Effective use of metallic Z-pins for composites' through-thickness reinforcement

https://doi.org/10.1016/j.compscitech.2019.02.024Get rights and content

Abstract

Z-pins offer effective through-thickness reinforcement for laminated composites. Various studies have however, shown that metal Z-pins are less effective at bridging Mode I delaminations than carbon-fibre composite Z-pins, due to poor interfacial bonding with the laminate. This is exacerbated by high thermal mismatch between the metallic Z-pins and the laminate. This study investigates inserting metallic Z-pins at angles offset from the laminate normal, to improve the Mode I bridging in composites. The effects on the apparent fracture toughness under pure and mixed Mode I/II loads using single pin specimens is investigated. Results show that, unlike orthogonally inserted metal Z-pins, inclined Z-pins exhibit high energy absorption throughout the mixed mode range. Double Cantilever Beam (DCB) tests show that the inclined metal Z-pins increase the Mode I apparent fracture toughness by a factor of 2 compared to traditional carbon fibre Z-pins. In End Loaded Split (ELS) tests, the Mode II apparent fracture toughness of inclined stainless steel Z-pins, although less than their uninclined equivalent, is greater than that of carbon fibre Z-pins.

Introduction

Z-pins are small rods inserted in the through the thickness of a laminate. They are used to improve the through-thickness properties of laminated composites. Z-pins most commonly take the form of thin pre-cured carbon fibre rods, inserted into an uncured pre-preg laminate. Metallic Z-pins are the most commonly used Z-pin type after composite Z-pins [[1], [2], [3], [4], [5], [6]]. They differ from composite Z-pins because of they are able to absorb large amounts of fracture energy via plastic deformation. In addition, metallic Z-pins can also possess additional properties, such as electromagnetism, which are potentially attractive for multifunctional laminates [7,8]. However, the use of metal Z-pins is hindered by their high specific density, high thermal expansion coefficients, and depending on the electro-potential value of the metal, high probability of galvanic corrosion with the carbon fibres in the laminate [9].

Metal Z-pins tend to have poor interfacial properties with composite materials, therefore, surface treatments are necessary to ensure high fracture toughness under Mode I loading [[1], [2], [3]]. In the literature, aggressive surface treatment techniques such as targeted forging on the Z-pin surface and aqua regia solutions have been investigated to increase the bond strength of metal Z-pins with the laminate [1,2]. Organosilane-coupling agents have also been shown to be effective at improving Mode I delaminations through the formation of polar bonds [3]. The same study [3] also demonstrated the importance of selecting Z-pin materials that can withstand the pull-out process, and more critically it highlighted that the use of surface treatments on metallic Z-pins, if poorly tailored with the laminate, may lead to low energy failure mechanisms.

Previous studies have shown that the use of inclined Z-pins is an effective means of increasing energy absorption of Z-pinned composites [4,10,11]. The benefits of inclined metallic reinforcements, particularly for pull-out applications, are well known in civil engineering applications where stainless steel rods are inserted in cement [12]. One of the earliest works on inclined metallic reinforcements was conducted at University of Oxford in 1974 [13]. A key finding from this study was that the benefit of increasing the peak load, and consequently the energy absorption of inclined metallic reinforcements loaded in Mode I, culminates at 45° [13]. The effects of high insertion angles of metal Z-pins under shear dominated loads are not well established beyond the study carried out by Cartie et al. [4]. In general, Z-pins loaded with the nap failed via pull-out while those loaded against the nap fail via rupture for insertion angles greater than 15°. It is possible that the rupture failure observed could be a result of strong interfacial properties of titanium Z-pins with the laminate (average frictional stress was 80% of the T300/BMI Z-pins). Cartie et al. [4] did not establish a clear pattern with respect to energy absorption under Mode I/II loads from their data set.

This study describes how the energy absorption of metallic Z-pins can be significantly increased without using any surface treatments. The Z-pins are inserted in composite laminates and tested under mixed-Mode I/II loads. The effect of insertion angle on the bridging performance is characterised on a sub-coupon level using single pin specimens and at coupon level using Double Cantilever Beam (DCB) and End Loaded Split (ELS) specimens. Finally, key highlights and recommendations are made, thereby completing the knowledge gap on the mixed mode response of inclined metallic Z-pins.

Section snippets

Materials and methods

The single pin specimens were manufactured from a quasi-isotropic laminate with a [0,+45,90,-45]4s layup in the top half and a [90,-45, 0,+45]4s layup in the bottom half. The laminate was made from carbon/epoxy IM7/8552 pre-preg tape, supplied by Hexcel. A layer of PTFE release film was inserted at the mid-plane of the laminate to create an artificial delamination. The specimens are designed to have a 90° ply angle mismatch at the mid-plane to avoid any nesting of the plies. The test specimen

Bridging behaviour of inclined metallic Z-pins

The Mode I/II load-displacement and energy absorption data for the single pin specimens are shown in Fig. 5 and Fig. 6. All the specimens tested failed via pull-out for all the mixed Mode load angles tested in this study. In Mode I, the maximum load during pull-out for the 0 degree stainless steel Z-pins is significantly lower than that of traditional carbon fibre Z-pins [14]. This is caused by poor interfacial properties between metallic Z-pins and the laminate, which results in a relatively

Conclusions

In this study, the fracture toughness of inclined metal Z-pins has been analysed using single pin, DCB and ELS specimens. Under Mode I loads, inclined stainless steel Z-pins in a ±θ degree configuration exhibit high energy absorption at low and high crack lengths due to a combination of enhanced friction due to snubbing and plastic deformation of the pins. Even at the low areal density analysed in this study, significantly high loads are required for Mode I delaminations to propagate through a

Acknowledgements

The authors would like to acknowledge Rolls-Royce plc and the Engineering and Physical Sciences Research Council (EPSRC) for their support of this research through the Composites University Technology Centre at the University of Bristol (UK) and Grant No. EP/G036772/1 respectively. Data from this study is not available, due to commercial sensitivity.

References (22)

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    For Mode II delamination resistance, plenty of studies were performed to explore the advantages of Z-pin by quasi-stati,9,14–19 fatigue10,11,20,21 and dynamic tests.11,12,22 M’Membe et al.9,15 reported that the inclined Z-pin can improve the axial bearing capacity of the laminate compared to the vertically inserted, i.e., 0° Z-pin. And as the insertion angle increases, snubbing effect23 becomes more obvious.

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Present address: Cranfield University, Cranfield MK43 0AL, United Kingdom.

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