Experimental and Analytical Study of Fatigue Crack Growth Behavior of Nuclear Graphite under Constant Amplitude Loading

ABSTRACT ISO-85 grade graphite is commonly utilized in the nuclear industry for reactor components subjected to cyclic fatigue loads. This study aims to predict the fatigue crack growth (FCG) behavior of ISO-85 graphite under constant amplitude (CA) loading using analytical methods, subsequently validating these predictions with experimental data. The FCG rate (FCGR) was characterized under CA loading at a stress ratio (R) of 0.1, utilizing standard single-edge-notch bend specimens. FCGR data were generated from the threshold to the high-ΔK regime. All fatigue testing was conducted at room temperature using a servo-hydraulic machine with a 35 kN capacity operating at a frequency of 2 Hz. Two FCGR tests performed at R = 0.1 showed excellent correlation across all growth regimes. The analytical approach involved deriving Newman’s FCG law based on the CA data, formulated in terms of the crack driving force, ΔK, and developing a MATLAB code to implement the model. The FCG behavior of the nuclear graphite under CA loading was predicted analytically at various reference loads, specifically 70 %, 72 %, and 74 % of the ultimate load. Additionally, experimental FCG behavior was determined at a reference load of 70 % of the ultimate load. Two tests were conducted at this load level, and the average experimental fatigue life (Nexpt) was approximately 53,875 cycles to fracture. The specimens exhibited an initially slow crack growth rate that accelerated as the crack length increased. The analytical predictions were conservative and reasonably well correlated with the experimental data at the 70 % ultimate load, with a predicted fatigue life (Npred) of approximately 47,638 cycles. The resulting FCG life ratio, Npred/Nexpt, was approximately 0.88.