Assessment of Life Prediction techniques Using C(T) Specimen Tests

Fracture mechanics-based Life Prediction is one of the practical methods in life estimation of aerospace structures. In this approach, it is assumed that one or more initial cracks exist in the structure. The main problem in the fracture mechanics-based life prediction is to determine the initial crack size for crack growth analysis. The length of this initial crack can be measured by Non Destructive Tests (NDT). However, due to the technology limitations, NDT is not yet able to detect small cracks. The inspected initial flaw size by NDT is larger than actual flaw size. Therefore, that may yield to a conservative design. In addition, the behavior of small crack growth is complicated and is dependent on microstructures of the material. Because of this complexity, the EIFS concept avoids small crack growth curve and uses an Equivalent Initial Flaw Size in large crack growth curve instead. To assess the efficiency of the EIFS concept in the life estimation of mechanical components, a novel approach is outlined in this paper. For this purpose, experimental tests are conducted on the C(T) specimens made of 4340 steel and the number of cycles required for the crack to grow from the end of the notch up to the fracture of the specimen are counted. In fact, the fatigue cycling for the pre-crack initiation is a part of cycle count procedure. This is because a small crack can exist at the end of the notch the length of which is to be computed using the EIFS techniques. Both back extrapolation method and Kitagawa-Takahashi diagram are used in order to estimate EIFS and their results are compared. Also, fatigue life of these C(T) specimens are predicted based on conventional methods using Brown-Miller critical plane criterion. The results are compared with the experimental results.