322 J. Carr et al. Fig. 30.5 Test rig including the mounting fixture, turbine blade, Whiffle tree, and pneumatic actuator Flapwise Mode 1 Flapwise Mode 2 Flapwise Mode 3 Flapwise Mode 4 Mode Pair FEA (Hz) EMA (Hz) % Diff MAC 1 14.56 14.47 -1.66 96.4 2 27.63 28.80 -4.05 98.8 3 53.04 54.22 -2.17 99.1 4 88.37 90.11 -1.93 98.9 Fig. 30.6 Correlation of free-free test to blade FEA model 30.3.2 Strain Gage Vs. DIC Strain Comparison: Static Testing For validation and calibration of the system and finite element model, a static test was performed on the blade [7]. The load was applied using a pneumatic actuator. Incremental loads of 50 lbs were applied up to a 200 lb load, and then incremental loads of 25 lbs were applied up to a 300 lb load. At each loading stage, data was taken using strain gages and DIC cameras; strains were computed using the Aramis system and the results were compared to the strain gage results. The DIC data can be used to generate a full-field strain plot over the surface of the blade. To compare the results of the strain gages and the DIC data, the results from one measurement point of the speckle pattern located in the center of the strain gages was compared to the strain gage data, as shown in the plot of Fig. 30.7. Overall, the results from the strain gage and DIC compare very well. The acceptable correlation of data from a static test validates the testing approach and permits dynamic testing to be performed with confidence. Using DIC techniques to measure strain provides full-field strain results on a structure, which is a large advantage over conventional discrete strain measurement techniques. Because strain gages only measure discrete points, unexpected strain values due to defects in the structure would only be captured if a strain gage were placed at that precise location.
RkJQdWJsaXNoZXIy MTMzNzEzMQ==