K.L. Alderson

The fabrication of microporous polyethylene having a negative Poisson's ratio

Abstract A novel thermoforming processing route has been developed that produces a microporous form of ultra high molecular weight polyethylene that demonstrates large negative Poissons ratios. The microstructure consists of nodules interconnected by fibrils. Poissons ratios as low as −1.2 have been obtained, depending on the degree of anisotropy in the material.

Auxetic polyethylene: The effect of a negative Poisson's ratio on hardness

Abstract Recent papers have described a particular form of ultra high molecular weight polyethylene (UHMWPE) which possesses a large negative Poissons ratio (ν) because of its complex microstructure. Classical elasticity theory predicts that this should lead to enhancements in certain mechanical properties and this paper examines one such property—that of ball indentation resistance or hardness. The hardness of auxetic UHMWPE with a negative ν, compression moulded UHMWPE and a microporous form of UHMWPE with a positive ν was measured. It was found that the presence of a negative ν resulted in enhancements of the hardness by up to a factor of 2 over conventional UHMWPE.

The static and dynamic moduli of auxetic microporous polyethylene

In this letter the elastic constants of an auxetic form of microporous ultra high molecular weigth polyethylene (UHMWPE) are reported

Failure mechanisms during the transverse loading of filament-wound pipes under static and low velocity impact conditions

Abstract The failure of filament-wound E-glass/epoxy resin pipes undergoing a transverse load and floor-supported along the pipes length has been examined using strain gauging and video camera methods. Elastic behaviour ends at low loads with the resin yielding, leading to delamination initiation. The failure process then continues with the growth of one favourable delamination, with subsequent smaller delaminations also occurring until a second major failure and load drop. Detailed microscopic examinations have shown that the first failure occurs regardless of pipe diameter, support condition and test types. The second failure, however, is strongly dependent on test type and support condition.

Strain-dependent behaviour of microporous polyethylene with a negative Poisson's ratio

Recent papers have discussed particular forms of expanded polytetrafluoroethylene (PTFE) and ultra-high molecular weight polyethylene (UHMWPE) that exhibit large negative Poissons ratios due to their complex microstructures. These consist of an open network of nodules interconnected by fibrils. Two geometric models have been developed to explain the effect in PTFE; one based on nodule translation and the other on nodule rotation. Data are presented for the strain-dependent variation of the Poissons ratio of UHMWPE under compression and the models are applied. The translation model alone accurately describes the variations in Poissons ratio as a function of compressive strain.

Low velocity transverse impact of filament-wound pipes: Part 1. Damage due to static and impact loads

Abstract Low velocity (up to 10 m/s) single-bounce impact and static tests have been conducted on glass-fibre reinforced filament-wound pipes in two support conditions — floor supported and end-cradled. Use of a simple backlighting technique has allowed the failure process of the pipes to be analysed and the damage area to be quantified. The failure process has two characteristic parts. Elastic behaviour ends at approximately the same load regardless of support condition with a definite failure associated with a concomitant drop in load. The process then continues with subsequent delaminations until a second major failure occurs, which is very dependent upon support condition.

The effect of the processing parameters on the fabrication of auxetic polyethylene

A novel fibrillated particulate microstructure has been fabricated in ultra high molecular weight polyethylene (UHMWPE) that produces a negative Poissons ratio (auxetic) material. The processing route involves compaction, sintering and extrusion of a UHMWPE fine powder. The first, compaction stage is examined in this paper in detail in order to ascertain the compaction conditions required to produce, as an end-product, an auxetic polymer and to assess the importance of this stage in the processing route. It was found that while part of the function of the compaction stage was to impart structural integrity to the processed polymers, the conditions for optimizing the production of auxetic UHMWPE were not identical to those for optimum structural integrity of the compact. Both sets of conditions were examined, with compaction pressure and temperature being the most important of the variables examined.

Low velocity transverse impact of filament-wound pipes: Part 2. Residual properties and correlations with impact damage

Abstract Transverse low velocity (up to 10 m/s) impact tests have been performed on filament-wound E-glass/epoxy resin pipes with a winding angle of ± 55° over a range of impact energies and velocities in two support conditions. Equivalent static tests were performed in both support conditions to see if any correlations existed, which would allow simple static tests to be used to predict the impact performance of the pipe. This was possible over a majority of the energy range considered for the cradled geometry, but not for the floor-supported geometry.

Dynamic analysis of filament wound pipes undergoing low velocity transverse impact

Abstract Transient load/time behaviour of GRP filament wound pipes under single-bounce, low-velocity (up to 10 m/s) impact testing in two support conditions has been modelled using two simple single-degree-of-freedom mass-spring models. Model A regards the pipe as a mass impacting a massless, simply-supported spring and gives a good first approximation to general behaviour. This is improved, particularly for the end-cradled support condition, by the use of a non-linear force constant. Model B regards the system as a massive, simply-supported beam (or ring) subjected to a ramp load and is used primarily to predict the period of pipe oscillation, Tn. Two versions of model B were employed, considering the pipe as a beam and a ring. The ring model was found to be the closest approximation to actual behaviour. This indicates that the behaviour of the pipe under impact is dominated by the ring mode of vibration.

Modelling of the transverse loading of filament wound pipes

Abstract E-glass/epoxy resin pipes have been examined for static and low-velocity impact loads. In order to quantify the results, two modelling methods have been employed. The first, a simple analytical solution, predicted the deflection to first failure very well. This was only slightly improved by the use of a commercially available finite element package. This also allowed good prediction of the stress states of the pipes under transverse load and provides a convenient means of assessing which layers delaminate first.