舰船玻璃钢夹芯板缺陷和损伤的影响英文文献和中文翻译(2)
Cosmetic repair (repair localised surface defects and prevent moisture ingress) • Permanent structural repair to original integrity, or to a specified percentage of the original strength • Repair and modification to give better performance than the original structure. Production Defects For production defects, corrective measures may be: • Rejection/replacement • Repair • No repair, no further action Normally only permanent repairs (cosmetic or struc-tural) are relevant for production defects. Defect/Damage Assessment If a defect or damage has been detected and its form, size and position determined, a decision on whether, when and how to repair should be based on a quantita-tive assessment of the influence of the defect or damage. For structures or structural elements that are not sub-jected to repeated loading that might cause fatigue fail-ure, the problem reduces to deciding whether the dam-aged structure has sufficient residual strength to with-stand future static or dynamic loads with an adequate margin of safety. To assess this it is important to estab-lish the strength of the structure with the appropriate type of defect or damage, for different sizes of the de-fect/damage. A defect or damage case may be consid-ered critical when the reduction in residual strength is unacceptably large. To assess the residual strength, it may be necessary to use a fracture mechanics approach to model damage growth, but in some cases simpler strength modelling approaches are sufficient. For structures that are subjected to many cycles of re-peated loading, it may be necessary to consider the additional damage that is induced in successive cycles of loading, and to estimate the residual life of the struc-ture. A decision on whether and when to repair should then be based on the adequacy of the residual life. The present study focuses on the residual strength ap-proach. In-Service Damage To decide which of the corrective measures should be taken the following input parameters are needed for the case of in-service damage: • Type of damage • Size/extent of damage • Location of damage in relation to the local structure (e.g. centre, edge or corner of a panel) • Location of damage in relation to the global struc-ture, i.e. where on the ship • Expected load or stress state at the damage location, in particular the utilisation level under the design loads, and frequency of loading •
Consequences of failure (including risk of progres-sive collapse) It is assumed that the type, extent and location of the damage have been established using inspection or moni-toring techniques. (However, there will always be un-certainty arising from the limitations in sensitivity or reliability of these techniques.) The assessment consists in establishing whether the stress levels expected at the damage location, taking account of appropriate safety factors, will be sufficient to cause failure of the damaged structure. If a criticality assessment is required for a specific ship or ship series, and extensive structural and load analysis models are available, the stress levels could in principle be deter-mined and criticality assessed for the exact damage type, size and location. However, in practice the as-sessment must normally be based on a series of prior analyses. Production Defects The principles are essentially as for in-service damage, but there are fewer corrective measures to choose be- tween. In general a higher proportion of defects will be repaired (or the components rejected) because the fin-ished product is usually expected to meet the full speci-fication; a reduced operational envelope or functionality will not normally be acceptable. Furthermore, repairs are generally easier to carry out during production. Assessment procedure: Principles The damage assessment procedure for cases where strength is the main consideration consists essentially of the following steps: 1. Estimate the strength reduction caused by the dam-age or defect. 2. Determine an allowable strength reduction based on the original design assumptions, operational enve-lope, etc. 3. Compare these. If the residual strength is smaller than the minimum allowable value, consider the possibilities for restricting the operational envelope and/or accepting an increased probability of failure until a repair can be conveniently effected. 4. If this does not help, carry out an emergency repair or take other emergency measures as necessary. In practice there are several possible ways of imple-menting this. Also, the way of estimating the strength reduction varies according to the type and extent of damage. The subsequent sections describe a possible scheme that lends itself to either manual or computer-aided implementation. It recognises three potential contexts in which the damage may have to be consid-ered in relation to the structure – the local, panel and global (ship) contexts, as illustrated in Fig. 1. Fig. 1: Damage illustrated in local, panel and global (ship) contexts. Damage levels It is convenient to pide damage into the following classes or levels, which determine the procedure to be followed for damage assessment. Level 1 damage: Small local damage Level 1 damage covers a small part of an inpidual panel so that its influence on the panel stiffness and on stresses at points on the panel some way from the dam-age can be neglected. In such cases, when considering uniform in-plane load-ing (e.g. in-plane compression) it is possible to find a value of the far-field stress or strain at which failure occurs, whose value is dependent on the damage size but not on the width or length of the panel. Examples are small impact damages. Small core/skin debonds may also lie in this category. Assessment of the influence of Level 1 damage on panel strength can be performed in three steps: 1. Determination of a local strength reduction factor Rl that quantifies the reduction in the far-field stress or strain at failure (for a given in-plane loading).
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