P.J. Hogg

The role of impact damage in post-impact compression testing

Abstract A ‘miniaturized’ post-impact compression strength ( pics ) test has been used to measure the properties of thermosetting (toughened epoxy) and thermoplastic (polyetheretherketone ( peek )) matrix carbon fibre-reinforced laminates. For low impact energies the superior performance of the peek matrix material is confirmed. In addition a correlation between damage width and pics has been found for both materials tested. When compared on this basis it is apparent that the two materials perform in a very similar manner during the compression part of the test. Following from this we conclude that the superior performance of the peek matrix material is almost entirely due to its resistance to the initiation and propagation of damage during impact.

The role of reinforcement architecture on impact damage mechanisms and post-impact compression behaviour

This review considers the link between the damage tolerance of composite laminates and the nature and organization of the fibre reinforcement. This embraces composites made from unidirectional prepregs through composites based on a variety of textile forms such as woven fabrics, multiaxial fabrics, braids and knits. The objective has been firstly to detail how the differing varieties of composite exhibit different properties under impact conditions and under subsequent loading after impact. This includes both fracture mechanisms and data such as energy absorption, and peak failure loads. The second objective is to describe the links that have been found between these properties and the specific fibre architectures and damage development processes in the various composite forms. The post impact compression properties are highlighted as this is the area of greatest interest by end-users. The review describes the different forms of textiles that are used for composite reinforcement, considers different impact conditions (e.g. low velocity and ballistics), general materials variables such as fibre and resin type, and ultimately looks at specific textile systems. Some consideration is also given to the value and role of numerical modelling in the field of damage formation and damage tolerance. Clear differences have been found in the literature between composites based on different textile forms in terms of damage states after impact and the consequences of this damage on subsequent properties. While the literature is clearly incomplete at this time there is sufficient information available to indicate that control of fibre organization by the use of textiles may be an effective method of optimizing composite properties for specific end use properties.

Mechanical characterisation of glass- and carbon-fibre-reinforced composites made with non-crimp fabrics

Abstract The labour-intensive requirements of manufacturing unidirectional prepreg tape laminates is driving industry to examine alternative processing routes that combine all the attributes of unidirectional prepreg tapes, but with reduced cost. One such technique that offers automated high-volume production with the potential for near-net-shape manufacturing is the textile process. The genre of particular interest in this investigation was the warp-knit (non-crimp) fabric. The ‘elastic’ macro-mechanical behaviour of the unidirectional prepreg tapes and non-crimp fabrics were predicted satisfactorily by using classical thin laminate theory. Mechanisms of failure in the non-crimp fabrics may generally be linked to crimp in the tows, but with subtle differences driving failure in tension and compression.

Composites in armor

Composite materials are traditionally regarded as materials that can save energy in large structures associated with transport. They are used to produce lightweight structures for fuel-efficient aircraft such as the new Boeing 787 Dreamliner; lightweight cars from Lotus, Ferrari and TVR; and high-speed trains, speedboats, and racing yachts. Now, however, some of the most interesting applications of composites are those where the materials are used to save lives and protect property by absorbing the energy of projectiles, impacts, and crashes.

Penetration impact resistance of hybrid composites based on commingled yarn fabrics

Abstract The penetration impact resistance of hybrid composites based on commingled yarn fabrics was investigated. The commingled yarn fabrics were composed of E-glass fibres (GF) and thermoplastic fibres blended together within the fibre bundles. Various thermoplastic fibres such as polypropylene (PP), polyamide (PA) and modified polyethylene terephthalate (mPET) were studied. Various resin matrices with different cure cycles were studied such as Quickcure polyester, Cycom X823 RTM epoxy, and Shell Epikote 828 epoxy resin. Depending on the crystalline melting temperature (Tm) of the thermoplastic fibres, the hybrid composites can be categorised as fibre-hybrid composites or matrix-hybrid composites. Fibre-hybrid composites refer to those in which the thermoplastic fibres remain in the fibre form after curing, for example the GF–PP and GF–PA hybrid composites. For matrix-hybrid composites, the thermoplastic fibres melt and dissolve into the thermosetting matrix during curing such as the GF-mPET hybrid composites. The results from the penetration impact showed that the total absorbed energy of the fibre-hybrid composites were significantly higher than for the plain glass composites. Plastic deformation in the thermoplastic fibres is the key factor that improves the absorbed energy of the hybrid composites. When the thermoplastic fibres dissolved into the thermosetting matrix as in matrix-hybrid composites, the total absorbed energy was similar to that of the plain glass fibre composites. This suggests that the total absorbed energy is dependent on the properties of the fibres rather than the matrix. However, the fibre-hybrid composites showed slight differences in the total absorbed energy for different matrices. The differences are thought to be related to the differences in the bonding between the thermoplastic fibres and the thermosetting matrix which have yet to be investigated.

