Preliminary evaluation of the performance of novel fibre reinforced peel stopper concept in sandwich structures

Sandwich structures exhibit high stiffness and strength to weight ratios [1], and they are used extensively for multiple applications for this reason. However they are very sensitive to localized stress concentrations occurring at load introductions and discontinuities between the face sheet and core, which may lead to the development of interface debonds and cracks. Particularly, interface cracks have been extensively studied due to their unique behavior and characteristics. Often the toughness of the face-core interface is lower than the toughness of the bonded materials causing the crack to propagate parallel to the interface. That way the crack path is imposed by the interface geometry. Thus interface cracks tend to propagate under mixed mode conditions, while both opening and sliding of the crack faces is observed. In addition, the difference in stiffness between the facesheet and core material creates a characteristic oscillatory singularity at the crack tip, which has been extensively studied together with its effect on propagation [2-3]. Jakobsen et al. [4] also derived explicit equations for stress intensity factors for an interface crack closing a tri-material wedge while studying crack deflection by core junctions. Deriving crack propagation properties for a wide range of bi-material interfaces has been feverously followed. D.Zenkert and M Burman [5-6] performed a series of quasistatic and fatigue tests to identify fracture toughness properties and power law coefficients for propagation in fatigue for different facesheet-core interfaces. Quispitupa and Manca [78] studied interface crack propagation between a wide range of PVC foams and glass reinforced resin polymer and derived power law curves for different phase angles of mode mixity. Together with the characteristics of crack propagation, the impact of cracks in sandwich structures has been investigated. D.Zenkert [9] used experimental tests and numerical tools to investigate the reduction in strength of sandwich beams with an initial face/core debond. Moreover, damage tolerance in sandwich structures has been researched by Zenkert and Hayman [10-11] for a wide range of applications in the industry. The importance of investigating the effect and severity of debonds in sandwich structures is underlined as well as the investigation of damage tolerance and ways to improve it. For that reason, several crack stopping devices have been proposed [12-13] to limit the severity of debond propagation in sandwich structures. A new concept for a peel stopper was proposed recently by Jakobsen et al. [14], using Polyurethane (PU) for the manufacturing of a special core insert. The new peel stopper approach was tested in quasistatic and fatigue loading conditions [15], and it was proven capable of achieving crack deflection away from the face-core interface. Furthermore it was able to arrest the crack and prevent it from kinking back into the face-core interface and continue propagating. The purpose of the current investigation is to test the performance of a new concept of fiber reinforced PU peel stopper under both quasistatic and fatigue loading conditions. The new peel stopper concept is fabricated in the shape of thin sheets to reduce the weight penalty associated with introduction the peel stopper device into a sandwich structure. Glass fibers were included in the PU material to increase the fracture toughness of the material and prevent crack kinking during fatigue loading conditions. In PRELIMINARY EVALUATION OF THE PERFORMANCE OF NOVEL FIBRE REINFORCED PEEL STOPPER CONCEPT IN SANDWICH STRUCTURES