Fatigue crack opening

Crack extension is often observed to vary significantly at the same nominal value of AK (= Y AOpos Tta) for different values of R-ratio. Elber [26] was the first to explain this observation for metals in terms of the crack closure phenomenon. He determined, by measuring specimen compliance, that fatigue cracks open and close at the crack tip at positive values of stress due to contact between crack surfaces behind the crack tip. For elastic fatigue conditions it is generally found that P p = P, and Kop = K, where P is the applied load. 

The fracture surfaces, revealed when the tube is broken open, are found to be smooth with a rippled appearance characteristic of fatigue. This type of behavior is sometimes known as leak before break. On the other hand, if the material lacks toughness, the propagation of the fatigue crack may be intermpted part way through the wall by the intervention of fast fracture, resulting in what is sometimes known as the break before leak mode of failure. 

The method can locate discontinuities open to the surface in porous materials. It is possible to detect porosity, surface cracks and through leaks, which may result from fatigue cracks, shrinkage porosity, cold shuts, grinding and heat-treat cracks, seams, forging laps and bursts as well as improper bonding between joined metals. 

The matrix fracture behavior can also be described by using stress intensity factors, K. This approach is more convenient than the /-integral in some cases particularly for short cracks and for fatigue.31,84 To apply this approach, it is first necessary to specify the contribution to the crack opening induced by the applied stress, as well as that provided by the bridging fibers. For a plane strain crack of length 2a in an infinite plate, the contribution due to the applied stress is85... 

Fig. 6.18 Schematic representation showing how a reduction in the bridging force by interface wear leads to matrix crack growth. The fibers carry the applied load across the matrix cracks, reduing the crack opening displacement and the net stress intensity (Klip) at the tip of matrix cracks. If the interfacial shear stress t decreases during fatigue, then the bridging stress p(x) decreases, leading to a reduction in Kp. This reduction increases Kfy, which can cause the further extension of a matrix crack.

Fig. 7 Development of fatigue cracks in an epoxy/glass contact under gross slip condition (1Hz, displacement amplitude 60 xm) (from [97]). White arrows indicate the occurrence of crack initiation and propagation at the edge of the contact under the action of tensile stresses. The lateral contact stiffness, K, is essentially a measurement of the elastic response of the epoxy substrate within the contact zone. Brittle crack propagation is associated to a drop in stiffness due to the additional accommodation of the imposed displacement provided by crack opening mechanisms...

The fatigue lines have been assumed recently to be stretch zones created at arrest of the jumping crack, and their width was correlated to the maximum craze width. According to the assumption the stretch zone being half the crack opening stretch and, hence, approximately also half the craze width, the line widths in rigid... 

Fig. 55. Corrosion fatigue crack growth rate in sensitized Type 304SS as a function of potential and frequency in 0.01m Na2S04 solution at AK = 20 MPa m. 250 °C. Open symbols denote transgranular mode of cracking, half-symbols denote mixed transgranular/ intergranular mode of cracking, and closed symbols denote intergranular mode of cracking [114]. Reproduced from Corrosion J. 40, 573 0984) by permission of the Editor.

From studies of service behavior and from extensive laboratory investigations, the well-established terms stress-corrosion cracking (SCC) and corrosion fatigue have been shown to relate to a continuum of failure modes classified as environment-sensitive fracture. In many environments, the addition of stress, with associated strains, introduces a variable that can result in brittle failure in the sense of very limited plastic flow in otherwise ductile materials such as the stainless steels. Environment-sensitive fractures propagate at an advancing crack tip at which, simultaneously, the local stresses can influence the corrosion processes, and the corrosion can influence the crack-opening processes. Since these processes proceed by kinetic mechanisms, they are time and stress dependent with the result that the crack propagation rate can become very sensitive to the stress application rates. Conventional SCC usually has been associated with static stress, but this is seldom realized... 

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