PSI - Issue 57

Haelie Egbert et al. / Procedia Structural Integrity 57 (2024) 179–190 Haelie Egbert et al. / Structural Integrity Procedia 00 (2019) 000 – 000

183

5

Figure 3. Gear STBF tooth image from high-speed camera setup The lens specifications (focal length and aperture) and camera specifications (sensor size and sensitivity) were chosen such that the field of view (FOV) and depth of focus were acceptable with the lighting system the test machine was able to accommodate. A Tamron SP 90 mm F/2.8 macro lens with maximum 1:1 image size reproducibility (magnification ratio) was chosen and paired with a Photron Nova S6 camera utilizing a square 1024 x 1024 pixel CMOS image sensor with color filters. Eq. (1) was used to calculate the FOV of this setup, where and are the horizontal and vertical dimension that the image spans, and ℎ are the camera sensor width and height and is the magnification of the lens. The Photron Nova S6 uses a square sensor of dimensions ,ℎ = 20.48 x 20.48 mm and the maximum magnification ratio of the Tamron lens is 1:1 at the closest focal distance. This produced a minimum FOV of 20.48 x 20.48 mm. 3.2 Camera Trigger and Image Phasing The camera used was equipped with 32GB of memory buffer before recording had to stop, which was not sufficient to collect even just one image per loading cycle for a long fatigue test. It was expected that no visible cracks will exist in the early stages of a test while large cracks will exist in the late stages. Therefore, it was desirable to capture images from as much of the late-stage test condition as possible, ending with the complete separation of the tooth. During recording, the camera records in a loop, filling the RAM and then writing over the stored data until triggered to stop. Therefore, the camera can be triggered at the end of the test (post-trigger) such that it keeps the stored images in the bufferfrom the time leading up to the trigger. The trigger was initiated from criterion on the real time applied force measurement from the load cell on the hydraulic ram. STBF test methodologies utilize stress ratios of > 0 to avoid impacts from the hydraulic ram if it potentially loses contact trying to control to zero applied force. As such, the actual load in a normally operating STBF test will never reach or drop below zero. When a tooth breaks, its load carrying capacity drops to zero and the load cell measures zero force which is then used to initiate the camera post trigger. A DAQ system, separate from the load frame controller, was used to record load cell and camera frame synchronization information as well as output the TTL trigger to the camera. The load cell and camera frame timing information were recorded forthe entire STB test. This allowed for phasingbetween camera image exposures and the applied load to be determined such that frames corresponding to the maximal load could be saved. Both load cell output and camera timing were collected at a rate of 20,000 samples per second. The camera was setup independently of the DAQ and triggering programs. All capture settings such as resolution, frames per second (fps), and length of recording were set manually before the test. STBF tests were performed at 30 Hz loading rate allowing for a 750 fps , wh , x y S F M = (1)