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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In Fig. 5b, 5d, point 1 and point 2 indicates the linear stage of load increase, after an initial nonlinear stage due to the overall closure of crack or void throughout the specimen. The lateral displacement control using AUSBIT was applied from the beginning, helping to stabilize the cracking process at least partly, as shown in Fig. 5a and 5c. The predefined rate of lateral displacement is consistent throughout the whole test. The load and vertical displacement increase nonlinearly with time before reaching peak load value. However, due to weak interface between layers, the failure process becomes abrupt slightly after peak load, although snap-back can still be captured before the occurrence of unstable failure. In both tests, the lateral displacement jumps up suddenly due to loss of control at certain stages after peak load. This can be due to the lack of sensitiveness of the loading machine compared to the rate of crack propagation along the weak interfaces. This is the issue that should be investigated and addressed later in the control of the test.

Fig. 6. Evolution of lateral strain, and failure pattern observed at the end of the test (0° loading angle).

Lateral strain obtained from DIC and its evolution during failure are plotted in Fig. 6. Once the load reaches the peak value at point 3, the strain distribution of the DIC contour shows that the highest strain (red region) starts concentrating in the middle vertical loading pathway of the specimen. The failure process becomes unstable beyond point 4, given the control is lost due to low strength and fracture toughness of the interface. As can be seen in the failure pattern, the crack in this case is almost straight, following the interface between two layers in the middle of the specimen. 3.3. Comparison of tensile strengths Based on the results of tested specimen, the peak load and tensile strength of some specified specimens have certain differences with lower values due to the inconsistent printing quality and the cracking paths. Fig. 1(a) shows the layer height and width has variations of the printed wall results in inconsistent printing quality. Therefore, the cracking paths of the fully fractured specimens did not always go through the weak interface between two printed layers. The specimen with lower peak load and strength were obtained from the specimens with cracking path along the whole weaker interface compared with the others were cracked within the layer in vertical direction. The cementitious mortar density of printed specimens is different due to the influence of printing process. The density of specimen has impact on both strength and stiffness of the specimen which eventually affects the tensile strength. The average tensile strengths of specimens underloading at different loading angles (90° and 0°) with similar average density are shown in Table 2. The preliminary results indicate that the tensile strength of specimens with 90° loading angle specimens is stronger than those with 0° loading angle. Although the results are not significantly different due to lack of data, this still shows the effect of weaker interlayer on strength. The results in Table 2 are indicative only, given they are preliminary ones showing both successes and challenges in control cracking processes in Brazilian disc testing. The repeatability of the tests and consistency of test results still