Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
65

Development of a BOCDA-SHM System
to Reduce Airplane Operating Costs
TAKASHI YARI*1

NOZOMI SAITO*2

KANEHIRO NAGAI*3

KAZUO HOTATE*4KIYOSHI ENOMOTO*5

Mitsubishi Heavy Industries, Ltd. (MHI) has developed a structural health monitoring(SHM) system for aircraft structures, using Brillouin optical correlation domain analysis(BOCDA). This includes an aircraft-mountable BOCDA measuring device, whose accuracy has
been verified in flight tests. This paper outlines the current status of the BOCDA-SHM system, andprovides a durability evaluation of the sensor installation part and a reliability analysis of thedamage diagnosis, which were conducted as a part of our efforts to enhance the overall reliabilityof the system.

|1. Introduction
In recent years, airlines have increased their demands for health monitoring of aircraft
structures in order to reduce aircraft operating costs. A SHM system is expected to meet theirdemands, since it facilitates a reduction in operating costs by shortening the period required foraircraft structural inspections. Among the various SHM systems, BOCDA-SHM system detects
structural damage and measures structural life by recording and analyzing variations in the strain
distribution of the overall aircraft structure, or the load history data for each operation.1
The basic concept of applying BOCDA measurements to aircraft structural health monitoringis illustrated in Figure 1.

Figure 1

Application concept of the BOCDA-SHM system

BOCDA can measure the strain distribution and dynamic strain at an arbitrary point on an
optical fiber. By placing optical fibers throughout an aircraft structure, damage can be diagnosed
from distributed strain variations, and structural life can be measured from the load history of thestructure. We built a prototype of an aircraft-mounted BOCDA measuring device, and havedemonstrated its accuracy in a business jet operating environment.2 To apply an SHM system thatmonitors structural health during operation, the reliability of the overall system and thetrustworthiness of its diagnosis must first be ensured. This paper describes a durability evaluationof the sensor installation part for an aircraft BOCDA-SHM system, as well as the development
status of the diagnostic technology for assessing structural damage.
*1Manager, Research Department,Aerospace Systems*3

Deputy General Manager, Research Department,Aerospace Systems*2*4Professor, Graduate School of Engineering, The University of Tokyo*5

The Materials Process Technology Center (Sokeizai Center)Research Department,Aerospace Systems

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)66|2. Development of the BOCDA-SHM system
2.1

Evaluation of sensor system durability

The durability of the SHM system can be ensured by confirming the durability of themeasurement devices and sensor installation part under operating conditions. Accordingly, weevaluated the durability of an optical fiber sensor installation on a structural surface in an aircraft
operating environment. The evaluation included the effects of major environmental factors, such astemperature, pressure, humidity, and lubricants. The environmental conditions for this evaluation
are listed in Table 1.
The test specimen for the evaluation was a carbon fiber-reinforced plastic (CFRP) plate (usedin aircraft structures), to which an optical fiber was bonded with epoxy adhesive, and protectedwith fuel tank sealant. The optical fiber sensor had a diameter of 125 μm, and was coated withpolyimid resin. We applied a tension load to this optical fiber to evaluate the durability of theinstallation part. The applied tension was 4 N, corresponding to a step-like strain change of 8000
μstrain. Figure 2 illustrates the test procedure. Each simulated environmental condition listed inTable 1 was applied to the sensor installation part, and the adhesion strength of the optical fiber
was evaluated. The results of this evaluation verify that the adhesive strength of the optical fiber isadequate under the conditions listed in the table, and thus the sensor installation part is durable inan actual operating environment.
Table 1 Environmental conditions for durability
evaluation of a sensor installation part
Test Item
Environmental conditions
High temperature environment+110±5oC, air
Low temperature environment-65±2oC, airTemperature cycle-65-110oC, airHigh pressure environment
200±10kPa
Low pressure environment
11.6±10kPa
Operating oil environment +70±2oC, skydrol immersion
Fuel environment+50±2oC, jet-A immersion
Humidity environment+70±2oC, 90±5%R. H.

Figure 2 Test procedure for evaluating
optical fiber adhesiveness

2.2

Adhesive delamination damage detection algorithm

The BOCDA-SHM system must evaluate a massive amount of data at a high spatial
resolution to measure the strain distribution on an optical fiber sensor (with a typical length ofseveral hundred meters) installed on an aircraft structure, and detect damage. To handle the large
quantity of data efficiently, it was necessary to develop a diagnostic algorithm that automaticallydetects the portion of the strain variation associated with structural damage.
We therefore developed an algorithm that diagnoses delamination expansion of an adhesive
member from the output of the BOCDA measurement system. We adopted theMahalanobis–Taguchi (MT) method to identify an abnormality in the large quantity ofmulti-dimensional data generated by the BOCDA measurements. The MT method defines theMahalanobis distance (MD) as the relative distance between an evaluation criterion and themeasured data of a diagnosis target, and evaluates a deviation from normal conditions (i.e., anabnormality) by uniformly evaluating the MD. When using this procedure, the unit space (anaggregation of data defined as normal) is first specified, and then the MD (the distance from theunit space) is calculated from various parameters in measured data during operation. A structuralabnormality is diagnosed from the size of the MD. Hence, this process does not require themodeling (i.e., model equations) of an a priori predicted multivariate quantity, and can be used forwide-range determination and monitoring of various conditions of the diagnosis target.3
Our developed algorithm for detecting adhesive delamination damage employs the strain
distribution and the distance-directed wavelet-transformed results as a diagnostic parameter.
Structural conditions are evaluated by calculating the MD from diagnostic parameters during
operation, after first specifying the unit space in terms of diagnostic parameters under healthy
conditions.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)67

