Fatigue initiation

YOSHIOKA,T.- MINORU,T. Clarification of Rolling Contact Fatigue Initiation using Acoustic Emission Technique. Lubrication Engineering, Vol.51, No.l, 1995, p.41-44. 

Figure 7.45 Chromium layer (arrow) at fatigue initiation site (F). (Reproduced from COM 1999 and 2005 with permission from the Metallurgy Society of CIM)...

It is concluded that the pin failed due to fatigue initiated at the outside cylindrical surface where wear and pitting corrosion occurred. The fracture initiated at a shallow circumferential groove and corrosion of the fracture occurred after the rupture. There was no evidence of stress-corrosion cracking. 

Fracture surfaces of the cyclically deformed fatigue specimens were observed at low magnification in order to identify the zones of fatigue initiation and final failure, at higher magnifications in order to identify the regions of microscopic crack formation and growth and microscopic plastic mechanisms. 

Figure 5.43 Chromium layer at fatigue initiation site. (Figure originally published in Reference 26. Reproduced with permission of the Canadian Institute of Mining, Metallurgy and Petroleum, www.chn.org.)...

The rate dependencies of the ferroelectric material properties are also reflected in the dynamics after fatigue. Initially, most of the domain system will be switched almost instantaneously [235], and only a small amount of polarization will creep for longer time periods [194]. A highly retarded stretched exponential relaxation was observed after bipolar fatigue treatment [235], and these observations correlated well with the thermally activated domain dynamics. If the overall materials response was represented in a rate-dependent constitutive material law 236], however, then a growing defect cluster size would retard the domain dynamics considerably. Hard and soft material behaviors were also representable as different barrier heights to a thermally activated domain wall motion, as demonstrated by the theoretical studies of Belov and Kreher [236]. 

Thus far, the postulated complex movements of the bellows-pipe assembly have been described such as might have occurred with a fatigue-initiated tear in the B4 bellows. The detailed finite-element analysis of the bellows and pipe bridge behavior supporting this interpretation is described next, and then the remainder of the section examines the discharge processes for the case postulated by the Court and that which would have resulted under the circumstances outlined above. We will thus compare the developments of the vapor cloud noted by witnesses and the consequences of its ignition. 

Table 1. Comparison between Fatigue Initiation and Propagation Lifetimes for Polyacetal and Nylon-6,6 ...

Therefore fatigue initiation occurs more readily in the impact-modified versions than in the unmodified versions crack propagation is slower in the former (30). 

Equiaxed a Strength Ductility Fatigue Initiation Resist. Low Cycle Fatigue Resist. Fracture Tbughness Fatigue Crack Growth Resist. Notched Fatigue Resist. 

Coarse prior P grains Fracture Tbughness Creep Strength DuctOity Low Cycle Fatigue Resist. Fatigue Initiation Resist. 

Fine prior P grains Strength Fatigue Initiation Resist. Ductihty Fracture Tbughness Notched Fatigue Resist. 

Mixed-mode grain size Strength Fatigue Initiation Resist. Fracture Tbughness... 

Corner cracks may also initiate under fatigue loading. In fact this may be the most common form of nucleation. Nonetheless, this problem seems to have received relatively little attention in the open literature. Lefebvre and Dillard [26,27] considered an epoxy wedge on an aluminum beam under cyclic loading. They chose corner angles (55°, 70° and 90°) that resulted in one singularity. A stress intensity factor based fatigue initiation envelope was then developed. 

F. P. Ford, Prediction of corrosion fatigue initiation in low alloy and carbon steel water systems at 288C, Proceedings of 6th International Conference for Environmental Degradation of Materials in Nuclear Power Systems—Water Reactors, San Diego, TMS, 1993. 

Fig. 1.77 Synthesis of the four Phases and three Stages of fatigue that characterize fatigue initiation and propagation...

Service fracture of 4130 steel shaft with sharp circumferential notch subjected to unidirectional bending fatigue. Fig. 2.19a [22]. Fatigue initiated on the lower part where metal fibers were in traction. Beach marks, due to oxidation when... 

---