16 2.4 Results and Discussion 2.4.1 Experimental Pull-Out Results Figure 2.5 provides the pull-out performance of the Pi-joints from the experiments. The general failure mode in the experiments for the Pi-joints consisted of an increasing load until failure, which led to the separation of the Pi-preform from the base. The failure was instantaneous and the direction of failure propagation could not be seen with the naked eye. A highframe rate camera would be required to confirm the propagation path of failure. Since symmetric failure initiation was observed, the delamination propagation was also expected to be symmetric. Instead an asymmetric failure was observed. Such a discrepancy may be caused by a combination of eccentricity in load application and manufacturing flaws. The use of a brittle adhesive, like the one used in this work, makes the joint highly susceptible to manufacturing flaws and less tolerant to damage. 2.4.2 Digital Image Correlation Results Digital Image Correlation was used to capture the displacement and strain on the cut surface of the Pi-joint as it was loaded in the tensile test. The images were processed using commercial software (Dantec Dynamics Istra 4D) to calculate the displacements and strains in the Pi-joint. A facet (or subset) size of 25 pixels was used, with an offset (or shift) of 15 pixels. The measured surface strain in the x-direction is shown in Fig. 2.7 at an applied load of 3 kN. The two red boxes indicate the areas for comparison with the FE results. 2.4.3 Numerical Simulation Results The finite element (FE) simulations successfully modeled the complete pull-out response of the damaged and undamaged Pi-joints and the results are shown in Fig. 2.5. Figure 2.6 shows the failure mechanisms observed in the FE simulation. The initial onset of failure occurs in the web/Pi region (see inset in Fig. 2.6) and continues asymmetrically on one side of the web. This onset of failure does not reduce the load carrying capacity as the load transfer continues mostly across the other bonded 0 2000 4000 6000 8000 10000 12000 14000 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00 Peak Pull-out Force (N) Top-Web Displacement (mm.) Experimental Response Simula on Predic ons Fig. 2.5 Comparison of the complete pull-out response of the experiment and the FE simulation C. Sebastian et al.
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