TITLE:
Automatic Damage Detection and Monitoring of a Stitch Laminate System Using a Fiber Bragg Grating Strain Sensor
AUTHORS:
Agus Trilaksono, Naoyuki Watanabe, Atsushi Kondo, Hikaru Hoshi, Yutaka Iwahori
KEYWORDS:
Carbon Fiber; Stitch Laminate; Damage Tolerance; Structural Health Monitoring
JOURNAL NAME:
Open Journal of Composite Materials,
Vol.4 No.1,
January
15,
2014
ABSTRACT:
There are numerous aspects and
questions related to the use of composite materials for primary structures in aircraft where the
structural integrity is the most important factor. This is especially true if the main concerns are that the material should have good
reliability and durability for the primary structural application. Composite
laminates are highly sensitive to out-of-plane failure due to their low inter laminar fracture toughness. An
alternate method to increase the damage resistance is through three-dimensional
fibrous reinforcement such as through-the-thickness stitching with a single
fiber as the thread. Recent studies have shown that the stitching of standard
laminates can enhance damage tolerance to levels obtainable with toughened
resin systems. However, for next-generation aircraft, material
improvement alone is not enough to assure or increase the safety and reliability of
the structure. Continuous damage monitoring during operation will become an
important issue in aircraft safety. Embed ding fiber Bragg grating
(FBG) technology into the composite structure as strain sensors could potentially
solve the above problem because the FBG can be used to detect and characterize the
damage before it reaches a critical stage. The model used to represent this
problem is a 6 × 6 Vectran stitched carbon/ epoxy laminate under tension loading, and the
real-time monitoring using the FBG strain sensors is combined with
acoustic emissions that were conducted during the test. A numerical laminate
theory using a rebar element and first-ply failure criterion is performed to
determine the preferred area on the specimen for the placement of the FBG
before manufacturing and testing. Experiments are presented
to determine the damage growth that was quantified with an ultrasonic (water immersion) c-scan. In this research, the FBG successfully detected and characterized the damage in the carbon/epoxy stitch laminate
caused in tension loading cases. The FBG is enhanced with acoustic emission
data and can quantitatively
predict the damage growth.