Failure mechanisms and fatigue strength reduction factor of a Cr-Ni-Mo-V steel welded joint up to ultra-long life regime

Abstract

Abstract. It is known that welded joint is much "weaker" than base metal due to discontinuities of geometry, materials and residual stresses. It seems current international design rules do not adopt a uniform approach to weld efficiency, which is often defined as the ratio of the strength of a welded joint to the strength of base metal, in their guidance for creep and fatigue design of welds. This appears to be a great barrier for the application of nuclear welded structures which has a prolonged design lifetime of 60 years. In this work, fatigue strength reduction factor of a Cr-Ni-Mo-V steel welded joint, machined from welded steam turbine rotors for nuclear power plant, was investigated by performing axially push-pull cyclic loads tests with both cross-weld and pure base metal specimens up to very high cycle fatigue regime under ultrasonic frequency at ambient temperature. The effects of residual stress, strain localization, and microdefects in mismatched steels on failure mechanisms of welds were discussed thoroughly. Results show that fatigue strength reduction factor is varied in the range of 0.95-0.975, and is found to be dependent on fatigue lifetime for the first time. It is indicated that variation of fatigue strength reduction factor are associated with transition of crack initiation from specimen surface in high cycle fatigue regime to interior micro-defects in very high cycle fatigue regime. Comparing existing codes and standards for fatigue design of welds with experimental data indicates the over-conservativeness of present code-based design method. This implies a micro-defect based fatigue design approach is required for long life safe and reliability of weldments.

Probability life calculation model of beam element application and study in welded structures reliability analysis

“…And this process is cycled 10000 times to generate the distribution samples of defects within the unit volume. In the probability simulation of this paper, referring to the research content of initial defects of welds in references, 27,39 the occurrence of initial defect size is concentrated around 0.1 mm, and the occurrence of the number of defects per unit volume is concentrated around 220. The distribution of defects per unit volume can be randomly selected from this 10 million samples.…”

The role of loading mode on the property degradation of adhesives at high temperatures

“…Adhesive bonding technology is assuming a more relevant role in the industry in the last few decades, finding greater use in high‐performance aerospace, automotive, and marine applications. Behind this growth are the very interesting advantages provided by this joining technique, especially when compared with traditional fastening methods such as welding and riveting 1–4 . A more uniform stress distribution combined with the flexibility to join many different types of materials are key advantages of adhesive bonding 5–9 .…”

“…Behind this growth are the very interesting advantages provided by this joining technique, especially when compared with traditional fastening methods such as welding and riveting. [1][2][3][4] A more uniform stress distribution combined with the flexibility to join many different types of materials are key advantages of adhesive bonding. [5][6][7][8][9] The use of adhesives in combination with other fastening techniques has also been explored by authors to improve the strength of the joints.…”

A review of thermal, microstructural and constitutive modelling of 9Cr steel for power plant applications: Towards a through-process model for structural integrity of welded connections

“…Plant components are designed to comply with various standards, such as R5 [11] or R6 [12], but these standards are generally somewhat conservative. For high-cycle fatigue applications, Zhu and Xuan [13] compared different design standards for pressure vessels and found that microstructure was not taken into account in several codes and that all were overly conservative. For example, the allowable stress amplitudes permitted by the standards ranged from 40% to 4.5% of the tested stress amplitudes for the same number of cycles to failure.…”