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CRC Press - Mechanical Engineering Handbook- Mechanics Of Solids Part 12

Tham khảo tài liệu 'crc press - mechanical engineering handbook- mechanics of solids part 12', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 1-110 Section 1 FIGURE 1.6.10 Trends of toughness degradations. Degrading factors Some chemical compositions Sharper notch Greater thickness Faster loading Lower temperature Higher yield strength Hostile chemical environment Liquid metal embrittlement Tensile residual stress Neutron irradiation Microstructural features Moisture Gases in solid solution Surface hardening Note The toughness can drop essentially to zero in some cases. Crack Propagation Crack growth may be classified as either stable subcritical or unstable critical . Often stable cracks become unstable in time although the opposite behavior cracks decelerating and even stopping is sometimes possible. Unstable cracks under load control are extremely dangerous because they propagate at speeds nearly 40 of the speed of sound in that particular solid. This means for example in steels a crack growth speed of about 1 mi sec. Thus warnings and even electronically activated automated countermeasures during the unstable propagation are useless. The only reasonable course is to provide by design and proper manufacture preventive measures such as ductile regions in a structure where cracks become stable and slow to grow allowing for inspection and repair. There are three kinds of stable crack growth each important in its own right with interactions between them possible. Under steady loads environmentally assisted crack growth also called stress corrosion cracking and creep crack growth are commonly found. Under cyclic loading fatigue crack growth is likely to occur. In each case the rate of crack growth tends to accelerate in time or with progressive cycles of load if the loads are maintained while the cracks reduce the load-bearing cross-sectional area. This common situation caused by increasing true stresses is illustrated schematically in Figure 1.6.11 where a0 is an initial flaw s size da dNand da dt are the fatigue and creep crack growth rates respectively and ac is the critical crack size. The rate of .