Fatigue damage indicators

There are a number of indicators of fatigue damage that have attracted interest in the literature. During the life of a component subjected to fatigue, the material can exhibit changes in modulus, permanent offset strain, shape of the hysteresis loops, and temperature rise of the specimen surface. Direct evidence of matrix crack density can be obtained by surface replication, while a more detailed analysis of microstructural damage requires scanning electron microscopy (SEM). 

In order to reduce the unit energy cost, it is essential to adopt a superheated steam cycle. Operating experience of elevated temperature components in FBR indicates that creep-fatigue damage can be well taken care of in the design. Thermo-hydraulic analysis needs to be detailed to have complete knowledge of thermal loading. 

Sain, T. and C. Kishen (2008). Damage indices for failure of concrete beams under fatigue. Engineering jracture mechanics 75, 4036—4051. 

Where a condition of combined fatigue stresses exists the application of uniaxial test data may not be directly relevant. Therefore, a rigorous design assessment will require a suitable test programme. However, as a conservative approach the damage indicator method given in the EUROCOMP Design Code will yield a safe result. 

A significant difference between the appearance of fracture surfaces of fatigue specimens tested at 75 and -300 F at the same stress is that the specimens tested at -300 F show more "rubbing" action. It appears that the fracture growth is not as rapid as that in the case of the specimens tested at 75 F. The extent of fatigue damage on the specimen prior to fracture indicates the combined effects of a slower crack propagation rate and the increased tensile properties. 

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