FATIGUE CRACK GROWTH MODEL FOR COLLINEAR SEMI-ELLIPTICAL CRACKS INCLUDING COALESCENCE AND RECHARACTERIZATION

摘要

The coalescence and recharacterization of two adjacent semi-elliptical cracks as they evolve into a single crack has for some time been an interesting and challenging research topic. The design codes which provide guidance on the treatment of fatigue crack growth and coalescence are the ASME Boiler and Pressure Vessel Code Section XI and the British Standards Code PD6439. This paper presents a computer prediction crack growth model which investigates the fatigue crack growth and coalescence of several collinear semi-elliptical cracks. So far the coalescence process which encompasses a significant portion of the total fatigue life is not clearly understood, therefore this investigation includes in the model empirical equations based on the crack shape development of coalescing cracks. Experimental results of fatigue crack growth and crack shape development, including the coalescence phase, is also presented for semi-elliptical surface cracks in steel plate specimens loaded in four point bending. In these tests the fatigue crack growth and crack shape development arc monitored using a novel mobile DCPD (direct current potential drop) probe arrangement. The results from these experimental tests are used to develop empirical relationships which describe the coalescence and recharacterization of adjacent cracks in the prediction model. The results from the prediction model are compared with the results obtained using the guidance rules given in the international codes.