Reliability of Damage Tolerance Composite Structure Using Fasteners as Disbond Arrest Mechanism

A FEA model for analyzing effectiveness of fastener as crack arrest mechanism has been built. The effect of the fastener in the sliding direction (Mode II) is modeled using fastener flexibility approach. It was shown that the fastener provide significant crack retardation capability in all load combinations considered (axial tensile and opening moment). The development of analytical techniques in this area is important to the design of bonded/cocured/co-bonded composite aircraft structures. A procedure for assessing the reliability of the fastener arrest mechanism has been demonstrated. A probabilistic approach is used because traditional damage tolerance methods for metallic structures are not completely applicable to composite structures. Probabilistic method can provide a more quantitative evaluation of reliability and safety of a structure or feature. I. Introduction HE use of composites in aircraft has enabled the use of bonded (or co-cured, co-bonded) structures, the main advantages of which are reduction of part counts and weight. The critical damage mode in this type of structure is disbond due to impact damage. Complete disbonding of components (e.g. skin-stringer) can cause failure at the structural level even though the individual components remain intact. Therefore, any bonded structures must demonstrate fail-safety by providing adequate disbond arrest capability to ensure safety. In addition, due to the probabilistic nature of impact damage occurrence, traditional damage tolerance methodology does not apply and thus is unable to provide meaningful reliability estimate for the structure. Damage tolerance methodology for metallic structures is based on assumed defect or crack which grows at a known rate, with proper inspection and maintenance schemes to detect and repair the damage before it becomes critical. However, impact damage and matrix crack growth are practically discrete events, rendering traditional methods ineffective. The probabilistic approach to damage tolerance [1-3] was developed to address this issue by taking into account the life-cycle discrete events of damage occurrence, peak load occurrence, inspection and repair. This paper will apply the probabilistic approach to damage tolerance to a practical and realistic problem in composite structures: disbond arrestment in fuselage skin-stringer by fasteners. The effectiveness of fasteners as disbond arrest mechanism, a relatively understudied area, will be analyzed using deterministic means. The reliability of the structural component, fuselage skin-stringer assembly, will be evaluated using probabilistic means. The goal of this paper is to demonstrate the analysis procedure needed to evaluate the reliability of a damage tolerant composite structure. II. Fastener Effectiveness as Disbond Arrest Mechanism Bonded structures (co-cured, co-bonded or bonded) are common in composite structures, especially thin structures, due to numerous advantages including weight, part count and assembly costs. The bond alone, which is the primary load path, seldom possesses necessary geometric or mechanical arrest capability. This can be a difficult problem when designing the structure to be damage tolerant. In aircraft structures, it is common to use fasteners on geometrically complex locations (e.g. fuselage skin-frame shear tie). These fasteners are co-located with the skinstringer bond, and thus also perform as disbond arrest mechanism, without the added cost and complexity alternatives such as z-pin and z-stitching. Disbond in Mode I is well understood [4] and typically less problematic because any mechanism arrest feature would be so effective that the laminates will fail in other modes first, e.g.