Measurement of process-induced strains in composite materials using embedded fiber optic sensors Page: 1 of 11
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Measurement of Process-Induced Strains in Composite
Materials Using Embedded Fiber Optic Sensors
Craig M. Lawrence and Drew V. Nelson
Department of Mechanical Engineering - Design Division
Jay R. Spingarn and Thomas E. Bennett
Sandia National Laboratories - Composites Laboratory ? y 2
Livermore, CA 94551
This paper presents the results of experiments to measure the internal strains and temperatures that are generated in
graphite/epoxy composite specimens during processing using embedded fiber optic strain sensors and thermocou-
ples. Measurements of strain and temperature, combined with a computational model, offer the potential for non-
destructive, real-time determination of residual stress in composites, and may be useful for process monitoring and
control. Extrinsic Fabry-Perot interferometer, Bragg grating strain sensors, and thermocouples were embedded in
graphite/epoxy composite laminates prior to cure. The specimens were cured in a press, and the internal strains and
temperatures developed during processing were monitored and recorded. The results are compared with expected
values, and limitations of the experimental technique are discussed.
Keywords: composite, residual stress, fiber optic sensor, Bragg grating, extrinsic Fabry-Perot Interferometer
Fiber reinforced composite materials are used in many aerospace, civil, and industrial applications. To produce
these materials, strong, stiff reinforcing fibers such as graphite or glass are embedded in a lightweight, compliant
matrix such as epoxy to create a material which takes advantage of the beneficial mechanical and thermal proper-
ties of both constituents. Composite materials are often selected over traditional engineering materials because they
may possess higher stiffness-to-weight and strength-to-weight ratios. Additionally, the mechanical and thermal
properties of composites can be customized through the appropriate selection of constituent materials, geometry,
and processing conditions.The increased use of composites in structural engineering applications has led to concern
about the reliability of these materials. In particular, residual stress introduced during fabrication is cited as one of
the most significant problems in the processing of composite structures . These stresses can cause warping, or
"spring-back," of the composite structure and can significantly degrade the strength of the material, resulting in
cracking, reduced fracture toughness and fatigue strength, and delamination. The primary causes for residual stress
in composites are thermal stresses due to the different coefficients of thermal expansion of the constituents, and
curing stresses which result from chemical shrinkage of the matrix material during processing of polymer matrix
Predicting residual stress in composites is difficult because of the large number of factors which contribute to the
stress, including thermal, chemical, and viscoelastic effects, as well as moisture absorption. Experimental determi-
nation of residual stress in composites is also difficult. Often, the stresses of interest are located below the surface
of the material, requiring the use of destructive techniques or non-standard measurement methods. To date, there is
no universally accepted "best" method to measure residual stresses in composite materials. Methods developed for
the measurement of residual stresses in metals have been extended and applied to composite materials. Most of the
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Lawrence, C.M.; Nelson, D.V.; Spingarn, J.R. & Bennett, T.E. Measurement of process-induced strains in composite materials using embedded fiber optic sensors, report, May 1, 1996; Livermore, California. (digital.library.unt.edu/ark:/67531/metadc667412/m1/1/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.