PSI - Issue 54

Isyna Izzal Muna et al. / Procedia Structural Integrity 54 (2024) 437–445 Isyna Izzal Muna, Magdalena Mieloszyk / Structural Integrity Procedia 00 (2019) 000 – 000

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Table 2. Tensile strength [MPa]

Group Intact

Experiment

Microscale (Unit cell)

Macroscale

226.14 ± 11.43 217.99 ± 8.44 221.21 ± 6.69

235.82 224.37 227.23

232.21 222.34 225.13

HS-A (65°C) HS-B (145°C)

Table 3. Young’s Modulus [GPa]

Group Intact

Experiment 28.65 ± 1.14 23.34 ± 2.18 20.44 ± 6.12

Microscale (Unit cell)

Macroscale

30.24

28.72 26.73 22.07

HS-A (65°C) HS-B (145°C)

27.965

25.23

4.1. NDT inspection with visual and THz spectroscopy The spectroscopy investigation was performed as NDT method for the treated sample subjected to 145 ° C temperature after tensile testing as shown in Figure 6. When compared to other conventional NDT techniques, THz spectroscopy NDT can offer better inspection and detection results for characterization of defects in the composite materials due to their superior qualities such as high temporal and spatial resolution, high penetrating ability, noninvasive nature, and nonionizing properties (Lu, 2022; Nsengiyumva, 2021). The THz approach is based on electromagnetic theory, and radiation signals in the terahertz frequency range can be generated by a current transient. The detector can accept a beam of THz pulses transmitted by the emitter's transmissive or reflecting optics. Since carbon fiber is a conductive material and has a high THz reflectance, it is typically impossible to test the CFRP sample. The polarization of the THz radiation input and the orientation of the fiber will have an impact on the THz radiation reflectivity because of the anisotropic conductivity of CFRP (Zhang, 2016). However, the sparse arrangement of the carbon fibers due to the matrix material made of PLA polymer make it possible to observe the internal structure of the sample (Mieloszyk, 2018). On the top sample surface, it can be seen that the visible defect is marked by the darker areas at the middle which may be caused by the inhomogeneity of the carbon fiber strand between polymer due to induced pull-out of the fiber toward the upper layer of the sample. The fiber is hanging as a result of fiber pullout affected by prolonged temperature exposure and post-tensile testing. It is also confirmed from the bottom surface that the dark marks were lying quite densely due to the CFRP thickness (thus the reflection cannot reach to the bottom) except in one strand of bright mark which denotes the visible polymer material as a result of fiber pull-out on the surface that generates the void on the lower layers. It must be noted that the THz scanning was performed with one time procedure, and the two surfaces measurement were obtained through data processing using Matlab.

a)

b)

Fig. 6. The reflective imaging results of a CFRP sample subjected to heat treatment at 145 ° C for 6 hours: ( a ) Bottom surface of the samples; ( b ) Upper surface