Delamination Arrest by Fasteners in Aircraft Structures Under Static and Fatigue Loading

Abstract

Laminated composite structures convey tremendous benefits of specific strength and fatigue properties in plane, but are highly susceptible to interlaminar failures such as delaminations and disbonds. The initiation of this failure mode further complicates the design as delaminations initiate along termination points of bonded interfaces, such as a skin-stringer, or are often driven by discrete damage events. As a result, the service life cannot be predicted in the same manner as for metallic aircraft where fatigue analysis is used to substantiate the damage tolerance of the aircraft. In order to provide a substantiation for the FAA damage tolerance requirement of composite bonded structures (FAR23.573), fasteners are subsequently installed. The delamination arrest by fasteners was studied in depth to create a detailed understanding of the process such that it could be accurately predicted under varying load conditions. The fastener itself provides crack arrest capability initially through mode I suppression, and subsequently through fastener joint shear stiffness and frictional load transfer. However, there is also important interplay between these three fastener parameters, the laminate fracture toughness, fatigue properties and the expected loading conditions which ultimately determines the fastener’s effectiveness in delamination arrestment. Tests were conducted utilizing multiple fasteners to understand the capability of fasteners to arrest delaminations generated by in plane loading both under static and fatigue loading, i.e. mode II propagation. The test results show that the fastener is capable of crack arrest and retardation, but is highly sensitive to inclusion of clearance in the system. Testing, in conjunction with analysis, also demonstrated that the load transfer through friction, which is generated by fastener installation torque, is a critical parameter in the arrestment of fatigue delaminations and plays a dominant role under a number of fatigue loading conditions. In conjunction with the testing and finite element analysis in Abaqus FEA, a high efficiency FEA based tool was developed in MATLAB which allowed for the more direct manipulation of the input parameters in order to best understand the influence of each. Comparisons between the predictive tool and experimental results showed agreement which indicated the ability of a highly simplified, one dimensional, model to capture the relatively complex fatigue delamination process, provided care was taken in establishing the input parameters.