Fiber yield strength

In each case the section is designed to keep the deflection to less than 2 in. in 16 in. for a design life of 5 years and the extreme fiber stress is kept to a value less than the yield strength of the material. The first step in the analysis is to determine the necessary section to resist the bending load using the short-term tensile and compressive strength and modulus values. The extreme fiber stress is calculated for these sections to determine that the chair will not break when deflected. 

Critical fiber length = [Ultimate or tensile strength times fiber diameter/2] times the fiber-matrix bond strength or the shear yield strength of the matrix— whichever is smaller... 

The tensile properties of isotactic polypropylene materials reinforced with continuous nylon fibers were measured. Less than 10 vol % of the fibers leads to an increased yield strength and yield elongation. As little as 3 vol % of the nylon fibers increased the elongation at necking from 10 to 20%. This retarded necking arises from the fiber-matrix debonding which delocalizes the microscopic yielding processes. 

Figure 15.12 shows the fatigue resistance of carbon and glass fiber filled poly(phenylene ether ketone).The flexural fatigue depends on tensile properties of the composite. The yield strength of the matrix and the quality of the interface affect the fatigue properties of composites. 

The fundamental difference between mechanical stresses and tliermal stresses lies in the nature of the loading. Thermal stresses as previously stated are a result of restraint or temperature distribution. The fibers at high temperature are compressed and those at lower temperatures are stretched. The stress pattern must only satisfy the requirements for equilibrium of the internal forces. The result being that yielding will relax the thermal stress. If a part is loaded mechanically beyond its yield strength, the part will continue to yield until it breaks, unless the deflection is limited by strain hardening or stress redistribution. The external load remains constant, thus the internal stresses cannot relax. 

FIGURE 3.31 Yield strength versus spin-line stress for spun polypropylene fibers. (From Nadella, H.P., Henson, H.M., Spruiell, J.E. White, J.L. J. Appl. Polym. Sci., 1977, 21, 3003. With permission.)... 

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