CORRELATION BETWEEN SHORT AND LONG FATIGUE CRACK GROWTH BEHAVIOR IN SAE 0030 CAST STEEL AND AISI 1020 HOT-ROLLED STEEL (CRACKS, FRACTURE MECHANICS, ENGINEERING)

Constant amplitude fatigue crack growth behavior was inves- tigated for physically short cracks (0.3 (LESSTHEQ) a < 1 mm) and for long cracks (a (GREATERTHEQ) 2 mm) using SEN type specimens in SAE 0030 cast steel and AISI 1020 wrought steel with load ratios of R = 0.05, 1/2, and -1 at room temperature. A crack closure model which con- sidered the residual plastic zone, fracture surface roughness and environmental effects was developed to correlate these two crack length ranges by introducing a transient crack length.;The effective stress intensity factor range based upon COD measurements gave better correlation between the short and long cracks but the differences were not totally eliminated at the near threshold region. The stress intensity factor ranges corrected by introducing a material constant (,o) also gave improved relationships between the short and the long cracks but still noticable differences existed. Thus it can be considered that the crack closure effect is one of the main factors which causes the differences between the short and long cracks but is not the only factor. The corrected effective stress intensity factor ranges by using a material constant (,o) together with the crack closure effect gave better correlation than that by the above two methods at R = 0.05 and -1, except R = 1/2. The equivalent stress intensity factor ranges which were corrected by transient crack length gave the best correlation between the short and the long cracks. Thus the transient crack length could be used as a parameter which can define the physically short cracks.;It was found that the short cracks grew faster than long cracks based upon applied (DELTA)K with da/dN (LESSTHEQ) 4 x 10('-8)m/cycle and the difference in growth rates between these two crack length ranges became greater with decreasing applied stress intensity factor range. The transient crack lengths where similitude with applied (DELTA)K existed were between 1 and 2 mm for all three load ratios and two steels. Applied threshold stress intensity factor ranges for short cracks were found to be about 40 to 55 percent smaller than the corresponding threshold stress intensity factor ranges for long cracks in both steels.