An Improved Magnetic Technique for the Non-Destructive Detection of Faults in Embedded Steel Reinforcement
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An Improved Magnetic Technique for the Non-Destructive Detection of Faults in Embedded Steel Reinforcement Peter W. Haycock1,3, Simon D. Brown2,3, Nicholas Tomlinson1 and Matthew J. Hocking3 1 Birchall Centre for Inorganic Chemistry and Materials Science, School of Chemistry and Physics, Lennard-Jones Laboratories, Keele University, Staffordshire, ST5 5BG, UK. 2 XmaS, ESRF, Oliver Lodge Laboratory, University of Liverpool, Oxford Street, Liverpool, L69 72E, UK. 3 Magnon Inspection, Science Park 1, Keele University, Staffordshire, ST5 5SP, UK. ABSTRACT There are many situations in which steel reinforcement within structural materials is vulnerable to corrosion. In some cases the result of such deterioration of the structural integrity can have significant safety implications. However, there is currently little by way of reliable non-destructive means of obtaining direct evaluation of the state of the embedded steel, which yields anything more than rudimentary information. This paper considers the possibilities of making use of the vectorial nature of the magnetization of steel to improve the qualitative and quantitative evaluation of the integrity of embedded steel reinforcement in vehicle tyres. Results are presented which show that enhanced non-destructive mapping of faults within tyres can be achieved by this means. INTRODUCTION Steel is used as a reinforcing material in a range of composite materials which have application in situations where structures are expected to be, or at risk of being, in tension. Being the reinforcing material embedded within a matrix, direct observation of the state of the steel is often impossible. In certain cases, indirect means can be used to assess the likelihood that corrosion of the steel has occurred. However, determination of the actual amount of corrosion that has taken place (and hence, importantly, the amount of good steel remaining) is largely an unsolved problem. One composite material in which corrosion to steel reinforcing materials has safety implications is vehicle tyres. Penetration of sharp objects through the outer rubber as far as the steel plies can occur without a deflation of the tyre. If the object is subsequently ejected from the tyre, the damage is unnoticeable, but comprises a small hole which may be sufficient to allow ingress of moist air. Under the conditions encountered within a tyre in-service, i.e. elevated temperature and pressure and a constantly changing stress distribution, this can lead to significant corrosion of the steel cords extending several centimetres from the initial hole. The result can be the familiar shedding of tyres on the road or failure of a tyre carcass during subsequent retreading. Around 25% of in-service failures of retread tyres are attributable directly to undetected rust. Such rust is not easy to find, since during the retreading process the rubber is removed to just above the top reinforcing ply and so the steel is not exposed for visual inspection. Latent rust which has not yet caused separation between the layers in the ty
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