In this application video, a combi TSA (ThermoESA) and DIC (3-cam 5Mpx FlexDIC) system are being used,
in parallel/synchronization, to measure the strain/stress concentration of an aluminum coupon (cut-out) in
a cyclic fatigue test at 2.6 Hz. Each system views the coupon from the opposite side however both are
synchronized in the image acquisition via using the testing rig output as a clock signal (Sync In for the DIC DAQ Controller).For the DIC system, a real-time overlay of the speckle pattern quality can be seen, indicating the standard
deviation with respect to the selected facet/subset size. Then after, the projection calibration is performed
which automatically acquires images of the calibration target and computes the projection calibration in
real-time, additionally providing live feedback of the quality. The recording procedure protocol for the test
uses a pre-defined template based on the Phase Shifting Method (PSM), which acquires images are phase
intervals at 90° (0° / 90° / 180° / 270°) assuming a sinusoidal signal. The method, as such, captures images
when the sample is at the peak (max) load (90°) and trough (min) load (270°). 400 images are acquired
throughout the test.Using thermoelastic stress analysis (TSA), a live in-situ measurement of the coupons’ thermoelastic response
can be seen on the surface (stress visualization). Using cross-correlation, the load (signal) and temperature
can be isolated, which results in lower noise with successive block-series acquisition.After acquisition of the measurement images, the correlation evaluation is performed whereby the central
camera is selected to be the reference camera, known as the Central Reference Camera Perspective (CRCP).
This approach defines the facet/subset generation on the camera with the lowest stereo angle, reducing
perspective distortion, and maximizing both in-plane sharpness and out-of-plane depth-of-field (DoF).
It also improves measurement accuracy by 30%, as opposed to using a 2-camera setup.In the graphical visualization, a 2D-overlay of the total displacement (in um) with rigid body motion
removed, can be seen. A maximum movement of 10 um can be seen, however the distribution is not
exactly symmetrical around the centre axis of the specimen. From the 2D-overlay, of the true principal
strain 1 (major strain) (in ustrain), the strain concentration around the feet of the tower can be seen.
Using two virtual point gauges, P1 is placed at the feet of the tower (e1 = 583 ustrain & e2 = -86 ustrain =
bulk 497 ustrain) and P2 is placed for reference TSA calibration at the top of the coupon
(e1 = 165 ustrain & e2 = -10 ustrain = bulk 155 ustrain)Calibration of the thermoelastic response to strain can be performed using known reference strain values
from the DIC measurement. The P2 DIC bulk strain of 155 ustrain is inserted and a point gauge, in the MiTE
software, is placed at the same location (P1) at the feet of the tower measuring 502 ustrain.As such, at location (P1) the measured bulk strain using DIC = 497 ustrain and TSA = 502 ustrain, yielding
a 5 ustrain difference.
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