Initiation and Propagation Characteristics of Multi-Source Fatigue Crack in Superalloy Disk

Yinglong Wu,Haijun Xuan

doi:10.1051/jnwpu/20203861291

Abstract

The multi-source fatigue crack growth characteristics of a turbine disk were studied via high-speed rotating low cycle fatigue test, fracture analysis and crack growth simulation analysis. The results show that the fatigue initiation life N0 of Superalloy disk and its simulation parts is less dispersive and similar, and the S-N0 curve of the material is not conservative to predict the N0 of the disk. The fracture of eccentric hole of disk is multi-source fatigue fracture, and the surface characteristic of the crack L/D decreases rapidly from 7.5 to 2, which is obviously different from the single source fatigue crack growth. Because of the long and thin stress distribution at the eccentric hole and the small dispersivity of GH698 crack initiation life, it's relatively easy to crack on the plane A of the eccentric hole, which is prone to multi-source fatigue. At the stage of a ≥ 1.23 mm, the reverse life of crack propagation fracture of 2# and 5# eccentric holes is consistent, and the error between the simulation value of single source fatigue crack propagation life and the reverse life is below 5%. In the stage of 0.61 mm ≤ a < 1.23 mm, the simulation crack growth rate of 5# eccentric hole of multi-source is close to that of fracture inversion. Accurate simulation analysis of multi-source crack growth needs to define the initial crack accurately, at the same time, consider the influence of existing fatigue damage on crack growth.

Highlights

The multi⁃source fatigue crack growth characteristics of a turbine disk were studied via high⁃speed rota⁃ ting low cycle fatigue test, fracture analysis and crack growth simulation analysis
The results show that the fatigue initiation life N0 of Superalloy disk and its simulation parts is less dispersive and similar, and the S-N0 curve of the material is not conservative to predict the N0 of the disk
Because of the long and thin stress distribution at the eccentric hole and the small dispersivity of GH698 crack initiation life, it′s relatively easy to crack on the plane A of the eccentric hole, which is prone to multi⁃source fatigue

Summary

Introduction

劳断口指定路径( 见图 6c) 、6f) ) 进行观察。指定路径从一处疲劳源开始。图 8 为 2#偏心孔断口 A 面指定路径典型位置的 SEM 照片。由分析可知: 疲劳源区未见冶金或加工缺陷;疲劳微裂纹在合金疲劳试样表面不同位置的晶界、孪晶界或者脆性碳化物处开始萌生,沿着径向方向扩展,主要表现为沿晶开裂形式,类解理河流花样。偏心孔 A 面 σθ 随 L 的变化曲线如图 10 所示。由该曲线及图 2 中 S - N0 中值曲线可得到的偏心孔 A 面不同位置处的萌生裂纹寿命 N0L。令 ΔN0L 为各处 N0L 相较于 σθ,max = 892 MPa 处 N0L 的增量。假设在 σθ = 892 MPa 处存在一半径为 0.38 mm 的初始裂纹,并令 ΔN1p 为假设初始裂纹下裂纹扩展至长度 L 时的循环数。 ΔN0L 和 ΔN1p 随 L 的变化如图 10 所示。由该图可知: L ≤ 18.5 mm 时,ΔN0L 比 ΔN1p 小,因此,相对于 σθ,max = 892 MPa 处裂纹扩展至偏心孔 A 面各处,各处各自萌生裂纹相对容易。因而,初始时偏心孔表面易出现多源疲劳,从而具有较高的 L / d。之后,各个疲劳源萌生的裂纹相互融合,形成一个大裂纹并继续扩展,此时,L / d 下降。 [4] ABIKCHI M, BILLOT T, CRÉPIN J, et al Fatigue Life and Initiation Mechanisms in Wrought Inconel 718 DA for Different Microstructures[ C] ∥13th International Conference on Fracture, Beijing, China, 2013

Results

Conclusion