Trade-Offs in Lighting for DIC Strain Measurements in Vibration-Based Fatigue Testing

Vibration based fatigue testing offers a fast and efficient method for determining the breaking point of a material. This method often uses a device, called a strain gage, to measure the deformation of a test sample at a single spot on the test sample. However, these strain gages often break before testing is complete. To overcome this premature break, a laser vibrometer is often used to measure the amount of movement or the speed of the test sample throughout the vibration test. The deformation measured by the strain gage is compared against the amount of movement or velocity of the test specimen as measured by the laser vibrometer to allow for future comparison after the strain gage breaks. As an alternative to strain gages, Digital Image Correlation (DIC) is a popular camera-based, non-contacting method that determines the movement and deformation across the entire test sample. In a vibration-based fatigue test, DIC is desirable because DIC can continue to monitor the deformation in the test sample even after a strain gage would have broken. However, one of the major challenges of DIC is maintaining sufficient lighting on a test sample throughout a vibration test. Large motions towards the cameras require small apertures for improved depth of field while fast moving test samples require short exposure times to reduce motion blur. In this work, the trade-offs between aperture, exposure time, gain (artificial brightness), and external lighting (ring lights and strobe lights) is examined to compare the different lighting sources and determine if artificially increasing the brightness of an image by using gain can successfully be used without negatively impacting the deformation measurement from DIC. This was accomplished by first creating a series of tests that vary the aperture, exposure time, and gain for two different light sources. Second, an analytical solution was applied to the DIC movement measurements to allow for calculation of the strain on the test sample. Third, the deformation was compared to the calculated deformation from the movement measured by the laser vibrometer to provide a common comparison point between the different lighting tests. The tests performed showed the strobe lights provided the most stable and accurate results, but the ring lights had the lowest amount of difference between measurements. Artificially brightening the image through gain typically had no impact on the calculated deformation or the variation between measurements, except in the case of extremely small apertures. In this case, artificial brightening proved to be beneficial to increase the accuracy and decrease the variation of the analytical solution.