Carbon-fibre non-crimp fabric laminates for cost-effective damage-tolerant structures

The emphasis today for the aerospace customer is no longer only on the primary goal of minimum weight, and hence performance, but principally on the cost of ownership. Textile manufactured composites, particularly of the warp-knit or non-crimp fabric type offer significant cost savings in terms of reduced labour time and higher deposition rates over the unidirectional prepreg tape. However attractive the bottom line on a balance sheet, the financial issues must be balanced against critical design considerations. A significant test for the non-crimp fabrics is how well they compare with the unidirectional prepreg tape in terms of the current aerospace design allowable limiting test, compression after impact.

High-temperature damage tolerance of carbon fibre-reinforced plastics: Part 1: Impact characteristics

Abstract The impact performance of a number of thermoplastic- and thermosetting-matrix carbon fibre composites is examined at room temperature, 80°C and 150°C. Tests were performed to examine the influence of matrix type and morphology on the ability of the composite to withstand penetration, absorb energy and sustain damage. A distinction is made between impacts at high energy where full penetration of the specimen takes place by the indentor, and low-energy tests where damage is introduced but the plate is not ruptured. The influence of test temperature on both types of impact event is examined for a restricted set of specimens consisting of an epoxy-matrix laminate, Fibredux 924C, and a polyaryl sulfone-matrix thermoplastic laminate, Radel 8320. Test temperature is shown to have little influence on through-penetration impact results, although high-temperature testing does increase the spread of delaminations in epoxy laminates subjected to low-energy impact tests.

A model for the reduction in compression strength of continuous fibre composites after impact damage

Abstract The post-impact compression behaviour of three different laminates has been measured experimentally. It was observed that normalized values of post-impact compression strength of dissimilar laminates were similar when considered as a function of damage width. This observation has been investigated using a finite element model. The model is based on the hypothesis that failure in post-impact compression occurs via the stress magnification arising from the presence of the damaged zone. The implied values of the modulus of the damaged zone are deduced. These values are found to be dependent on the damage width. The model provides an explanation for the failure in compression of impacted plates that does not invoke fracture mechanics concepts and is therefore compatible with experimental results that indicate comparable performance from laminates that exhibit different GIC and GIIC values.

Moisture effects on the toughness, mode-I and mode-II of particles filled quasi-isotropic glass-fibre reinforced polyester resin composites

An experimental programme is presented for the effect of moisture on the toughness, mode-I and mode-II of aluminium tri-hydrate and polyethylene filled and unfilled quasi-isotropic glass–fibre reinforced epoxy–vinylester resin (GFRP) composites. Specimens were exposed in water at room temperature (20°C) for a period of 8 months and the effect of moisture content on toughness, GIc and GIIc values were obtained at an interval of every 2 months. Some samples were exposed in hot water at 40°C temperature to accelerate the uptake of moisture and produce saturated composites. The results indicate that equilibrium moisture content and diffusion coefficients increase with increase of weight of filler content in GFRP composites which is linked to an increase in microscopic cracking. Also mode-I, toughness of all composites increased with an increase in moisture uptake, mode-II toughness was relatively unaffected. Aluminium-tri-hydrate filled GFRP composites showed a higher moisture uptake, which resulted in higher values of both mode-I and mode-II, toughness than the polyethylene filled and unfilled GFRP composites.

Influence of Fibre Architecture on Impact Damage Tolerance in 3D Woven Composites

Abstract3D woven composites, due to the presence of through-thickness fibre-bridging, have the potential to improve damage tolerance and at the same time to reduce the manufacturing costs. However, ability to withstand damage depends on weave topology as well as geometry of individual tows. There is an extensive literature on damage tolerance of 2D prepreg laminates but limited work is reported on the damage tolerance of 3D weaves. In view of the recent interest in 3D woven composites from aerospace as well as non-aerospace sectors, this paper aims to provide an understanding of the impact damage resistance as well as damage tolerance of 3D woven composites. Four different 3D woven architectures, orthogonal, angle interlocked, layer-to-layer and modified layer-to-layer structures, have been produced under identical weaving conditions. Two additional structures, Unidirectional (UD) cross-ply and 2D plain weave, have been developed for comparison with 3D weaves. All the four 3D woven laminates have similar order of magnitude of damage area and damage width, but significantly lower than UD and 2D woven laminates. Damage Resistance, calculated as impact energy per unit damage area, has been shown to be significantly higher for 3D woven laminates. Rate of change of CAI strength with impact energy appears to be similar for all four 3D woven laminates as well as UD laminate; 2D woven laminate has higher rate of degradation with respect to impact energy. Undamaged compression strength has been shown to be a function of average tow waviness angle. Additionally, 3D weaves exhibit a critical damage size; below this size there is no appreciable reduction in compression strength. 3D woven laminates have also exhibited a degree of plasticity during compression whereas UD laminates fail instantly. The experimental work reported in this paper forms a foundation for systematic development of computational models for 3D woven architectures for damage tolerance.