Browsing by Author "Friend, Clifford M."

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    Bonded Repair of Composite Structures; A Finite Element Approach
    (2009-10-28T17:17:53Z) Odi, A. R. A.; Friend, Clifford M.
    This thesis addresses the issues surrounding the application of the finite element method to analyse composite structure repairs with an emphasis on aircraft applications. A comprehensive literature survey has been carried out for this purpose and the results are presented. A preliminary study and a comparative study of different modelling approaches have been completed. These studies aim to explore and identify the problems in modelling repairso n simplec ompositep anelsw ith speciala ttention given to adhesivem odelling. Three modelling approaches have been considered: Siener's model which is an extension of the traditional plane strain 2D model used for adhesively bonded joints, Bait's model which is a promising new approach and a full 3D model. These studies have shown that these methods are complementary providing a different insight into bonded repairs. They have also highlighted the need for a new modelling approach which will provide an overall view of bonded repairs. Improved modelling approachesh ave been developedf or externallyb onded patch and flush repairs. These models enable the study of adhesive failure as well as composite adherendf ailures.T hesea pproachesh aveb eena ppliedt o real repairs and the predicted results compared to experimental data. Four case studies have been conducted: external bonded patch repairs to composite plates, a scarf joint for bonded repairs, a flat panel repaired with a scarfed patch and a repaired curved panel. These case studies have shown that bonded repairs to composite structures can be analyseds uccessfullyu sing PC-basedc ommercialf inite elementc odes.
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    Comparative study of joining methods for a SMART aerospace application
    (Cranfield University, 2007-04-19) Chau, Eric T F; Friend, Clifford M.; Allen, David; Webster, John; Clark, Daniel; Goffin, Keith
    The adaptive serrated nozzle (ASN) is one of the most promising concepts to help reduce the noise level generated by aero-engines. Shear between a hot air stream and ambient air at the nozzle exit creates noise. The serrated nozzle is designed to protrude into the air stream causing mixing between the two air streams reducing the noise level. Adaptive control system using shape memory alloy (SMA) actuators deploy the protrusion only when required in order to maximise fuel efficiency. The successful joining of NiTi shape memory alloy (SMA) to the titanium parent structure is critical to the development of the adaptive serrated nozzle. However, joining of SMAs to dissimilar metals is widely known as extremely difficult if not impossible. This research provides a preliminary study into the potential of using SMAs in large engineering applications such as the ASN and the development of viable joining methods for joining SMA to titanium based alloy. Five most favourable conventional joining methods were selected for experimental investigation. Results proved that the successful joining of SMA to dissimilar alloys was extremely difficult, joint failures were mainly due to the formation of brittle intermetallics at joint interfaces. The formation of these intermetallics occurs irrespective of the type of joining method and level of heat input employed. However, it has been shown that the formation of these intermetallics can be suppressed by the manipulation of the joint composition. A marked improvement in joint performance has been achieved for joints that contained no more than 25 at% nickel. Joint improvement has also been achieved through the addition of titanium at the joint, although further research is necessary to investigate the effect of titanium addition to joint performance.
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    Enhanced radiopacity austenitic stainless steel foil
    (Cranfield University, 2007-12-12T15:52:58Z) Craig, C. H.; Friend, Clifford M.; Edwards, M. R.
    Austenitic stainless steel designed for implant applications is used to fabricate balloon expandable coronary stents. The alloy was not designed for this purpose but has been found to work well except for relatively low radiopacity in the energy range used for stent deployment, typically 80kV to 100kV. Stents made of more dense elements such as tantalum exhibit high radiopacity in this energy range. Low radiopacity is due to a combination of tubular stents having a thin wall (strut) thickness (less than 0.13mm) and the alloy being comprised of low-density elements, approximately 2/3 iron and 1/3 chromium and nickel. To retain the desired thickness and increase radiopacity, alloy density may be increased by partial substitution with dense element(s). The new alloy must maintain the biocompatibility, corrosion resistance, non-ferromagnetic structure, strength, ductility, and fatigue- and fracture-resistant characteristics that made the original alloy attractive to stent designers. Coronary stents are subject to intensive review by regulatory authorities prior to being approved for human use, thus stent designers are hesitant to depart from accepted standards in selecting new alloys. Revising an existing alloy is the preferred approach to achieve subtle feature changes. A set of criteria was set that maintained chromium, nickel, and molybdenum within prescribed compositional ranges and diminished iron to its minimum level, allowing platinum to be substituted for approximately 1/3 the total elemental weight (wt%). Above 20wt% platinum, undesirable precipitates were found. An alloy containing 20wt% platinum, in the form of foil and at a thickness of 0.127mm, was found to be free of precipitates not found in the base or original alloy and to provide approximately 20% radiopacity increase at 80kV and 15% radiopacity increase at 100kV, exceeding minimum programme goals at 80kV and equaling those at 100kV.
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    The Stability of NiTi shape memory alloys and actuator applications
    (Cranfield University; College of Defence Technology; Department of Materials and Medical Sciences, 1999-10) Morgan, Neil; Friend, Clifford M.
    Recent research and development in the area of shape memory alloys (SMA) continues to yield novel and unique results. Of the alloys that display the effect NiTi alloys have found the most commercial interest resulting in a number of niche applications, particularly in the medical device industry. An analysis of the market and strategic issues associated with diffusing NiTi SMA's into mainstream design use and commercial application is presented. It is concluded that for SMA's to become commercially viable, R&D must be carried out on design relevant properties and particular the cyclic stability. To address this need a systematic investigation into the durability and stability of martensitic transformations in commercial NiTi actuator alloys has been completed. A factorial design on experiments approach is employed to study: alloy type, thermal processing, heating/cooling rates, applied loads and prior cold work level. In addition, the structural integrity and internal stress effects of repeated actuation against applied external stress are considered and compared with previously published data. For the first time, this work analyses the main interactive effects and individual significance of processing and operating variables on transformation stability and actuator output. The results can therefore be used as a guide to optimizing the processing and operating conditions for long term actuator stability.