T

curve for the material (Fig. 3.11). The test frequency for plastics is typically 30 Hz, and test temperature is typically conditioned and tested in an environment of 23°C (73°F). The behavior of viscoelastic materials is very temperature and strain rate dependent. Consequendy, both test frequency and test temperature has a significant effect upon the observed fatigue behavior. The fatigue testing of TPs is normally terminated at 107 cycles.

S-N curve provides information on the higher the applied material stresses or strains, the fewer cycles the specimen can survive. It also provides the curve that gradually approaches a stress or strain level called the fatigue endurance limit below which the material is much less susceptible to fatigue failure. A curve of stress to failure vs. the number of cycles to this stress level to cause failure is made by testing a large number of representative samples of the material under cyclical stress. Each test made at a progressively lowered stress level. This S-N curve is used in designing for fatigue failure by determining the allowable stress level for a number of stress cycles anticipated for the product. In the case of materials such as metals, this approach is relatively uncomplicated. Unfortunately, in the case of plastics the loading rate, the repetition rate, and the temperature all have a substantial effect on the S-N curve, and it is important that the appropriate tests be conducted.

There is the potential for having a large amount of internal friction generated within the plastics when exposed to fatigue. This action involves the accumulation of hysteretic energy generated during each loading cycle. Because this energy is dissipated mainly in the form of heat, the material experiences an associated temperature increase. When heating takes place the dynamic modulus decreases, which results in a greater degree of heat generation under conditions of constant stress. The greater the loss modulus of the material, the greater the amount of heat generated that can be dissipated. TPs, particularly the crystalline type that are above their glass-transition temperatures (Tg), will be more sensitive to this heating and highly cross-linked plastics or glass-

reinforced TS plastics (GRTSs) are less sensitive to the frequency of load.

If the TP's surface area of a product is insufficient to permit the heat to be dissipated, the plastic will become hot enough to soften and melt. The possibility of adversely affecting its mechanical properties by heat generation during cyclic loading must therefore always be considered. The heat generated during cyclic loading can be calculated from the loss modulus or loss tangent of the plastics.

Damping is the loss of energy usually as dissipated heat that results when a material or material system is subjected to fatigue, oscillatory load, or displacement. Perfectly elastic materials have no mechanical damping. Damping reduces vibrations (mechanical and acoustical) and prevents resonance vibrations from building up to dangerous amplitudes. However, high damping is generally an indication of reduced dimensional stability, which can be very undesirable in structures carrying loads for long time periods. Many other mechanical properties are intimately related to damping; these include fatigue life, toughness and impact, wear and coefficient of friction, etc. Measuring damping capacity is equal to the area of the elastic hysteresis loop divided by the deformation energy of a vibrating material. It can be calculated by measuring the rate of decay of vibrations induced in a material.

This dynamic mechanical behavior of plastics is important. The role of mechanical damping is not as well known. Damping is often the most sensitive indicator of all kinds of molecular motions going 011 in a material. Aside from the purely scientific interest in understanding the molecular motions that can occur, analyzing these motions is of great practical importance in determining the mechanical behavior of plastics. For this reason, the absolute value of a given damping and the temperature and frequency at which the damping peaks occur can be of considerable interest and use.

High damping is sometimes an advantage, sometimes a disadvantage. For instance, in a car tire high damping tends to give better friction with the road surface, but at the same time it causes heat buildup, which makes a tire degrade more rapidly. Damping reduces mechanical and acoustical vibrations and prevents resonance vibrations from building up to dangerous amplitudes. However, the existence of high damping is generally an indication of reduced dimensional stability, which can be undesirable in structures carrying loads for long periods of time.

To improve fatigue performance, as with other properties of other properties use is made of reinforcements. RPs are susceptible to fatigue.

Figure 3,1 2 High-performance fatigue properties of RPs and other materials

Figure 3,1 2 High-performance fatigue properties of RPs and other materials

4130 2024-13 7075-T6 Epoxy Fiber Fibai Fib«:' Fiber/ Fiber.'

Epoxy Epoxy Epoxy Epoxy Epoxy

"E" -S" (Thorn»! 300) (Kevlai 49) (HTS)

4130 2024-13 7075-T6 Epoxy Fiber Fibai Fib«:' Fiber/ Fiber.'

Epoxy Epoxy Epoxy Epoxy Epoxy

"E" -S" (Thorn»! 300) (Kevlai 49) (HTS)

However, they provide high performance when compared to unreinforced plastics and many other materials (Fig. 3.12). With a TP there is a possibility of thermal softening failures at high stresses or high frequencies. However, in general the presence of fibers reduces the hysteretic heating effect, with a reduced tendency toward thermal softening failures. When conditions are chosen to avoid thermal softening, the normal fatigue process takes places as a progressive weakening of the material from crack initiation and propagation.

Plastics reinforced with carbon, graphite, boron, and aramid are stiffer than the glass-reinforced plastics (GRP) and are less vulnerable to fatigue. (E-glass is the most popular type used; S-glass improves both short- and long-term properties.) In short-fiber GRPs cracks tend to develop easily in the matrix, particularly at the interface close to the ends of the fibers. It is not uncommon for cracks to propagate through a TS matrix and destroy the material's integrity before fracturing of the fabricated product occurs. With short-fiber composites fatigue life can be prolonged if the fiber aspect ratio of its length to its diameter is large, such as at least a factor of five, with ten or better for maximum performance.

