Damage tolerance of repaired composite sandwich structures

Abstract

Advanced polymer composite sandwich structures are increasingly being used in a variety of primary, secondary and tertiary structures in all forms of engineering endeavour. Whilst the key benefits of composite materials are widely recognised, high strength-to-weight and stiffness-to-weight ratios, their limited industrial use stems from their poor delamination resistance when subjected to impact damage. Any form of delamination damage can cause severe reductions in stiffness and strength, and may lead to catastrophic failure of the structure. To overcome this perceived limitation various repair designs have evolved to ensure that the stiffness and strength of the original structure is restored in an economically viable way. These methods, often industry and component specific, have started to overcome the designer's fear of applying composite materials to primary engineering structures. The work presented here describes investigations into the damage tolerance performance of repairs contained within an advanced polymer composite sandwich structures. The composite materials considered were representative of the materials used by the Royal National Lifeboat Institution (RNLI) in the manufacture of their high performance rescue craft. The impact damage tolerance of a standard 3-degree tapered scarf repair design, commonly employed in the marine and aerospace communities, was considered within this project. A design methodology, incorporating a Fracture Mechanics Assessment, Strength Based Assessment and a Repair Structural Integrity Assessment has been developed to assess the damage tolerance performance of the repaired structure. Through the application of a failure mode, effect and criticality analysis (FMECA), critical defects at critical locations have been identified for further evaluation. A detailed approach for the damage tolerance assessment of damage tapered scarf repairs is proposed through the application of flowcharts and damage tolerance tables. This detailed route map helps the end user to make an informed decision about the criticality of the defect, i.e. whether the defect is benign and hence can be considered damage tolerant.

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