doi: 10.1111/j.1460-2695.2012.01715.x Identification of fatigue crack growth mechanisms in IN100 superalloy as a function of temperature and frequency B. S. ADAIR 1 , W. S. JOHNSON 2 , S. D. ANTOLOVICH 2 and A. STAROSELSKY 3 1 George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA, 2 School of Materials Science and Engineering, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332, USA, 3 Staff Engineer, Pratt & Whitney, East Hartford, CT, 06108, USA Received in final form 26 June 2012 ABSTRACT A study is undertaken to investigate the fatigue crack growth rate properties of polycrys- talline IN100 through the identification of crack growth mechanisms as a function of temperature, frequency and K . An additional goal is to determine the stress free activa- tion energy of IN100. Constant amplitude, load controlled tests are performed at room temperature (22 ◦ C), 316 ◦ C, 482 ◦ C and 649 ◦ C under two different loading frequencies of 20 and 0.33 Hz. These specimens are then analysed via scanning electron microscopy (SEM) to determine failure mechanisms. SEM shows that, as temperature increased from room temperature to 649 ◦ C, the fracture mechanism transitions from transgranular to intergranular. The fracture mechanism is shown to transition from intergranular to trans- granular at elevated temperatures as da/dN increases as a result of growing K . Scanning electron microscopy shows that, as frequency decreases from 20 to 0.33 Hz at 649 ◦ C, the fracture mechanism transitions from transgranular to intergranular. Keywords failure mechanisms; fatigue crack growth (FCG); polycrystalline superal- loy; scanning electron microscopy (SEM); stress free activation energy; temperature and frequency effects. NOMENCLATURE a = crack length b = specimen thickness CCD = charge-coupled device CT = compact tension da/dN = change in crack length divided by change in number of cycles E = Young’s modulus/elastic modulus FCGR = fatigue crack growth rate G = crack extension force K = stress intensity factor n = time exponent P = applied load P H = hydrostatic pressure P/M = powder metallurgy Q = activation energy Q o = stress free activation energy Q th = thermal activation energy R = load ratio RFC = retirement for cause Correspondence: B.S. Adair. E-mail: benadair@gatech.edu c  2012 Blackwell Publishing Ltd. Fatigue Fract Engng Mater Struct 00, 1–11 1