In most GRPs debonding can occur after even a small number of cycles, even at modest load levels. If the material is translucent, the buildup of fatigue damage can be observed. The first signs (for example, with glass-fiber TS polyester) are that the material becomes opaque each time the load is applied. Subsequently, the opacity becomes permanent and more pronounced, as can occur in corrugated RP translucent roofing panels. Eventually, plastic cracks will become visible, but the product will still be capable of bearing the applied load until localized intense damage causes separation in the components. However, the first appearance of matrix cracks may cause sufficient concern, whether for safety or aesthetic reasons, to limit the useful life of the product. Unlike most other materials, GRPs give visual warning of their fatigue failure.

Since GRPs can tend not to exhibit a fatigue limit, it is necessary to design for a specific endurance, with safety factors in the region of 3 to 4 being commonly used. Higher fatigue performance is achieved when the data are for tensile loading, with zero mean stress. In other modes of loading, such as flexural, compression, or torsion, the fatigue behavior can be more unfavorable than that in tension due to potential abrasion action between fibers if debonding of fiber and matrix occurs. This is generally thought to be caused by the setting up of shear stresses in sections of the matrix that are unprotected by some method such as having properly aligned fibers that can be applied in certain designs. An approach that has been used successfully in products such as highperformance RP aircraft wing structures, incorporates a very thin, high-heat-resistant film such as Mylar between layers of glass fibers. With GRPs this construction significantly reduces the self-destructive action of glass-to-glass abrasion and significantly increases the fatigue endurance limit.

Fatigue data provides the means to design and fabricate products that are susceptible to fatigue. Ranking fatigue behavior among various plastics should be conducted after an analysis is made of the application and the testing method to be used or being considered. It is necessary to also identify whether the product will be subjected to stress or strain loads. Plastics that exhibit considerable damping may possess low fatigue strength under constant stress amplitude but exhibit a considerably higher ranking in constant deflection amplitude and strain testing. Also needing consideration is the volume of material under stress in the product and its surface area-to-volume ratio. Because plastics are viscoelastic, this ratio is critical in that it influences the temperature that will be reached. At the same stress level, the ratio of stressed volume to area may well be the difference between a thermal short-life failure and a brittle long-life failure, particularly with TPs.

Like in metal and other material in any design books, factors should be eliminated or reduced such as sharp corners or abrupt changes in their cross-sectional geometry or wall thickness should be avoided because they can result in weakened, high-stress areas. The areas of high loading where fatigue requirements are high need more generous radii, combined with optimal material distribution. Radii of ten to twenty times are suggested for extruded parts, and one quarter to one half the wall thickness may be necessary for moldings to distribute stress more uniformly over a large area.

Figure 3.13 Carbon fiber-epoxy RPs fatigue data

Figure 3.13 Carbon fiber-epoxy RPs fatigue data

Numbtr ol Cyctes to Failure, H

In evaluating plastics for a particular cyclic loading condition, rhe type of material and the fabrication variables are important. As an example, the tension fatigue behavior of unidirectional RPs is one of their great advantages over other plastics and other materials. In general the tension S-N curves (curves of maximum stressed plotted as a function of cycles to failure) of RPs with carbon, boron, and aramid fibers are relatively flat. Glass fiber RPs show a greater reduction in strength with increasing number of cycles. However, RPs with high strength glass fiber are widely used in applications for which fatigue resistance is a critical design consideration, such as helicopter blades.

Fig. 3.13 shows the cycles to failure as a function of maximum stress for carbon fiber-reinforced epoxy laminates subjected to tension and compression fatigue. The laminates have 60% of their layers oriented at 0°, 20% at +45°, and 20% at -45°. They are subjected to a fluctuating load in the 0° direction. The ratios of minimum stress-to-maximum stress for tensile and compressive fatigue are 0.1 and 10, respectively. One observes that the reduction in strength is much greater for compression fatigue. However as an example, the RPs compressive fatigue strength at 107 cycles is still considerably greater than the corresponding tensile value for aluminum.

Metals are more likely to fail in fatigue when subjected to fluctuating tensile rather than compressive load. This is because they tend to fail by crack propagation under fatigue loading. However, the failure modes in RPs are very different and more complex. One consequence is that RPs tend to be more susceptible to fatigue failure when loaded in compression.

Fiber reinforcement provides significant improvements in fatigue with carbon fibers and graphite and aramid fibers being higher than glass fibers. The effects of moisture in the service environment should also be considered, whenever hygroscopic plastics such as nylon, PCs, and others are to be used. For service involving a large number of fatigue cycles in TPs, crystalline-types offer the potential of more predictable results than those based on amorphous types, because the crystalline ones usually have definite fatigue endurance. Also, for optimum fatigue life in service involving both high-stress and fatigue loading, the reinforced high-temperature performance plastics like PEEK, PES, and Pi are recommended.

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