Fitness-for-Service Analysis Procedure – Determine Material Toughness and Crack Propagation Properties
1. Obtain appropriate KIC, COD, or J-integral data at ambient and operating temperature by one of the following:
a. Testing of samples of the actual material used for construction. (This would provide the most accurate data but is rarely obtainable.)
b. Published data for the “generic” material of construction. (The lower bound trend curve should be used.)
c. Estimate from CV-impact data for the actual or “generic” material of construction. (Correlations have been made that can be used to predict fracture toughness from CV-impact data.
However, there is considerable scatter in the correlations and they should be used only if fracture toughness data are not available.) The effect of the process environment, especially hydrogen charging upon fracture toughness, should be considered. Some data are available to indicate the degree to which the toughness is reduced by hydrogen, and a more complete data base is being obtained by industry task groups.
2. If the flaw is subjected to internal pressure or temperature fluctuations, obtain crack growth rate data (i.e., growth/cycle vs. stress intensity range/cycle) by one of the following:
a. Testing of samples of the actual material used for construction. (This is not a common test performed with the material used for construction and cannot usually be obtained.)
b. Published data for the “generic” material of construction. (The lower bound trend curve should be used.)
The process environment can have a significant effect on crack growth rate and should be taken into consideration. The growth of surface cracks can be accelerated by corrosion fatigue, and hydrogen charging can accelerate the growth of both surface and internal cracks. Little test data are available for crack growth rates in simulated process environments, and estimates may have to be made based upon knowledge of the general behavior of the material in the environment.
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