Failure Mechanics of a Composite Laminate Embedded with a Fiber Optic Sensor

A fiber optic sensor (FOS) embedded perpendicular to the reinforcing fibers causes an ‘eye’-shaped defect. The length of the eye is about 16 times the fiber optic radius (RFOS) and the height is about 2RFOS. The eye contains fiber optics in the center surrounded by an elongated resin pocket. Embedding FOS causes geometric distortion of the reinforcing fiber over a height equal to 6-8 RFOS. This defect causes severe stress concentration at the root of the resin pocket, the interface (in the composite) between the optical fiber and the composite, and at 90 to the load direction in the composite. The stress concentration is calculated by the finite element modeling of a representative micrograph. The FE results agreed reasonably with the analytical and the experimental data in the literature for a similar problem. The stress concentration in the axial direction is about 1.44 and in the transverse direction at the interface is 0.165 and at the resin pocket is 0.171. Under tensile loading, the initial failure is by transverse matrix cracking (fiber splitting) at the root of the resin pocket, then it leads to final fracture by fiber breakage. Under compression loading, the failure initiation is by interfacial cracking due to large transverse tensile stress and the final fracture is by compression. The fracture stress calculated from the analysis using the maximum stress criteria agreed reasonably with the test data.

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