W.J. Cantwell

The impact resistance of composite materials : a review

Abstract In this paper the impact response of continuous fibre-reinforced composites is reviewed. An attempt is made to draw together much of the work published in the literature and to identify the fundamental parameters determining the impact resistance of continuous fibre-reinforced composite materials. The effect of varying the properties of the fibre, matrix and interphase are examined as well as the role of target geometry and loading rate on the dynamic response of these materials.

Comparison of the low and high velocity impact response of CFRP

Abstract A series of low and high velocity impact tests has been conducted on a wide range of cfrp laminates to examine the initiation and development of damage under these two widely differing loading conditions. For conditions of low velocity impact loading the size and shape of the target determines its energy-absorbing capability and therefore its impact response. High velocity impact loading by a fast moving projectile induces a localized form of target response and the level of damage incurred does not, therefore, appear to be governed by the areal size of the component. The effect of such loading on the residual tensile strength has also been assessed. High velocity impact loading by a small projectile is generally more detrimental to the integrity of a composite structure than low velocity drop-weight impact loading.

The mechanical properties of fibre-metal laminates based on glass fibre reinforced polypropylene

This paper investigates the quasi-static and impact properties of a novel fibre/metal laminate system based on a tough glass-fibre-reinforced polypropylene (GFPP). Initial testing has shown that excellent adhesion can be achieved by surface treating the aluminium and incorporating an interlayer based on a maleic-anhydride modified polypropylene copolymer at the interface between the composite and aluminium plies. Single cantilever beam tests have indicated that the fracture energy of these systems is extremely high over a wide range of loading rates. Subsequent testing of a number of laminates has shown that the tensile properties of these layered systems are, as expected, strongly dependent on the volume fraction of composite. Low-velocity impact testing on three different stacking sequences has indicated that these materials offer excellent resistance to dynamic loading. In addition, an examination of polished cross-sections of the impact-damaged region have shown that the incident energy is absorbed through plastic deformation in the aluminium layers and localised micro-cracking within the composite plies.

Embedded fibre Bragg grating sensors in advanced composite materials

Abstract Fibre Bragg grating (FBG) sensors have been embedded in a number of advanced composite materials and fibre/metal laminates (FMLs). The post-fabrication FBG spectra were studied to examine the influence of manufacturing variables (such as composite stacking sequence and resin flow during processing) on the final profile of the spectrum and the functionality of the FBG sensor. Distortion and broadening of the width of the FBG spectra were observed in several of the specimens. As a result of a strong non-uniformity of the strain field caused by local asymmetric loading of the sensor, pronounced splitting of the spectra into multiple peaks was noted for FBG sensors embedded in angle-ply configurations. In contrast, the FBG spectra for the unidirectional specimens exhibited a single well-defined peak. Tensile tests carried out on these specimens showed excellent linearity within the test regime. However, for specimens exhibiting a multi-peak spectrum, it was observed that these specimens showed a tendency to produce strain anomalies during the loading event.

The low velocity impact response of foam-based sandwich structures

Abstract The low velocity impact response of a range of foam-based sandwich structures has been investigated using an instrumented falling-weight impact tower. Initially, the rate-sensitivity of the skin and core materials was investigated through a series of flexure and indentation tests. Here, it was shown that the flexural modulus of the skins and all 11 foam materials did not exhibit any sensitivity to crosshead displacement rate over the conditions studied here. In addition, it was shown that the indentation response of the sandwich structures could be modelled using a simple indentation law, the parameters of which did not exhibit any sensitivity to loading rate. Low velocity impact tests on the sandwich structures resulted in a number of different failure modes. Here, shear fracture was found to occur in the PVC/PUR systems based on brittle core materials. In contrast, buckling failures in the uppermost composite skin were observed in the intermediate modulus systems, whereas initial damage in the higher modulus PVC/PUR systems took the form of delamination within the top surface skin. It has been shown that a simple energy-balance model based on the dissipation of energy during the impact event can be used to successfully model the elastic response of foam-based sandwich structures. The energy-balance model is particularly useful since it can be used to establish the partition of energy during the impact process.

