Executive Summary : | The reliability and economical operation of safety critical structures and machines, such as airframes and aeroengines, depend on periodic non-destructive inspections. The optimum periodicity of inspections requires reliable estimates of service duration required for pre-existing defects to grow to detectable crack size, but not yet to dangerous dimensions. Fracture Mechanics considerations dictate that a fatigue crack will grow if associated stress intensity range, K, exceeds threshold stress intensity range, Kth, a material property similar to fatigue limit. Conventional test techniques, stipulated by standards institutions such as ASTM, ISO, and Bureau of Indian Standards BIS, involve a test procedure that involves progressive decrease of applied K through load shedding until crack arrest. However, fatigue cracks in engineering practice can grow at much lower K than Kth obtained using prevailing standard testing practice. A new test procedure that eliminates the "side effects" of load shedding has been developed by Dr. Sunder and his team at BISS Research, which has been tested for various materials, including those obtained by additive manufacturing. The new test procedure and its incorporation into industrial design software are a viable proposition.
Over the past few years, the participants of a proposed project have actively worked together to harmonize their experimental test equipment and software, allowing for the successful execution of the project without unnecessary delays. Their joint experience with similar test equipment and research practice makes it likely that the project will conclude successfully without unexpected stumbling blocks or additional time or financing requirements. References include the Aircraft Structural Integrity Program General Guidelines, the Standard Test Method for Measurement of Fatigue Crack Growth Rates, the Effect of Various Programmed Overloads on the Threshold for High-Frequency Fatigue Crack Growth, and the Combined Action of Crack Closure and Residual Stress under Periodic Overloads. |
Co-PI: | Dr. R Sunder, BISS Division ITW India Private Limited, Bangalore, Karnataka (560058), Dr. Vikram Jayaram, Indian Institute of Science, Bangalore, Karnataka (560012) |