PSI - Issue 38

Sara Eliasson et al. / Procedia Structural Integrity 38 (2022) 631–639 Author name / Structural Integrity Procedia 00 (2021) 000 – 000

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The transistor switch triggering the HSC was operated by a microcontroller and Fig. 3 illustrates the process. The trigger condition was controlled by a cycle count that was dependent on a threshold value (Fig. 4). The microcontroller received an analog output signal from the load cell of the fatigue test rig and used this signal to determine if the load was above the set threshold value. This allowed for counting the number of load cycles.

Fig. 3. Trigger mechanism flow chart.

Table 1. High Speed Camera settings.

Component

Settings

Camera

Dantec Dynamics Nano Sense MKIII

Lens

Nikon AF-S 60/2-8 ED

Capture Distance Trigger Settings Ex. of Trigger Frequency (cycle interval: trig cycle)

500-610mm

Circular, 115-20/35-40 pre/post trigger every 200 cycles 1000 – 10000: every 500 cycles 10000 – 50000: every 1000 cycles 50000 – 250000: every 5000 cycles 250000 – End: every 10000 cycles 0 – 1000:

Fig. 4. Trigger description during cyclic loading.

3.2.1. Data Handling Sampling 150 images for each trigger event in a two million cycles fatigue test resulted in large sets of data that could not be processed manually. There were two steps to extract the DIC data. First the peak images from the HSC were extracted using a 2D DIC Python code (André, 2017). The next step of the analysis, calculating the strains for the peak images, was conducted using the open source 2D DIC Matlab software, Ncorr (Blaber, et al., 2015). From the Ncorr data the average strain over the surface of the specimen was calculated. In Fig. 5 the axis system for an HSC image and a typical result image for the strain in y-direction is presented. When analyzing the images with Ncorr the Region of Interest (ROI) must be chosen carefully. Effects of splinters on the edges during the test can radically change the strain field if this is included in the analysis. Therefore, the ROI was made slightly narrower than the full width of the specimen. The speckle pattern was also evaluated for damage