Study of Short Crack Growth in Shot Peened 300M Steel

Author:  Amrita Bag, Dorian Delbergue, Martin Lévesque, Philippe Bocher, Myriam Brochu
Source:  ICSP-13
Doc ID:  2017073
Year of Publication:  2017
Introduction: Shot peening can enhance the fatigue life of metals as a result of strain hardening and induced compressive residual stresses (CRS) which either delay the initiation of short fatigue cracks or cause an arrest of the already existing short cracks within the CRS layer. Some studies show that shot peening does not delay the initiation but the propagation of micro cracks and that strain hardening contributes very little in arresting cracks, as compared to crack closure by CRS[1][2]. Short crack growth (including crack initiation) has been found to account for more than 70% of the fatigue life of a component [3] and therefore could better reflect the effect of peening on fatigue life. These short cracks are usually of the order of a few micrometres to a millimetre in length and have been found to exhibit crack growth rates which are much higher than conventional long Linear Elastic Fracture Mechanics (LEFM) cracks mainly due to the absence of small scale yielding and closure observed in long cracks [4]. Short cracks can be classified into two broad types based on their size and the applied stress namely: microstructurally short cracks (MSC) or Stage I cracks and physically short cracks (PSC) or Stage II cracks as schematized in Fig.1. The MSC are usually of the order of microstructural features such as the grain diameter. They exhibit an initial accelerating–decelerating growth pattern as they advance and the crack tip plastic zone encounters microstructural barriers. Gradually, the plastic zone size increases, thus increasing the crack’s resistance to the barriers. After traversing 2-3 grains [4], the effect of microstructure diminishes and crack transitions to Stage II. Its growth is then governed by Elastic-plastic Fracture Mechanics (EPFM); provided the crack is still smaller than 25 times the plastic zone size. As the crack grows further, its size exceeds this value, and after covering approximately 10 grains [4], the crack behaviour merges with that of a long LEFM crack. Shot peening has been found to primarily affect Stage I and Stage II EPFM cracks [5].

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