John P. Dear

The failure of fibre composites and adhesively bonded fibre composites under high rates of test

The failure of fibre composites and adhesively bonded fibre composites under high rates of test, up to rates of about 15 m s−1 were studied in detail. The present paper. Part I of the series, considers the experimental aspects of the mode I fracture of the fibre composite materials and joints. Part II will analyse the dynamic effects which are invariably associated with high-rate tests, and will show how these effects influence the observed behaviour of the test specimens. Part III will report the results from mode II and mixed-mode I/II tests on the fibre composite materials.

The calculation of adhesive fracture energies from double-cantilever beam test specimens

The purpose of this letter is to demonstrate that, when using the DCB test specimen to study joints that consist of bonded polymeric fibre-composite substrates, careful attention needs to be paid to the method used to analyse the experimental data

Impact damage processes in reinforced polymeric materials

This study investigates the impact toughness of two grades of Sheet Moulding Compound (SMC) and one grade of Glass Mat Thermoplastic (GMT) material. A servo-hydraulic impact machine and a drop-weight impact facility are used to obtain and relate different degrees of deformation and penetration damage of the two grades of SMC and GMT materials to load displacement records. The servo-hydraulic impact data provides a step-by-step damage history of drop-weight impact penetration of these materials. The impact energy absorbed by the different composite materials is determined for different degrees of deformation and penetration of the specimens. Notable in the findings is that, in some cases, the onset of through-thickness damage can occur before there is visual evidence of surface damage at the point of contact between the striker and the specimen.

Modelling the behaviour of composite sandwich structures when subject to air blast loading

Large-scale glass fibre reinforced polymer (GFRP) and carbon fibre reinforced polymer (CFRP) sandwich structures (1.6 m x 1.3 m) were subject to explosive air blast (100 kg TNT equivalent) at stand-off distances of 14 m. Digital image correlation (DIC) was used to obtain full-field data for the rear-face of each deforming target. A steel plate of comparable mass per unit area was also subjected to the same blast conditions for comparison. The experimental data was then verified with finite element models generated in Abaqus/Explicit. Close agreement was obtained between the numerical and experimental results, confirming that the CFRP panels had a superior blast performance to the GFRP panels. Moreover all composite targets sustained localised failures (that were more severe in the GFRP targets) but retained their original shape post blast. The rear-skins remained intact for each composite target with core shear failure present.

The effects of chlorine depletion of antioxidants in polyethylene

For many polyethylene products, their working life-span depends on retention by the material of its antioxidant. In dry air, when the working and environmental stresses remain within defined limits, the life-span of the material can be many decades. Immersed in water, for example, the diffusion and loss of antioxidant from the materials surface can increase. Also, some types of aggressive agents, if in the water, can enter the materials surface and migrate into the material to increase the depletion of the antioxidant population. Researched in this study, is the depletion of antioxidants in polyethylene when exposed to water containing different chlorine concentrations. This research relates to the world-wide use of polyethylene pipes in water treatment and distribution networks.

Compressive strength after blast of sandwich composite materials

Composite sandwich materials have yet to be widely adopted in the construction of naval vessels despite their excellent strength-to-weight ratio and low radar return. One barrier to their wider use is our limited understanding of their performance when subjected to air blast. This paper focuses on this problem and specifically the strength remaining after damage caused during an explosion. Carbon-fibre-reinforced polymer (CFRP) composite skins on a styrene–acrylonitrile (SAN) polymer closed-cell foam core are the primary composite system evaluated. Glass-fibre-reinforced polymer (GFRP) composite skins were also included for comparison in a comparable sandwich configuration. Full-scale blast experiments were conducted, where 1.6×1.3 m sized panels were subjected to blast of a Hopkinson–Cranz scaled distance of 3.02 m kg−1/3, 100 kg TNT equivalent at a stand-off distance of 14 m. This explosive blast represents a surface blast threat, where the shockwave propagates in air towards the naval vessel. Hopkinson was the first to investigate the characteristics of this explosive air-blast pulse (Hopkinson 1948 Proc. R. Soc. Lond. A 89, 411–413 (doi:10.1098/rspa.1914.0008)). Further analysis is provided on the performance of the CFRP sandwich panel relative to the GFRP sandwich panel when subjected to blast loading through use of high-speed speckle strain mapping. After the blast events, the residual compressive load-bearing capacity is investigated experimentally, using appropriate loading conditions that an in-service vessel may have to sustain. Residual strength testing is well established for post-impact ballistic assessment, but there has been less research performed on the residual strength of sandwich composites after blast.

