A closure model for predicting crack growth under creep-fatigue loading

Creep and fatigue are two important phenomena that lead to deformation and ultimately failure of components. While creep involves steady loads, fatigue consists of alternating loads applied to a component. Generally, failure of structural components by creep-fatigue crack growth occurs when mechanical loading and elevated temperature act concurrently, conditions that are common in power plants, chemical plants and the aerospace industry. Such knowledge has contributed to much research on creep-fatigue crack growth at elevated temperatures. Regardless, comprehending the mechanics of crack growth and predicting the measured rates of crack growth during creep-fatigue loading is still an outstanding issue. Contemporary research has recently established that previous publications overlooked the influence of creep loading on the mechanics of crack-tip opening and closing during fatigue loading. Consequently, a recent study has proposed the adoption of the strip-yield modeling for tackle this problem. In fact, over the last few decades, several strip-yield models have been developed for fatigue crack growth, thermomechanical fatigue crack growth and creep crack growth. Unfortunately, until recently, there have been no strip-yield models developed for creep-fatigue crack growth.

In a recent publication, Professor Gabriel Potirniche from the Mechanical Engineering Department at University of Idaho developed a Strip-Yield Model for Creep-Fatigue Crack Growth (SYM-CFCG) using the weight function method to capture the displacement fields around a crack tip. In this model, a strip-yield methodology was used to simulate creep-fatigue crack growth and to quantify the influence of hold time on plasticity-induced crack closure during creep-fatigue crack growth. His work is currently published in International Journal of Fatigue.

Technically, Professor Gabriel Potirniche computed the creep fatigue crack growth rates by summing the growth rates during the creep and fatigue portions of each loading cycle. In his approach, the loading interaction effects on crack growth rates were accounted for by modeling the decrease of crack-tip opening stress (S_op) with longer hold times. Overall, he analyzed several specimen geometries commonly used in laboratory testing of crack growth, including: the middle-tension test, single-edge notch test, double-edge notch tension and compact-tension test specimens. Lastly, he used the SYM-CFCG to compute crack growth rates in a nickel-base superalloy and austenitic 316 stainless steel.

The author reported that the longer the hold time, the larger the creep deformation and crack opening displacements in the near crack-tip region. Consequently, he observed that the aforementioned effect leads to a decrease in the crack-tip opening stress/load and faster crack growth rates during the subsequent fatigue cycle. In other words, the opening stress dropped to the minimum stress in the cycle for longer hold times, which implied that the crack was fully open at minimum load.

In summary, the study presented a novel model to study the influence of creep hold times on fatigue crack growth during creep-fatigue loading. Basically, the presented model aimed at advancing the idea that a decrease of plasticity-induced crack closure is experienced by the crack during fatigue loading when a longer hold time is applied each creep-fatigue cycle. Remarkably, Professor Gabriel Potirniche demonstrated that enhanced creep at the crack tip and the related stress relaxation in the crack-tip plastic zone lead to increased crack opening displacements and diminished compressive stresses for the entire crack surface at minimum load in the cycle. In an interview with Advances in Engineering, he further pointed out that his new model could predict the loading frequency effect and the synergistic action of creep and fatigue loading on crack growth rates. He further stated that his approach offered a template for how crack growth predictions could be performed under variable creep-fatigue loading with short to moderate hold times.