PSI - Issue 68

A. Jiménez et al. / Procedia Structural Integrity 68 (2025) 603–609 Adriano Jiménez et al. / Structural Integrity Procedia 00 (2025) 000–000

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An ultrasonic test machine (Pundit Lab, Proceq, Switzerland) was used for the tests, with the following settings: frequency of 24 kHz, pulse duration of 20.8 µs, and automatic excitation voltage. Measurements were taken using a couplant gel to ensure proper contact between the transducers and the ceramic pieces. For each part, both direct measurements (with transducers on opposite prism faces) and semi-direct measurements (with transducers on adjacent prism faces) were recorded.

3. Results 3.1. Mechanical tests

The mechanical tests revealed that discontinuities from the LDM manufacturing process create internal joints in the composite material, along which fractures typically occur. In compression tests, failures are more common along vertical joints (between printing lines) and less frequent along horizontal joints (between layers). In contrast, solid specimens show no fracture pattern indicating anisotropy. For the three-point bending tests, fractures tend to follow weak planes formed by both vertical and horizontal joints. However, when tensile stress aligns with the printing lines, the material lacks weak planes, resulting in irregular fractures like those seen in solid specimens. 3.1.1. Three point bending tests Fig. 3 presents the force/displacement curves from four three-point bending tests on different specimens, with labels (X, Y, Z) indicating the orientation relative to the printing path. A curve for a solid specimen is also included. All curves display a linear force-displacement relationship until fracture, showing the elastic behavior typical of brittle materials like ceramics. The similar slopes across all tests suggest that the elastic constants might be comparable in the three orthogonal directions. However, since the displacement data comes from the crosshead movement, it does not accurately reflect the specimen's deformation. More precise deformation data will be obtained from the compression tests using strain gauges. A significant drop in flexural strength is seen in specimens where the loading plane aligns with the X direction (Fig. 4; left), as these specimens show discontinuities from print paths and layers in the loading plane. Flexural strength decreases by 69% to 76% in the Y direction and 46% to 63% in the Z direction, both compared to the X direction. Interestingly, specimens with print paths perpendicular to the load (X direction) tend to be even stronger than solid specimens, which show a flexural strength reduction between 0% and 18%. This strength is likely due to the enhanced homogeneity in the X direction from the manufacturing process.

Fig. 3. Force / Displacement curves from 3 point bending tests