Damage assessment and damage tolerance of FRP

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The paper is based largely on experience from the recently completed saNDI Project “Inspection and Repair of Sandwich Structures in Naval Ships” [1,3,4] and ...
Damage assessment and damage tolerance of FRP sandwich structures Brian Hayman Section for Structural Integrity and Laboratories Det Norske Veritas AS Høvik, Norway (Proposed as keynote lecture) ABSTRACT A review is made of approaches to the assessment of production defects and in-service damage for sandwich structures having fibre reinforced polymer (FRP) face sheets. Particular attention is paid to the following aspects: • • •

The range of defect and damage types that may have to be considered. Relevant defect and damage models and the failure mechanisms these take account of. How these models can be used in an assessment of criticality with regard to local and global structure, and thereby provide a basis for deciding on corrective measures. The case of a naval ship is used as an illustration (Fig. 1) [1].

A review is made of information that can usefully be assembled during the design and production phases to enable defects and damage to be rapidly assessed. Challenges resulting from limitations in inspection techniques are discussed. The concept of damage tolerance is discussed in the light of the above and of developments that have been taking place in the aircraft industry. It is argued that the most suitable and economical approach to achieving damage tolerance is dependent on the application. In particular, a distinction is made between products that are produced in large series and those that are produced either one-off or in very short series. The differing challenges posed by relatively simple components and more complex structures are also addressed, as are differences associated with operational environment, and with ease of inspection and typical periods between inspections. More specifically, in the aerospace industry [2], damage tolerant design is generally achieved by a rather strict regime that ensures that components that have critical functions are either part of a redundant system or capable of withstanding one or more specified damage events, or both. This is supplemented by requirements that ensure that defects and damage that are not visible and not readily detected by other available inspection methods will not lead to immediate failure under extreme design loads or to unacceptable growth within the period between successive inspections. Such regimes are difficult or impossible (or too costly) to apply in their entirety to ships and many other structures, so alternative approaches are necessary. In the case of ships, recent research has resulted in a comprehensive set of defect and damage models, as described earlier, that can be used at the design stage to ensure damage tolerance at an

appropriate level. These models and associated methodologies can also compensate for less rigorous built-in damage tolerance by ensuring that, if damage does occur, it can be assessed rapidly and appropriate corrective actions taken so that overall safety is not compromised. The paper is based largely on experience from the recently completed saNDI Project “Inspection and Repair of Sandwich Structures in Naval Ships” [1,3,4] and the interaction between aerospace and maritime research environments this brought about.

Fig. 1 Scheme for damage assessment for a naval ship built in FRP sandwich SELECTED REFERENCES [1] [2] [3] [4]

Hayman, B. and Zenkert, D. The Influence of Defects and Damage on the Strength of FRP Sandwich Panels for Naval Ships, PRADS 2004, Travemünde, Germany, 2004. Tomblin, J. et al. Review of Damage Tolerance for Composite Sandwich Airframe Structures, Office of Aviation Research Report DOT/FAA/AR-99/49, 1999. Hayman, B. Inspection and Repair of Sandwich Structures Based on Damage Tolerance Principles, 6th Intnl. Conf. on Sandwich Construction, Fort Lauderdale, Florida, USA, 2003. Hayman, B. Defect and Damage Assessment for Ships Built in FRP Sandwich, RINA Conference on High Speed Craft, London, England, November 2004.