Stress, types three-point flexural/bending test

Under three point bend loading of a composite (beam), cracks may be developed due to tensile stresses at the lower stratus of the specimen as well as compression stresses at the upper one, or due to interlaminar shear. The type of failure depends on the ratio of span to depth (L/D). Short beam specimens usually fail in shear and long ones by tensile or compression stresses. For interlaminar shear strength (ILSS) tests, a L/D = 5 was chosen (ASTM-D-2344-76). In case of flexural strength tests, this ratio was fixed to 40 (DIN 29971). 

Flexural properties are measured most often in plastics by using a three-point bending jig attached to a universal -type tensometer. The test consists of measuring the stress developed at the surface of a prescribed test piece supported near each end and loaded at the centre (Figure 9.15). The test piece is deflected at a set rate (standard cross-head separation speed), dependent, as in tensile testing, on the type of plastic to be tested. 

Three point bending tests were carried with Instron 2710 testing machine equipped with 5kN head type at a loading rate Imm/min. Specimens were of approximate dimensions 60 mm x 9.5 mm x 1.5 mm and the span length was 45 mm. Flexural stress (of) and flexural strain (ef) were calculated based on ISO standards [19] using the following formula (2) and (3). 

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