The mechanical properties of the surface of steel gears used in turbojet and turboprop engines are generally improved via thermochemical treatments such as carburizing. These treatments induce microstructural modifications and compressive residual stresses in the sub-surface and a global increase of the hardness of the surface. Hence, the complexity of the fatigue design of a gear tooth root is due to the interaction between the mechanical loading gradient and different properties gradients. This complex situation can be studied via the analysis of notched specimens subjected to bending. This work is dedicated to the fatigue crack propagation of semi-elliptical cracks in such specimens made of carburized steel.
The crack growth of an artificial defect machined at the root of the notch is derived from optical measurements made on the surface of the specimen all along the test, and from fracture analyses performed on specimens after different number of cycles. These measurements are used to derive the evolution all along the test of the value of the stress intensity factor via weight functions proposed by Shen et al. – while taking into account residual stress field due to carburizing and crack shape change during its propagation –, and the parameters of a Paris’ law.
Finally, it is proposed to predict the probability distribution of the fatigue life of the material from Monte Carlo simulations assuming this deterministic Paris’ law and an initial crack size following a Fréchet distribution. The comparison between the numerical results and the experimental evidence validates this probabilistic approach.
|Wednesday 29th November
S05-2 - Contact fatigue & Fatigue in transmission system
133 - Characterisation of cracks growth and fatigue life distribution estimation of carburized steels