Fatigue crack growth rate at material and geometry transitions in glass-epoxy composites

S Goutianos,B.F Sørensen

doi:10.1016/j.compstruct.2021.114445

S Goutianos, B.F Sørensen

Open Access

https://doi.org/10.1016/j.compstruct.2021.114445

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Abstract

An element test specimen with ply drops, intended to be representative of a composite structure of varying thickness such as the main laminate in a wind turbine rotor blade structure, is used to investigate the fatigue damage initiating from a ply drop under cyclic tension–tension loading. The focus is to measure the growth rate of delamination cracks propagating from a thin towards a thicker section. Several delaminations initiate from tunneling cracks - cracks between the ply drops and resin reach areas - after very few load cycles. All except one delamination crack propagate for a number of cycles but eventually stop growing. The only delamination crack that continued to grow has the characteristic that its growth rate increases as it propagates to thicker sections of the element specimen. The experimental findings are supported by finite element results.

Highlights

The external shape of many large lightweight composite structures such as 3 wind turbine rotor blades is dictated by aerodynamic considerations
Fatigue crack initiation and growth from ply drops The damage initiation from a ply drop and the subsequent growth of cracks / delaminations with cycles will be first shown with a series of optical images
An element specimen with several ply drops can provide insight into the dam630 age evolution initiated from ply drops under cyclic loading

Summary

Introduction

The external shape of many large lightweight composite structures such as 3 wind turbine rotor blades is dictated by aerodynamic considerations. Crack initiation and delamination between the dropped and continuous ply has been experimentally observed and predicted by numerical models [1, 6, 9, 10, 15, 17] The formation of such a delamination probably cannot be suppressed by any conventional ply drop design. Understanding the sequence of damage initiation from a ply drop and the damage evolution is important in designing a composite structure of complex shape and forms the basis for defining an inspection/monitoring plan. This approach potentially leads to larger weight savings

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