Equivalent flexural modulus

The equations are first expressed in terms of Ef, the equivalent flexural modulus, and then in terms of the moduli of the constituent composites Epc and E c) and the secondary composite volume ratio (T c). These equations were used to generate the parametric nondimensional plots shown in Figures 6.8-6.10. 

As mentioned in Section 3.8, use can be made of composite beams. In a sandwich beam, such as that shown in Fig. 3.26, the core usually has lower values of Young s and shear modulus than the thin faces. The Young s modulus of the faces and core will be denoted by and E, respectively. One approach to stress analysis in these sandwich beams, is to transform the cross-section into a geometry with an equivalent flexural rigidity but consisting of a single material. This transformation is shown in Fig. 4.10 in which the core is replaced by the same material as the faceplates but with a width bE IE ). In sandwich beams, the faces are usually much thinner than the cores and the equivalent flexural rigidity can be written approximately as... 

We normally perform flexural testing over a limited range of strain, generally only sufficient to determine the sample s flexural yield point. From an end use point of view, this is a reasonable place to halt the test. If we are using a polymer in an application where it is exposed to flexural loads, such as car body panels or bulk liquid storage tanks, it is generally considered to have failed at the point where it no longer deforms reversibly. We obtain a value of flexural modulus from the force versus deformation curve, much as we do for the tensik equivalent. 

Waferboard, a more recent wood constmction product, competes more with plywood than particle board. Waferboard and strand board are bonded with soHd, rather than Hquid, phenoHc resins. Both pulverized and spray-dried, rapid-curing resins have been successfully appHed. Wafers are dried, dusted with powdered resin and wax, and formed on a caul plate. A top caul plate is added and the wafers are bonded in a press at ca 180°C for 5—10 min. Physical properties such as flexural strength, modulus, and internal bond are similar to those of a plywood of equivalent thickness. 

The elastic modulus, measured using the pulse echo method, was about 180 GPa, and the fracture strength determined by the three-point flexure test was about 800 MPa in the grain-alignment direction. This strength, which is relatively high for the large porosity of 25%, is almost equivalent to that of commercially available... 

The majority of tests to evaluate the characteristics of plastics are performed in tension or flexure hence, the compressive stress-strain behavior of many plastics is not well described. Generally, the behavior in compression per ASTM D 695 is different from that in tension, but the stress-strain response in compression is usually close enough to that of tension so that possible differences can be neglected (see Fig. 3-18). The compression modulus is not always reported, since defining a stress at a strain is equivalent to reporting a secant modulus. However, if a compression modulus is reported, it will generally be an initial modulus. 

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