PSI - Issue 64

Valentina Tomei et al. / Procedia Structural Integrity 64 (2024) 901–907 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

905

5

Before tensile test

0 10 20 30 40 50

normal stress σ [MPa]

After tensile test

1

0

0.025 0.05 0.075 0.1

strain ε [-]

H_Ag_dog-bone

H_dog-bone

2

(a) (b) Figure 4. (a) normal stress-strain curves for H_dog-bone samples and H_Ag_dog-bone samples; (b) picture of failure H_Ag_dog-bone samples after the Accelerated Aging process (top) and failure mode (bottom). The normal stress-strain curves (where stress was calculated by dividing the applied force by the cross-sectional area of the samples, and strain by dividing the displacement by the length of the sample) revealed that two samples exhibited lower strength compared to the others. These samples were characterized by failure occurring at the point of narrowing the variation of width of cross-section (refer to Figure 4 b-2). It is important to note that shape variation is a factor that could influence the visual appeal of printed elements intended to replicate architectural components of monuments. 3.3. Effect of the presence of the optical fiber Figure 5 compares the experimental responses of samples with and without optical fiber. From the figure it is evident that the inclusion of optical fiber resulted in a slight decrease in strength, an increase in pre-peak stiffness, and post-peak softening. Furthermore, the orientation of the fiber optic within the samples significantly impacted their behavior. Indeed, as shown in Figure 5, samples with filaments aligned longitudinally (H_Fh_dog-bone sample) exhibited greater strength and lower ultimate strain compared to samples containing diagonally oriented filaments (H_Fd_dog-bone sample). Furthermore, regarding samples with optical fiber, it is possible to observe a rise in the average elastic modulus, potentially attributable to the fiber's contribution to stiffness, alongside a slight decrease in both ductility and strength compared to H_dog-bone specimens. Additionally, there's a gradual degradation in strength (softening) characterizing the post-peak behavior of samples with optical fiber, likely stemming from the multi-phase printing process. The above outcomes underline the influence of the presence of fiber optic inside samples on the tensile behavior of sample. Finally, Figure 5b shows the post-test images of the samples. Interestingly, fractures in samples with optical fibers tended to occur near the ends, unlike those without fibers, which typically fractured closer to the center. Figure 6 presents the strain vs time curves provided by FBG sensor (strain sensitivity: 1.2 pm/με) for two samples with fiber optic horizontally placed. Both plots indicate a linear strain increase until a curve drop occurred within the strain range of 0.01-0.014. This drop corresponds to strain levels reached at time instants of 25 and 29 seconds for samples N.1 and N.2, respectively. The subsequent curve branches maintain an almost horizontal shape, suggesting minimal strain variation over time. This indicates that the optical fiber slipped within the sample, rendering it unable to accurately measure subsequent strains after slippage. Specifically, it was observed for both samples that this phenomenon occurred at approximately 80% of their tensile strength. Despite, with the current monitoring system, it is not possible monitoring strains until collapse, FBG sensors demonstrate effective monitoring across a wide range of strain, which maximum value is above the values expected to be considered dangerous in case of SHM (structural health monitoring) applications. Consequently, these results affirm the validity of producing 3D printed components