1-110 Section 1Crack Propagation Crack growth may be classified as either stable subcritical or unstable critical.. Often stable cracks become unstable in time, although the opposite beh
Trang 11-110 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 a0 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 1.6.10 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.
K∝stress crack length