“…Direct methods are based on measuring the crack length visually, optically, or by employing X‐ray radiographic testing, while indirect crack length measurements are based on monitoring a change in a physical or mechanical property of the material due to the extension of the crack. For example, the change in the material electric resistance or material stiffness, due to crack growth, has led to the development of electrical potential technique and the compliance methods for crack growth measurement, respectively. In the same manner, other methods such as infrared and thermal testing, acoustic emission, eddy current, and ultrasonic have been successfully employed to monitor the defect size.…”