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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A detailed and accurate three-dimensional finite element stress analysis was conducted on countersunk rivet holes in a plate subjected to tension loading. The analysis included a wide range of countersunk depths, plate thicknesses, countersunk angles and plate widths. The study confirmed some of the previous results, addressed their differences, provided many new results, and investigated countersunk angle and width effects. Using the detailed FE results and the limiting conditions, a design equation for stress concentration was developed and verified.

Countersunk rivets are used to join components to achieve aerodynamic or hydrodynamic surfaces. At countersunk holes, three-dimensional stress and strain concentrations occur. Previously, the present authors developed a three-dimensional equation for the stress concentration factor K t through a detailed finite element analysis. This paper extends the study to include an equation for three-dimensional strain concentration factor K te using a similar approach. The resulting equation was verified by finite element analysis for a wide range of countersunk hole configurations and plate sizes. Results showed that the maximum strain concentration is at the countersunk edge. The developed equation is within 5 per cent of the finite element results for all practical cases. It was also found that the K te and K t expressions are similar and K te ¡ K t. The maximum difference between the two is 8 per cent (for n 5 0.3) or n 2 for straight-shank holes and about n 2 /2 for countersunk holes. The proposed equation is a valuable tool for strain-based design of structural elements.