Friction and Adhesion in Rigid Surface Indentation of Nitrile Rubber

When a rigid body in the form of a plane strain indentor is forced into an elastomer, the asperities on the surface of the indentor are filled by the softer material. As depth of ingress increases, the rubber displaced into the indentor asperities exhibits stick-slip behaviour. The rubber adheres to the rigid body and if the depth of ingress is held at a maximum, the level of adhesion remains constant despite short-term load relaxation occurring in the rubber. This text describes the influence of a range of factors on indentation forces and adhesion in rigid indentation of hydrogenated nitrile rubbers. Blocks of rubber in four hardness grades were subjected to plane strain indentation using mild steel plate indentors. The edges forced into the elastomers were radiused to produce ingress of a semi-circular profile into the blocks and this allowed subsequent finite element modelling of the indentor as a continuum. During physical testing, indentation rates and indentor surface finish were varied and load/displacement characteristics, adhesion and short-term load relaxation were measured. The correlation between indentation loads at the common maximum depth of ingress and the adhesion theory of friction for different surface finishes was examined. Nonlinear finite element stress analyses, employing adaptive meshing, alternative friction algorithms and competing strain energy density functions were used to model the indentation process and comparisons of surface profiles with test results are included

Contradictions in Irish Academic Research

The conditions that govern academic research vary greatly from country to country and research in the Republic of Ireland was and remains markedly different from that of its larger European neighbours and the United States. Despite the quality of its education system and the excellent reputation of its universities, until recently Ireland had relatively low levels of academic research. Pinnacles of excellence could be found in certain disciplines, but state funding was low and issues relating to industrial collaborations, international partnerships, commercialisation and the exploitation of Intellectual Property (IP) rarely arose. Even today the Irish Government’s spending on academic research, though only slightly less than the European average based on GNP, is dwarfed by the Research and Development (R & D) budgets of individual multinational companies. Nonetheless, rapid economic growth has led to a heightened awareness of the need for strategically planned research. The ‘Lisbon Objective’ proposes to make Europe “the most dynamic knowledge-driven economy in the world by 2010”. Consequently research is heavily influenced by this policy and so a range of unfamiliar problems are posed for managers of Irish academic research. Key to successful operational planning and growth is the need to reconcile a number of contradictions at the heart of R & D in Third Level Institutes[1]. [1] ‘Third Level Institute’ is a term used in Ireland to describe any higher education institut

A Rheological Model of the Dynamic Behaviour of Magnetorheological Elastomers

A rheological model is described that was developed to simulate the dynamic behaviour of magnetorheological elastomers (MREs). The viscoelasticity of the polymer composite, magnetic field induced properties and interfacial slippage between the matrix and particles were modelled by analogy with a standard linear solid model, a stiffness variable spring and a spring-Coulomb friction slider respectively. The loading history and rate dependent constitutive relationships for MREs were derived from the rheological model. The hysteresis loop from shear strain - shear stress plots, which determines the shear modulus and loss factor, were obtained from substituting cyclic loading into these constitutive relationships. The dynamic behaviours of MREs were simulated by changing parameters in the rheological model to influence MREs’ performances. The simulation results verified the effectiveness of the model

Prediction of Compressive Creep Bhaviour in Flexible Polyurethane Foam Over Long Time Scales and at Elevated Temperatures

Compressive creep gradually affects the structural performance of flexible polymeric foam material over extended time periods. When designing components, it is often difficult to account for long-term creep, as accurate creep data over long time periods or at high temperatures is often unavailable. This is mainly due to the lengthy testing times and/or inadequate high temperature testing facilities. This issue can be resolved by conducting a range of short-term creep tests and applying accurate prediction methods to the results. Short-term creep testing was conducted on viscoelastic polyurethane foam, a material commonly used in seating and bedding systems. Tests were conducted over a range of temperatures, providing the necessary results to allow for the generation of predictions of long-term creep behaviour using time-temperature superposition. Additional predictions were generated, using the William Landel Ferry time-temperature empirical relations, for material performance at temperatures above and below the reference temperature range. Further tests validated the results generated from these theoretical predictions

A Critique of Singaporean Internal Tertiary Education Programmes offered by Private Colleges: A Brief Comparison with Ireland

