THERMO-MECHANICAL DESIGN STUDY OF FIBER OPTIC PRESSURE SENSOR DIAPHRAGMS
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This thesis parametrically explores the nonlinear design sensitivity of a fiber optic pressure sensor (FOPS), based on selected thermo-mechanical failure mechanisms expected in the sensor diaphragm. The product under study is a miniature FOPS that can be embedded in, or installed on, a structure for pressure monitoring applications. The field operating conditions considered in this study are defined in terms of temperature and pressure The FOPS probe has a Fabry-Perot cavity, with the fiber tip and a miniature diaphragm acting as the two mirrors. The cavity length changes when the diaphragm deflects under pressure. However, due to field operating conditions, several failure mechanisms may affect the structural and optical characteristics of the sensor, such as nonlinear displacement of the diaphragm, cracks in the diaphragm, buckling of the diaphragm, high residual stresses in the optical fiber and deformations and failure in the epoxy sealant between the optical fiber and the steel casing. With the aid of nonlinear thermomechanical finite element analysis, this article investigates conflicting design constraints due to sensitivity and selected failure mechanisms in the sensor, e.g. nonlinear diaphragm deformation, diaphragm fracture and diaphragm buckling. The study is divided into three parts. The first part of this study considers the mechanical loading due to external pressure which the FOPS will experience and gives design guidelines based on the nonlinear diaphragm deflection and stress predictions. The second part accounts for the thermo-mechanical loading in which the FOPS is placed in a temperature drop and the resulting nonlinear in-plane compressive stresses and diaphragm deflections are analyzed. In the third part of this study the combined effects of pressure and thermal loadings are examined for more realistic application conditions. Design guidelines for both simultaneous and sequential changes in temperature and pressure are examined, to represent different working environments.