Browsing by Author "Mahmood, K."

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    The influence of imposed strain rate on fracture of surface oxides.
    (Elsevier Science B.V., Amsterdam., 1993-01-01T00:00:00Z) Hancock, P.; Nicholls, John R.; Mahmood, K.
    The mechanical properties of chromium rich scales formed on 304 stainless steel have been investigated as a function of deformation rate and operating temperature. At 900C at slow strain rates < 10-6 per second no cracking was observed at strains up to 10%. At rapid strain rates in excess of 10-4 per second oxide cracking was found to be independent of strain rate and controlled by the fracture toughness of the oxide. In the intermediate region, with strain rates between 10-4 per second and 10-6 in the temperature range 700 to 950C, the behaviour is determined by the creep deformation and fracture mode of the oxide. The mechanism of surface oxide failure is examined and an equation to predict cracking density over the full range of both monotonic tensile and creep fracture modes is sugg
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    Influence of strain rate on oxide fracture
    (Cranfield University, 1988-10) Mahmood, K.; Hancock, P.
    The ability of metals and alloys to form and retain protective oxide scales is crucial to their stability at elevated temperatures for extended times. Hence the identification of factors that promote or limit the integrity of oxides on high temperature materials has been the subject of intensive investigations. In the present study the mechanical properties of this chromiwm.-rkh scale on 304 stainless steel foil has been investigated in relation to the deformation rates in the substrate. It was shown that heavy cold working (up to 90%) delays the onset of breakaway oxidation and results in a very adherent scale. The cracking behaviour of the scale was found to be strain rate and temperature dependent under slow strain rate conditions when the substrate deforms by creep. No strain rate dependence was observed over the temperature range 700-900°C when faster strain rates (> 10- S sec -1) were applied. The transition between these two responses was found to vary only slightly with temperature between S.Ox10- S sec- 1 and 7.Sx10- S sec -1 ,increasing as the temperature is raised. A new method has been described for determining the fracture behaviour of oxide scale by estimating the composite defect size. From a knowledge of the onset of scale cracking, determined in Sl(U usi ng (h~ acoustic emission technique, it was possible to correlate the measured intercrack spacing with the fracture toughness from which the tensile properties of the scale can be evaluated.