David Nash

On erosion issues associated with the leading edge of wind turbine blades

The increasing developments in wind turbine technology, coupled with an unpredictable operating environment, present significant challenges regarding erosion issues on the leading edge of the blade tips. This review examines the potential degradation posed by the different environmental variables, with specific emphasis on both rain droplet and hailstone impact on the blade leading edge. Drawing on both the insights from experimental results and recent field data from the literature, the mechanisms of leading edge erosion are discussed. Meteorological tools that may enable rain and hailstone erosion prediction are addressed, as well as potential experimental and numerical approaches that may provide insight into the nature of impact and erosion on the blade surface.

Finite element model creation and stability considerations of complex biological articulation: The human wrist joint

The finite element method has been used with considerable success to simulate the behaviour of various joints such as the hip, knee and shoulder. It has had less impact on more complicated joints such as the wrist and the ankle. Previously published finite element studies on these multi-bone joints have needed to introduce un-physiological boundary conditions in order to establish numerical convergence of the model simulation. That is necessary since the stabilizing soft tissue mechanism of these joints is usually too elaborate in order to be fully included both anatomically and with regard to material properties. This paper looks at the methodology of creating a finite element model of such a joint focussing on the wrist and the effects additional constraining has on the solution of the model. The study shows that by investigating the effects each of the constraints, a better understanding on the nature of the stabilizing mechanisms of these joints can be achieved.

On the experimental testing of fine Nitinol wires for medical devices

Nitinol, a nickel titanium alloy, is widely used as a biocompatible metal with applications in high strain medical devices. The alloy exhibits both superelasticity and thermal shape memory behaviour. Basic mechanical properties can be established and are provided by suppliers; however the true stress-strain response under repeated load is not fully understood. It is essential to know this behaviour in order to design devices where failure by fatigue may be possible. The present work develops an approach for characterising the time varying mechanical properties of fine Nitinol wire and investigates processing factors, asymmetric stress-strain behaviour, temperature dependency, strain rate dependency and the material response to thermal and repeated mechanical loading. Physically realistic and accurately determined mechanical properties are provided in a format suitable for use in finite element analysis for the design of medical devices. Guidance is also given as to the most appropriate experimental set up procedures for gripping and testing thin Nitinol wire.

A three-dimensional finite element model of maximal grip loading in the human wrist

Abstract The aim of this work was to create an anatomically accurate three-dimensional finite element model of the wrist, applying subject-specific loading and quantifying the internal load transfer through the joint during maximal grip. For three subjects, representing the anatomical variation at the wrist, loading on each digit was measured during a maximal grip strength test with simultaneous motion capture. The internal metacarpophalangeal joint load was calculated using a biomechanical model. High-resolution magnetic resonance scans were acquired to quantify bone geometry. Finite element analysis was performed, with ligaments and tendons added, to calculate the internal load distribution. It was found that for the maximal grip the thumb carried the highest load, an average of 72.2 ± 20.1 N in the neutral position. Results from the finite element model suggested that the highest regions of stress were located at the radial aspect of the carpus. Most of the load was transmitted through the radius, 87.5 per cent, as opposed to 12.5 per cent through the ulna with the wrist in a neutral position. A fully three-dimensional finite element analysis of the wrist using subject-specific anatomy and loading conditions was performed. The study emphasizes the importance of modelling a large ensemble of subjects in order to capture the spectrum of the load transfer through the wrist due to anatomical variation.

An investigation of web crushing behaviour in thin-walled beams

An initial investigation of the collapse behaviour of webs of thin-walled beams under concentrated loads applied through one or both flanges is carried out. The analysis methods given in some current design specifications with respect to web crushing are outlined and compared. Theoretical examinations of the behaviour of channel sections under single or two flange loading are derived and compared with the predictions of different design specifications. The theoretical investigations make use of simple Energy Analysis as well as finite strip and finite element examinations. A design approach which incorporates the European column curves is set out.

