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Abstract The paper describes a methodology for determining the influence of both production defects and in-service damage on the performance of composite sandwich panels in ship structures, and for deciding whether, when and where replacement or repair is needed. The approach is based on quantitative modelling of the re-spective defect and damage types. Keywords Composite; sandwich; ships; defects; damage; repair. Introduction Sandwich construction, with fibre-reinforced polymer (FRP) skins separated by a lightweight core, has been used extensively in hulls and superstructures of high speed and naval craft. Currently the inspection and repair procedures for such structures are not generally based on a quantitative description of the growth of defects and damage. Without such an approach it is difficult to establish limits for acceptable defect and damage sizes. 58680
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Sometimes in-service damage is repaired unnecessarily, and in some cases the repair may even reduce the structural performance more than the original damage. Studies performed in the UK to establish the damage tolerance of stiffened, single-skin GRP ship structures are reported by Sumpter et al. (1997) and by Elliott and Trask (2001). These have enabled repair criteria to be established for the Royal Navy’s Hunt Class vessels. The aim of the current work has been to develop a simi-lar approach for sandwich structures in naval ships. The work has been performed within a European collabora-tive project (SaNDI). The paper begins with a review of the defects that are known to arise during production of FRP sandwich panels for naval and other ships. A similar review is made of the various types of damage that may occur in such structures while in service. Also reviewed are the options available for corrective action. Attention is then turned to rational ways of assessing the influence of the defect or damage and deciding whether, when, where and how a repair should be effected. At-tention is focused on retention of adequate local and global strength, though other functionality considera-tions can readily be built into the methodology. Some examples of defect and damage models are discussed, and strength reduction curves for some relevant defect and damage cases are presented. The paper concludes with a brief discussion of the chal-lenges being faced with regard to the detection of such defects and damage and the determination of their loca-tion and extent. Defect and Damage Types For single-skin laminates and skin laminates of sand-wich structures, production defects include dry zones, voids, delaminations, wrinkles, misalignment of fibres, and poor curing (giving reduced physical properties).
For sandwich structures, core/skin debonds must be considered in addition, and also voids and inclusions in the core, and lack of bond (edge-to-edge and face-to-face) between core sheets. In-service damage may include various types of contact damage (quasi-static contact as well as impact), heat damage and numerous types of damage resulting from overloading, such as core fracture or crushing, skin/core debonds, laminate rupture, delamination either within a laminate or at a secondary lamination, and failure at equipment fastenings. Impact damage may be confined to the impacted face laminate (with or without penetra-tion of the laminate), or may involve crushing and/or cracking of the core. In extreme cases, penetration of the entire sandwich may occur. More substantial dam-age cases include the removal of whole panels or as-semblies by fire or collision. For naval vessels some types of damage may be caused by weapon effects, such as air blast, underwater explosions, and fragment or missile hits. Corrective Measures The purpose of defect/damage assessment is to decide on corrective actions. The main choices available are outlined below. In-Service Damage For damage detected in service, available corrective measures may involve some or all of the following: • Perform immediate emergency repair (followed by a permanent repair later), possibly with stipulation of operational restrictions until a permanent repair is effected. • Perform permanent repair at sea; this is not usually an option as conditions at sea are rarely conducive to such repair. • Proceed to safe harbour or dry dock and perform repair (or make new assessment). Possible interim operational restriction. • Proceed to dry dock and strengthen/modify struc-ture. Possible interim operational restriction. • Repair at next scheduled survey, possibly monitor-ing development in the interim. • No repair, no further action. It is extremely important to assess the cause of damage before deciding on corrective actions, and in particular to establish whether the damage is a result of inadequate design. The decision on corrective action may also depend on whether or not a monitoring system is in-stalled. For naval vessels it may depend on whether the ship is in a war- or peace-time situation. The repairs themselves may be broadly pided into the following categories: • Emergency, temporary or simplified repair •