EFFECT OF LOADING FREQUENCY ON FATIGUE-CRACK GROWTH UNDER HIGH-TEMPERATURE

The effect of loading frequency on the fatigue crack growth under high temperature is studied. The high temperature fatigue crack growth tes ts for pure titanium (99.5%) are carried out under three loading frequ encies (2, 6, 20 Hz). An elasto/viscoplastic constitutive equation whi ch fully couples strain to continuum damage is used to analyse fatigue crack growth at high temperature. A damage criterion is employed for predicting the crack growth which is related to the actual material fa ilure ahead of the crack tip. By means of FEM, the mechanical fields ( sigma(ij) epsilon(ij) D) and the dependence of crack growth rate on lo ading frequency and viscoplastic strain at the crack tip are investiga ted in detail. The numerical simulation results of the fatigue crack g rowth are shown and compared with the experimental ones. The results i n the present study show that: (1) the stress at the crack tip element decreases rapidly with the cyclic numbers. There is no stable state, of stress distribution in the fatigue process when material damage is taken into account. (2) The crack tip stress is lower than the peak st ress and the peak stress takes place at a location away from the crack tip. No stress singularity is present at the crack tip. (3) The fatig ue growth rate dl/dN is closely related to the Delta epsilon(y)(vp) ob tained by numerical analysis. If dl/dN is plotted against Delta epsilo n(y)(vp), the relation can be expressed by a straight line in logarith mic coordinates for any loading frequency. Copyright (C) 1996 Elsevier Science Ltd.