Optical in situ investigations of overload effects during fatigue crack growth in nodular cast iron

An innovative pulsed reflection microscope (PRM) is used for optical in situ investigations of overload effects during fatigue crack growth in nodular cast iron. The optical system is based on the principle of a reflected-light microscope and can be integrated in any standard testing machines. Using the PRM the microstructural processes near the crack tip are visualized when an overload is applied. With the help of the presented optical system, the microstructural phenomena responsible for the acceleration effects in nodular cast iron are elucidated for the first time. Furthermore, scanning electron microscopy is used a posteriori to investigate the crack profiles and the fracture surfaces of the fatigue crack for the different stages of the overload event.