SEDL / STP / STP1263-EB / STP16454S

Thermal Mechanical Fatigue Crack Growth in Titanium Alloys: Experiments and Modelling

Dai, J
Stress Engineer, Bombardier/Canadair, Ville St-Laurent, Québec

Marchand, NJ
R & D Engineer, AMRA Technologies, Montreal, Quebec

Hongoh, M
Project Engineer, Stress Analysis Group, Pratt and Whitney Canada, Longueuil,

Pages: 23    Published: Jan 1996

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Abstract

Strain controlled thermal-mechanical fatigue crack growth (TMFCG) tests were conducted on two titanium alloys, namely Ti-6A1-4V and Ti-6Al-2Sn-4Zr-6Mo, to evaluate the effect of phase angle between strain and temperature on the TMFCG rates. Three fracture mechanics parameters were used to correlate the data: the ΔK, ΔK^e and ΔK^eff. A fractographic study of the specimens tested under TMF was carried-out to identify the mechanisms responsible for cracking in these two titanium alloys. Hence, specimens tested under in-phase (ε^max at T^max), out-of-phase (ε^min at T^max) and counter-clockwise diamond (90° out-of-phase) conditions were compared to specimens tested under isothermal conditions (T^min and T^max) for different ΔK^eff levels. The dominant TMF cracking mechanisms were mechanical fatigue (crack tip plasticity) and oxygen-induced embrittlement. The ΔK^eff was found to be the only parameter to properly correlate all the data obtained under various testing conditions. A model is developed to predict the TMFCG rates based solely on isothermal data. The model uses a linear summation of the contributions to crack growth of the two dominant mechanisms which are active at the minimum and maximum temperature of the cycle. A discussion on the applicability of the model to predict the fatigue lives of actual components is discussed.