Fatigue Crack Growth Prediction in a Nickel-Base Superalloy Under Spectrum Loads Using FRANC3D

In this study, the fatigue crack growth behavior in an aero-engine material under a standard spectrum load sequence was predicted using FRANC3D and compared with available experimental results. Finite-element model of a compact tension (CT) specimen of GTM720 nickel-base superalloy was created using HYPERMESH pre-processor and an initial through thickness elliptical crack of 2.0 mm in length from the notch root was inserted in the FRANC3D environment. The local model (cracked region) along with global model (entire CT specimen) was then analyzed for fatigue crack growth behavior under a standard cold-TURBISTAN spectrum load sequence using FRANC3D in conjunction with MSC NASTRAN. Bilinear fatigue crack growth (FCG) law for the material GTM720 was derived from the earlier experimental constant amplitude fatigue crack growth rate data and used in the analysis. The stress intensity factor was calculated by M integral method. The SIFs at the crack front determined for an applied static load of 15 kN was closely matching with analytically determined SIFs. The predicted fatigue crack growth behavior trend was similar to experimental results. The total crack growth life predicted was about 42 blocks and conservative as against 46 blocks observed in earlier experimental work. The FRANC3D prediction was almost similar to earlier prediction made using in-house codes as well. Use of other crack growth laws and incorporation of load interaction effects during prediction may further improve the prediction accuracy.