EFFECT OF FIBER LENGTH ON THE TENSILE STRENGTH OF UNIDIRECTIONALLY ARRAYED CHOPPED STRANDS

A sheet consisting of unidirectionally arrayed chopped strands (UACS) with predetermined fiber length can be realised by cutting slits into prepreg at regular intervals in the fiber direction. UACS can be multidirectionally stacked and formed in complex shapes with minimal loss of mechanical properties compared to its original prepreg. This work aims to predict tensile strength of unidirectional composites with a stagger-structure of chopped strands made by regularly stacking UACS. The author has previously identified that final failure is caused by unstable growth of delamination from stress concentration at the tip of bundles through in-situ observation during tensile tests, and proposed an analytical equation to predict its strength using the interlaminar fracture toughness in Mode II. The equation showed good agreement with experiments in the case of relatively long fiber length, but could not explain a large knock down of tensile strength with decreasing fiber length in experiments. In the present study, a newly-revised equation utilizes the stress-strain curve's non-linearity to account for the effect of fiber length, relying on interlaminar shear yield stress obtained from compression interlaminar shear (CILS) tests, in addition to interlaminar fracture toughness in Mode II obtained by end-notched flexure (ENF) tests. It was confirmed that this closely reflects the relation between tensile strength and fiber length, without applying any fitting parameters.