Evaluation using Digital Image Correlation (DIC)

During tensile testing, deformations and crack development were recorded on one side of each specimen using an optical full-field deformation measurement system ARAMISTM 12M by GOM. This system makes use of Digital Image Correlation (DIC) technique with a stereoscopic camera setup, i.e. two charge-coupled device (CCD)-cameras with 12-megapixel resolution (Figure 6.7). DIC is an accurate non- contact measurement technique which has been proven to be an applicable method for the crack opening measurement of concrete structures (Corr et al., 2007, McCormick et al., 2010, Skarzynski et al., 2013).This technique involves the sequential mapping of the deformation of a defined speckled surface area using a series of digital images captured during loading. It is to say that displacements are calculated by mapping the same pixels between a discretized subset of pixels from an undeformed reference and deformed digital image as per Figure 6.8 (Pan et al., 2009). The quality of the applied random speckled pattern can influence the results, as it acts as the deformation tracer from image to image. For additional details related to the underlying calculations involved in DIC, refer to e.g. Pan et al. (2009).

Figure 6.7 Setup of DIC method for tensile tests.

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75 Within the speckled region of the TRC specimen under testing, the following features were investigated using DIC in this work (see Figure 6.9):

> Virtual extensometers were defined at relevant reference points.

• Strains within a defined measurement area, i.e. extensometer 1 and 2, (Figure 6.10) and successive crack opening at a defined location could be evaluated (Figure 6.11).

> Cross-sectional cut was defined along the length of the specimen.

• Major strain versus cross-sectional length could be plotted which illustrates the successive formation and location of cracks by major strain peaks (Figure 6.12).

Figure 6.9 Illustration of selected features identified in the DIC system.

When comparing the reinforcement stress and the average strain extracted from the virtual extensometers, it is clear from Figure 6.10 that there was a pull-out failure of the TRC specimens reinforced by 3D carbon textiles. The stress-strain curve presented for the carbon yarn illustrates the strain hardening and rupture limits. It is to say that if the bond or anchorage length would be improved, the stress-strain curve of the specimens would shift towards the slope of the carbon yarn curve, as depicted in Figure 6.2. A similar behaviour would also be the case for the 2D carbon textiles.

Figure 6.10 Reinforcement stress and average strain curve depicted for TRC specimens with 3D carbon textiles versus carbon yarn.

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The use of virtual extensometers at prescribed crack locations is illustrated in Figure 6.11. As the loading increases, a given crack opening in terms of displacement can be followed successively. Additionally, the location of the crack formation along the length of the specimen can be monitored by means of a cross-sectional cut as shown in Figure 6.12. These two methods can be combined to retrieve detailed output data which could be useful for e.g. validation of models and design in SLS.

Figure 6.11 Load versus average strain (a) and monitoring of successive crack openings with virtual extensometers (b).

Figure 6.12 Load versus average strain (a) and major strain versus length in x- direction for selected load levels (b).

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6.4 Summary

The tensile behaviour of TRC specimens reinforced by carbon textile reinforcement was investigated using uniaxial tensile tests (Objective 1). The use of digital image correlation (DIC) to further document and investigate the test results was explored in this work. The principle outcomes and associated commentary are:

> It can be concluded from these results that the bond and end anchorage are critical factors that influence the tensile behaviour of TRC.

• Pull-out failure was observed for all tested specimens.

• A significant difference was noted for the crack development particularly related to the 3D carbon textile in orientation O2 compared to observations in flexure (Section 5.2.2). It appears as if the utilisation of the 3D textile (O2) is additionally influenced by the curvature of the specimen in flexure such that the bond was enhanced for small slips. The tensile capacity and COE are however overestimated due to the available anchorage length and clamping in the tensile test.

• The interfacial bond between the carbon textiles and matrix can be improved by e.g. alternative curing conditions or casting methods, finer concrete matrix, surface coatings (Section 4.4, Appendix B). • The anchorage can also be improved by e.g. increasing the anchorage

length in the test setup or increasing the strength of the specimen in the anchorage zone via epoxy or additional reinforcement.

> The correlation of the tensile and flexural behaviour was compared using the coefficient of efficiency (COE).

• The COE was applied to calculate an estimated bending moment resistance which proved to be comparable to the experimental average results from Section 5.2.2 with a difference ranging between 11-47 %. • Underlying differences in the results can be primarily attributed to the

differing end anchorage and clamping in the tensile and flexure tests which alter the boundary conditions.

> DIC proved to be a valuable and accurate method to measure the strains and crack development of tested TRC specimens.

• These data can be useful when attempting to validate models or concerning the design in SLS.

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