Optical fiber sensors are very useful to monitor the internal strain and temperature in composites during manufacturing and assembly as well as in practical operations. The authors have been using both multi-point and distributed strain monitoring techniques to characterize the internal state of composite structures. This paper reports some recent developments of life cycle monitoring and quality control of aerospace composite structures. Specifically, distributed sensing for large-scaled parts, through-thickness strain monitoring for complex-shaped parts, and direction-dependent cure shrinkage monitoring are described, highlighting wide applicability of embedded optical fiber sensors for intelligent process monitoring and quality assessment of composite parts.
Thermal strain significantly affects stability of fiber optic gyroscope (FOG) performance. This study investigates thermal strain development in a lightweight carbon fiber reinforced plastic (CFRP) FOG under thermal vacuum condition simulating space environment. First, we measure thermal strain distribution along an optical fiber in a CFRP FOG using a Brillouin-based high-spatial resolution system. The key strain profile is clarified and the strain development is simulated using finite element analysis. Finally, several constituent materials for FOG are quantitatively compared from the aspect of the maximum thermal strain and the density, confirming the clear advantage of CFRP.
Life cycle health monitoring technology for composite airframe structures based on strain mapping is proposed. It
detects damages and deformation harmful to the structures by strain mapping using fiber Bragg grating (FBG) sensors
through their life cycles including the stages of molding, machining, assembling, operation, and maintenance. In this
paper, we firstly carried out a strain monitoring test of CFRP mock-up structure through the life cycle including the stage
of molding, machining, assembling, and operation. The experimental result confirms that the strain which arises in each
life cycle stage can be measured by FBG sensors embedded in molding stage and demonstrates the feasibility of life
cycle structural health monitoring by using FBG sensors. Secondly, we conducted the strain monitoring test of CFRP
scarf-repaired specimen subject to fatigue load. FBG sensors were embedded in the scarf-repaired part of the specimen
and their reflection spectra were measured in uni-axial cyclic load test. Strain changes were compared with the pulse
thermographic inspection. As a result, strain measured by FBG sensors changed sensitively with debonded area of repair
patch, which demonstrates that the debondings of repair patches in scarf-repaired composites due to fatigue load can be
detected by FBG sensors.
This paper proposes structural health monitoring technology based on the strain mapping of composite airframe
structures through their life cycles by FBG sensors. We carried out operational load tests of small-sized mockup
specimens of CFRP pressure bulkhead and measured the strain by FBG sensors. In addition, we confirmed strain change
due to stiffener debondings. Moreover, debonding detectability of FBG sensors were investigated through the strain
monitoring test of CFRP skin-stiffener panel specimens. As a result, the strain distribution varied with damage
configurations. Moreover, the change in strain distribution measured by FBG sensors agrees well with numerical
simulation. These results demonstrate that FBG sensors can detect stiffener debondings with the dimension of 5mm in
composite airframe structures.
This talk describes the embedded optical fiber sensor systems for smart aircraft composite structures. First, a summary of
the current Japanese national project on structural integrity diagnosis of aircraft composite structures is described with
special emphasis on the use of embedded small-diameter optical fiber sensors including FBG sensors. Then, some
examples of life-cycle monitoring of aircraft composite structures are presented using embedded small-diameter optical
fiber sensors for low-cost and reliable manufacturing merits.
The objective of this work is to develop a system for monitoring the structural integrity of composite airframe structures by
strain mapping over the entire lifecycle of the structure. Specifically, we use fiber Bragg grating sensors to measure strain
in a pressure bulkhead made of carbon fiber reinforced plastics (CFRPs) through a sequence of lifecycle stages (molding,
machining, assembly, operation and maintenance) and detect the damage, defects, and deformation that occurs at each stage
from the obtained strain distributions. In previous work, we have evaluated strain monitoring at each step in the FRP
molding and machining stages of the lifecycle. In the work reported here, we evaluate the monitoring of the changes in
strain that occur at the time of bolt fastening during assembly. The results show that the FBG sensors can detect the
changes in strain that occur when a load is applied to the structure during correction of thermal deformation or when there
is an offset in the hole position when structures are bolted together. We also conducted experiments to evaluate the
detection of damage and deformation modes that occur in the pressure bulkhead during operation. Those results show that
the FBG sensors detect the characteristic changes in strain for each mode.
