1-110 Section 1 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 a 0 is an initial flaw’s size, da/dN and da/dt are the fatigue and creep crack growth rates, respectively, and a c is the critical crack size. The rate of stable crack growth is controlled by the stress intensity factor. This will be discussed later. Design and Failure Analysis Using Stress Intensity Concepts The concept of stress intensity of cracked members is highly useful and practical. Three major possi- bilities are outlined here with respect to the essential framework of (1.6.7) FIGURE 1.6.10Trends 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. K∝stress crack length . 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. 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. stress intensity of cracked members is highly useful and practical. Three major possi- bilities are outlined here with respect to the essential framework of (1.6.7) FIGURE 1.6.10Trends of toughness