Paper

Health monitoring of large composite structures by using fiber optic distributed strain sensors-Experiences at America's Cup 2000-

Hideaki MURAYAMA' Kazuro KAGEYAMA and Isao KIMPARA Department of Environmental and Ocean Engineering, University of Tokyo 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan:murayama@ygg. naoe. t. u-tkyo. ac. jp

Akiyoshi SHIMADA and Hiroshi NARUSE NTT Access Network Service Systems Laboratories

1-71 Hanabatake, Tukuba-city, Ibaraki 305-0805, Japan: shimada@ansl. ntt. co. jp

(Received 21, July 2000 Accepted 30, November 2000)

Health monitoring technologies of a structure made of polymer matrix composites have lately become a subject of special interest. International America's Cup Class (IACC) yachts are specific boats whose most components made of carbon fiber reinforced plastic (CFRP) and that are regulated by the IACC rule. A

Brillouin optical time domain reflectometer (BOTDR) is a fiber optic distributed sensor that can measure strain and temperature along an optical fiber. We equipped IACC yachts that were the Japanese entry in

America's Cup 2000 with these sensors. Strain distributions of the yachts measured with BOTDR were used for structural health monitoring and assessments of the structural integrity were continued during races.

Keywords: America's Cup, IACC yacht, sandwich structure, fiber optic distributed sensor, BOTDR, health monitoring

I. INTRODUCTION The America's Cup is one of the world's most

famous yacht races. A yacht used in these races is called International America's Cup Class (IACC) yacht and its design is regulated by the IACC rule. The yacht is about 24m long with a mast about 35m high above the water surface. Designers try to develop as a fast yacht as possible following the rule and they also attempt to make their yacht lighter and stronger. The race for design superiority is at the core of America's Cup competition.

In terms of structural design of IACC yachts, today's advanced composite materials offer the beneficial properties of high specific stiffness (stiffness/density) and high specific strength (strength/density) for stiff lightweight structures. Most components of an IACC yacht are made of carbon fiber reinforced plastic (CFRP), either with single skinned or sandwich structures. The sandwich structure usually consists of two faces (called skin) which are kept separated by a core. The hull, the deck and the bulkhead, which are the key components in an IACC yacht structure, consist of the sandwich structures with CFRP skins and an aluminum honeycomb core and they are joined by glue.

In a large yacht assembled using CFRP sandwich composite materials, it is difficult to make assure that a manufactured structure fulfills designer's expectation. Although dynamic forces (wind and waves) influence the structural conditions of an IACC yacht, the yacht is usually designed to have the adequate stiffness and strength against a static load rather than a dynamic load. Because static forces loaded through wires (called stays) that support the mast are considerable and sometimes reach more than 50 tons during races. Especially, the longitudinal stiffness of the yacht and the strength of the adhesive joint between the hull and the bulkhead are very important subjects for the designer.

In the past America's Cup, the Japanese syndicate inspected the yacht structure by using strain gauge and obtained adequate confidence that the product was in accordance with requirements. But this method required many sensors and huge effort. Additionally, the sensors and the measurement system were disposable. So we attempted to develop a new and easier method for quality assurance by using distributed fiber optic sensors and to reuse the sensing system for assessing structural integrity of IACC yachts.

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With the rapid progress of distributed or quasi-distributed fiber optic sensors in recent years, their applicability for structural health monitoring has increased. We can obtain comprehensive structural information easily by using them. In addition, they have enough robustness and durability for marine structures. A Brillouin optical time domain reflectometer (BOTDR) that can measure strain and temperature in arbitrary regions of a sensing fiber is a fiber optic distributed sensor based on Brillouin scattering.2 The optical effect depends solely on the fiber material, in that the bare fiber itself acts as sensing element without any special processing or preparation.3 A BOTDR needed 3-6 minutes acquisition time. This means that we can't measure dynamic deformation using the BOTDR. However, we are able to assess structural properties (stiffness and symmetricalness) and integrity using strain data for an IACC yacht that is only subjected to a dead load. Because static forces mainly influence the structural conditions as mentioned above.

In this paper, we describe a strain measurement system using a BOTDR developed by Nippon Telegraph and Telephone Corporation (NIT) and a health monitoring technique applied to the IACC yachts of Nippon Challenge syndicate entered in America's Cup 2000. Sensing fibers were fixed on the hull in the longitudinal direction and on the adhesive joint between the hull and the bulkhead supporting the mast in the transverse direction. They were used to measure the deformation of the yacht and the measurement results were compared with the results calculated by finite element analysis. We successfully measured strain distributions of the yacht that was subjected to a dead load on shore. The measurement results were in good agreement with the calculated results. Therefore, we could judge that the structural properties of the manufactured yacht was in accordance with designer's requirements. Additionally, we undertook a structural health monitoring of the yachts using the strain distribution data.

