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INSPECTION, EVALUATION AND REPAIR OF HYDRAULIC STEEL STRUCTURES Episode 8

Tham khảo tài liệu 'inspection, evaluation and repair of hydraulic steel structures episode 8', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | EM 1110-2-6054 1 Dec 01 Figure 6-18. Relation between dimensions of a discontinuity and the parameter ã for surface discontinuities. Extracts from British Standards Institution 1980. Complete copies of the standard can be obtained by post from BSI Publications Linford Wood Milton Keynes MK14 6LE c. All steels have microscopic discontinuities and welded structures always contain larger discontinuities due to the welding process. Thus the main concern in fatigue assessment of welded structures is to determine the crack-propagation life before the critical crack size is reached that results in brittle fracture. The life of a structural component that contains a crack is governed by the rate of subcritical crack propagation. d. Fatigue analysis methods described in paragraphs 6-7 and 6-8 are based on extensive analyses of test results from numerous specimens. Variation in test data is large and inherent uncertainty exists in defining load and strength parameters. Therefore fatigue life predictions should be used as a means to evaluate a reliable service life not to actually predict when a structure will fail. Fatigue analysis is needed when the remaining structure life and the crack growth rate are necessary for developing the inspection and maintenance scheduling for a distressed structure as discussed in paragraph 6-11. An example of the estimation of fatigue life from Sr-N curves for a gate with a vibration problem is given in Chapter 7. 6-7. Fatigue Crack-Propagation The fatigue crack-propagation behavior for metals is shown in Figure 6-21. Figure 6-21 is a plot log10 scale of the rate of fatigue crack growth per cycle of load da dN versus the variation of the stress-intensity factor AK The parameter a denotes crack length Nthe number of cycles and AK the stress-intensity factor range KImax to KImn Based on Figure 6-21 fatigue-crack behavior for steel can be characterized by three regions. Barsom and Rolfe 1987 describe these regions in more detail. 6-14 EM .