On the Drilling of Natural Fiber Composites and Their Hybrids

Very little research has been conducted on the mechanical properties and fatigue behaviours of open-holed flax and hybrid carbon/flax laminates. The aim of this research is to fill in the knowledge gaps on the damage evolution, static and fatigue behaviours of open-hole flax and carbon/flax hybrid materials machined by conventional drilling (CD) and abrasive water jet (AWJ) machining, while also studying the effect hybridization of flax with carbon has on open hole fiber reinforced epoxy laminates. The specimens were manufactured using woven carbon fiber (C) and unidirectional flax fiber (F) in 6 configurations: unidirectional flax (F[0]), 0°-90° cross ply flax (F[0/90]), ±45° angled ply flax (F[±45]), woven carbon and unidirectional flax (CF[0]), woven carbon and 0°-90° cross ply flax (CF[0/90]), and woven carbon and ±45° angled ply flax (CF[±45]). The specimens were subjected to monotonic tensile, load-unload, and step-wise fatigue tensile tests until failure. The damage evolution, plastic strain, and surface fracture were examined for each type of testing. Microscopic observations of the walls of AWJ machined holes revealed severe damage in the form of peel-up, push-out, and secondary delamination. On the other hand, CD samples exhibited only push-out delamination at the exit side of the hole. For both the hybrids and pure flax layups the unidirectional specimens have the highest strength followed by the cross-ply layups with the angle ply layups having the lowest. It was determined that in all cases the pure flax specimens accumulated more damage during quasi-static load-unload testing than their hybrid counterparts. It was also found that in all cases CD specimens have a higher high cycle fatigue strength (HCFS). Finally, a 3D progressive fatigue damage model was developed and successfully used to predict the fatigue life and failure mechanisms of non-holed hybrid carbon/flax specimens. Key Words: Natural Fibers; Hybrid Carbon/flax composites; Delamination; Fatigue; 3D progressive fatigue damage model