SAMPE Journal - November/December 2011 - (Page 6)

Feature Article Life Cycle Structural Health Monitoring of Airframe Structures: Strain Mapping Using FBG Sensors Takahashia, K. Sekine, H. Takeya Mitsubishi Electric Corporation, Hyogo, Japan Y. Iwahori Japan Aerospace Exploration Agency, Tokyo, Japan N. Takeda The University of Tokyo, Chiba, Japan Y. Koshioka R&D Institute of Metals and Composites for Future Industries, Tokyo, Japan Abstract The purpose of this research is to develop structural health monitoring technology based on the strain mapping of composite airframe structures through their life cycles. We conducted the evaluation tests for damage detection in CFRP pressure bulkheads using fiber Bragg grating (FBG) sensors and verified the damage detection ability and accuracy of FBG sensors on various modes of damages and deformation. As a result, damages and deformation can be detected as the changes in strain distribution measured by FBG sensors. Moreover, these changes agree well with numerical simulation. These results demonstrate that the strain mapping by FBG sensors can be used for damage detection of the CFRP pressure bulkhead. Introduction Carbon fiber reinforced plastics (CFRPs) have been widely used for airframe structures because of their excellent mechanical properties such as high specific stiffness and strength. However, CFRPs also have complicated damage propagation behavior compared to metallic materials, which makes the damage tolerance design difficult. Moreover, some conventional non-destructive inspection (NDI) techniques used for metallic materials is difficult to apply directly to CFRP composites. This situation may lead to the increase of maintenance cost of composite aircrafts. This is why structural health monitoring (SHM) techniques have been widely studied1-4. Among them, fiber Bragg grating (FBG) sensors are promising SHM techniques because of their advantages such as the insusceptibility to electromagnetic noises, multipoint measurement. Moreover, FBG sensors indicate their “present” strain condition with their reflection spectra. That is to say, even if they are disconnected optically, re-connection allows us to measure the strain change since they are built into structures. Therefore, not only the strain applied in flight, the strain accumulated in manufacturing process, which includes curing, trimming and assembling, can be measured by FBG sensors. The accumulated strain in manufacturing process needs to be evaluated because it may lead to defects and failures of structures. In this research, taking advantage of these characteristics of FBG sensors, we develop the structural health monitoring system for composite airframe structures by strain mapping with FBG sensors through their life cycles including manufacturing, operation and maintenance. CFRP pressure bulkhead was selected as a target to which our SHM system is applied. In our previous paper5, we reported the results of strain measurement in curing process of CFRP structures by using FBG sensors. In this paper, we conducted operational load tests of scale-down specimens of CFRP pressure bulkhead simulating the differential pressure load in flight and accidental enforced deformation such as tail strikes in takeoff, and measured the strain of the structures under loadings by using FBG sensors. In addition, the strain changes due to damages and deformation were investigated. Strain Monitoring of CFRP Pressure Bulkhead in Operational Load Test by FBG Sensors Principle of the Strain Measurement and Damage Detection by Using FBG Sensors Figure 1 is the schematic of an optical fiber with an FBG sensor illustrating the principle of strain measurement with FBG sensors. Optical fibers consist of a core and a cladding. FBG is a grating which is written in a core with the periodical change in refractive index. When FBGs are illuminated SAMPE Journal, Volume 47, No. 6, November/December 2011 Figure 1. Schematic of an optical fiber with an FBG sensor. 6

Table of Contents for the Digital Edition of SAMPE Journal - November/December 2011

SMPE Journal - November/December 2011
Table of Contents
President’s Message
Technical Director’s Corner
Life Cycle Structural Health Monitoring of Airframe Structures: Strain Mapping Using FBG Sensors
Europe News & Views
SAMPE Europe’s SEICO 2012, Paris
12th Japan International SAMPE Symposium and Exhibition – JISSE-12
SAMPE Books & CD’s Order Form
SAMPE Proceedings
SAMPE Tech 2012 Call for Papers
Pitch-Catch (Low Frequency) Bondtesters
The SAMPE Foundation
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Materials & Products
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SAMPE Journal Editorial Calender
Welcome SAMPE’s Newest Members
SAMPE 2012
SAMPE 2012 Exhibition
Industry News
SAMPE Asia 2012|Kuala Lumpur, Malaysia
Cost-Effective, Phase-Based Thermography Method for Ultrasonic Investigation of Defects in CFRP Structures
Tech Tidbits
Upcoming SAMPE Events
Chapter Meetings Dates and Contacts
Advertiser’s Index
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SAMPE Journal - November/December 2011