Effect of Core Thickness on Load Carrying Capacity of Sandwich Panel Behavior Beyond Yield Limit

Introduction

Researchers are continuously looking for new, better and efficient construction materials. The main goal of these researches is to improve the structural efficiency, performance and durability. Light-weight, excellent corrosion characteristics and rapid installation capabilities created tremendous opportunities for sandwich panel structure in industry. Sandwich panel normally consists of a low-density core material sandwiched between two high modulus face (usually made of metal) skins to produce a lightweight panel with exceptional stiffness as shown in Figure 1. The face sheets act like the flanges of I-beam, while the core carries the shear forces. The faces are typically bonded to the core to achieve the composite action and to transfer the forces between the components.

Figure 1.

Illustration sandwich plate geometry

Work on the theoretical description of sandwich structure behavior has started on the early years of the second half of the 20th century. Plantema (1966) published the first book about sandwich structures, followed by a book written by Allen (1969), and more recently a book by Zenkert (1995). Although Triantafillou and Gibson (1987) developed a method to design for minimum weight sandwich panel, and reported the failure mode map of sandwich construction, however they didn’t consider the post yield state of the sandwich structure.

Mercado and Sikarskie (1999) reported that the load carried by sandwich structures continues to increase after core yielding. Knowing that the core could not carry additional load after yield, this increasing load carrying capacity of post yield sandwich structure initiates the postulation that the additional shear load was transferred to the face sheets. To account for the above-mentioned phenomenon, Mercado et al. (2000) developed a higher order theory by including a bilinear core material module. This theory yields a fairly accurate prediction on the deflection of a foam cored sandwich beam in four point bending (Mercado et al., 2000). In addition, this theory does not take into account the core compression under localized load, or any geometric non-linearity. The classical sandwich beam theory assumes that in-plane displacements of the core through its depth are linear. In other words, it is assumed that the core thickness remains constant and cross-sections perpendicular to the neutral axis remain plane after deformation. This assumption is generally true for traditional core material such as metallic honeycomb (Frostig et al., 1992). However, this assumption is not suitable for soft, foam-based cores, especially when the sandwich structure is subjected to a concentrated load (Thomsen, 1995). With a much lower rigidity compared to metallic honeycomb, foam-based cored sandwich structures are susceptible to localized failure. Insufficient support to the face sheets due to core compression near the application points of concentrated loads can lead to failures such as face sheet/ core delamination, face sheet buckling, and face sheet yielding. This localized non-linearity is reported by many researchers such as (Thomsen, 1993, 1997), Caprino (2000) and Gdoutos et al. (2001) but the shear distribution at localized failure points was not well defined. Miers (2001) investigated the effect of localized strengthening inserts on the overall stiffness of a sandwich structure. This localized strengthening increased the rigidity of the sandwich structure, but the addition of high stiffness inserts complicates the manufacturing process of sandwich structure. The two most popular theories that include these localized effects are the superposition method (Zenkert 1997) and high order theory (Frostig et al., 1992).