Delamination suppression in tapered unidirectional laminates with multiple ply drops using a tape scarfing technique

Tharan Gordon,Xiaodong Xu,Luiz Kawashita,Michael R Wisnom,Stephen R Hallett,Byung Chul Kim

doi:10.1016/j.compositesa.2021.106627

Tharan Gordon, Xiaodong Xu + Show 4 more

Open Access

https://doi.org/10.1016/j.compositesa.2021.106627

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Abstract

Composite laminate thickness tapering is essential for weight efficient structures. This is achieved by terminating specific plies, but these can in turn act as sites for delamination initiation. One area of importance is the spacing between adjacent ply drops, which can have a significant impact on the delamination stress. In this work, a novel tape scarfing method that applies a tapered profile to dropped ply ends was used on unidirectional tapered laminate specimens with multiple ply drops. The effect of ply drop spacing on the delamination stress of both conventional and scarfed plies in tension was studied experimentally. The results showed that the scarfed ply drop configurations can completely suppress delamination in certain configurations. The scarfed ply drops also retained higher performance with smaller ply drop spacings. This indicates that in using scarfed plies, conventional laminate design rules could be relaxed and the weight of the structure could be further reduced. The underlying failure mechanisms were also investigated using both analytical and numerical methods. A simple stress-based formula for estimating the delamination stress of scarfed plies was introduced and shown to be consistent with both experimental and numerical results, which could be used (and further developed) to make tapered laminate design easier.

Highlights

Within large aircraft laminated composite material components, there are likely to be a multitude of ply drops present at a variety of distances from one another, to achieve a weight efficient structural performance
All the conventional specimens failed by delamination
Along with the shear yield stress of the composite, τy (82.5 MPa [11]), the critical distance of conventional ply drop interaction, Sc, is estimated to be approximately 3.4 mm in the current samples. This value is consistent with the experimental observations: it is below the 5 mm spacing, which saw the absence of ply drop interaction and had approximately the same delamination onset stress as that of a specimen with a single ply drop (2238 versus 2376 MPa [8]), and above the 2 mm spacing which saw a performance knockdown to 1491 MPa

Summary

Introduction

Within large aircraft laminated composite material components, there are likely to be a multitude of ply drops present at a variety of distances from one another, to achieve a weight efficient structural performance. Effort has been made previ ously to characterise the relationship between successive ply drop-off spacing or staggering distance (Δply-drop in Fig. 1) and laminate delam ination behaviour. This parameter is of interest to understand for a number of reasons: it has a bearing on the required accuracy of the manufacturing method employed for a certifiable strength of component and it may impose a design constraint on how quickly a thick pad up region for example, can be tapered down to a thinner one.

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Conclusion