M.J. Clifford

Rotating periodic orbits of the parametrically excited pendulum

Rotating orbits in the parametrically excited pendulum are considered. The location of subharmonic orbits shows that there is in general no easily defined lower bound on the forcing amplitude, p, below which rotating orbits cannot exist. This is particularly important if the parametrically excited pendulum is considered in terms of escape from a potential well.

Finite element modelling of fabric compression

The mechanical behaviour of woven fabric under compression is investigated using 3D finite element analysis in conjunction with a nonlinear mechanical model for the yarn. The FE model captures the main fabric compression response, including geometric and material nonlinearities, yarn interactions and hysteresis. It is found that the behaviour of fabric in compression is governed by the stiffness of the yarn cross-section and the transverse–longitudinal shear modulus. The stiffness along the yarn direction has no noticeable effect. The model is sufficient to simulate the known responses of a fabric as well as to predict the behaviour of novel fabrics based on the properties of the component yarns and yarn interactions.

Modelling the effect of yarn twist on the tensile strength of unidirectional plant fibre yarn composites

The structural potential of plant fibres as reinforcing agents can only be realized when the highest reinforcement efficiency is employed. Hence, aligned plant fibre composites are of interest. However, due to the short length of technical plant fibres, the reinforcement needs to be in the form of staple fibre yarns, which have a twisted structure. Although twist facilitates yarn processability, it has several detrimental effects on the composites produced from such twisted yarn reinforcements; one of which is fibre obliquity and misalignment. This results in a drastic drop in composite mechanical properties. No analytical model currently exists to accurately predict the effect of yarn twist on aligned plant fibre composite tensile strength. In this paper, a novel mathematical model is developed. The model is based on (i) a modified rule of mixtures for plant fibre composites, (ii) well-defined structure-property relationships in an idealised twisted staple fibre yarn and (iii) the Krenchel orientation efficiency factor. The developed model includes a corrected orientation efficiency factor of cos2(2α), where α is the yarn surface twist angle. The model is validated with extensive experimental data from Goutianos and Peijs (Goutianos S, Peijs T (2003) Adv Compos Lett 12(6):237) and is found to be a near-perfect fit (R2 = 0.960). Experimental data from other studies are also used for further verification.

Finite element modelling of fabric shear

In this study, a finite element model to predict shear force versus shear angle for woven fabrics is developed. The model is based on the TexGen geometric modelling schema, developed at the University of Nottingham and orthotropic constitutive models for yarn behaviour, coupled with a unified displacement-difference periodic boundary condition. A major distinction from prior modelling of fabric shear is that the details of picture frame kinematics are included in the model, which allows the mechanisms of fabric shear to be represented more accurately. Meso- and micro-mechanisms of deformation are modelled to determine their contributions to energy dissipation during shear. The model is evaluated using results obtained for a glass fibre plain woven fabric, and the importance of boundary conditions in the analysis of deformation mechanisms is highlighted. The simulation results show that the simple rotation boundary condition is adequate for predicting shear force at large deformations, with most of the energy being dissipated at higher shear angles due to yarn compaction. For small deformations, a detailed kinematic analysis is needed, enabling the yarn shear and rotation deformation mechanisms to be modelled accurately.

Automated geometric modelling of textile structures

An automated approach (TexGen) for modeling the geometry of textile structures is presented. This model provides a generic approach to the description of yarn geometry and yarn interlacement for all types of weaving. One feature of this model is that the shape and size of the cross sections may change locally; this is exploited in the functions for interference correction, which modify the textile according to geometric considerations to avoid penetration of yarns. Another feature of this model is that it acts as a pre-processor for finite element simulations by generating a mesh, definition of contact, materials orientation and boundary conditions, thus providing an automatic procedure. This paper describes the modeling techniques, algorithms and concepts implemented in TexGen and examines the functionality of their implementation for a range of two-dimensional/three-dimensional commercial fabrics. Comparisons between the images of real fabrics and modeled fabric structures confirm that the software is capable of modeling sophisticated fabric architectures, including twisted yarns with varied yarn cross sections. Accurate input measurements of fabric geometry are critical for successful results. The paper also discusses directions for further development of the approach to overcome current limitations.

