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8.7 Fatigue strength

Fatigue strength is the strength of material under cyclic loading. It has unit of stress, [MPa]. Fatigue strength is usually two or more times less than ultimate tensile strength. Fatigue is characterized by fatigue strength sR, [MPa] and fatigue life N, [cycles]. R is cycle parameter, equal to the ratio of minimum and maximum stress. During the initial cycles a notched ductile steel specimen can sustain stress exceeding the ultimate tensile stress. Fatigue strength at N=105 of notched specimen is usually lower than the unnotched. The figure shows approximate values of fatigue strength for carbon steel welded joints. The fatigue strength varies over a wide range. Geometry of weld affects the fatigue strength. A machined weld demonstrates greater fatigue strength. The numbers shows percentage of fatigue strength of a uniform plate under tension. Fatigue cracks can start from all defects, but only one crack becomes dominate and results in failure. Lack of fusion on the surface is a case where the fatigue crack grows fastest. Initial manufacturing defects in welds decrease fatigue strength. The critical stress is sufficiently smaller than the static one. There are two main mechanisms of fatigue crack growth: I for small weld sizes s and II for large s. For large values of s the parameter does not affect the fatigue life of the joint. A large angle q corresponds to high fatigue strength. The effect of stress increase is higher for fatigue strength than for tensile strength. Imperfections such as offset d decreases fatigue strength of butt-welds. It creates high stress concentration, fatigue crack is initiated faster for a weld with an offset. Fatigue strength decreases for greater cross-section due to larger number of surface defects and lower ability to deform plastically. Loading with negative cycle parameter R leads to increased local plastic deformation and faster crack initiation. Fatigue strength is lower at negative cycle parameter R.

8.8 Fracture

There are three basic fractures connected with welded structures: 1) brittle fracture: fast crack propagated from a welding defect into a heat-affected zone, usually occurring within a second; 2) fatigue fracture: fatigue crack growing slowly from welding defects under cyclic loading; 3) corrosion fatigue: a crack propagated by joint action of corrosion (local embrittlement) and cyclic loading. For complex structures, the trajectory of a fatigue crack can be curved. A fatigue crack initiates from or tends to zones of maximum tensile stress. The strongest welded structures have the smallest concentration of welding defects and residual stress in the most highly loaded zones. Some welding defects can be observed at the weld surface. Defects in welds have different geometry and location: A. Hot cracks are usually curved and open. B. Cold cracks are usually straight. C. Lamellar cracks are perpendicular to the thick plate surface. Scale effect. For wider welds there is a higher probability of weld defects and fatigue cracks. Fatigue life decreases with weld size increase.

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