Mode-I fracture toughness of thin CF/PA6 UD composites identified in a DCB test with stiffening al beams in the presence of debonding

Abstract

Compression molding of unidirectional (UD) carbon fiber-reinforced polyamide6 (CF/PA6) prepreg tapes is conducted to manufacture thin 4-ply UD laminates. Two cooling rates of 43 °C/min and 760 °C/min are obtained using different press machines. Isothermal cooling through the thickness of the laminates is ensured by the small laminate thickness and validated by thermal cycle recording at different locations. Different cooling rates result in different degrees of crystallinity [1], and their effects on mode-I fracture toughness are studied in the current work. To prevent excessive bending and yielding when testing thin specimens in DCB mode, stiffening beams should be bonded to the specimens [2], [3]. Hence, aluminum (Al) stiffening beams are bonded to both sides of the thin laminates. The surfaces of the CF/PA6 and Al adherends are ground and degreased before bonding. Various types of adhesives are tried, including epoxy-based and methacrylate adhesives. Combinations of different mechanical treatments of the surface, such as sandblasting (SB) or sandpaper grinding (SP) with different grit sizes, are tried to promote CF/PA6-Al adhesion. A double cantilever beam (DCB) experiment is performed on specimens following the ASTM D5528 standard. The results for all cases showed premature failure at CF/PA6-Al interfaces and test failure at small displacements (Fig. 1a) even before mid-plane crack initiation. In the load-displacement curves, several drops are visible, caused by the defects in the adhesive layer. In other words, defects in the adhesive layers could result in local drops of the force followed by lower specimen stiffness. The post-mortem surface of the CF/PA6 in Fig. 1b shows no adhesive on the composite surface. Instead, the adhesive is attached to the Al beam. This observation indicates adhesive failure during the DCB test due to the poor wettability of the composite substrate