Drivers' exposure to carbon monoxide in Nottingham, U.K.

We present the preliminary results of a 7-week program to monitor the levels and time integral exposure of carbon monoxide experienced by drivers in Nottingham U.K. The results show that the levels are within the World Health Organisations guidelines for 1 h exposure. The relationship between vehicle speed and carbon monoxide concentration is also investigated and is found to be not significant.

Local aspects of vehicular pollution

Abstract We consider the local aspects of vehicular pollution by the use of l:10 scale models. The model automobiles were placed in a low-velocity wind tunnel to simulate a queue of slow-moving city traffic and tracer gasses were then injected into the air flow though the tail pipes fitted to two of the models. Measurements show that the exhaust gases are entrained in the wake of the vehicle from which they were emitted, and are dispersed mainly by the passage of the following car. The spatial distribution of tracer gas along and across the models suggests that the level of pollution received by a commuter in a slow-moving heavy traffic may be strongly influenced by the pollution produced from the car immediately in front.

Rigid-body modelling of shaken baby syndrome.

Abstract Recent reassessment of the literature on the shaken baby syndrome (SBS) has revealed a lack of scientific evidence and understanding of all aspects of the syndrome. In particular, studies have been unable to clarify the mechanisms of injury, indicating that impact, rather than shaking alone, is necessary to cause the type of brain damage observed. Rigid-body modelling (RBM) was used to investigate the effect of neck stiffness on head motion and head-torso impacts as a possible mechanism of injury. Realistic shaking data obtained from an anthropometric test dummy (ATD) was used to simulate shaking. In each study injury levels for concussion were exceeded, though impact-type characteristics were required to do so in the neck stiffness study. Levels for the type of injury associated with the syndrome were not exceeded. It is unlikely that further gross biomechanical investigation of the syndrome will be able to significantly contribute to the understanding of SBS. Current injury criteria are based on high-energy, single-impact studies. Since this is not the type of loading in SBS it is suggested that their application here is inappropriate and that future studies should focus on injury mechanisms in low-energy cyclic loading.

Void Content Measurements in Commingled E-Glass/ Polypropylene Composites Using Image Analysis from Optical Micrographs

The principal interest of measuring the level of internal porosity in composite materials lies in the possibility of relating void content with macro and micro-structural properties and hence with composite manufacturing quality. In the particular case of commingled E-glass/polypropylene composites, processing parameters, such as moulding pressure and material temperature, have been demonstrated to influence void content, which is in turn affecting mechanical and impact properties of the material. It is of paramount importance therefore to dispose of a simple and reliable method for void content determination on these composites, which are increasingly used e.g., in automotive industry, for their energy absorbing properties and their low cost. In this paper, void measurements in E-glass/polypropylene commingled laminates are performed by analysing images from optical micrographs. Voids were classified, due to their size and orientation, as microvoids, intratow voidage, coplanar voids and extensive voids. In practice, optical microscopy of specimens was able to detect void contents of less than 1% using a statistical point-counting technique. Computerised analysis of the micrographs was also possible, based on the grey scale difference existing between voids and the rest of the composite image (matrix, reinforcement). Possibilities and limitations of this approach are highlighted by discussing the relation between the sample of the material considered and the obtained accuracy, which depends also on image filtering and appropriate

Locating oscillatory orbits of the parametrically-excited pendulum

A method is considered for locating oscillating, nonrotating solutions for the parametrically-excited pendulum by inferring that a particular horseshoe exists in the stable and unstable manifolds of the local saddles. In particular, odd-periodic solutions are determined which are difficult to locate by alternative numerical techniques. A pseudo-Anosov braid is also located which implies the existence of a countable infinity of periodic orbits without the horseshoe assumption being necessary.