The high velocity impact response of composite and FML-reinforced sandwich structures

The high velocity impact response of a range of novel aluminium foam sandwich structures has been investigated using a nitrogen gas gun. Tests were undertaken on sandwich structures based on plain composite and fibre-metal laminate (FML) skins. Impact testing was conducted using a 10 mm diameter projectile at energies up to that required to achieve complete perforation of the target. High velocity impact tests on the sandwich structures resulted in a number of different failure modes. Delamination and longitudinal splitting of the composite skins were observed in the unidirectional glass fibre/polypropylene-based systems. In contrast, the woven glass fibre/polypropylene-based sandwich structures exhibited smaller amounts of delamination after high velocity impact testing. In addition, the aluminium foam in both systems exhibited a localised indentation failure followed by progressive collapse at higher impact energies. The ballistic limit of all of the sandwich structures was predicted using a simple analytical model. It has been shown that the predictions of the model are in good agreement with the experimental data. Finally, it has been shown that these novel systems offer excellent energy absorbing characteristics under high velocity impact loading conditions.

Impact perforation of carbon fibre reinforced plastic

Abstract Low and high velocity impact tests have been undertaken on a series of CFRP laminates in order to examine the perforation process in a fibre reinforced composite. By sectioning and polishing many of the specimens a characteristic conical-shaped fracture zone has been highlighted, the basic form of which does not appear to vary with fibre stacking sequence or target thickness below 4 mm. The effect of varying certain geometrical parameters on the energy required to perforate the composite targets has also been examined. For conditions of low velocity impact loading where the structural response of the target is important, the areal dimensions of the target determine the perforation threshold energy. Conversely, under high velocity impact loading where the target response is highly localized, the perforation threshold appears to be independent of the areal geometry of the structure. By accounting for the dissipation of energy during the impact process, a perforation model has been developed in order to predict the influence of target thickness and specimen size on the perforation threshold. The accuracy of the model was then assessed by comparing its predictions with the available experimental evidence.

Detection of impact damage in CFRP laminates

Abstract The initiation and propagation of damage in carbon fibre composites subjected to impact loading has been investigated. High velocity impact tests were conducted on a variety of stacking configurations using a nitrogen operated gas gun. The damage processes were characterised using X-radiography, ultrasonic C-scanning, optical microscopy and the deply technique. The merits and weaknesses of applying such damage detection techniques to the monitoring of impact damage in composites are discussed. The consequence of the various fracture mechanisms on residual tensile strength is also considered.

The low velocity impact response of an aluminium honeycomb sandwich structure

Abstract The low velocity impact response of two aluminium honeycomb sandwich structures has been investigated by conducting drop-weight impact tests using an instrumented falling-weight impact tower. Initially, the rate-sensitivity of the glass fibre reinforced/epoxy skins and aluminium core was investigated through a series of flexure, shear and indentation tests. Here, it was found that the flexural modulus of the composite skins and the shear modulus of the aluminium honeycomb core did not exhibit any strain-rate sensitivity over the conditions investigated here. In addition, it was found that the indentation characteristics of this lightweight sandwich structure can be analysed using a Meyer indentation law, the parameters of which did not exhibit any sensitivity to crosshead displacement rate. The impact response of the aluminium honeycomb sandwich structures was modelled using a simple energy-balance model which accounts for energy absorption in bending, shear and contact effects. Agreement between the energy-balance model and the experimental data was found to be good, particularly at low energies where damage was localised to the core material immediate to the point of impact. The energy balance was also used to identify energy partitioning during the impact event. Here, it was shown that the partition of the incident energy depends strongly on the geometry of the impacting projectile.

Geometrical effects in the low velocity impact response of CFRP

Abstract The influence of specimen geometry on the low velocity impact response of a series of CFRP laminates has been studied. Damage initiation and development in a range of composite beams have been assessed using a number of techniques including optical microscopy and ultrasonic C-scanning. Two forms of damage initiation have been highlighted, a top surface contact failure in short thick targets and a lower surface flexural failure in long thin laminates. The subsequent development of damage has been shown to depend strongly upon the energy-absorbing capability of the structure, and this has been further highlighted by assessing the geometrical dependence of the perforation threshold. Finally, in order to assess the impact response of a more representative structure, a series of drop-weight impact tests have been conducted on a range of circular panels.