Steel crack propagation

In the case of the fibrous laminate not much work has been done, but it has been observed that a significant loss of stiffness in boron—aluminum laminate occurs when cycled in tension—tension (43,44). Also, in a manner similar to that in the laminated PMCs, the ply stacking sequence affects the fatigue behavior. For example, 90° surface pHes in a 90°/0° sequence develop damage more rapidly than 0° pHes. In the case of laminates made out of metallic sheets, eg, stainless steel and aluminum, further enhanced resistance against fatigue crack propagation than either one of the components in isolation has been observed (45). 

Fig. 2-22 Effect of stress intensity range dJC on crack propagation for a notched specimen of X60 steel in 3.5% NaCl solution at 0.1 Hz.

Embrittlement embrittlement and for improperly heat treated steel, both of which give intergranular cracks. (Intercrystalline penetration by molten metals is also considered SCC). Other steels in caustic nitrates and some chloride solutions. Brass in aqueous ammonia and sulfur dioxide. physical environments. bases of small corrosion pits, and cracks form with vicious circle of additional corrosion and further crack propagation until failure occurs. Stresses may be dynamic, static, or residual. stress relieve susceptible materials. Consider the new superaustenitic stainless steels. 

The stress corrosion resistance of maraging steel has been evaluated both by the use of smooth specimens loaded to some fraction of the yield strength and taking the time to failure as an indication of resistance, and by the fracture mechanics approach which involves the use of specimens with a pre-existing crack. Using the latter approach it is possible to obtain crack propagation rates at known stress intensity factors (K) and to determine critical stress intensity factors (A iscc) below which a crack will not propagate (see Section 8.9). 

A further estimation of the corrosion resistance of maraging steel can be obtained from data on the rate of crack propagation. Although the rate of crack propagation has been found to be a function of stress intensity in some alloys, for many alloys and heat treatments there is a range of stress... 

Table 3.30 Crack propagation rates for a number of high strength steels ...

Stewart, A. T., Effect of hydrogen on fatigue crack propagation in steels , Proc. Int. Conf. on Mechanisms of Environment Sensitive Cracking in Materials, University of Surrey, 4-7 April 1977, The Metals Society, pp. 400-11 (1977)... 

James, L. A., Fatigue crack propagation in Austenitic stainless steel . Atomic Energy Review 14, 37-86 (1976)... 

Ritchie, R. O., Near threshold fatigue crack propagation in steels . International Metals Reviews, Nos. 5 and 6, pp. 205-230 (1979)... 

The NACE publication Corrosion Fatigue gives a comprehensive account of all aspects of the subject, and in this work a review of the application of fracture mechanics for studying the phenomenon has been presented by McEvily and Wei , whilst Kitagawa has given a detailed account of crack propagation in unnotched steel specimens. This work should be consulted for details of testing and interpretation of results. 

Figure 4. Comparison of the Fracture Appearance of the Unexposed Heats of Type 21-6-9 Stainless Steel (a) Conventionally Forged, and (b) HERF. Arrows Indicate Crack Propagation Direction.

Figure 9. Fracture Appearance of Conventionally Forged Type 21-6-9 Steel after Exposure to Hydrogen Gas (34MPa) (a) 100 X (b) 500X. Arrow Indicates Direction of Crack Propagation.

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