Influence of resin pockets on mechanical properties of optical fiber-embedded smart composites

Optical fiber smart composites fabricated by embedding Fiber Bragg Grating (FBG) sensors between composite laminates possess the capability of real-time structural health monitoring, which can satisfy the urgent requirements for lightweight and intelligent design of aerospace structures. Nevertheless, resin pockets generated around embedded FBG sensors disrupt the structural continuity of composites and degrade their overall mechanical performance. Given that the influence mechanism of resin pockets on the strength of optical fiber smart composites remains unclear, this paper takes resin pockets as the research focus and investigates the effect of their geometric area on composite strength. Standard specimens with resin pockets of varying areas were fabricated by adjusting the optical fiber embedding angle and preforming pressure. Combined with mechanical tests, the degradation mechanism of mechanical properties induced by resin pockets was analyzed, and the correlation between the geometric area of resin pockets and material strength was revealed. The results show that the area of resin pockets decreases with the increase of preforming pressure, while increases as the optical fiber embedding angle rises. Serving as the initiation site of structural damage, resin pockets lead to strength attenuation: every 1 μm² increase in their area causes the tensile, flexural and compressive strengths to decrease by 4.44×10⁻⁴ MPa, 5.44×10⁻⁴ MPa and 11.2×10⁻⁴ MPa, respectively. It is noteworthy that the preforming pressure has an optimal range. Increasing preforming pressure can reduce the size of resin pockets and mitigate strength degradation, whereas an excessive pressure above 0.3 MPa will aggravate fiber buckling and further reduce the compressive strength.