Theory And Experiment Study Of Thermal Deformation Measurement Method Based On Optical Coherence Tomography

High performance composite structures have been widely used in aerospace engineering,of which thermal deformation need to be measured dynamically.But conventional methods cannot measure depth-resolved deformations inside materials with high sensitivity.Phase-contrast spectral optical coherence tomography(PC-SOCT)is a novel approach that can be used for measuring micro deformations inside transparent/semitransparent materials.However,there are also several issues need to be overcome before applying the method into the thermal deformation investigation:1)PC-SOCT is an interferometry based optical path difference measurement method.When the refractive index changes with sample deformation,the deformation cannot be measured precisely by the conventional PC-SOCT;2)the light paths of illumination and imaging of the conventional PC-SOCT are of coaxial structure,therefore,only out-of-plane deformation(the deformation along optical axis direction)can be measured.In this thesis,the principle of PC-SOCT is firstly introduced,and then the solutions of the above problems are provided as follows:1)Considering the multilayer structure of composite materials,and the relationships among the optical path difference changes,displacements,and the variations of refractive index,a novel physical model was proposed.The depth-resolved thermal deformation inside the materials can be accurately measured by PC-SOCT while the temperature-refractive index variations of the materials have been predetermined.In the thesis,an optical setup with depth range of 4.3mm and depth resolution of 10.7?m has been established.Besides,a silicone rubber sample with known thermal expansion coefficient has been measured to validate the method.2)To reduce the measurement error and the complexity of the measurement procedure,a method based on PC-SOCT for measuring temperature dependent refractive index variation is proposed.The method is realized by monitoring the displacement of the substrate while the thermal deformation measurement,from which the sample thickness,refractive index,and temperature-refractive index variation can be measured simultaneously.In addition,the method can also immune the vibration of the system and the air temperature induced optical path difference.The sensitivity of the method can reach to 10-5/?.3)A dual-channel PC-SOCT method is proposed for measuring in-plane and out-of-plane deformations.By employing a dual-channel observation structure,the deformation of the tested section can be observed from two directions symmetrically.Combining the geometrical relations of the optical path,the in-plane and out-of-plane deformations can be evaluated.Further,the method uses depth multiplexing,which needs only one shot before and one shot after the object deformation to acquire the deformations simultaneously.