Flexural strength definition

Thus the effects of the rate of application of stress and the ambient temperature must be recognized when polymers are used as structural materials, and definite rates and temperatures must be specified for tests, such as those for tensile and flexural strengths cited in Chapter 3. A knowledge of the structure of polymers is essential for the understanding of these effects, which differ from the effects of stress and temperature on all other materials of construction. 

The tensile strength at -320 F of the most amorphous Kel-F material we could make was approximately 26,000 psi, while that of the most crystalline was approximately 15,000 psi. The more amorphous samples had a measurable elongation (-+5 ) at -320 F, whereas the crystalline samples showed a typical brittle failure. Flexural, torsional, compressive and impact results also show a definite change in properties with a change in crystallinity. 

Figure 2 Cost-related (specific) flexural modulus of major thermoplastics versus cost-related (specific) impact strength. (For definitions and abbreviations, see Figure 1). Impact strength notched, Izod.

For any given application, certain mechanical properties will be of more importance than others. It is therefore, essential to identify and rank the most relevant properties and formulate or purchase the least expensive composite material that satisfies the requirements. The key mechanical properties for most applications are modulus (tensile or flexural), yield strength, impact strength and possibly HDT. A distinction is often made between reinforcing and nonreinforcing fillers, but unfortunately, the term reinforcement is rarely defined explicitly. Fibres are usually considered to reinforce and isotropic fillers are not, with platy fillers somewhere in between. As shown later, it is not appropriate to define reinforcement in terms of particle shape, because that definition breaks down with variations in anisotropy and particle size. In agreement with Ram [59], the definition of reinforcement as the simultaneous improvement of both modulus and yield strength will be used in this chapter... 

It is useful at this point to consider the definition of an additive as given by the EC (European Commission) an additive is a substance, which is incorporated into plastics to achieve a technical effect in the finished product, and is intended to be an essential part of the finished product. Since the very early stages of the development of the polymer industry, it was realized that useful materials could only be obtained if certain additives were incorporated into the polymer matrix, in a process normally known as compounding. Additives confer on plastics significant extensions of properties in one or more directions, such as general durability, stiffness and strength, impact resistance, thermal resistance, resistance to flexure and wear, acoustic isolation, etc [86]. 

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