Enhancement of the fatigue strength assessment of welded components by consideration of mean and residual stresses in the crack initiation and propagation phases

Abstract

The starting point of the investigations presented in this paper is the simplified consideration of mean and residual stresses as well as the complete disregard of the complex failure mechanisms in the common fatigue standards and guidelines. An improved fatigue life assessment for welded components, which includes the crack initiation and propagation phase is the scope of this paper. The crack initiation phase is considered using the strain-life approach. The crack propagation life, starting from a technical crack of a depth of a = 0.5 mm, is derived by the application of linear-elastic fracture mechanics. In this two-phase approach, the mean and residual stresses are considered by using damage parameters for the initiation phase and effective stress intensity factors for the propagation phase. With this approach, a better understanding can be obtained of the influence of mean- and residual stresses and their influence on the course (slope k and knee point Nk) of the S-N curve. With this information to hand, a more reliable fatigue assessment can be achieved than is possible with the common standards and guidelines. With the proposed two-phase approach, higher permissible stresses can be allowed in the design phase, leading to resource-efficient, lightweight constructions

Crack Initiation and Propagation Fatigue Life of Ultra High-Strength Steel Butt Joints

“…The threshold crack depth defining the line between crack initiation and crack propagation life is set to a th = 0.5 mm for all investigated specimens [3,4,63]. According to Equation (5), the corresponding surface crack length is 2c = 2.9 mm, giving a rather low crack aspect ratio of a/c = 0.345.…”

Fatigue crack initiation assessment of welded joints accounting for residual stress

“…54 Furthermore, because the transition size is generally small, the observation of such small cracks needs extensive efforts, making the fatigue tests complex. One of the reasons may be that the definition of the transition crack size between the crack initiation and propagation is ambiguous and various definitions exist.…”

“…One of the reasons may be that the definition of the transition crack size between the crack initiation and propagation is ambiguous and various definitions exist. 54 Furthermore, because the transition size is generally small, the observation of such small cracks needs extensive efforts, making the fatigue tests complex. Therefore, limited crack initiation life data is available.…”

Evaluation of Thermal Fatigue Life and Crack Morphology in Brake Discs of Low-Alloy Steel for High-Speed Trains

“…The crack initiation stage can improve the accuracy of crack life assessment, which was already established by recent experimental studies [ 14 , 15 ]. Baumgartner et al [ 16 ] describe the crack initiation phase with stress or strain-based concepts using an experimental approach in which a 0.5 mm crack size is treated as the crack initiation stage. Heikki Remes et al [ 17 ] studied the crack initiation period by using the strain-based approach.…”