Research On The Crossing-Double-Crack Fatigue Propagation

Fatigue crack propagation is one of the major failures of structures in engineering. The multiple cracks may exist in structures because of various reasons. It is more complicated to study the multiple-crack problems owing to the crack interactions. In this thesis, finite element numerical simulations and experiments were conducted on the propagation behaviors of the crossing-double-crack in the Q345R plate. The main work and conclusions were drawn as follows:Firstly, fatigue tensile experiments were performed on the specimens with pre-existed mixed mode single cracks or crossing-double-cracks. It was found that new cracks were initiated at the tips of the pre-existed mixed mode single cracks. After the initiation, the new cracks grew in the direction which is perpendicular to the applied load. With the propagation of the cracks, the mode I stress intensity ?KIincreases and crack mode transferred from the I+II mixed mode to mode I. For the crossing-double-cracks, the new cracks were initiated at the crack tip with the larger mode I stress intensity factor ?K?. The mode I stress intensity factor AKI increases with increasing of ? which is the angle between the applied loading direction and the longer crack. Or in other words, the crack is more easily initiated and the crack propagation rate is faster when the angle ? is bigger.Secondly, the interactions between the long cracks and the short cracks were studied. It is found that in the crack configurations studied in this thesis, the long crack propagation is seldom influenced by the short crack. The mode I stress intensity factor ?K? at the tip of the short crack decreases with the long crack propagation. But the mode ? stress intensity factor ?K? is increases which means that the short crack mode changes from the mixed mode crack to the mode ? crack.Thirdly, the fatigue crack propagation rate was calculated for the mixed mode single crack and crossing-double-crack propagation. The results indicate that the mode I stress intensity ?K? plays a leading role when the crossing-double-crack propagates. Therefore, the propagation rate of the crossing-double-crack da/dN can be expressed as the function of the ?K?, It turns out that the relationship between the crack propagation rate and mode I stress intensity factor for the crossing-double-crack conforms to the Pairs formula. This result is in accordance with that of the mixed mode single crack.Finally, metallographic and fractography analyses were carried out. Metallographic analysis shows that the fracture mode for both mixed mode single crack and crossing-double-crack is transgranular fracture with no obvious secondary cracks. Fractography analysis shows that big differences were found at the different stage. Clear fatigue striations were observed at the propagation area. The distance between the fatigue striations becomes greater and greater with the crack propagation. There exists a larger number of dimples, which implies the material is good at plasticity.