This study proposes a novel hierarchical sensing system for detecting impact damage in composite structures. In the
hierarchical system, numerous three-dimensionally structured sensor devices are distributed throughout the whole
structural area and connected with an optical fiber network through transducing mechanisms. The distributed devices
detect damage, and the fiber optic network gathers the damage signals and transmits the information to a measuring
instrument. This study began by discussing the basic concept of the hierarchical sensing system through comparison with
existing fiber-optic-based systems and the impact damage detection system for the composite structure was then
proposed. The sensor devices were developed based on Comparative Vacuum Monitoring (CVM) system and Brillouin
based distributed strain sensing was utilized to gather the damage signals from the distributed devices. Finally a
verification test was conducted using a carbon fiber reinforced plastic (CFRP) fuselage demonstrator. Occurrence of
barely visible impact damage was successfully detected and it was clearly indicated that the hierarchical system has
better repairability, higher robustness, and wider monitorable area compared to existing systems.
A crack arrester has been recently developed to suppress crack propagation along the interface between the facesheet and
the core of foam core sandwich structures. The crack arrester is a semi-cylindrical stiff material inserted into the
interface. The crack arrester decreases an energy release rate at the crack tip by suppressing local deformation around the
crack. If the arrested crack can be instantaneously detected, damage tolerance of foam core sandwich structures is
dramatically improved. This study establishes an innovative crack detection technique using metal wires and fiber Bragg
grating (FBG) sensors embedded at both edges of the arrester. Specific strain distribution induced by arresting the
interface crack is first memorized by the metal wire and the consequent residual strain is then picked up by the FBG
sensor as a damage signal. This study began by simulating sensor response to evaluate the feasibility of the proposed
technique. A verification test was then conducted, confirming the spectral change of the FBG can indicate propagation
direction and tip location of the arrested crack.
This study proposes novel hierarchical sensing concept for detecting damages in composite structures. In the hierarchical
system, numerous three-dimensionally structured sensor devices are distributed throughout the whole structural area and
connected with the optical fiber network through transducing mechanisms. The distributed "sensory nerve cell" devices
detect the damage, and the fiber optic "spinal cord" network gathers damage signals and transmits the information to a
measuring instrument. This study began by discussing the basic concept of the hierarchical sensing system thorough
comparison with existing fiber optic based systems and nerve systems in the animal kingdom. Then, in order to validate
the proposed sensing concept, impact damage detection system for the composite structure was proposed. The sensor
devices were developed based on Comparative Vacuum Monitoring (CVM) system and the Brillouin based distributed
strain sensing was utilized to gather the damage signals from the distributed devices. Finally a verification test was
conducted using prototype devices. Occurrence of barely visible impact damage was successfully detected and it was
clearly indicated that the hierarchical system has better repairability, higher robustness, and wider monitorable area
compared to existing systems utilizing embedded optical fiber sensors.
In this research, the authors developed various detection techniques for particular damages, such as debonding and impact damage, in sandwich panels consisting of composite face-sheets and aluminum honeycomb core with small-diameter optical fiber sensors. First, two methods for debonding detection were established taking advantage of the behavior of fillets formed on the adhesive layer between the core and the skin. One method uses the fracture of optical fibers, and the other one uses the shape recovery of the reflection spectrum from a fiber Bragg grating (FBG) sensor because of the release of thermal residual stress in the fillets. Secondly, as for impact damages, chirped FBG sensors were applied to monitor the change in strain distribution of the face-sheet due to the dent caused by the impact loadings. Furthermore, a newly developed MEMS-optical spectrum analyzer (MEMS-OSA) was introduced to identification of impact points and damages. This system could measure the reflection spectrum at very high speed. The change in the form of the reflection spectrum during the impact loading was found to be different depending on the impact energy and the impact location, and this tendency was confirmed by theoretical simulations using the change in the strain distribution obtained by foil strain gauges. These results show that the high speed measurement of the reflection spectrum by MEMS-OSA has a potential to identify the impact location and damage magnitude through the comparison with theoretical simulations.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.