Application of fracture mechanics to failure of surface oxide scales

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Abstract
A new simple method is described for determining the dimensions of a ‘composite defect’ in surface scales by summing the actual defects present in the scale using well established techniques of linear elastic analysis. From such estimates, fracture toughness can be assessed and the stress intensity factor for growing surface oxide scales can be calculated. The technique also enables an estimate to be made of the residual internal stress of growing surface oxide scales. Data are presented for the stress intensity of the oxide scales on Nimonic 75 (which is a Ni–Cr alloy), Armco iron, and dilute titanium alloys, at temperatures in the range 20–850°C. Such data can be used to predict oxide stability on components at high temperature when subjected to applied stress or thermal cycling. MST/854