We conducted an adhesive delamination test on a specimen that imitated a composite
reinforced panel, in order to evaluate the damage detection characteristics of this algorithm. The
test specimen was 1,000 mm long, 400 mm wide, and approximately 1.5 mm (8-ply) thick, and hada strap (in the form of a stringer) that was 400 mm long, 50 mm wide, and approximately 1.5 mm(8-ply) thick secondarily bonded to its center. The optical fiber sensor, which was over 700 mm
long, was installed 5 mm from and parallel to the strap (Figure 3).
Strain variation was evaluated from BOCDA measurements by applying a tension load to thepanel. The BOCDA measurements were taken with 30 mm spatial resolution. Adhesive
delamination expansion occurred at a tension of around 130 kN, and a variation of the strain
distribution was observed in the vicinity of 0.2 m from the origin of measurements (Figure 4). We
calculated the MD distribution by applying our developed MT diagnostic algorithm to thisvariation of the strain distribution. The calculated MD distribution is shown in Figure 5. The
results indicate that the variation was detectable, since an increment in the MD value was observedat the locations where the delamination expansion occurred.

Figure 3 Overview of the test specimen for evaluating adhesive delamination detection

Figure 4 Variations of strain distribution
associated with adhesive delamination
damage expansion

2.3

Figure 5 Results of applying the adhesive
delamination
damagedetection
algorithm

Development of a damage detection algorithm

The optical fiber sensors used in the BOCDA-SHM system can be embedded in CFRP
materials, and offer the potential for detecting damage inside the CFRP. Accordingly, weembedded an optical fiber in a CFRP material to evaluate the detectability of delamination damage.
Figure 6 shows an overview of the test specimen for evaluating delamination damage
detection. The specimen was 350 mm long, 200 mm wide, and approximately 1.5 mm (8-ply) thick,and artificial delamination damage was installed on the central layer of the plate. An optical fiber
sensor approximately 600 mm long was embedded in the same layer as the artificial delamination,
and was drawn out from the edge of the CFRP.
We inserted a metal blade with a thickness of 1 mm in the artificially created delamination tocause the delamination to expand gradually, and took BOCDA measurements at a spatial resolution

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)
68of 30 mm. Figure 6 depicts the gradually expanding delamination fronts A, B, and C as dottedlines. The Brillouin frequency shift (BFS) corresponding to the strain distribution, and the Brillouingain spectrum (BGS) at each measurement point were measured for the evaluation. The BGS
indicates the condition of the measurement point; the spectrum tends to be narrower when the strain
distribution is uniform within the spatial resolution (a precipitous peak), and wider when the strain
distribution fluctuates within the spatial resolution (a mild peak). The BOCDA measurement
results are shown in Figure 7. The BFS variation in the vicinity of 0.2 m, associated with theexpansion of delamination damage, is shown in Figure 7(a). The BGS at the corresponding positionis shown in Figure 7(b). The shape of the BGS changed with the expansion of delamination
damage. Figure 7(b) indicates that the residual strain created during CFRP curing was released inthe vicinity of the delamination damage front, where the distribution became non-uniform.
However, uniformity was restored beyond the area of delamination damage. The expansion ofdamage inside a CFRP material involves the release of residual strain, and results in a non-uniform
strain distribution for an embedded optical fiber.

Figure 6 Overview of the test specimen for evaluating delamination damage detection

Figure 7

Measurement results of the delamination damage detection test

To evaluate the conditions of such delamination quantitatively, we developed a diagnostic
algorithm based on the MT method, using the BGS profile as a parameter. In this diagnostic
algorithm, the BGS half value width, center frequency of the half value, 3/4 value width, and center
frequency of the 3/4 value are adopted as evaluation parameters. The MD distributions of the crack
fronts A and C, calculated by the MT diagnostic algorithm, are shown in Figure 8. The figure
reveals that the MD value increased with the expansion of delamination at the delaminated location
or delamination front, and thus an abnormality (i.e., structural damage) can be detected. However,the MD value also increased at a point where a sensor was not embedded, due to disturbance in thevicinity of the measurement, and approximately 60 mm of distance error was detected between theactual delamination point and the point where the MD increased. Improved stability of damage
detection and improved location accuracy remain challenges for future research.

Mitsubishi Heavy Industries Technical Review Vol. 48 No. 4 (December 2011)69

Figure 8

Results of applying the delamination damage detection algorithm|3. Conclusion
In this paper, we outlined the results of an optical fiber sensor durability test and thedevelopment status of a diagnostic algorithm, as a part of our efforts to develop a health monitoring
system for the overall structures of actual aircraft. We ensured that the sensor installation part isdurable in an aircraft operating environment, and devised an MT-based algorithm for diagnosing
the delamination and adhesive delamination that are typical damage conditions in an aircraftstructure. In light of these achievements, we are planning to downsize the aircraft structural healthmonitoring device, evaluate the damage diagnosis possibilities quantitatively, and reduce theapplication cost of the sensor system.
This research was conducted as a part of the project “Aerospace Industry Innovation Program– Advanced Material & Process Development for Next-Generation Aircraft Structures” under thecontract with the material process technology center (Sokeizai center), founded by Ministry ofEconomy, Trade and Industry (METI) of Japan. We express our appreciation to those involved inthis project.

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