A digital image analysis method for monitoring crack growth in metal fatigue testing

Abstract

Metal fatigue tests are an everyday occurrence that updates existing fatigue libraries, ensuring that structures and components do not fail when in use. The American Society for Testing and Materials (ASTM) provides standard tests whereby certain material properties are obtained by the exact same method for each test, providing designers the information to prevent premature failure. The Fatigue Crack Growth Rate (FCGR) of a standard specimen provides information for situations where a crack may exist in components. The critical size of the crack determines when it is safe to use a component and when to discard it. Testing methods relating to fatigue crack growth propagation rates vary with respect to requirements and conditions. A wide variety of test methods can be utilised to find reliable data. One such a method uses a travelling microscope. It has been extensively used with success, but requires constant stoppages for measurements and user attention to make interval measurements. Alternative measurement methods have solved these disadvantages but have generally been of the contact and indirect types. Contact to the specimen may in some cases influence the results negatively, while indirect methods generally require previously obtained data to calibrate the results. The presented digital image analysis method has in principle the same functioning as that of the travelling microscope whilst eliminating constant user attention and stoppages. The process was automated and provided a cost saving alternative to similar products available on the market. The standard test method for measurement of fatigue crack growth rates as outlined by ASTM E647 (2002) was employed to provide standardised results. The designed and assembled test facility was put to test when a FCGR test was conducted. The set-up consisted of an lnstron 1603 Electromagnetic Resonance machine, a Nikon D70 and a PC with acquired and custom written software. The digital image analysis method provided crack growth measurements with a difference of less than 1% from the actual values. Furthermore, the end result provided a Paris equation for a mild steel specimen.