Repairs to Damage Tolerant Aircraft

Abstract

The results of displacement compatibility analysis, representing a variety of repair doubler and lap splice configurations, are presented with a view to illustrating how structural repairs can degrade the fatigue initiation life and damage tolerance capability of primary transport aircraft structure. Examples show that fatigue initiation life is directly related to the peak loads induced in the first fastener rows at the edges of repair doublers. Design of repairs to an equal or better static strength capability and the associated static strength analysis will not normally highlight these peak loads which can result in considerable degradation of structural fatigue life. Critical fastener loads, based on displacement compatibility analysis accounting for fastener flexibility, are parametrically presented for a variety of skin and doubler thicknesses. Suggestions are made on how repair designs can be modified to improve fatigue initiation life and subsequent fatigue crack detectability particularly in the event of multiple-site damage (MSD). The importance of riveting quality during repairs, often not up to initial manufacturing standards, is discussed with respect to fatigue initiation life. A simplified but conservative method to generate crack growth curves is discussed with a view to easing the analytical burden for the small modifiers. It is hoped that this information, together with conservative fatigue Sn data, will help the many small repair and modification stations gain an appreciation of the fatigue and damage tolerance quality of structural repairs. It is pointed out in the paper that the FAA regulations were amended in December, 1978 for new transport category aircraft and in May, 1981 for aging transport aircraft to include a damage tolerance philosophy. This means that any repair to a transport category airframe, which may effect threshold, frequency and type of inspection of principal structural elements, must be evaluated for it’s damage tolerance capability.