Effect of loading frequency on fatigue crack growth under high temperature

Abstract

The effect of loading frequency on the fatigue crack growth under high temperature is studied. The high temperature fatigue crack growth tests for pure titanium (99.5%) are carried out under three loading frequencies (2, 6, 20 Hz). An elasto/viscoplastic constitutive equation which 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 failure ahead of the crack tip. By means of FEM, the mechanical fields (σij, εij, D) and the dependence of crack growth rate on loading frequency and viscoplastic strain at the crack tip are investigated in detail. The numerical simulation results of the fatigue crack growth are shown and compared with the experimental ones.The results in 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 stress 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 fatigue growth rate dl/dN is closely related to the Δεyvp obtained by numerical analysis. If dt/dN is plotted against Δεyvp, the relation can be expressed by a straight line in logarithmic coordinates for any loading frequency.