Crack closure effects play an important role in dominating small crack propagation behaviors, which were rather less well investigated, especially at high temperature in the air. Based on photomicroscopy and digital image correlation, small crack growth behaviors and growth rates are investigated both at 600℃ and RT in air for a Powder Metallurgy superalloy, then the crack displacement fields are measured. Two max. stress levels and two stress ratios are considered in order to understand their effects on small crack growth behaviors. The experimental results reveal the crack growth behaviors ranged from 80 m to ~1000m. With the help of EBSD at the grains of the crack growth path, links of this particular growth behaviors with the microstructure features, such as the orientation, grain boundary, are discussed. Using DIC-measured crack opening displacement with the crack growth, the roles of oxides and roughness induced crack closure in early small crack propagation at high temperature are analyzed. Finally, a crack closure model is proposed including the combined effects of oxide-induced, roughness-induced and plasticity-induced crack closure (OICC, RICC and PICC).
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