Low Cycle Fatigue Crack Propagation Resistance of Materials for Large Welded Structures

The crack propagation properties for ultrafine-grained Zn -22 wt % Al alloy during low cycle fatigue (LCF) in the superplastic region and the non-superplastic region were investigated and compared with the corresponding results for several other materials. With the Zn - 22 wt % Al alloy, it was possible to conduct LCF tests even at high strain amplitudes of more than ±5%, and the alloy appeared to exhibit a longer LCF lifetime than the other materials examined. The fatigue life is higher in the superplastic region than in the non-superplastic region. The rate of fatigue crack propagation in the superplastic region is lower than that in the other materials in the high J-integral range. In addition, the formation of cavities and crack branching were observed around a crack tip in the supereplastic region. We therefore conclude that the formation of cavities and secondary cracks as a result of the relaxation of stress concentration around the crack tip results in a reduction in the rate of fatigue crack propagation and results in a longer fatigue lifetime.

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The Fatigue and Fracture Analysis of Steam Turbine Rotor Shafts Containing Defects

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.795.254 ◽

2019 ◽

Vol 795 ◽

pp. 254-261

Author(s):

Shang Wang ◽

Wei Qiang Wang ◽

Ming Da Song ◽

Hao Zhang

Keyword(s):

Fatigue Crack ◽

Crack Propagation ◽

Steam Turbine ◽

Fatigue Crack Propagation ◽

Low Cycle Fatigue ◽

Turbine Rotor ◽

Initial Surface ◽

Cycle Fatigue ◽

Steam Turbine Rotor ◽

Fatigue Crack Propagation Life

In this study, the assessment and calculation methods for the crack propagation life of steam turbine rotor shafts containing defects are presented. The analytic methods for estimating the average stress and the alternating stress amplitude of the steam turbine rotor shafts are introduced. The defects on/in the rotor shafts were regularized by the method of fracture mechanics, and the high cycle fatigue crack propagation life and low cycle fatigue crack propagation life of the rotor shafts are estimated from Paris formula. Taking the 60MW turbine rotor shafts containing an initial surface defect and an initial internal defect as the examples respectively, the crack propagation life of them were calculated. The results indicated that the assessment method for the crack propagation life can preliminarily be both used to estimate the safety-operating life and to analyze the fracture reason of a steam turbine rotor shaft containing defects. This paper can provide reference for periodic maintenance and safety evaluation of turbine rotor shafts.

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Enlarged Zn, Mg Contents with a same Zn/Mg Ratio Improve Fatigue Crack Propagation Resistance of Al-Zn-Mg-Cu Alloys with T7651 State

Materials Science Forum ◽

10.4028/www.scientific.net/msf.1003.3 ◽

2020 ◽

Vol 1003 ◽

pp. 3-10

Author(s):

Kai Wen ◽

Bai Qing Xiong ◽

Hua Zhou ◽

Xi Wu Li ◽

Zheng An Wang ◽

...

Keyword(s):

Fatigue Crack ◽

Crack Propagation ◽

Fatigue Crack Propagation ◽

Aging Treatment ◽

Fracture Morphology ◽

Precipitate Size ◽

Propagation Resistance ◽

Crack Propagation Resistance ◽

Cu Alloys ◽

The Difference

The fatigue crack propagation of Al-Zn-Mg-Cu alloys could be influenced by the content of main alloying element. In the present work, two Al-Zn-Mg-Cu alloys with a same Zn/Mg ratio were treated by two stage over-aging aging treatment and typical T7651 states were extracted via mechanical properties. Fatigue crack propagation of the two alloys were tested and the related precipitation characteristics and fracture morphology were observed. The results showed that the alloy with higher Zn, Mg contents possessed a better fatigue crack propagation resistance compared with the alloy with lower Zn, Mg contents. The corresponding fracture morphology also showed the difference of fatigue striation, which provided an additional support. The precipitation observation demonstrated that the both alloys possessed GPII zone, η' phase and η phase while the alloy with higher Zn, Mg contents had a larger average precipitate size and a larger proportion of large size precipitates compared with the alloy with lower Zn, Mg contents. Cut and bypass mechanisms of dislocation-precipitate interactions were used to explain the difference of fatigue crack propagation between the two alloys.

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Comparison of the Fatigue Crack Propagation Resistance of α+β and β Titanium Alloys

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.378-379.117 ◽

2008 ◽

Vol 378-379 ◽

pp. 117-130

Author(s):

Matteo Benedetti

Keyword(s):

Fatigue Crack ◽

Crack Propagation ◽

Titanium Alloys ◽

Fatigue Crack Propagation ◽

Propagation Resistance ◽

Microstructural Parameters ◽

Crack Propagation Resistance ◽

Microcrack Propagation ◽

Small Cracks ◽

The Relationship

The present paper tries to summarize the relationship between microstructure, extrinsic mechanisms and fatigue crack propagation resistance of α+β and β titanium alloys. Emphasis is placed on microstructural parameters, which can be varied by processing, and their effects on the material inherent fracture properties, governing the resistance against microcrack propagation. Moreover, the resistance against macrocracks as well as small cracks in the presence of notch plasticity has been discussed on the basis of secondary extrinsic mechanics such as crack front geometry, crack bridging and crack closure.

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