On the Use of a Woven Mat to Control the Crack Path in Composite Sandwich Specimens With Foam Core

Abstract

In the last couple of decades the use of sandwich structures has increased tremendously in applications where low weight is of importance e.g. ship structures, where sandwich panels are often built from fiber reinforced faces and foam cores. An important damage type in sandwich structures is separation of face and core (debonding). Debonds can arise as a result of defects from production when an area between face and core has not been primed sufficiently resulting in a lack of adhesion. In use, impact loading, e.g. due to collision with objects, can result in formation of a debond crack, followed by growth due to continued loading. With debonds present the structure might fail under loads significantly lower than those for an intact sandwich structure [1, 2]. A debond crack in a foam cored sandwich can propagate self similarly or kink away from the interface into either the face or core. Whether or not kinking occurs is governed by the stress state at the crack tip, e.g. described by the mode-mixity of the complex stress intensity factor and the properties of the face, core and adhesive [3]. The criticality of an existing crack can be highly dependent on the crack propagation path, since the fracture toughness of the face, core and interface are often very different. As the crack propagates in the interface or laminate the fibers in the face laminate can form a bridging zone behind the crack tip. This can increase the fracture toughness significantly since the bridging fibers provide closing tractions between the separated crack surfaces [4, 5]. The outline of a crack propagating under large scale bridging in a sandwich structure can be seen in Figure 1.

A Modified TSD Specimen for Fracture Toughness Characterization - Fracture Mechanics Analysis and Design

“…Conversely, zero or a negative shear stress, w < 0, will generally promote interface growth, although cases have been reported where negative shear leads to crack kinking into the face sheet [5,6]. The face sheets in the specimens examined by Berggreen et al [5] and Lundsgaard-Larsen et al [6] were composite laminates. Kinking into the face sheet is not physically possible for metal faces.…”

“…It should be pointed out that positive shear stress ahead of the crack tip, w > 0, see Figure 2, tends to promote kinking of the crack into the core [3,4]. Conversely, zero or a negative shear stress, w < 0, will generally promote interface growth, although cases have been reported where negative shear leads to crack kinking into the face sheet [5,6]. The face sheets in the specimens examined by Berggreen et al [5] and Lundsgaard-Larsen et al [6] were composite laminates.…”

Investigation of sandwich crack stop elements under fatigue loading

“…However, the CPJ concept is interesting, but not very suitable for an integral lightweight construction manufactured by the use of one-step liquid composite moulding (LCM) technologies. Lundsgaard-Larsen et al 13 – 16 and Lundsgaard-Larsen 17 designed not a real crack stopper but a simple and effective method to decrease the sandwich interface crack propagation. They placed chopped strand mats (CSMs), continuous filament mats (CFMs) and woven mats at the face/core interface as sources for fibre bridging.…”