Damage creep

In many high-strength alloys this creep damage appears early in life and leads to failure after small creep strains (as little as 1%). In high-temperature design it is important to make sure ... 

Surface cracks in boiler tubes, creep damage in power plant components and precipitate analysis in components subjected to high temperature and stress have been successfully assessed by this technique and necessary further inspection scheduled, depending upon the severity of the observed defect. The following is an example of the type and severity of defects and the necessary action taken in the context of power plant operations. 

Fig. 2.3 Elevated fracture toughness of alumina/20 vol.% SiC whisker composite showing a linear response up to temperatures of 1000°C. At temperatures 1100°C, creep damage results in apparent increases in toughness.

Of particular interest in the present chapter is the effect of test atmosphere on creep and creep damage mechanisms. While there are undoubtedly several factors that can promote creep cavitation and contribute to the observed changes in stress exponent and activation energy, the fact remains that the strain rates are substantially higher in air than in inert atmospheres, as shown in Fig. 8.12. This phenomenon is a direct consequence of the topotactic oxidation reaction of SiC whiskers exposed at the surface. As described by Porter and Chokshi,38 and subsequently by others,21,22 at high temperatures in air, a carbon-condensed oxidation displacement reaction occurs in which graphitic carbon and silica are formed at the whisker interface via... 

K. Chan and R. Page, Creep Damage in Structural Ceramics." J. Amer. Cer. Soc.. 

Murakami S. 1983. Notion of continuum damage mechanics and its application to anisotropic creep damage theory. Engng. Mater.Technol (105) 99-105. 

The microstructure of the tube indicated the transformation to ferrite with spheroidized carbides. Metallographic examination was also performed at the 8 00 orientation of the tube to look for creep damage, if any. The microstructure was the same, irrespective of the specimen location, and there was no evidence of creep. 

Chan, K.S. and Page, R.A. (1993) Creep damage development in structural ceramics, J. Am. Ceram. Soc. 76, 803. A review of creep behavior. 

Creep damage begins with microscopic voids in the material. In time the voids link up into fissures, and finally cracks. The voids, fissures and cracks form at grain boundaries. Creep consists of three distinct stages, primary, secondary and tertiary. Descriptions are as follows ... 

Fig. 6.97 High-magniflcation view of creep damage in an internal section of a tensile sample fractured of ARCO at 82 MPa and 1250 C a near a microcrack and b typical general cavitation damage. Bars = 10 pm [93], With kind permission of John Wiley and Sons...

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