Abstract: Through the fatigue experiment on CT speciment made of polycarbonate and the photoelastic experiment conducted at the same time, the writers find that residual stress field is formed in the neighborhood of crack tip along with the growth of fatigue crack, through it is the problem of law stress fatigue. The unloading residual stress field is gave corresponding to the number of cycles in the neighborhood of crack tip. Thus the existence of close effect is revealed and its measurement criterion is put forward. The result of the experiment shows that close effect will reduce the stress intensity factor. So the original theoretical calculating formula for stress intensity factor is no longer applicable. A simple and practical revised scheme is proposed.

Key words: Close effect, Fatigue crack, Residual stress, Stress intensity factor