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Wednesday, March 19, 2014

Flexural Testing for Fibre Reinforced Plastic

The stress-strain behavior of polymers in flexure is of interest to a designer as well as a polymer manufacturer. Flexural strength is the ability of the material to withstand bending forces applied perpendicular to its longitudinal axis. The stresses induced by the flexural load are a combination of compressive and tensile stresses. This effect is illustrated in Figure xx. Flexural properties are reported and calculated in terms of the maximum stress and strain that occur at the outside surface of the test 

Many polymers do not break under flexure even after a large deflection that makes determination of the ultimate flexural strength impractical for many polymers. In such cases, the common practice is to report flexural yield strength when the maximum strain in the outer fibre of the specimen has reached 5 percent.

For polymeric materials that break easily under flexural load, the specimen is deflected until a rupture occurs in the outer fibres. There are several advantages of flexural strength tests over tensile tests. If a material is used in the form of a beam and if the service failure occurs in bending, then a flexural test is more relevant for design or specification purposes than a tensile test, which may give a strength value very different from the calculated strength of the outer fibre in the bent beam

The flexural specimen is comparatively easy to prepare without residual strain. The specimen alignment is also more difficult in tensile tests. Also, the tight clamping of the test specimens creates stress concentration points. One other advantage of the flexural test is that at small strains, the actual deformations are sufficiently large to be measured accurately. There are two basic methods that cover the determination of flexural properties of plastics. Method 1 is a three-point loading system utilizing centre loading on a simple supported beam. A bar of rectangular cross section rests on two supports and is loaded by means of a loading nose midway between the supports. The maximum axial fibre stresses occur on a line under the loading nose.