Session: 17-01-01: Research Posters

Paper Number: 150509

150509 - Quantifying How Additional Mass Added to Vibration Fatigue Testing Assembly Impacts Strain Response. 

High cycle fatigue commonly occurs in machine components that are frequently exposed to small strains over many cycles. When designing parts for machines that will see millions of cycles of small strains, engineers use S-N diagrams to estimate the number of cycles a material will survive at a specific strain. A current commonly-used method to develop S-N diagrams is axial loading, which often tests on the order of 40 Hz. This method is very time consuming, requiring on the order of 70 hours to accumulate 10^7 cycles, representing only one data point for an S-N diagram. Axial methods also produce overly conservative fatigue life for materials in fully reversed bending (e.g. rotating blades in gas turbine engines).

One solution that addressed the issues of the axial loading method is vibration-based fatigue methods, which operate at much higher frequencies and can complete the same number of cycles much more quickly. One such method of particular interest uses a cantilevered plate and an electro-mechanical shaker to excite the plate to a particularly beneficial mode for rapid fatigue testing. This mode causes bending in the free edge of the plate opposite from the clamp, thereby generating a meaningful strain to fatigue the plate at the center of the free edge. Depending on the size of the plate and a few simple material properties, this method can test in the range of 500-2000Hz greatly improving the speed of testing. At these speeds a 10^7 cycle test takes only a few hours to complete. In addition, the data points resulting from these tests are produced from fully reversed bending giving more accurate results for components experiencing high cycle fatigue from vibrations.

To reduce material costs, the Air Force Research Lab (AFRL) developed a revised testing assembly that uses relatively small, expendable inserts clamped into the free edge of a reusable carrier plate. However, this method requires an especially strong shaker table to impose meaningfully high strains, which for certain materials may demand prohibitively high force that risks maxing out the shaker. Shakers strong enough to perform these tests are not cheap and not every lab has access to them. This work seeks explore if strategically adding mass to the corners of the assembly is a viable solution to make the AFRL testing method more available to labs with less powerful shakers. To ensure adding mass will work as a solution, we will first ensure that after adding mass to the assembly, the frequency of the desired mode remains sufficiently isolated from other neighboring modes. Next, the impacts of added mass on the resonant frequency of the two-stripe mode will be explored to ensure that testing speed is not too greatly reduced. Finally, the strain generated in the specimen and carrier plate will be determined. This will ensure that the strain in the specimen remains greater than the strain in the plate so that the plate can be reused for multiple tests. It will also be used to determine if the strain per exerted force of the shaker is increasing with added mass.

Presenting Author: Jacob Heninger Utah State University

Presenting Author Biography: Jacob Heninger is a first year Ph.D student at the vibrational fatigue lab at Utah State University. His current research focus is using added mass to make vibration-based fatigue testing more accessible to all sizes of electo-mechanical shakers.

Authors:

Jacob Heninger Utah State University
Jeffrey Wagner Utah State University
Tate Adams Utah State Universtiy
Brandon Furman Utah State University
Ryan Berke Utah State University