11 Characterization of Interface Debonding Behavior Utilizing an Embedded Digital Image Correlation Scheme 85 Fig. 11.2 Front view of tensile load test sample with embedded glass bead and digital image correlation speckle pattern tracking pattern [8], a number of different tracking particles were used in a series of initial samples in order to down-select to a suitable tracking pattern. It was found that Cospheric 27–32 μm black paramagnetic polyethylene microspheres served as ideal tracking particles for this experiment setup. 11.2.2 Mechanical Testing A global tensile stress condition was imparted upon the sample through the use of a conventional load frame, as conducted in a typical tensile test. An MTS Exceed E42.503 (5 kN) load frame with a 1kN load cell was used to apply a constant velocity extension while also measuring a continuous tensile load to the sample. The load frame was controlled by MTS TestSuite Elite software. The extension of the load frame cross head was held at a constant rate of 0.5 mm/s in order to apply a constant global engineering strain rate to the sample. The sample that was preloaded at a value of 5 N of tension to properly orient the sample is the grips and position the camera. The glass particle and surrounding tracking pattern was imaged with the use of a Point Grey Grasshopper 2 camera with a resolution of 2048 by 2048 pixels. A Rokinon 2.8/14 mm optical lens was used in concert with the camera to capture highly detailed images of the region of interest which corresponds to a field of view of approximately 10 by 10 mm with an approximate standoff distance of 0.5 m. Illumination of the sample was supplied by the use a Visual Instrumentation Corp. 900420 LED module lamp. Given the transparent nature of Sylgard 184, lighting may be accomplished by direct illumination or by backlighting. After attempting both methods, direct lighting, where the illumination source was on the same side of the sample as the camera, was found to allow for greater success with the DIC analysis. Due to the load frame configuration, the load is applied to the sample with the controlled displacement rate of an upper grip, while the lower grip is held stationary. This configuration subsequently results in a displacement of the glass particle nominally equal to one half of the displacement of the crosshead controller of the load frame. This behavior in turn makes it difficult to continuously observe the glass particle and surrounding tracking pattern at high resolution. In order to counter this effect, the camera was mounted to a Thorlabs LTS150 motorized stage in the vertical direction and controlled with Thorlabs Kinesis software. The displacement of the stage was then programmed to travel at a constant rate equal to one half that of the travel rate of the crosshead controller. An image of the experimental setup is shown in Fig. 11.3. Images were captured using Point Grey FlyCapture 2 software, and the loading conditions were measured such that the imaging and loading information of the sample were recorded as a function of common time. This methodology was successful in capturing the relative translation of the tracking particles with respect to the embedded glass particle and clearly captured the delamination process between the binder and particle. The captured images of a sample before and after delamination are shown in Fig. 11.4. 11.3 Data Analysis and Results To determine the strain state of the binder material near the glass inclusion throughout the experiment, the acquired images were processed using Correlated Solutions Vic-2D Digital Image Correlation software, version 7. A parameterization study
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