A centrally cracked thin circular disk, Part II: mixed mode fatigue crack propagation

Abstract

Under mixed mode plane stress condition, the fatigue crack growth trajectory has been determined by performing experiments on the center cracked thin disk specimen of aluminum 2024. With two mixed mode loading ratios, the subsequent propagation of the main crack has been performed by producing a series of kinks and forks. For practical applications such as the case of a flaw or a crack in aircraft or ship structure in which the loading axis (with respect to the crack) frequently changes, a zig-zag or complicated crack trajectory is very important in both experimental and theoretical studies. In order to apply any specific crack growth criterion for the analysis of the experimental results, a detailed analysis for the stresses near the crack front and the extent of yielding was given in Part I ( Paul and Khan, 1998 ). This is due to the reason that the center crack specimen shows a significant difference in the size of the plastic zone, distribution of stresses and crack tip opening displacement with the result obtained from boundary layer solution even at the load level lower than that specified by ASTM for small scale yielding in plane strain condition ( Larsson and Carlsson, 1973 , Al-Ani and Hancock, 1990 , O'Dowd and Shih, 1991 ). Betegon and Hancock (1991), Al-Ani and Hancock (1990), O'Dowd and Shih (1991) have shown that due to the loss of triaxiality of stresses at the near crack tip, the dominance of J (RICE's J -integral, 1968) is lost in this type of specimen under small scale yielding to fully plastic state. Thus, in the description of the elastic-plastic crack tip stress field , J loses the credit to be a similitude parameter. The complex crack trajectory has been analyzed on the basis of the strain energy density criterion for the prediction of propagated crack path under subsequent changes in orientation of the crack to the loading axis or equivalently saying the changes in loading direction with respect to the crack. For the prediction of the number of load cycles, Sih's strain energy density concept extended to fatigue crack growth studies by Sih and Barthelamy (1980) has been followed. These have been performed by calculating the appropriate stress intensity factors on the basis of the solution presented for kinked and curved crack by Cotterell and Rice (1980) . In order to calculate the stress intensity factors on successive kinks the procedure followed by Suresh (1983) for a doubly kinked crack has been followed. Some of the merits and demerits of the criterion have been discussed. The predicted number of load cycles corresponding to the values observed in the experiments at the different positions on the trajectory are not in close agreement.