Keywords: fatigue, fibre-reinforced plastics, layer-based concept, energy-based concept, strength degradation, stiffness degradation, failure mechanisms, sequence effects, composites.

This paper presents a novel fatigue analysis procedure for fibre-reinforced polymer-composite laminates composed of unidirectional (UD) laminae. It consists mainly of a continuum mechanics approach on the macro scale which has been embedded in the commercial finite element code AbaqusR as a material routine for shell elements.

The present applied fatigue analysis procedure is based on the linear damage accumulation rule according to Miner [1] and does not take the multiphase and complex failure behaviour of fibre-reinforced plastics (FRP) into account, a proposal is made for a failure mode dependent analysis which makes use of Puck's failure mode concept [2]. The present concept combines Puck's concept with continuous stiffness degradations caused by fatigue loading and strength degradations. For general applicability the procedure is a layer-wise approach and overcomes therewith the need for experimentally testing the fatigue behaviour of each laminate lay-up.

In contrast to other published fatigue analysis concepts the present procedure couples strength and stiffness degradation and distinguishes between tensile and compressive loading. In order to determine the fatigue-related continuous degradation an energy-based hypothesis is used, which was originally applied to structures made of reinforced concrete [3]. As a result of the high number of load cycles, structures made of FRPs are usually exposed to, a cycle-by-cycle analysis is avoided and a block-wise approach or a cycle-jump strategy is preferred. The concept concentrates on shell elements since the structures to be investigated are generally shell-like and these elements are computationally efficient.

The current, intermediate state of the concept is already tested and shows promising results. By applying different loading sequences to an open-hole structure interesting sequence effects could be investigated.

1

M.A. Miner, "Cumulative damage in fatigue", Journal of Applied Mechanics, 67, A159-A164, 1945.

2

A. Puck, "Festigkeitsanalyse von Faser-Matrix-Laminaten - Modelle für die Praxis", Carl Hanser, München, 1996. also in M. Knops, "Analysis of Failure in Fiber Polymer Laminates: The Theory of Alfred Puck", Springer, Germany, 2008.

3

D. Pfanner, "Zur Degradation von Stahlbetonbauteilen unter Ermüdungsbeanspruchung" ("Degradation of Structures Made of Reinforced Concrete under Fatigue Loading"), PhD-Thesis, Ruhr-Universität Bochum, 2000.

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