The prediction of the remaining life of a high-temperature engineering structure that contains cracks is an inportant technological problem. Engineering structures that operate at elevated temperatures often fail prematurely under creep conditions by the growth of macroscopic cracks. Since creep crack growth is a potential failure mode, it is very important to understand the correct mechanisms by which such creep cracks grow. The present investigation has resolved certain questions about creep crack growth, especially those involving the controlling loading parameter and the effect of damage on crack propagation. The new findings contribute significantly to the understanding of creep crack growth in metals and to the prediction of lifetime of high temperature components.
The mechanisms of creep crack growth in simple metals and alloys have been investigated using both mechanical and microstructural approaches. The mechanical approach establishes the relevance of fracture mechanics to creep crack growth; the microstructural approach focuses on the microscopic mechanisms of crack growth. The primary focus of these studies has been to identify crack tip parameters which can successfully characterize the time rate of crack growth in the presence of creep deformation.
Mechanisms of high temperature crack growth were studied in several aspects. Creep crack growth in 3.5Ni-Cr-Mo-V steel has been studied by using compact tension specimen under constant load. The crack tip parameters evaluated are the stress intensity factor; K, the $C^*$-integral and the Ct parameter. The creep crack growth rate in compact specimens is best characterized by the Ct parameter. Attempts to correlate creep crack growth rate to stress intensity parameter, K and the $C^*$-integral were unsuccessful.
The crack growth rate depends weakly on temperature and varies as a $Ct^{\frac{n}{n+1}}$. These dependencies can consistently be explained by a micromechanistic model based on creep-constrained cavitation of grain boundaries ahead of the crack tip.
