On the mechanism of fatigue failure in the superlong life regime (N>10⁷ cycles). Part II: influence of hydrogen trapped by inclusions

Abstract

High cycle fatigue fracture surfaces of specimens in which failure was initiated at a subsurface inclusion were investigated by atomic force microscopy and by scanning electron microscopy. The surface roughness Ra increased with radial distance from the fracture origin (inclusion) under constant amplitude tension–compression fatigue, and the approximate relationship: Ra ≅ CΔK 2I holds. At the border of a fish‐eye there is a stretched zone. Dimple patterns and intergranular fracture morphologies are present outside the border of the fish‐eye. The height of the stretch zone is approximately a constant value around the periphery of the fish‐eye. If we assume that a fatigue crack grows cycle‐by‐cycle from the edge of the optically dark area (ODA) outside the inclusion at the fracture origin to the border of the fish‐eye, we can correlate the crack growth rate da/dN, stress intensity factor range ΔKI and Ra for SCM435 steel by the equation and by da/dN proportional to the parameter Ra . Integrating the crack growth rate equation, the crack propagation period Np2 consumed from the edge of the ODA to the border of the fish‐eye can be estimated for the specimens which failed at Nf > 107. Values of Np2 were estimated to be ∼1.0 × 106 for the specimens which failed at Nf ≅ 5 × 108. It follows that the fatigue life in the regime of Nf >107 is mostly spent in crack initiation and discrete crack growth inside the ODA.

Effect of Hydrogen on Fatigue Strength of High‐Strength Steels in the VHCF Regime

“…The transition from a short to ultralong life appears usually as a horizontal step in the S-N curve, and the horizontal step usually corresponds to the conventional fatigue limit. [6][7][8][9][10][11][12][13] In the vicinity of a nonmetallic inclusion at the fracture origin, a dark area was often observed by optical microscopy (OM) inside a fisheye mark for specimens with a long fatigue life. Murakami et al named this area the ''optically dark area'' (ODA).…”

“…Murakami et al named this area the ''optically dark area'' (ODA). [10][11][12] This feature appeared as a very rough area when observed by scanning electron microscopy (SEM); Shiozawa et al named it the ''granular bright facet'' (GBF). [14,15] Many workers tried to explain the formation mechanism of ODA and obtained some interesting results.…”

“…[14,15] Many workers tried to explain the formation mechanism of ODA and obtained some interesting results. [10][11][12][14][15][16][17] Murakami et al considered that the formation of ODA might be related to hydrogen trapped by inclusion. [10][11][12] Hydrogen trapped by inclusion was observed by high-resolution tritium autoradiography, as shown in Figure 1, [18] and it seems that hydrogen trapped by inclusion should play an important role during the fatigue cracking process.…”

Effect of Nitriding on Very High Cycle Fatigue Properties of Developed High Speed Steel

“…(1)~ (17) ．これは高応力振幅・短寿命域における表面破壊か ら，低応力振幅・長寿命域における内部介在物を起点とした内部破壊に破壊機構が変化することによって生じ， 通常 10 6 ～10 7 回程度で現れる疲労限度が消失する現象である．したがって，機械・構造物の安全性や信頼性を確 保した疲労設計と保守・管理技術の確立のためには，10 7 回を越える超高サイクル域で生じる内部破壊の機構を解 明することが重要な課題であり，これまでに多くの研究が行われてきた (18) ．超高サイクル域で生じる内部破壊の 起点となった介在物周囲には凹凸の大きな粒状の領域，GBF（Granular Bright Facet）が観察され，塩澤らはこれ までに GBF 領域形成の機構として「微細炭化物の離散剥離説」を提案してきた (19), (20) ．また，この領域の形成が 超高サイクル域の疲労破壊を支配することを指摘してきた．内部介在物周囲に形成される特異な領域およびその 形成機構は塩澤らによるものの他，村上ら (21), (22) は ODA と命名して介在物周囲にトラップされた水素の影響と繰 * 原稿受付 2010 年 12 月 2 日 *1 学生員，富山大学大学院理工学教育部（〒930-8555 3190） *3 正員，フェロー，富山大学大学院理工学研究部 *4 正員，金沢工業大学基礎教育部（〒921-8501 石川県石川郡野々市町扇が丘 7-1） *5 (株)不二越マテリアル事業部（〒931-8511 3-1-1） E-mail: shiozawa@eng.u-toyama.ac.jp …”

Fatigue crack initiation in nickel-based superalloys studied by microstructure-based FE modeling and scanning electron microscopy

“…Consequently, crack formation was attributed to localized micro-plastic deformation. On the a Corresponding author: markus.fried@mtu.de other hand, Murakami et al [5] have described crack initiation starting from inclusions, growing to microcracks until critical stress concentrations induced crack propagation. A large portion of the total specimen life at low load levels is consumed by the crack initiation period.…”