Flexural Testing of Composite Sandwich Structures with Digital Speckle Photogrammetry

Composite sandwich structures are finding increasingly widespread use in fields ranging from aerospace and wind turbines to sports applications such as skis and surfboards. The high specific stiffness that composite sandwich structures can provide lends them well to these applications. However, the operational environment of these structures is frequently aggressive and often results in damage during service. The extent and effect of damage incurred is an important factor in the design and maintenance of composite sandwich structures. Failure of an individual component can be catastrophic for the rest of the structure. The purpose of this investigation was, firstly, to ascertain whether DSP was a viable technique for determining strain fields within composite sandwich structures. Secondly, to determine whether four point flexure would give rise to pure flexure between the central rollers, and if not, to understand what load conditions were present. This investigation was also carried out with a view to extend the investigation into the effect of defects on composite sandwich structures manufactured by RIFT. The grounds for selection of composite sandwich structures normally lie in their flexural performance. Reliable and accurate quantitative testing methods for evaluating the flexural performance of sandwich panels are needed if composite sandwich structures are to be used safely and effectively. In addition, methods to determine the effect of damage and defects on flexural behaviour of sandwich structures is particularly important for designing the repair and maintenance regimes of composite sandwich components.

Rapid Bidirectional Reorganization of Cortical Microcircuits

Mature neocortex adapts to altered sensory input by changing neural activity in cortical circuits. The underlying cellular mechanisms remain unclear. We used blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) to show reorganization in somatosensory cortex elicited by altered whisker sensory input. We found that there was rapid expansion followed by retraction of whisker cortical maps. The cellular basis for the reorganization in primary somatosensory cortex was investigated with paired electrophysiological recordings in the periphery of the expanded whisker representation. During map expansion, the chance of finding a monosynaptic connection between pairs of pyramidal neurons increased 3-fold. Despite the rapid increase in local excitatory connectivity, the average strength and synaptic dynamics did not change, which suggests that new excitatory connections rapidly acquire the properties of established excitatory connections. During map retraction, entire excitatory connections between pyramidal neurons were lost. In contrast, connectivity between pyramidal neurons and fast spiking interneurons was unchanged. Hence, the changes in local excitatory connectivity did not occur in all circuits involving pyramidal neurons. Our data show that pyramidal neurons are recruited to and eliminated from local excitatory networks over days. These findings suggest that the local excitatory connectome is dynamic in mature neocortex.

Development of a method for fast crack testing of high-grade polymers

A technique is described which takes advantage of the good thermal insulation properties of polymers and their tendency to become embrittled when cooled to low temperatures and so make crack initiation in an embrittled part of a specimen a simple process. In many tests, the practice is to initiate a crack in a specimen of polymer or similar material by using an impacting device that also applies dynamically a three-point bend. A problem with this approach can be in determining how much the crack initiation and propagation is due to the strong transient forces relative to the bending or other dynamic loading. As the toughness, dynamic and other properties of materials are improved and hence higher impact velocities are required, so this factor becomes more difficult to resolve. Thus, there is increasing interest in different arrangements for testing these new materials that avoid transient impact problems, and also to provide better information concerning the threshold load for crack propagation and related factors. On-specimen gauges are used for some of these studies.

Simple method of determining the fracture resistance for rapidly propagating cracks in polymers

A recently developed method has been used to obtain fracture toughness data for rapidly propagating cracks from small specimens of polymers. By varying the load on the specimen, the crack speed can be changed and this load-crack speed relationship has been used in conjunction with a mass-spring model to obtain fracture toughness, R, and the limiting crack speed in the specimen, CL. A relationship between R and CL is suggested and shown to describe the data.