13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Creep-Fatigue Crack Growth Using Digital Image Correlation Jeffrey L. Evans 1,* 1 Department of Mechanical and Aerospace Engineering, University of Alabama in Huntsville, Huntsville, Alabama 35899, USA * Corresponding author: jeff.evans@uah.edu Abstract Creep-fatigue crack growth tests are cyclic tests that have a hold period at the maximum load. This hold period, or dwell, is the period during the test when the time-dependent mechanisms operate. The length of the dwell period has been shown to affect the fatigue crack growth rate. During the hold period, crack tip blunting (due to creep deformation) and crack extension can be measured using 2-D digital image correlation (DIC). DIC requires a random speckle pattern on the flat surface being investigated. The speckle pattern is a unique pattern and, as the surface is strained, locations on the pattern will move to new positions relative to a previous strained condition. The images are compared and an algorithm calculates the strain. The camera can be positioned to view the crack tip and measure crack extension and crack opening displacement (COD). This provides a means by which the crack tip creep deformation can be determined during the hold period, providing valuable information regarding the crack tip kinetics and crack driving force. This paper will evaluate this technique for creep-fatigue crack growth measurements and provide preliminary data for a Ni-base alloy. Keywords Creep-Fatigue, Crack Growth, Digital Image Correlation 1. Introduction A number of important industrial applications require materials to be subjected to elevated temperatures, cyclic stresses, and potentially aggressive environments. Creep-fatigue occurs when a combination of these conditions are present. During the dwell period of a creep-fatigue cycle, several damaging effects at the crack tip can take place causing an increase in the crack growth rate. The crack tip driving force therefore could be influenced by a number of active processes, such as oxidation, oxygen diffusion, and creep deformation [1]. Understanding the kinetics of these various processes is essential to predicting the behavior of a material under creep-fatigue conditions [2]. Some studies and techniques have been proposed to evaluate the rates of these processes [3]. It is quite challenging to experimentally disassociate the different processes and arrive at the exact contribution provided by each process. Oxygen diffusion along the grain boundaries is clearly a culprit in embrittling some materials such as nickel-base alloys [4]. This diffusional process is, in part, driven by the crack tip stress. Rapid stress relaxation, therefore, has been proposed to reduce the crack growth rate [5]. Quantifying the re-distribution of stresses and strains at the crack tip is critical to predicting the crack growth rate during creep-fatigue. One technique developed in the last 25 years that could be used to help interrogate the crack tip during creep-fatigue is digital image correlation [6]. This noncontact experimental technique allows for the observation of the strains using an image analysis technique [7]. The surface of a test specimen has to be prepared, having a unique speckle pattern. The random speckles are then used as reference points for determining amounts of displacement and strain measurements to be calculated with respect to a starting reference image. This technique has been used to determine crack tip displacements during creep crack growth as well as fatigue crack growth. This paper gives some preliminary results using a 2-D digital image correlation system to determine the crack tip displacements and crack growth rates for the nickel alloy Hastelloy X at elevated temperature with creep-fatigue conditions.
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