Referring to ASTM E 8M – 96 [8], the standard small cylindrical specimen size for tensile tests is 12.5mm in diameter. Depending on the stress amplitude during fatigue testing and the presence of mean stresses, a small amount of crack growth will occur before specimen failure, probably in the order of a few millimetres. This means however that no exact “initiation” size can be known to allow comparison with experimental data from large specimens. Therefore, any correlation between the ASME code and the experimental work presented here must remain purely qualitative. Namely, the pseudostress developed at a notched carbon steel specimen exposed to RTS can be related to number of cycles to initiation in a linear manner similar to that shown by the ASME fatigue curve. Reciprocally this means the ASME fatigue curve can be used as a guideline for determining approximate times to develop small (<1mm) deep cracks in carbon steel exposed to RTS. Crack Growth A number of observations can be made from the crack growth data obtained by relating the change in stress intensity factor ∆K to the propagation rate da/dN. • For all of the plotted points, the ASME water curves provide a conservative result (see Fig. 1). For some points this conservatism is more than an order of magnitude. • From the trend of the data represented it seems that deceleration of all observed cracks is occurring. • In specimens without steady state primary loaidng, crack growth rates begin decreasing immediately after crack initiation. • In contrast, after initiation, cracks in the specimens with large steady state primary loading experience an increase in crack growth rate as the R-ratio increases. Then, after a period of rapid growth (at around 1 mm/1000 cycles), the rate decreases rapidly. It is possible that this high rate of crack growth, followed by the quick drop off may correspond to the start of an environmentally influenced diffusion controlled crack growth. CONCLUSIONS Results obtained from a test program developed to simulate repeated thermal shock conditions produced in operating thermal power station have shown: 1. The application of a primary stress had little or no affect on crack initiation lifetime during repeated thermal shock below the creep range. A reduction of dissolved oxygen levels in the quenching water from 8ppm to 2ppm also did not significantly affect initiation times. ASME Boiler and Pressure Vessel Code, Section VII, Division 2 [2] fatigue data may be used as a guideline for determining approximate times to develop small (<1mm) deep cracks in notched carbon steel specimens exposed to repeated thermal shock. 2. Environmental and primary load interaction is highly influential in the growth of thermal shock cracks. Cracks with a low R ratio (no primary stress) show signs of rapid deceleration and cracks with high R ratio show signs of deceleration after a period of environmentally enhanced growth. The ASME provided crack growth curves for a water environment are conservative in all cases. ACKNOWLEDGMENTS This work has been completed with the assistance of an Australian Research Council grant with contributions from HRL Technology Ltd, Optima Energy, Western Power, Pacific Power and the Electric Power Research Institute of USA
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