Effect of stress concentrations on fatigue of composite and metallic structures

by Ross Henry Falen

Institution: Missouri University of Science and Technology
Year: 2014
Record ID: 2029677
Full text PDF: http://hdl.handle.net/10355/43389


"The complex shapes of hydrokinetic turbine blades can include part features such as a fillet, step, or hole. Situations can arise where two part features, such as a hole and a fillet, may be in close proximity which can introduce stress concentrations within the blade structure, adversely affecting the structure's life. Because the interaction between the part features isn't well known, fatigue data is needed to determine the proper analysis. In this thesis, two separate topics are discussed and investigated. The first topic deals with stress concentrations in hydrokinetic turbine blades. Several blade designs were tested and improved upon to increase blade strength and stiffness, insuring that the blade failed due to material limits. These test results were then compared with those from a finite element (FE) model replicating the physical test. Fatigue performance was also tested with an accompanying unloading stiffness test to determine the loss of stiffness within the blade. There was a good agreement between the failure loads determined from the tests and the FE model. The associated strain values had major discrepancies but followed similar trends, suggesting a strain gauge calibration error. For the associated blade geometry, the results indicate that fatigue does not play a significant role in the degradation of the blade life cycle. The second topic deals with a unique interaction between a hole and a fillet in a flat aluminum alloy specimen. The interaction between the two part features was analyzed to determine the stress concentration modification factor. For the associated geometry, the stress concentration modification factors increased as the distance between the hole and fillet decreased. For fatigue analysis of a hole interacting with a fillet, a conservative modification factor of 1.15 is recommended." – Abstract, page iii.