Composites Part A: Applied Science and Manufacturing
ReviewReview of z-pinned composite laminates
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.
References (95)
- et al.
Review of applications of advanced three-dimensional fibre textile composites
Composites A
(1999) - et al.
Improving the delamination resistance of CFRP by stitching – a review
Compos Sci Technol
(1994) - et al.
Properties and failure mechanisms of z-pinned laminates in monotonic and cyclic tension
Composites
(2006) - et al.
In-plane properties of composite laminates with through-thickness pin reinforcement
Int J Solids Struct
(2006) - et al.
Mechanisms of compressive failure in 3D composites
Acta Metallurgica et Materialia
(1992) - et al.
The potential of knitting for engineering composites – a review
Composites
(2000) The mechanics of z-fibre reinforcement
Compos Struct
(1996)- et al.
The effect of thermal mismatch on z-pinned laminated composite structures
Compos Struct
(2004) - et al.
Mechanisms of crack bridging by composite and metallic rods
Composites
(2004) - et al.
Experimental study on z-pin bridging law by pullout test
Compos Sci Technol
(2004)
Mixed mode delamination of polymer composite laminates reinforced through the thickness by z-fibres
Composites
Finite element analyses of mode I interlaminar delamination in z-fibre reinforced composite laminates
Compos Sci Technol
Numerical study of the mode I delamination toughness of z-pinned laminates
Compos Sci Technol
Mode II delamination toughness of z-pinned laminates
Compos Sci Technol
Damage mechanisms for angled through-thickness rod reinforcement in carbon-epoxy laminates
Composites A
Concepts of bridged mode II delamination cracks
J Phys Mech Solids
Bending effect of through-thickness reinforcement rods on mode I delamination toughness of DCB specimen. I. Linearly elastic and rigid-perfectly plastic models
Int J Solids Struct
Mode I DCB testing of composite laminates reinforced with z-direction pins: a simple model for the investigation of data reduction strategies
Eng Fract Mech
The estimate of the effect of z-pins on the strain energy release rate, fracture and fatigue in a composite co-cured z-pinned double cantilever beam
Compos Struct
Effectiveness of z-pins in preventing delamination of co-cured composite joints on the example of a double cantilever test
Composite
Cohesive modelling of delamination in z-pinned reinforced composites
Compos Sci Technol
On the effect of stitching on mode I delamination toughness of laminated composites
Compos Sci Technol
Further validation of the Jain and Mai models for interlaminar fracture of stitched composites
Compos Sci Technol
Prediction of stiffness and stresses in z-fibre reinforced composite laminates
Composites A
Tensile properties and failure mechanisms of z-pinned composite lap joints
Compos Sci Technol
Simulation of pin-reinforced single-lap composite joints
Compos Sci Technol
3D reinforcement of stiffener-to-skin T-joints by z-pinning and tufting
Eng Fract Mech
Failure of transversely stitched RTM lap joints
Compos Sci Technol
Improvement of bearing strength of laminated composites
Compos Struct
Microbuckle initiation in fibre composites: a finite element study
J Mech Phys Solids
A mechanistic approach to the properties of stitched laminates
Composites A
Compression–compression fatigue in 3D woven composites
Acta Metallurgica et Materialia
Three-dimensional fabrics for composites
3D Fibre Reinforced Polymer Composites
Analysis and Design of Structural Bonded Joints
Automated method of transverse reinforcement of composites by short fibres
Mechanika Kompozitnykh Materialov
A method of calculating technological regimes of transversal reinforcement of composites with short-fibre microparticles
Mekhanika Kompozitnykh Materialov
Transversal reinforcement of composite materials using ultrasonic vibrations
Mekhanika Kompozitnykh Materialov
Ultrasonic fastening system and method
Patent WO 98/29243
z-Pins strengthen the Super Hornet, save weight and cost
The Integrator
Manufacture and performance of z-pinned composites
Safety pins
Racecar Engineering
Fibre insertion process for improved damage tolerance in aircraft laminates
J Adv Mater
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