The objective of the proposed project is to develop physics based models which include the effects of surface condition, i.e. surface roughness in combination with residual stress state, on fatigue life. The final objective is the development of an engineering fatigue model to assess the benefits of various surface treatments on gear fatigue life so that performance of rotorcraft powertrains can be improved. Many efforts have sought to improve helicopter gear performance, but a main obstacle has been the lack of a software tool to accurately predict fatigue life to shorten the necessary endurance testing of transmissions and other critical powertrain assemblies. A quantitative materials engineering approach is proposed, using an internal state variable mechanical model that will capture the complicated mechanical behavior associated with residual stress formation, as well as the cumulative strain hardening/softening associated with cyclic fatigue loading. A set of related micro- and macro- models will be developed to model surface and microstructural interactions that result in cyclic softening or hardening. In addition, the use of simplified coupon geometries to represent complex part geometries and service stress states will be achieved through judicious application of finite element based models and statistical methods such as the response surface.
Benefit: The primary benefit to the Navy and helicopter OEMs will be the utility of having an engineering based design tool to correlate gear surface quality levels with projected life and load carrying capacity. The development this fatigue life predictive engineering tool will greatly shorten the time for determining processing and material effects on fatigue life. Cost of development will also be significantly reduced. These are critical metrics for development of new rotorcraft propulsion systems. The proposed technology also has myriad cross over potential for any gear components where service life and transmission power are critical. Ground vehicle applications for military and commercial vehicles, heavy trucks, automobiles and locomotive engines are all examples of additional vehicle systems which will benefit from the ability to improve gear design and processing by the proposed technology.
Keywords: Gear Surface Rougness, Gear Surface Rougness, gear steels, Modelling, fatigue life, Loading Stress Response, superfinishing, Surface Roughness
