Creep microcrack

The final stage of creep is known as tertiary creep at which time the creep rate increases rapidly culminating in failure. This acceleration in creep is due mainly to the formation of voids and microcracks in the material which form along the grain boundaries causing the fracture path to be predominantly intercrystalline. 

Interfacial cavitation, microcracking, diffusion and crack branching due to the viscous flow of the glass phase during creep-fatigue. 

Microcracks are also known to occur in the elevated temperature creep of alumina with little or no preexisting glass phase. For example, the development of microcracks during creep fracture of two hot-pressed aluminas, which were free of grain boundary glass films, was studied by Wilkinson et al.52 who employed tensile and four-point bend specimens. They found that the concentrations and morphology of the cavities and microcracks were strongly... 

The creep of concrete will be related to the presence of C-S-H gel as well however, in this case the microcracks in the paste-aggregate interface will be also important [130],... 

The mechanisms responsible for fracture in structural ceramics at elevated temperatures have been reviewed [154]. Sensitivity to flaws or microstructural inhomogeneities which nucleate microcracks are among the failure mechanisms. The flaws which control failure under creep conditions are different from those responsible for fast fracture at room temperature. A common feature is the development of cracks through gradual damage accumulation, depend on the microstructure. The role of cracks in the deformation and fracture behavior of polycrystalline structural ceramics have been reviewed [155]. 

To determine if the composite would provide stress oxidative stability after the matrix is microcracked, stepped stress-mpture tests were carried out. The 0/90° cross-ply Nicalon/BN/SiC/BSAS composites, containing 40 volume% of fibers, were tested in standard dead weight creep machines at 1100°C and 1200°C in air. No testing was done at... 

For a more detailed representation of all data including the smaller events, AE locations in the vertical profile SI are given in Fig. 11.20. The location of profile SI which is a cross-section through the cavities is indicated in the plan view of Fig. 11.19. Data from a time interval of one month - about 10,000 events - from a spatial interval of 20 m thickness were projected into the cross-section. The AE locations in Fig. 11.20 show a dense accumulation of events between the comers of the higher and lower rooms. The reason for the high microcrack activity are "bandlike" stmc-tures of high shear stress leading to creep deformation of the rock salt accompanied by dilatancy (Spies et al. 2004). 

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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