Test method evaluation for fiber‐reinforced molding materials

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Abstract
Standard test methods ASTM D‐651 for determining tensile properties and ASTM D‐790 for flexure properties are evaluated. Heterogeneous fiber distributions due to molding flow conditions are shown to yield test specimens wherein fiber orientation near surfaces is highly collimated while orientations in central regions are oriented approximately transverse to the specimen axis. In addition, the converging and diverging flow fields in the dog bone (ASTM D‐651) specimen with end gate are shown to yield a variation in fiber orientation along the specimen length. Finite element analysis of the dog bone (ASTM D‐651) reveals that stress concentrations and a non‐uniform stress distribution are introduced due to the required gripping arrangement of the test standard. The flexural specimen is decomposed into two elements of a collimated and transverse fiber orientations and a mathematical model is developed which reflects the composite behavior of the beam specimen. Finally, it is concluded that the D‐651 and D‐790 test methods do not reveal intrinsic design data for fiber reinforced molding materials.

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