PSI - Issue 61

Zili Huang et al. / Procedia Structural Integrity 61 (2024) 252–259 Huang et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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Fig. 2. (a) overall testing setup of MTS loading frame and DIC technique; (b) loading direction relative to printing direction.

Adelaide University Snap-Back Indirect test (AUSBIT) was applied in this study, where the indirect lateral displacement control could stabilize the instant cracking under the diametrical compression load (Verma et al. 2019a; Verma et al. 2021a). Compared with the conventional indirect tensile test, AUSBIT enables to capture a complete strain response with the post-peak behaviour (Verma et al. 2021b). In this study, the vertical displacement rate was set as 0.2 mm/min and the load set point is 1 kN for early stage of loading, before switching to lateral control. The lateral deformation was used to control the loading over the cracking period with the lateral displacement rate of 0.5 μm/min . The tensile strength of the material can be obtained using the peak load as per Equation 1 (Verma et al. 2021b): (1) where P is peak load (kN), D is the diameter (mm), and t is the thickness (mm) of the disc specimen. Given the layer-by-layer deposition of cement mortar in 3D printing, the interfaces between layers are usually the weakest link, governing the strength and fracture properties of 3D-printed cement mortar. In this sense, in Brazilian disc testing, loading direction relative to printing direction has strong effects on the results. This also affect the stability and controllability of the tests, given the disc specimen in Brazilian disc testing is expected to be weaker if loading direction is parallel with printing direction (making an angle of zero degree; Fig. 2b). On the other hand, the specimen is expected to be stronger if the angle between the loading and printing direction is 90 degrees. In relation to the above two configurations (0° and 90°, Fig. 2b, right), the obtain strengths in these two cases correspond to strength of the interface (for 0° loading angle) and cement mortar layer (for 90° loading angle). This is one of the advantages of using Brazilian disc testing for investigating the behaviour of 3D-printed cement mortar, given the ease in specimen preparation and also loading condition. The disadvantage is that this kind of test is usually very unstable, with abrupt fracture in a fraction of a second, and therefore it is much more difficult to control the fracturing process in the case of for 0° loading angle. The use of AUSBIT in this case can help stabilise the fracturing process, giving more time for advanced instrumentation based on DIC. 3. Result and discussion 3.1. Discontinuity perpendicular to the applied load (90° loading angle) The AUSIT approach is applied for controlling the lateral deformation for three specimens which are tested under 90 degree loading angle. The evolutions of lateral displacement and vertical displacement with time are shown in Figs. 3a, 3c, 3e, while Figs. 3b, 3d and 3f show the load-displacement curves. The points on the load-displacement curves