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In a previous report, a fatigue crack propagation model was analyzed by considering fatigue crack propagation as a sequence of fatigue crack initiation events. This permitted correlation to be made between the fatigue crack growth resistance and low cycle fatigue properties of metals. Four steels were tested for low cycle fatigue behavior at the University of Illinois. Good correlation was obtained between the theoretically predicted crack propagation rate based on the measured low cycle fatigue properties of the four metals and experimentally observed crack propagation rates that were reported in the literature. Further, good estimates of crack propagation rates can be obtained on the basis of static tensile properties of the metals alone where cyclic properties are not available. (Author).
The fatigue crack initiation behavior of chromium-molybdenumvanadium steel in high-temperature air and steam was studied in respect with the cyclic frequency and oxygen concentration in the surrounding atmosphere. Fatigue tests were conducted at 565°C in air and steam at frequencies of 10, 1, and 0.1 cpm. The number of cycles to crack initiation was determined by the potential drop technique. It was found that the fatigue life to crack initiation is significantly reduced by decreasing frequency. At a low cycling frequency, the oxygen concentration determines the reduction of cycles to crack initiation. In the case of crack initiation related to inclusions, the reduction of number of cycles to crack initiation is remarkable.
The fatigue crack initiation life of AISI 1018 steel was investigated using compact tension specimens having sharp notch root radii. The data were analyzed using two methods for predicting initiation in strain cycling experiments. Also, another approach in which initiation is related to the stress intensity factor was developed. The next phase, that of propagation, was studied using AISI 1018 steel and a new high strength steel HY-180. The crack propagation data obtained for both steels tested in air can be described accurately by the power law first suggested by Paris, da/dN = C(.delta. K)/sup n/, where a is the crack length, N the number of cycles, and C and n are material constants. However, the exponent n was found to be two times larger for AISI 1018 steel than HY-180 steel.
The clarification of uncertain factors of rolling contact fatigue (RCF) life variation is expected to lead us to better understanding of the RCF mechanism and further improvement of the service life of bearings. The objective of this study is to clarify the effect of defect location on RCF life. Artificial cavities, pores, and drilled holes were introduced to the specimens as a flaking origin under RCF for simplification on the presumption that their physical properties and interfacial rubbing between cavity and matrix were ignorable. The RCF test resulted in flakings initiated from the pore located right below the center of the track, when a specimen included numerous pores. Their RCF lives were simply determined by fracture mechanical parameters, size of pore, and orthogonal shear stress range parallel to rolling direction. On the other hand, RCF life was prolonged when the drilled hole in a specimen was located near the contact edge, although the resultant flakings appear the same irrespective of defect location. Therefore, defect location is one of the important factors for RCF life variation. The following were found through a further verification experiment and finite-element analysis: (1) A crack initiates from a drilled hole surface because of principal tensile stress at early stages of RCF irrespective of location of the hole. (2) Both of the orthogonal shear stresses, parallel and perpendicular to rolling direction, lead to the three-dimensional propagation of crack. (3) The weakening/damaging effect from a hole near the contact edge is less than that from a hole near the center of the contact track.