Fatigue initial crack

The toughness of wood is important in design for exactly the same reasons that that of steel is it determines whether a structure (a frame building, a pit prop, the mast of a yacht) will fail suddenly and unexpectedly by the propagation of a fast crack. In a steel structure the initial crack is that of a defective weld, or is formed by corrosion or fatigue in a wooden structure the initial defect may be a knot, or a saw cut, or cell damage caused by severe mishandling. 

The simultaneous action of cyclic stress alternating tensile and compressive and corrosive attack is known as corrosion fatigue. Corrosive attack can be in the form of pitting. These pits function as notches, acting as stress risers and initiate cracks. Once a crack is formed, the probability of pipe failure is enhanced by further corrosion as corrosion is accelerated by action of stress. The tip of the crack deep within the fracture, the area under the greatest stress, is anodic to the wider part of the crack. As corrosion progresses, the metal at the tip of the crack goes into the solution, the crack deepens and eventually penetrates the wall of the tube. 

P.J. Laz, B.M. Hillherry Fatigue life prediction from inclusion initiated cracks. Int. J. Fatigue 20, 263-270 (1998)... 

X.S. Wang et al Low-cycle fatigue small crack initiation and propagation behaviour of cast magnesium alloys based on in-situ SEM observations. Phil. Mag. 86, 1581-1596 (2006)... 

The CCT specimens have been fatigue pre-cracked at room temperature to produce an initial crack length about 1.7 mm. Then specimen has been loaded to give K values at range of 14 MPa m/4 to 15 MPa-m A Test ended after estimated crack length has reached about 1.5 mm. 

In aluminum alloys exposed to aqueous chloride solutions, corrosion fatigue cracks frequently originate at sites of pitting or intergranular corrosion. Initial crack propagation... 

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. 

If NDI techniques can detect initial cracks as small as 10 in., then using Ui = 10 in., the estimated resulting fatigue life would be about 5 x 10 cycles. If, on the other hand, the initial crack size must be estimated from proof testing, say at 150 ksi, the assumed initial crack size for use in the fatigue analysis must be equal to the critical size for the proof stress (cr r), Eqn. (7.14). 

CF cracks are always initiated at the surface, unless there are near-surface defects that act as stress concentration sites and facilitate subsurface crack initiation. Crack initiation takes place independently of fatigue limit in air as it can be decreased or eliminated through the increase of dissolution rates at anodic sites. Localized corrosion such as pitting favors fatigue crack initiation through stress concentration and a local acidic environment. The two main mechanisms of CF are anodic slip dissolution and HE (80). 

Strength degradation with decreasing stress rate in brittle materials is known to occur by slow crack growth (delayed failure or fatigue) of an initial crack, and is expressed as ... 

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