Powder Epoxy for One-Shot Cure, Out-of-Autoclave Applications: Lap Shear Strength and Z-Pinning Study

Abstract

Large composite structures manufactured out-of-autoclave require the assembly and bonding of multiple parts. A one-shot cure manufacturing method is demonstrated using powder epoxy. Lap shear plates were manufactured from powder epoxy and glass fiber-reinforced plastic with four different bonding cases were assessed: secondary bonding using standard adhesive film, secondary bonding using powder epoxy, co-curing, and co-curing plus a novel Z-pinning method. This work investigates the lap shear strength of the four cases in accordance with ISO 4587:2003. Damage mechanisms and fracture behavior were explored using digital image correlation (DIC) and scanning electron microscopy (SEM), respectively. VTFA400 adhesive had a load at break 24.8% lower than secondary bonding using powder epoxy. Co-curing increased the load at break by 7.8% compared to powder epoxy secondary bonding, with the co-cured and pinned joint resulting in a 45.4% increase. In the co-cured and co-cured plus pinned cases, DIC indicated premature failure due to resin spew. SEM indicated shear failure of resin areas and a large amount of fiber pullout in both these cases, with pinning delaying fracture phenomena resulting in increased lap joint strength. This highlights the potential of powder epoxy for the co-curing of large composite structures out-of-autoclave.

Polymer/carbon pin through thickness reinforcement

“…The initial samples using a 12-layer 5HS lay-up with 0-90 stacking sequence demonstrated a stick-slip propagation and the shallow pin angle resulted in sub-optimal mode 1 performance. In a collaborative work with Edinburgh University [20], samples for lap-shear tests were constructed. The short overlap of the pinned region necessitated a change in the methods used.…”

“…Finally, is the investigation of alternate pin construction, in particular the polymer element. One candidate for this which may prove beneficial is the use of the powder epoxy investigated by Noble et al [20]. An Epoxy based pin, if combined with a laminate constructed of the same epoxy, might allow for the construction of through thickness reinforced preforms that can act as a ‘dry pre-preg’ without the storage requirements associated with conventional pre-preg materials.…”

Influence of Line Processing Parameters on Properties of Carbon Fibre Epoxy Towpreg

“…This feature provides versatility in the production phase by allowing the potential of separating the impregnation and curing of the composite parts. In addition, it is possible to co-cure different composite parts using powder-epoxy [13] instead of using adhesives [7]. Finally, as the powder epoxy is solid and stable at room temperature, storage costs are substantially lower than standard prepreg systems.…”