Keywords: Z-pin, mixed mode, composites, cohesive law, T-joint, stitch, delamination, double cantilever beam, end notched flexure.

Z-pinning methods have increasingly been used for improving the delamination toughness of composite laminates. A considerable number of extensive numerical calculations have been carried out to analyse the failure of Z-pinned composite laminates. For this purpose, non-linear spring elements have mostly been utilised to analyse mixed mode delamination phenomena [1,2,3,4]. In this paper, a new methodology for modeling Z-pins is reported, in which a coupled cohesive zone model is developed to incorporate both the interlaminar failure and the failure of Z-pins themselves. Cohesive elements of different constitutive behaviour have been coupled into a single layer. The traditional bilinear cohesive law has been used to simulate delamination, and a new mixed mode cohesive law has been introduced to represent the failure response of Z-pins. The bridging response of the Z-pins was captured using a recently developed micro-mechanics model [5].

The double cantilever beam (DCB) test for measuring the mode I fracture toughness, and the end notched flexure (ENF) test for measuring the mode II fracture toughness, have been simulated for both unpinned and Z-pinned composite laminates. In addition, unpinned and Z-pinned composite T-joints have been tested under bending loading, and their failure modes are here discussed. The modelling strategy has additionally been used to analyse the failure of the aforementioned Z-pinned composite T-joints. A comparison between the numerical simulation and the experimental results has been carried out on the basis of an extended numerical model, able to capture the failure behaviour of composite T-joints.

This paper shows that continuum cohesive elements are suitable for the modelling Z-pinned composite laminates. The modelling methodology presented here is easy to carry out and flexible enough to account for different Z-pin configurations and laminate constructions. In addition, it is believed that other through-thickness-reinforcements can also be taken into account using the coupled cohesive zone model.

1

W. Yan, H.Y. Liu, Y.W. Mai, "Numerical study on the mode I delamination toughness of z-pinned laminates", Composites Science and Technology, 63, 1481-1493, 2003. doi:10.1016/S0266-3538(03)00167-2

2

W. Yan, H.Y. Liu, Y.W. Mai, "Mode II delamination toughness of z-pinned laminates", Composites Science and Technology, 64, 1937-1945, 2004. doi:10.1016/j.compscitech.2004.02.008

3

M. Grassi, B.N. Cox, X. Zhang, "Simulation of pin-reinforced single-lap composite joints", Composites Science and Technology, 66, 1623-1638, 2006. doi:10.1016/j.compscitech.2005.11.013

4

G. Allegri, X. Zhang, "On the delamination and debond suppression in structural joints by Z-fibre pinning", Composites: Part A, 38, 1107-1115, 2007. doi:10.1016/j.compositesa.2006.06.013

5

H. Cui, Y.L. Li, S. Koussios, A. Beukers, "Bridging micromechanics of Z-pin in mixed mode delamination", Composite Structures, 93, 2685-2695, 2011. doi:10.1016/j.compstruct.2011.06.004

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