Automated Fatigue Crack Growth Tracking System with Image Processor.

An automatic fatigue crack tracking system with an image processor has been newly developed. This system consists of a microscope with a CCD camera, an image processor, a position measuring system, an automatic stage which moves the microscope, a personal computer and a fatigue testing machine. The tracking procedures fall into the following four distinct steps. Step 1 : When a maximum load is applied to the cracked specimen, the system obtains the image around the crack tip and stores the image to the frame memory. Step 2 : After appropriate stress cycles, the system obtains the image at the same position again. Step 3 : The image processor freezes the motion and subtracts the stored image from the image obtained in Step 2. Then the subtracted image is emphasized by adding the offset brightness level. Step 4 : The dark image which is obtained by preceding procedures, is tracked with a template by the position measuring system. The amount of shift of the template represents the length of fatigue crack extension. The system moves the microscope by a distance equal to the length of the crack extension. We applied this system to the fatigue crack propagation tests on steel and Si3N4 ceramics, and confirmed the reliability of our data.