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Modeling the Effect of Damage in Composite Structures: Simplified Approaches by Christos Kassapoglou

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Chapter 6Fatigue Life of Composite Structures: Analytical Models

6.1 Introduction

Under repeated loading, even if the load magnitudes are significantly lower than the corresponding static strength of the structure, the stiffness and the strength degrade. After a sufficient number of cycles, the structure fails. Fatigue is a process via which the load-carrying ability of a composite structure diminishes as a function of cyclic loading. This happens because some type of damage is created and grows reducing both the stiffness and the strength. Damage creation and evolution in composites occur at different scales starting, for example, from very low scales where tiny voids coalesce into microcracks or matrix crazing occurs. As cycling continues, damage grows or triggers other types of damage, for example, matrix cracks may transition to delaminations. At some point, the extent and the type of damage are such that the structure cannot sustain any further cyclic loading and failure occurs. Usually, the final failure is associated with fibre breakage but is not uncommon to have structure failing because the number and the size of delaminations have reduced the bending stiffness to a point that the deflections are excessive and the structure can no longer perform its function.

Because damage may start at very low-length scales, in the order of a fibre diameter or less, it may go undetected for a long time. Only after it has evolved or grown to a damage type and size measurable by the ...

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