Embedded fiber-optic strain sensors for process monitoring of composites.

摘要

A new class of mechanical structures, termed "smart" or "adaptive" structures, has been proposed by engineers for use in aerospace, civil, and industrial applications. These structures integrate sensors and actuators directly into the materials from which they are formed, and are envisioned to have the ability to monitor themselves during manufacturing, assess their structural integrity, adapt to changing conditions, and perform self-repair. Two of the key enabling technologies for smart structures are fiber optic sensors and composite materials. Fiber optic sensors are capable of responding to a variety of environmental stimuli, such as temperature and strain. These small sensors can be embedded within polymer-matrix composite materials to form the basic building block of a smart structure.; In the first part of this research, the ability of fiber optic sensors to monitor residual stresses generated during the processing of composites is investigated. A new measurement technique is described--the embedded fiber optic sensor (EFOS) method--in which residual stresses are computed from measurements of internal strain and temperature using a viscoelastic, cure-dependent process model. The EFOS method has the advantage that it is non-destructive and provides information on residual stress development during cure in real-time. Experiments were performed to test the method, and the resulting residual stress measurements compared favorably with prior theoretical predictions and measurements by a destructive technique. The EFOS method was also used to accurately predict the residual-stress induced warpage in a non-symmetric composite sample.; In the second part of this work, the development of a multi-parameter fiber optic sensor is presented which is created by forming two Bragg gratings at widely spaced wavelengths in polarization-maintaining optical fiber. The spectra of the light reflected from this sensor contains four peaks which may be used, in principle, to determine axial strain, two components of transverse strain, and temperature in a single fiber. A theoretical model of the sensor was developed, and several sensors were fabricated and tested to calibrate their performance. Additional experiments were performed to verify the ability for the multi-parameter sensor to simultaneously measure two and three independent components of strain.