Sandwich Structures With Smart Composite Face Skin
Authors: Konka Hari Prasad , Wahab M. A. , Lian Kun 
Published: August 2012

Sandwich structures are one of the very important classes of composite structures that have been studied quite extensively in the past few years. The concepts of sandwich structures have been widely used in the aerospace, automobile, marine, and civil engineering applications; because it is suitable and amenable to the development of light-weight structures with high in-plane and flexural stiffness. A typical sandwich structure is usually comprised of two stiff face skins, which are separated by a thick, lightweight, and compliant core. The primary function of the face skin sheets in a sandwich structure is to provide required bending and in-plane shear stiffness and to carry edge-wise bending and in-plane loads. The composite face skins are usually made from resin impregnated glass fiber or a laminate of unidirectional fibers (prepregs), graphite prepregs, aluminum alloys or many other refractory metal alloys. In this study, smart composite face skins comprise of the composite layers with embedded Piezoelectric Fiber Composite Sensors (PFCS). The functions of PFCS as an embedded sensor inside the composite sandwich structure are threefold: (i) to detect all loading conditions acting on to the structure, (ii) to detect the damages while in-service under dynamic loads, and finally, (iii) to monitor the pre-existing damages in the structure so that their severity can be ascertained to avoid eventual catastrophic or premature failures. The PFCS are generally an ideal choice for this type of sandwich structures applications, as they are highly flexible, easily embeddable; their high compatibility to the composite manufacturing techniques; and more importantly, they produce significantly less interfacial stresses when embedded inside the composite structures. This research is focused on examining the effects on the structural integrity of the composite sandwich structure (with glass micro-balloons syntactic foam core and resin infused glass fiber face skins) with PFCS embedded inside face skin. In-plane tensile, and tension-tension fatigue tests are performed to evaluate the strengths/behavior of the composites containing embedded PFCS. The tensile tests showed that both the average ultimate strength and the modulus of elasticity of the tested laminate with or without embedded PFCS are within 7%. The Stress-Life (S-N) curves obtained from fatigue tests indicates that the fatigue lives and strengths with and without the PFCS are close to each other as well. Then carefully planned experiments are conducted to investigate the ability of the embedded PFCS to monitor the stress/strain levels and detect damages in composite sandwich structure. Experiments were performed to explore the ability of the embedded PFCS (MFC and PFC) to detect the damages in the structures using modal analysis method. Results from these experiments shows that the PFCS are effective in detecting the initiations of damages like delamination inside these composite sandwich structures through changes in natural frequency modes. Hence a smart composite face skin can be an effective method to monitor the health of the composite sandwich structures’ in-service conditions.