Singapore has a similar population to Ireland and gross domestic products (GDPs) for both countries are comparable. However, culturally and in other respects, the two republics are very different. Unemployment in Ireland is six times higher than in Singapore where GDP has almost doubled since 2007 but has fallen by nearly 11% in Ireland in the same time-span. One aspect of life in both countries transcending cultural differences is the importance placed on tertiary education. The Irish and Singaporean people share a deep commitment to education and the imperative of building and maintaining a knowledge economy is central to both societies. Employment in higher education in Singapore is more tenuous than in Ireland and it is common for a lecturer who is considered to have underperformed to not have a contract renewed. Irish higher education adheres closely to European policies and practices, particularly in adoption of quality assurance procedures. Consequently, it is expected that similar bachelor degree programmes in Ireland, for example in Mechanical Engineering or Physics, will be quality assured to a consistent standard across the higher education system. Quality assurance (QA) differs markedly in both states. For instance, QA in the National University of Singapore (NUS) is typically managed by internal committees, augmented by ‘Visiting Committees’. The multitude of courses offered by private colleges in partnership with UK universities are subject to the QA procedures of the respective universities and hence are regulated by the UK Quality Assurance Agency for Higher Education (QAA). However, an internal tertiary programme may or may not follow a strict QA procedure. If it does, the QA policies are drafted by the institution’s staff and are based on the requirements of Singapore’s Council of Private Education (CPE). Hence it is possible that the standard set may be inconsistent with the standard demanded by overseas QA regulations. In cases where QA procedures are not implemented, there can be little confidence among stakeholders in respect of the quality of the programme and standards may be arbitrary. Quality assurance of Irish higher education is coordinated through Qualifications and Quality Ireland (QQI), a state body responsible for the review of institutions who usually operate their own quality assurance systems. In some cases, QQI takes direct responsibility for quality assurance within smaller institutions, while also taking direct responsibility in other education sectors, such as the further education sector. There is a widely held perception amongst stakeholders in higher education in Singapore that internal programmes in tertiary education offer lower quality than the external tertiary programmes which are affiliated to well-established, mainly British, universities. Many factors have influenced this view, leading to its reinforcement and wide acceptance. These factors are discussed here and a simple but telling case study is offered. The practices of private colleges running their own internal Diploma and Advanced Diploma programmes are evaluated by observing specific situations. Comparisons are made with similar practices of colleges that run external Diploma and Advanced Diploma programmes under the auspices of UK universities and practices followed in private colleges and institutes of technology in Ireland. In conclusion, the need for uniform quality procedures across the Singaporean higher education system is highlighted and some necessary steps in achieving this requirement are advanced

Characterisation, Modelling and Simulation of Flexible Polyurethane Foam

Flexible polyurethane foam is an open-celled polymeric material that exhibits strain rate and temperature effects. It has found various applications in areas including the packaging, medical, sports, aerospace and aeronautical industries. Polyurethane foam is ubiquitous in seating applications and finds particular use in specialised wheelchair seating where customised seating solutions are required which can provide proper comfort and support without the risk of developing pressure ulcers. Proper seating design is critical for users if this problem is to be avoided, but a lack of quantitative knowledge of this material’s behaviour has limited its effectiveness. The objectives of the work presented here are twofold. Firstly to characterise the behaviour of the materials and secondly to develop a validated numerical model which can be used to increase understanding of in-service behaviour. Three commonly used foams having different densities and viscoelastic properties were subjected to compression in a uni-axial test machine fitted with a custom-built temperature chamber. The results of these tests were analysed and are presented to aid in the characterisation of these materials. The effects on the stiffness of foam of different additives, densities, strain rates and temperatures were noted. A material model was developed to simulate indentation, in which compression and shear were the predominant modes of deformation. The results from the uni-axial characterisation tests were employed to determine material constants for Ogden’s constitutive model for compressible materials. Simple shear tests were also conducted with a custom-built dual lap shear tester and material constants were then determined for this mode of deformation. A curve-fit was developed which was a compromise between both modes of deformation to provide increased material model robustness. To validate the accuracy of the developed model, uni-axial indentation of standard polyurethane seating foam was then modelled using Finite Element (FE) code. Results show a high degree of accuracy

Testing, Modelling and Validation of Numerical Model Capable of Predicting Stress Fields Throughout Polyurethane Foam

Wheelchair seating systems are specialised for a number of reasons as users can have impaired mobility, which increases the possibility of pressure build up. These areas of high pressure frequently occur in the trunk region under the bony prominences known as the Ischial Tuberosities (IT), pressure ulcers may occur consequently. Polyurethane foam has been in use for some time in wheelchair seating systems as it exhibits good pressure relieving capabilities in most cases. However, little quantitative research has gone into foamed polymers, in comparison with conventional elastomeric materials. This lack of knowledge can ultimately lead to more time being spent in fitting, increased possibility of refitting and potentially an increase in trunk region pressures leading to the development of ulcers. Test results were used to accurately validate a Visco-Hyperfoam material model. Accurately simulating an indentation procedure using FE software verified the validation of the material model

The Implementation of a Visco-hyperelastic Numerical Material Model for Simulating the Behaviour of Polymer Foam Materials

Polyurethane foam has been in use for some time in wheelchair seating systems as it offers good pressure relieving capabilities in most cases. However, little characterisation work has gone into seating foam materials by comparison with conventional elastomeric materials. Accurate material models could allow better prediction of foam in-service behaviour, which could potentially improve seating design practises. The objective of this work was to develop an approach for the validation of hyperelastic and viscoelastic material model parameters used to simulate polyurethane foam behaviour. Material parameters were identified from relevant test procedures and implemented in a Finite Element simulation of an ISO foam indentation procedure. Physical test results were compared to results predicted using the identified material parameters. Simulations suggest a good overall agreement between test and model results