Effect of Fabric Stabilization Medium on the Interlaminar Shear Strength of Unidirectional Carbon/Epoxy Laminates

Dry unidirectional fiber preforms need to be stabilized by stitched or adhesively bonded materials to maintain a fixed fiber position and alignment during infiltration, compaction, and cure processes for composite laminates. However, these stitching and binding materials can have deleterious effects on the quality of manufactured laminates, such as in-plane-wavy fibers, resin rich areas, and a weak interface between the stabilization material and the matrix of the composite. Little information is available on the interlaminar shear strength (ILSS) of unidirectional carbon/epoxy composites with various types of stabilizing materials within individual plies. The objective of the current investigation is to experimentally evaluate the effect of ply-wise stitching and thermoplastic adhesive binding on the ILSS of unidirectional carbon/epoxy composites. One stitched fabric and two thermoplastic-bound fabrics were evaluated. Four laminates of two different sizes were manufactured using resin film infusion (RFI). Laminate quality was evaluated based on void and fiber volume fractions and ILSS from short beam shear (SBS) testing. Laminates with stitches demonstrated a lower ILSS (34.3 MPa) than those with binder (49.6 MPa). Manual removal of the binder before processing provided the highest ILSS (52.7 MPa). Laminates manufactured with stitched and bound fabrics had interlaminar failure modes connected to the stabilization materials. Laminates fabricated with the stitched fabric exhibited lower fiber volume fractions (55-56%) than those fabricated with binder (62%) and removed binder (66%). This result is attributed to the greater resistance of the stitched fabrics to compaction, in comparison to the binder and no binder fabrics. Void volume content was nearly the same in all four laminates-between 1% and 2%.