Fracture behaviour at tri-material junctions of crack stoppers in sandwich structures

Abstract

Abstract:Inspired by a previously published peel stopper design for foam cored composite sandwich structures, three novel markedly lighter peel stoppers were evaluated with respect to their ability to deflect and arrest propagating face debond cracks. Of the three novel peel stopper configurations, C1, C2 and C3, C1 was similar to the previous design, whereas C2 and C3 were modified with layers of glass fibre fabric extending from the peel stopper tip into the face sheet (C2) or into the face sheet/core interface (C3). The previous peel stopper was validated under mode II dominated conditions, but the novel designs were investigated under mode I dominated crack propagation conditions, which are of higher practical relevance. Both quasi-static and fatigue loading scenarios were investigated. The mechanisms controlling crack propagation at the internal peel stopper tip were studied using Thermoelastic Stress analysis (TSA) and Finite Element (FE) analysis. The TSA has revealed significant new information about the local stress fields in the vicinity of the tri-material junction (peel stopper tip) as well as the fracture process zone. Configuration C1 was unable to deflect debond cracks consistently, albeit it did so in most cases, whereas it was incapable of achieving crack arrest. C2 and C3 both performed better in that they consistently demonstrated the ability to deflect propagating cracks, whereas only C2 could arrest the cracks consistently as well. Detailed fracture mechanics analyses confirmed and explained the experimental observations.

Interfacial Crack Arrest in Sandwich Panels with Embedded Crack Stoppers Subjected to Fatigue Loading

“…This study concerns the fatigue testing of sandwich panels with GFRP face-sheet and PVC foam core that have been fitted with embedded peel stoppers of the improved design introduced in [26,27,28]. The ability of the peel stoppers to enhance the damage tolerance of sandwich panels, i.e.…”

“…Following on from and inspired by the work presented in [26][27][28], a new and significantly lighter design of the peel stopper was proposed and implemented in sandwich beams [26][27][28]. Initially, the conditions under which the novel peel stopper was able to deflect a propagating face-sheet/core interface crack were investigated in [26].…”

“…Initially, the conditions under which the novel peel stopper was able to deflect a propagating face-sheet/core interface crack were investigated in [26]. This included testing of sandwich beams made of glass reinforced composite face-sheets (GFRP) and PVC foam core with embedded peel stoppers subjected to fatigue loads using the Mixed Mode Bending test (MMB) [26].…”

“…Initially, the conditions under which the novel peel stopper was able to deflect a propagating face-sheet/core interface crack were investigated in [26]. This included testing of sandwich beams made of glass reinforced composite face-sheets (GFRP) and PVC foam core with embedded peel stoppers subjected to fatigue loads using the Mixed Mode Bending test (MMB) [26]. Three design variations of the peel stopper were tested showing that a design with glass fibres extruding from the tip of the peel stopper and extending into either the face-sheets or the face-sheet/core interface improved the interface crack deflection capabilities.…”

“…The peel stopper was made using a two-component Polyurethane (PU) resin (PERMALOCK 2K PU-9004) which was reinforced by UD glass fibres along its length as devised in [26]. The use of Polypropylene for the mould tool allowed easy de-moulding of the cast PU peel stoppers without the need of a release agent.…”