Design limits for composites tend to be much lower than their capability due to poor impact strength . Improving toughness will reduce the need for excessive safety factors and thus enable a great reduction in weight for high performance composite products. For this reason there is a large body of literature focusing on methods to improve composite toughness. The aim of this literature research is to find the relative effectiveness of all the available toughening methods and to determine which methods are most appropriate for each manufacturing process. Further aims include finding novel production processes and fibre/matrix combinations that yield good strength, stiffness and toughness. This knowledge will inform later design decisions. 108 papers were categorised, of which 95 have been summarised. Figure 16 shows a diagram of the category breakdown with the number of papers in each area. The literature review is summarised below.
Figure 15: A summary of research categories relevant for composite materials in rock climbing.
The main toughening methods are based on using different forms and combinations of fibres. The methods are: stitching, weaving, knitting, z-pinning, hybridising, outer protective layers and interlayers. Z-pinning, knitting and 3D-weaving appear to offer the very promising benefits - with z-pinning showing improvements in mode I and II toughness of up to an order of magnitude . In addition, damage area is reduced and compression after impact strength is increased . Z-pinning is where narrow composite pin-reinforcements are inserted perpendicular to the composite surface - giving the structure greater out-of-plane strength (figure 17).
Figure 17: Z-pins visible after pullout .
3D woven laminates are more compliant than unidirectional tape based laminates; this means that more of the impact energy is absorbed by the response of the structure rather than through delamination . In addition, the weave pattern helps to reduce the propagation of shear and delamination cracking. Knitted reinforcement (figure 18) has been compared with woven reinforcement - it was found that, after impact, the knitted laminates adsorbed more energy (and sustained more damage) but their residual tensile properties remained higher .
Figure 18: Various knitting patterns. Knitted composites maintain greater tensile properties after impact compared to their woven counterparts .
The toughness of the matrix material has been shown to have a very strong influence on the toughness of the composite [37, 38]. Using a tougher matrix increases the damage initiation load by increasing G IIC .
Using a toughening agent, for example rubber particle toughening, would normally improve toughness at the expense of stiffness. Adding alumina nanoparticles to an epoxy resin can improve both toughness and stiffness . Adding 1-2% alumina particles was shown to introduce new energy dissipating mechanisms - shear yielding of the matrix, particle pullout and crack pinning. Adding calcium silicate microparticles into the nanoparticle composite improved wear resistance by a factor of three, the impact energy stayed above that for the neat matrix but the strain to break decreased significantly.
Combining different types of fibre in a composite is called hybridization. The aim of hybridization is generally to combine two types of fibres, with very different properties, to attain the benefits of both. For example, a high modulus fibre might be used in combination with tougher fibres to give a composite both stiffness and impact resistance. It has been shown that, in laminated composite, alternating layers of glass and carbon fibre reduces notch sensitivity compared to pure glass or pure carbon fibre laminates . The effect is amplified if the layers are ordered such that carbon fibre is on the outside. Furthermore, this layering improves the compressive strength after impact as the damage area and crack lengths are reduced.
For woven hybrid composites it has been shown that interply are tougher than intraply hybrids in low velocity impact . In an interply hybrid each layer containing only one fibre type - but fibre types alternate in successive layers. In an intraply hybrid each layer contains a mixture of both fibre types. The interply hybrid showed a 9-67% higher specific energy adsorption, a 5-45% lower peak load and an 8-220% higher ductile index.
Hybridization in random oriented short fibre reinforced composites has been shown to significantly improve tensile strength and modulus, flexural strength and modulus and marginally improve toughness . Thermotropic liquid crystalline polymer (TLCP) fibres were added to a carbon fibre reinforced PEEK matrix (figure 19), the TLCP fibres were around an order of magnitude smaller than the carbon fibres (0.3-1.5 µm compared to 7 µm). The TLCP fibres also reduced the melt viscosity - enabling easier processing.
Figure 19: Adding smaller TLCP fibres to a short fibre reinforced matrix improves mechanical and processing properties .
Short shaped copper fibres have been shown to improve toughness . Shaping the fibres affects the fibre/matrix interface and, when combined with a tough matrix, shaped fibres perform better than their straight equivalents. Another toughening method, which makes use of interlaminar fibres, has been shown to reduce damage area and severity [43, 44]. Short fibres are sprinkled in between layers before curing; this increases the out of plane strength.
The volume fraction of fibres is a major determining factor for composite mechanical properties. For injection moulded polypropylene, with long discontinuous glass fibre reinforcement, there is an optimum percentage of reinforcement for maximising strength and toughness . Stiffness was found to increase linearly with fibre content up to the maximum possible of 73% weight; whilst the maximum content for strength and toughness was found to be 40-50% weight.
Consolidation temperature and cooling rates have been shown to affect toughness [46-48, 49]. Rapid cooling increases matrix toughness but reduces tensile strength due to reduced crystallinity - a compromise must be met to achieve sufficient strength whilst optimising toughness. Similarly, a lower consolidation temperature increases matrix ductility - improving composite toughness.
The properties of the interphase (the region of resin close to the reinforcement fibres) can be very different to those of the bulk resin . The interphase plays a crucial role in determining the overall strength and toughness of a composite. Its properties can be tailored to optimise strength and toughness by using appropriate sizings (surface treatment on the glass fibres) to affect the fibre/matrix bond.