PSI - Issue 77

Magdalena Mieloszyk et al. / Procedia Structural Integrity 77 (2026) 256–263 M.Mieloszyk & S.Bhadra / Structural Integrity Procedia 00 (2026) 000–000

262

7

Fig. 5. (a) GFRP after thermal exposure showing fibre–matrix debonding with an interfacial gap at the ply edge, (b) Matrix crack traversing a resin-rich / raster boundary region.

Fig. 6. (a) Region exhibiting matrix reflow, (b) Fibre pull-out and interfacial cavities indicative of reduced interfacial shear strength.

chanical loading. Also, Figure 6(b) presents glass fibres experiencing pull out upon mechanical loading, occurring because of the slipping of the fibre on the matrix material, indicating interfacial shear failure rather than cohesive matrix fracture in the inner plies. Such pullout is consistent with a reduction in the interfacial shear strength because of matrix softening, mismatch in the thermal expansion and local stress concentration at the raster boundaries. The pull-out phenomenon reduces the load transfer e ffi ciency of the fibres, diminishing the tensile strength of the GFRP specimen. These microstructural observations align with THz spectroscopy-detected discontinuities and changes in the internal structure and help explain any attenuation changes observed in FBG strain results after environmental conditioning.

4. Conclusion

The paper presents a comparison of GFRP material manufactured using two popular methods: standard (infusion) and AM (mFDM). The analyses were focused on the internal structure of samples with embedded FO, as FO-based sensors are applied in the designs of smart structures. It was presented that the internal structures of GFRP elements strongly depend on the used methods. Infusion results in laminates with layers with almost equal thickness. Their surfaces have di ff erent roughness related to the roughness of the material attaching to them during the manufacturing process. Embedded FOs influenced the structure locally in the form of a local increase of the sample thickness due to the thickness of the FO covered by resin (called resin pocket). AM samples contain consecutive layers of fibres and polymer. Embedding FO results in the local change of the thickness of the sample - all glass fibres 3D-printed over FO are moved up. Exposition of the GFRP sample to elevated temperature results in its structural changes. The positives are a re duction in the amount of voids and better adhesive joints between fibres in one layer and between them. However, such di ff erences in thermal properties of GFRP components (fibre and polymer) result in internal stresses that cause material degradation observed as matrix cracking or fibre–matrix debonding.