For a single-sided girth weld in a pipeline, a fatigue crack may initiate from either outside or inside of a pipe and the crack may propagate through weld metal WM , heat affected zone HAZ or parent metal PM depending on weld profile and service conditions.
Figure 3 schematically shows the possible notch positions for consideration. Figure 3 Schematic showing three notch positions from either outside or inside the surface of a girth welded pipe. A notch is often introduced using electric discharge machining EDM , with a wire diameter typically of 0.
Figure 4 Example of fracture surface of a FCG test specimen, showing the final crack front before brittle fracture. The data scatter for the former is associated with the sensitivity of many variables in this regime, including microstructures, residual stresses, changes in crack tip geometry crack branching , mechanisms associated with crack closure and environment.
After a test is completed, the specimen is broken following immersion in liquid nitrogen depending on the material to reveal the fracture surfaces. One example is shown in Figure 4. Nine crack length measurements across the crack width with an even spacing are carried out to obtain the average final crack size which is used to calibrate the crack sizes estimated by either the DCPD or compliance method. Afterwards, it is important to check the validity of the FCGR data obtained.
The Standards 1, 2 specify several validity criteria including crack front curvature and minimum un-cracked ligament small plastic deformation ahead of a crack. In regards to the requirement for the minimum un-cracked ligament, the yield strength of the metal, where a crack propagates through, is required.
This criterion only needs to be checked for the data obtained at high K max. For testing in a marine environment for offshore application, a loading frequency of 0. For some applications, for example, lateral buckling of offshore pipelines, the loading frequency is typically very slow. Frequency scanning FS testing is often used to determine the effect of loading frequency on FCGR in corrosive environments.
Annex A2. Annex A3. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
Background information on the ration-ale for employing linear elastic fracture mechanics to analyze fatigue crack growth rate data is given in Refs 1 5 and 2. Attention needs to be given to the proper selection and control of these variables in research studies and in the generation of design data. However, data on the influence of thickness on fatigue crack growth rate are mixed.
Thickness effects can also interact with other variables such as environment and heat treatment. The potential influence of specimen thickness should be considered when generating data for research or design. The effect can be significant when test specimens are removed from materials that embody residual stress fields; for example weldments or complex shape forged, extruded, cast or machined thick sections, where full stress relief is not possible, or worked parts having complex shape forged, extruded, cast or machined thick sections where full stress relief is not possible or worked parts having intentionally-induced residual stresses.
Specimens taken from such products that contain residual stresses will likewise themselves contain residual stress. While extraction of the specimen and introduction of the crack starting slot in itself partially relieves and redistributes the pattern of residual stress, the remaining magnitude can still cause significant error in the ensuing test result. Residual stress is superimposed on the applied cyclic stress and results in actual crack-tip maximum and minimum stress-intensities that are different from those based solely on externally applied cyclic forces or displacements.
The test analyzes the crack growth rate and reports the resistance of materials to stable crack extension under this cyclic loading. Fatigue crack growth rates, from the very slow threshold regime to the fast growth rate region, are typically developed using constant load or K-controlled parameters.
Fatigue crack growth test reports include the following:. Request a Quote Contact Us. You folks are top-notch in my book. Copyright Laboratory Testing Inc. ASTM E covers the determination of steady-state fatigue crack growth rates from near-threshold to Kmax controlled instability using either compact type, C T , or center-cracked-tension, M T , specimens. Results are expressed in terms of the crack-tip stress-intensity factor range? K , defined by the theory of linear elasticity.
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