Combined external load tests for standard and compact flanges

Abstract The recognised standard method of gasketed flanged joint design contained within most pressure vessel codes is that based on the Taylor Forge procedure (Trans ASME 59 (1937) 161). This has, as its basis, bolt load calculations, which are designed to apply sufficient load to both seat and initialise the gasket, and to ensure sealing via a gasket when the operational pressure load is present. The flange ring and hub transmit the bolt load to the gasket and must therefore be stiff and flat. However, there are many real situations where additional loads arise through external pulling and bending. This is commonly seen in piping systems and other flanged pressure equipment. Although the codes do not specifically address the ‘combined load’ problem, the normal method for considering this additional load is to form an equivalent pressure. This over-pressure is calculated by making the stress generated in the pipe or vessel wall, by the external load, equal to a longitudinal pressure stress which may be tensile or compressive, depending on the nature of the load. This results in an over-pressure which can therefore be added to the operating pressure. For bending loads, no account is taken of the variation around the circumference, or the change in gasket seating width, which will vary as the flange faces rotate. In order to assess the effects of external loading on flanges, a combined load test rig has been constructed and a number of bolted flange assemblies examined including standard ANSI joints and compact VERAX VCF joints (Fig. 1a and b). These assemblies have been strain gauged and tested for a variety of load conditions. Tests have been carried out using hydraulic fluid as the main pressurising medium. The results of the individual tests and the combinations of load are presented and discussed.

A parametric study of metal-to-metal full face taper-hub flanges

The paper presents the results of a parametric study, using finite element analysis, of the behaviour of full face metal-to-metal taper-hub flanges. The important stress values in the flange have been obtained for a range of flange thickness, taper-hub thickness and length, when the shell/flange component is subject to internal pressure. The influence of the pre-stress in the bolts is examined. The results obtained have been compared with the predictions from the appropriate sections of the ASME, BS and the new European Unfired Pressure Vessel Standard (Draft BS: prEN 13445).

Surface sensitivity study of non-gasketed flange joint

Abstract From analytical contact pressure studies of gasketed and non-gasketed flanged pipe joints, it is known that good contact maintained over consistently large areas occurs in a no-gasketed flange joint compared with a gasketed joint, where contact is only on the seal ring portion of the gasket. However, the presence of contact pressure and a good flange surface finish does not in itself eliminate the problem of leakage. Rather, it is the surface flatness in combination with contact pressure and surface finish that is necessary to generate effective contact at the inside diameter of the flange. This occurs providing there are no gaps due to deformation arising from the welding process to the mating pipe as flow is proportional to gap height. However, the question remains as how to ensure adequate surface flatness as non-gasketed flange surfaces are sensitive to damage, which may be present in the form of surface deformation, scratches and dents or bulges. A detailed experimental study is therefore carried out to measure the surface finish and surface flatness post manufacture and welding of such flanges. Results of actual flanges were compared with solid circular plates of the same dimension with inserted shims to provide gaps in order that flow rate/gap height ratio be studied. Manufacturing issues with respect to monitoring and controlling deformations are discussed and recommendations are suggested for flange redesign, appropriate welding approaches and manual handling of non-gasketed flanges.

Load transfer through the radiocarpal joint and the effects of partial wrist arthrodesis on carpal bone behaviour: a finite element study

A finite element model of the wrist was developed to simulate mechanical changes that occur after surgery of the wrist. After partial arthrodesis, the wrist will experience altered force transmission during loading. Three different types of partial arthrodesis were investigated — radiolunate, radioscaphoid, and radioscapholunate — and compared with the healthy untreated wrist. The results showed that the compressive forces on the radiocarpal joint decreased compared with the untreated wrist with both radiolunate and radioscaphoid fusions. The load transmission through the midcarpal joints varied depending on arthrodesis type. The forces in the extrinsic ligaments decreased with the fusion, most noticeably in the dorsal radiotriquetral ligament, but increased in the dorsal scaphotriquetral ligament. From the results of the study it can be concluded that the radioscapholunate fusion shows the most biomechanically similar behaviour out of the three fusion types compared with the healthy wrist. The modelling described in this paper may be a useful approach to pre-operative planning in wrist surgery.

Modelling Rain Drop Impact of Offshore Wind Turbine Blades

The effects of rain and hail erosion and impact damage on the leading edge of offshore wind turbine blades have been investigated. A literature review was conducted to establish the effects of exposure to these conditions and also to investigate the liquid impact phenomena and their implications for leading edge materials. The role of Explicit Dynamics software modelling in simulating impact events was then also established. Initial rain impact modelling is then discussed with the results showing good agreement with theoretical predictions both numerically and with respect to the temporal and spatial development of the impact event. Future development of the rain model and a proposed hail model are then detailed. Planned rain impact and erosion testing work is addressed which will be used to validate, inform and compliment the ongoing modelling efforts.