On the monitoring and implications of growing damages caused by manufacturing defects in composite structures

• Published in: Journal of physics. Conference series Vol. 628; no. 1; pp. 12096 - 12103
• Main Authors: Schagerl, M, Viechtbauer, C, Hörrmann, S
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 09.07.2015
• Subjects: Aircraft design; Aircraft structures; Composite structures; Crack initiation; Crack propagation; Damage assessment; Damage detection; Damage tolerance; Fatigue failure; Fiber reinforced plastics; Fiber reinforced polymers; Fracture mechanics; Inspection; Laminar composites; Manufacturing; Manufacturing defects; Physics; Residual strength; Safety; Service life; Static loads; Structural damage; Structural strength
• DOI: 10.1088/1742-6596/628/1/012096
• ISSN: 1742-6588

Damage tolerance is a classical safety concept for the design of aircraft structures. Basically, this approach considers possible damages in the structure, predicts the damage growth under applied loading conditions and predicts the following decrease of the structural strength. As a fundamental result the damage tolerance approach yields the maximum inspection interval, which is the time a damage grows from a detectable to a critical level. The above formulation of the damage tolerance safety concept targets on metallic structures where the damage is typically a simple fatigue crack. Fiber-reinforced polymers show a much more complex damage behavior, such as delaminationsin laminated composites. Moreover, progressive damage in composites is often initiated by manufacturing defects. The complex manufacturing processes for composite structures almost certainly yield parts with defects, e.g. pores in the matrix or undulations of fibers. From such defects growing damages may start after a certain time of operation. The demand to simplify or even avoid the inspection of composite structures has therefore led to a comeback of the traditional safe-life safety concept. The aim of the so-called safe-life flaw tolerance concept is a structure that is capable of carrying the static loads during operation, despite significant damages and after a representative fatigue load spectrum. A structure with this property does not need to be inspected, respectively monitored at all during its service life. However, its load carrying capability is thereby not fully utilized. This article presents the possible refinement of the state-of-the-art safe-life flaw tolerance concept for composite structures towards a damage tolerance approach considering also the influence of manufacturing defects on damage initiation and growth. Based on fundamental physical relations and experimental observations the challenges when developing damage growth and residual strength curves are discussed.