School of Industrial and Manufacturing Sciences (SIMS)
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Browsing School of Industrial and Manufacturing Sciences (SIMS) by Supervisor "Irving, Phil E."
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Item Open Access Interior frontal crash protection for passenger cars at high delta-v - possibilities and limitations(2005-03-19) Mellander, Hugo; Irving, Phil E.The purpose of this study was to analyse the possibilities to design an effective interior restraint system for the driver of a passenger car subjected to fully distributed frontal crashes in the speed range of 56 to 80 km/h (35 to 50 mph). In order to limit the research area the issue of structural integrity of the occupant compartment will not been addressed in this study. In other words it has been assumed that the frontal crush zone is large enough to absorb the kinetic energy and that the occupant compartment is not heavily deformed. A computer model of a driver restraint system including an airbag, a seat belt, knee padding, seat and a steering wheel with column has been developed with a 50%-ile crash test dummy as the driver. The computer model results have been validated by means of mechanical sled tests. A decelerating sled has been used for all the mechanical testing. The sled has been equipped with a mock-up of the frontal section of the occupant compartment of a passenger car including the steering wheel, steering column, seat and the driver restraint system consisting of an airbag, a knee restraint and a three-point seat belt. The crash pulse at §9 56 km/h, used in simulation and experimentation, has been selected to be representative for a mid size car from the nineties. To assess a realistic crash pulse for a high velocity fully distributed frontal crash with a mid size car from that period a crash test was performed at 80 km/h. A number of input parameters describing the restraint system have been selected for a two level variation analysis using design of experiment technique (fractional factorial design at two levels). The restraint configurations defined in the matrix have been run through the computer model at #G 56 and 80 km/h impact velocities. The variables with the greatest effect on the dummy response have been identified. Some of the restraint system configurations have also been tested on the mechanical sled. The results show that, for a given crash pulse, it is possible to design an interior restraint system with a low risk of injury at 80 km/h impact given that no significant intrusions into the occupant compartment occur. The importance of tuning the characteristics of the restraint systems to the condition at hands i.e. the crash severity to achieve improvements in driver protection is also demonstrated.Item Open Access Investigation of helicopter loading spectra variations on fatigue crack growth in titanium and aluminium alloys(1999-04) Buller, Richard Gregory; Irving, Phil E.An investigation has been made into the effect of omitting small, vibratory load cycles from a helicopter load spectrum on the fatigue crack growth rates of high strength titanium (Ti-lOV 2Fe 3A1) and aluminium (7010 T73651) alloys. The investigation is made in the light of new requirements for the damage tolerance design of transport helicopter structures that have normally been designed to safe life criteria. The work aims to improve the damage tolerance design of helicopter structures by understanding the contribution of the vibratory load cycles to fatigue crack growth damage. The experimental work consisted of two parts that considered fatigue crack growth under simple overload type loading and complex fatigue load sequences using compact tension specimens. Simple overload and underload tests were run under near-threshold, plane strain crack growth rates typical of those experienced in helicopter components. These were supplemented by crack closure measurements made using a strain gauge adhered close to the crack tip. Fatigue crack growth rate retardation was observed after an overload and this was reduced if a tensile underload was subsequently applied. The experimental evidence suggested that observed crack growth transient behaviour could be explained by a residual stress field mechanism ahead of the crack tip with closure only serving in a secondary role to modify the applied external loading. A fatigue load sequence was developed for a helicopter rotorhead component and included representations of manoeuvre loads superimposed with the high mean stress, vibratory load cycles. A technique of progressively omitting small load cycles of increasing range from this sequence was used to determine the effect of these cycles on the fatigue crack growth. It was found that the these cycles of 16% range caused up to 80% of the total crack length damage and that the observed crack growth rate of the cycles was three times greater than that predicted by a conservative fatigue crack growth model. These are significant observations because vibratory cycles are usually considered to be non-damaging under a safe life design to which most current transport helicopters have been certified to. It was proposed that the accelerated growth rate of these cycles was caused by frequent underloads in the rotorhead loading sequence. A residual stress field model was invoked to explain this behaviour. The results are used to provide guidance for damage tolerant design of helicopter structures.Item Open Access Safety factors and risk in fatigue substantiation of helicopter components(2005-01) Toulas, N.; Irving, Phil E.Helicopter dynamic components are substantiated against fatigue using the safe life methodology. The advance of usage monitoring systems suggest the extension of the safe life according to measured usage, leading to maintenance credits. The question is whether the extended life is associated with a different probability of failure due to elevation of safety factors embedded within the safe life methodology. The fatigue substantiation process was studied and its conservatisms were identified. Both S-N curve behaviour and service loading have been investigated. Three components were studied. One was a location on the lift frame, the others were a main rotor blade linkage and a rotor pitch change link. The variation in manoeuvre loads and usage during normal operation of a UK military helicopter was statistically estimated. Service loads were measured using strain gauged components and a data recording system. Usage was monitored by manual identification and recording of manoeuvres throughout the helicopter flight. It was found that usage variability is very great, with coefficients of variation in excess o f 100% for the majority of manoeuvres. It was found that usage measured in service was significantly more benign for all types of mission, than that assumed in design. Mechanical test results showed significant errors in damage accumulation and mean stress models for life prediction. Monte Carlo simulations demonstrate that calculated probability of failure is dominated by material and modelling errors; usage and manoeuvre load variability playing a minor role.Item Open Access Smart characterisation of damage in carbon fibre reinforced composites under static and fatigue loading conditions by means of electrical resistivity measurements(1996-02) Thiagarajan, C.; Irving, Phil E.This thesis investigates the smart damage sensing capabilities of the electrical resistivity measurement technique in carbon fibre reinforced composite materials. The static and fatigue damage mechanisms of unidirectional and cross-ply laminates are reviewed. The electrical resistivity measurement investigations of carbon fibre, epoxy resin and carbon fibre reinforced composites are described. Theoretical models related to damage and electrical conduction modelling are reviewed. The material properties, laminate fabrication, specimen design, tab design and electrode design details are presented. The experimental test set-up and test programme details are presented. The details of the formulation and algorithms of a proposed new random resistor network model and the three-dimensional unit cell assemblage model are described. The results related to the applicability and the validation of these models are presented. The influence of electrode effects on the electrical resistance are investigated. The relevant results are presented. The electrical properties of carbon fibre reinforced composites independent of specimen geometry and electrode configuration are established. The dependence of electrical resistance on the specimen geometry was investigated and the relevant results are presented. The smart damage sensing capabilities are demonstrated using computational simulation studies using the proposed new three-dimensional model. The strain sensing and damage sensing capabilities of electrical resistivity measurement technique under static and fatigue loading conditions are presented. The dependence of electrical resistance to the applied stress and fatigue life of the samples subjected to fatigue loading is presented. The smart sensing capabilities are highlighted based on the experimental and theoretical results. The static and fatigue behaviour of epoxy 914 and epoxy 920 based CFRP laminates is investigated. The failure mechanisms are established using the electrical resistance and other non-destructive test methods.