Review
Review of z-pinned composite laminates

https://doi.org/10.1016/j.compositesa.2007.08.016Get rights and content

Abstract

This paper reviews published research into polymer composite laminates reinforced in the through-thickness direction with z-pins. Research into the manufacture, microstructure, delamination resistance, damage tolerance, joint strength and mechanical properties of z-pinned composites is described. Benefits of reinforcing composites with z-pins are assessed, including improvements to the delamination toughness, impact damage resistance, post-impact damage tolerance and through-thickness properties. Improvements to the failure strength of bonded and bearing joints due to z-pinning are also examined. The paper also reviews research into the adverse effects of z-pins on the in-plane mechanical properties, which includes reduced elastic modulus, strength and fatigue performance. Mechanisms responsible for the reduction to the in-plane properties are discussed, and techniques to minimise the adverse effect of z-pins are described. The benefits and drawbacks of z-pinning on the interlaminar toughness, damage tolerance and in-plane mechanical properties are compared against other common types of through-thickness reinforcement for composites, such as 3D weaving and stitching. Gaps in our understanding and unresolved research problems with z-pinned composites are identified to provide a road map for future research into these materials.

Introduction

Many novel techniques have been developed to reinforce polymer laminate composites in the through-thickness direction as a solution to the problems of poor impact damage tolerance, low through-thickness mechanical properties, and weak strength of bonded joints. The most common through-thickness reinforcement techniques are 3D weaving [1], [2], [3], stitching [2], [3], [4] and braiding [2], [3]. More specialist techniques include embroidery [3], tufting [5] and z-anchoring® [6]. These techniques are effective at increasing the delamination resistance and impact damage tolerance. 3D weaving and stitching are also effective at increasing the ultimate strength and damage tolerance of composite joints [7]. However, these techniques are only suitable for textile laminates made using a dry fabric preform which contains the through-thickness reinforcement prior to resin infusion. None of the techniques can be used for the through-thickness reinforcement of prepreg laminates. Attempts to reinforce uncured prepreg result in excessive fibre damage that degrades the in-plane mechanical properties. This is a serious limitation because many highly-loaded composite components, including many aircraft structures, are made using prepreg laminates.

The only technique capable of reinforcing prepreg laminates in the through-thickness direction in large commercial quantities is z-pinning. z-Pins1 act as fine nails that lock the laminate plies together by a combination of friction and adhesion (Fig. 1). Thin metal rods were first used to reinforce laminates in the 1970s, although the pins were inserted individually using a labour-intensive manual process that is not practical for large-scale production [8]. Tomashevkii and colleagues developed an automated process during the 1980s for inserting thin wire fibres through laminates [9], [10], [11]. At the same time, Aztex Inc. (Waltham, USA) developed the UAZ® (Ultrasonically Assisted Z-Fibre™) process for the rapid insertion of a large number of thin fibrous or metal pins [12], [13]. UAZ®is now the most common process for the z-pinning of laminates in large quantities. z-Pins are made using high stiffness, high strength material such as titanium alloy, steel or fibrous carbon composite with a diameter of 0.2–1.0 mm. Only a relatively small volume fraction of z-pins is needed to significantly enhance the through-thickness properties and damage tolerance. Pin contents typically range from 0.5 to 4.0 vol% (which is approximately equivalent to about 8–70 z-pins/cm2), and only rarely are higher pin contents necessary.

z-Pinning is an effective and simple method to increase the delamination resistance, impact damage tolerance and joint strength of prepreg laminates. Pins may be used for the wide-area reinforcement of damage tolerant panels or used in selective areas requiring local reinforcement, such as structural bonds, stiffener attachments, stress concentrations and holes. A variety of aerospace composite structures have been reinforced with z-pins to demonstrate the application of the technology, including joints and stiffened panels, however, to date the use of z-pinned composites in aircraft is limited. The only current aerospace application of z-pins is in the F/A-18E/FSuperhornet, which are used to replace titanium fasteners in the air inlet ducts and engine bay doors [14]. This provides a good cost saving (US$83,000) and modest weight reduction (17 kg) per aircraft [15]. Partridge et al. [15] estimate that the automated insertion of z-pins in an aircraft structure reduces the manufacturing cost by 70% compared to drilling and then installing metal fasteners. Furthermore, the time to insert z-pins compares favourably with the time needed to install metal fasteners into composite structures. Despite the projected cost savings, z-pins are not used in aircraft other than the Superhornet, although they are being considered for the Joint Strike Fighter. z-Pins are also used to reinforce the composite roll-over bars on Formula 1 racing cars, although this is the only non-aerospace application of z-pinning to be reported [16].

A review of recent progress in the development and characterisation of z-pinned composite laminates is presented in this paper. Most published works have examined the effect of z-pins on carbon/epoxy tape laminates because of their potential use in aerospace structures, although the research findings are generally applicable to any composite application. The manufacturing processes used to make z-pinned laminates are described together with changes to the microstructure of the laminate due to pinning. Following this, the benefits of z-pinning in terms of improved delamination resistance, impact damage tolerance, through-thickness stiffness, joint strength and bearing strength are outlined. Improvements to these properties due to z-pinning are compared against other through-thickness reinforcement techniques, such as 3D weaving and stitching. The detrimental effects of z-pinning on the in-plane mechanical properties are then described. Reductions to the elastic properties, strength and fatigue performance due to z-pinning are examined, and compared to changes to the in-plane properties of composite materials with other types of through-thickness reinforcement. The paper concludes with an assessment of the outstanding issues that must be resolved by research and development before the full potential of z-pinning in aerospace applications can be realised.

Section snippets

Manufacture of z-pinned composites

Various methods are used to manufacture z-pinned laminates, with the most common method being the UAZ® process that involves inserting z-pins into an uncured prepreg stack using an ultrasonic tool, as shown in Fig. 2 [13], [15], [17].

Fibre waviness, crimp and breakage

The microstructure of laminates is changed in several ways by z-pinning, which may have beneficial and adverse effects on the damage tolerance and mechanical properties. Understanding the microstructural changes is essential to understanding the property changes to laminates caused by z-pinning. One obvious change to the microstructure is fibre waviness near the z-pins, as shown in Fig. 4. The waviness occurs because the fibres, which are very thin compared to the z-pins, are forced aside

Interlaminar fracture toughness

An important benefit of z-pinning is improved delamination toughness that increases the impact resistance, damage tolerance and joint strength. Delamination toughness is without doubt the most studied property of z-pinned laminates [17], [20], [29], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48], [49], [50], [51], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63]. A large body of theoretical and experimental research has

Disadvantages of z-pinning

Most research into z-pinned laminates has focussed on the benefits, such as improved delamination toughness, impact damage tolerance and joint strength. The adverse effect of z-pinning has received less attention, particularly reductions to the in-plane mechanical properties. Several studies into the effect of z-pinning on the elastic modulus, strength and fatigue life of laminates have been performed, although much remains to be done.

Concluding remarks

This paper has presented an overview of published research and development work into z-pinned composite materials. Over the past 10 years there has been significant progress, with benefits such as improved delamination resistance, damage tolerance, through-thickness stiffness and joint strength being demonstrated. The detrimental effects of z-pinning on the in-plane mechanical properties, such as lower elastic modulus, strength and fatigue performance, have also been investigated. In general,

Acknowledgements

The author acknowledges the many valuable discussions with Dr Paul Chang and Dr Brian Cox and the Australian Research Council (Grant No. DP0211709) for funding support.

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