Mathematical modeling and validation of stress-intensity factor solutions for cracks emanating from countersunk holes using finite elements

Date
2011
Authors
Cronenberger, Jody O.
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Abstract

Cyclic load bearing structures are very susceptible to fatigue damage, which may lead to the catastrophic failure of structures over time. Metallic structures such as those used in aircraft are particularly vulnerable to this type of damage. As aircraft fleets around the world age and operate well beyond their original design life, it is becoming increasingly important to implement damage tolerance analysis methods that can accurately predict the fatigue life for these structures. A significant amount of research has been conducted that focuses on techniques used to model crack growth for a number of different configurations. However, limited amount of research has been conducted to develop an easy-to-implement and validated method for estimating crack growth for cracks growing from a countersunk hole, which is a commonly used structure in aircraft and prone to fatigue issues. This thesis focuses on developing, verifying and validating a solution database method for estimating stress intensity factors (SIFs) and geometry correction factors for common aerospace countersunk hole geometries. The results can easily be used in conjunction with linear elastic fracture mechanics theory to predict fatigue crack growth for countersunk hole geometries with remote tension loading.

Numerical analysis is performed using a finite element analysis that employs higher order elements. The numerical results are validated through a series of constant amplitude and marker band experimental tests then verified by performing a comparison with several published solutions. An error analysis is conducted to investigate errors due to modeling, experimental measurements and KI stress intensity extraction techniques. The effect of adding a countersink to a straight through hole is investigated. Finally, a series of KI stress intensity factors are calculated and a multidimensional interpolation method is presented that can be used to estimate KI stress intensity factors for most common countersunk holes found on aircraft structures.

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Keywords
countersunk hole, finite element, linear elastic fracture mechanics, stress intensity factor
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Department
Mechanical Engineering