Typical Shear Strength Behavior

Here we have conducted experiments to develop an understanding of how the commercial size interacts with the matrix in the glass fiber-matrix interphase. Careful characterization of the mechanical response of the fiber-matrix interphase (interfacial shear strength and failure mode) with measurements of the relevant materials properties (tensile modulus, tensile strength, Poisson s ratio, and toughness) of size/matrix compositions typical of expected interphases has been used to develop a materials perspective of the fiber-sizing-matrix interphase which can be used to explain composite mechanical behavior and which can aid in the formulation of new sizing systems. 

The measured tensile strength Oo- using a molded ASTM tensile specimen was used to define the boundary value at 0 = 0, i.e. o(0) = Gir. In Fig. 12.27, each material exhibits a parabolic-shaped curve suggesting transverse tensile behavior typical of the shear failure in the fiber-matrix interface. Based on Lees (32), the minimum tensile strength value for each curve approximates o(45°) for the given composite material. By substituting the value of o(45°) into Eq. (12.9), the effective interfacial shear strength value was obtained x = sin (45°) cos (45°) g(45°) = (0.707)(0.707) 0(45°) = o(45°)/2. 

Inert fillers, as well as reducing cost, may increase the density of the compound, lower the shrinkage, increase the hardness, and increase the heat deflection temperature. Reinforcing fillers typically will increase the tensile, compressive, and shear strength increase the heat deflection temperature lower shrinkage increase the modulus and improve the creep behavior. 

Several experiments will now be described from which the foregoing basic stiffness and strength information can be obtained. For many, but not all, composite materials, the stress-strain behavior is linear from zero load to the ultimate or fracture load. Such linear behavior is typical for glass-epoxy composite materials and is quite reasonable for boron-epoxy and graphite-epoxy composite materials except for the shear behavior that is very nonlinear to fracture. 

PET. The behavior of crystalline PET at plane strain can be explained if its yield locus is similar to that of PS and PMMA (9, 10) where a craze locus intercepts the shear yield locus. The transition at plane strain to a craze locus would account for the brittleness. This transition, which takes place quite sharply at W/t = 23 (W/b = 8), is probably the cause for the low impact strength (< 1 ft-lb/inch) of the Vs-inch thick notched bars. The plane strain brittleness can be avoided if the geometric constraints can be removed, such as making the notch less sharp or making the test bar thinner. In fact, unnotched bars of PET, equivalent to having an infinite notch radius, are quite tough. The notch sensitivity of PET is typical of crystalline polymers. 

Mechanical testing is the determination of the behavior of a material caused by some applied loading. The material is loaded in its bulk form via a mechanical testing machine (i.e., MTS, Instron, etc.) and its properties are evaluated. Typically these include the elastic modulus or stiffness, the yield strength, the fracture stress or ultimate strength, the elongation, and Poisson s ratio. These properties depend on the mode of loading, such as tension, compression, shear, or flexure. 

The characteristic mechanical property of the amorphous polymers is high strength and a brittle up to ductile deformation behavior. The reason for this behavior is the formation of localized deformation zones under load, such as crazes, deformation bands, or shear bands [12]. The typical type of deformation seen in the amorphous brittle, glassy polymers is the craze. Crazes are often visible with the naked eye in reflected light see Fig. 1.4. The word craze recalls a macroscopic crack-like appearance craze comes from an old English word. 

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