earlier helps in ranking materials, because it represents more realistic conditions that are encountered by actual parts in certain applications. Fatigue tests measure the ability of plastics materials to withstand repeated stresses or other cyclic phenomena. Example applications are a snap-action or snap-fit latch that is frequently opened and closed, a gear tooth, a bearing, and a structural component subject to vibration or to repeated impacts. Cyclic loading can cause mechanical deterioration and progressive fracture, leading to failure in service. Typical fatigue tests are carried out on machines designed to subject a cantilever test piece to reversing flexural loading cycles at different stress levels. ASTM Test D671 is of this type, with a cycle rate of 30 Hz. The number of cycles to failure is recorded for each stress level. Data are normally presented in plots of stress (S) versus the log of the number of cycles (N) called S-N curves for specific cycle rates and environmental temperatures. Tensile impact tests mount the test specimen on the swinging arm. Attached to the test specimen is a cross-piece that is arrested by a notched anvil as the bar swings down, allow ing the energy stored in the arm to break the specimen under tension as it passes through the notch. Another impact test used for plastics allows a weighted, round-ended cylindri- cal “dart” to fall on a flat disk of the plastics to be tested. This test is good for ranking mate - rials because it represents conditions encountered by actual parts in certain applications. Thermal Properties.— Melting temperatures of highly crystalline thermoplastics are sharp and clearly defined, but amorphous and liquid-crystalline materials soften and be - come more fluid over wider temperature ranges. Melting points have greater significance in molding and assembly operations than in product design, which usually deals with the product’s service temperatures well beneath the melting range. Glass transition is a reversible change that occurs in an amorphous polymer or in amor phous regions of a partly crystalline polymer when it is heated from a very low temperature into a certain range, peculiar to each polymer, characterized by a rather sudden change from a hard, glassy, or brittle condition to a flexible or rubbery condition. Physical proper ties such as coefficient of thermal expansion, specific heat, and density usually undergo distinct changes in their temperature derivatives at the same time. The symbol for the glass-transition temperature is T g .
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