II. PRINCIPLE OF BOTDR A lightwave transmitted through an optical fiber is

scattered by nonlinear interaction with acoustic wave and this is called Brillouin scattering. The sensor system makes use of Brillouin scattering within a single mode fiber to allow measurement of both strain and temperature. The magnitude of the Brillouin frequency shift vB is related to the crystal structure of the fiber and as such it is influenced by temperature changes and applied strains.4 The

coefcients of vB change,∂vB/∂T and∂vB/∂ ε are

given by

(1)

(2)

respectively. The propagation delays of light

traveling in the fiber are related to the distance x

along the fiber. So the width W of the transmitted

pulse signal gives the special resolution ox as

(3)

where Vc is the velocity of light in the fiber. We

obtained 1m spatial resolution from 10 ns pulses.

The sensor accuracy of a BOTDR developed by

NTT was assumed to be±60μ ε.5 Figure l shows

Fig. 1. Principle of BOTDR

how to measure strain r from the Brillouin spectrum.

III. FINITE ELEMENT ANALYSIS Finite element analysis is one of the indispensable

components for structural design of an IACC yacht. We used a commercial FEA code

(PRO/MECHANICA, Parametric Technology Corporation) and the calculated strain distributions of the IACC yacht were compared with the results measured by the BOTDR. Figure 2 shows the finite

Fig. 2. The finite element model

element model with all structural components.

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IV. DISTRIBUTED STRAIN MEASUREMENT OF IACC YACHTS Nippon Challenge syndicate built two yachts (Asura and Idaten) for America's Cup 2000. We equipped the yachts with the sensing fibers. They consist of two parts; one was fixed to the hull in the longitudinal direction and the other was adhered in the circumferential direction near the joint between the hull and the bulkhead supporting the mast. In this paper, the sensing fibers will be called the longitudinal sensing fiber and the circular sensing fiber respectively. A. System integration The sensing fibers were installed in the yachts under construction. We used 0.25mm diameter optical fiber with a coating and adhered on the inner surface of the hull. Then we covered the sensors with CFRP sheets to-protect them from various potential sources of damage including machine tools and shoes. Without such protection, the boat builders and crew could not have walked on the yachts without damaging the sensors. The longitudinal sensing fiber covered with CFRP sheets in Idaten is shown in Fig. 3. We used a CAD application in order to calculate the actual positions and the length of the sensing fibers. Figure 4 shows the locations of the sensing fibers in Idaten. The longitudinal sensing fiber was set from a to i, and the circular sensing fiber set from j to m in Fig. 4. The lengths between the gauge marks are listed in Table 1.

Fig. 3. The installation of the sensing fiber under

construction

The longitudinal sensing fiber was fixed to the

hull and it traveled back and forth twice in the

longitudinal direction. Two lines, a-b and g-h, were

located above the neutral axis of the yacht and

another two lines, c-d and e-f, were located below it.

The neutral axis means the line along which the

stresses are nothing in longitudinal bending. The

circular sensing fiber traveled back and forth along

the boundary between the hull and the bulkhead supporting the mast. A line, j-k, was fixed to the hull and another, 1-m, was fixed to the fringe of the bulkhead. We connected the loose fibers about 10m long to the sensing fibers with fusion splices. Internal stresses of the loose fibers were always relieved.

Fig. 4. The sensing fiber in the IACC yacht

Table 1. The lengths of the sensing fibers

B. IACC yacht loading conditions and

measurement setup The static forces mentioned in the introduction are

principally classified into the two tensile forces, namely forestay/backstay tension and sidestay tension. The forestay and the backstay are wires that

extend from the head of the mast to the bow and the stem respectively. The sidestay is a wire that extends

from the head of the mast to the right side or the left side of a yacht. These static forces can be applied to

a yacht on a slipway during maintenance periods as well as at sea. When applying the forces on a slipway,

a particular stress or strain distribution occurs in the

yacht's structure. For example, the forestay tension can bring about a strain distribution in longitudinal bending of the yacht. Figure 5 shows the

measurement setup at the base camp of Nippon Challenge and the static forces that were applied to

the IACC yacht. During the measurements, the three loading

conditions were used. Under the first condition, the

forestay tension was set at 0 tons in Fig. 5. For the second, it was set at 4 tons. Under the both

conditions, the sidestay tension was 30 tons. The

mast was removed from the yacht in the third case. Then, of course, the forestay and sidestay tension

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were both zero. In this paper, we refer to these three loading conditions as Ot, 4t and nomast, respectively.

A difference in the strain distributions under different loading conditions represents a deformation

caused by the tensile forces. For example, the difference between the strain distributions under 4t

and Ot represents a bending state of the hull structure because of the forestay/backstay tension, while the

difference between the strain distributions under Ot

and nomast represents a deformation caused by the

sidestay tension. In the former case, the longitudinal

sensing fiber was suitable for monitoring the deformation. The circular sensing fiber was suitable in the latter case. All measurements were carried out

after sunset because BOTDR strain measurement results were affected by temperature changes.

Fig. S. The measurement setup and the static forces of the yacht on the slipway

C. Results of strain measurement and FEA In Fig. 6, the solid line is the difference between

the measured strain distributions under 4t and Ot and the broken line is the calculated result. We can see the state of the hull that was bent by the forestay and the backstay in Fig. 6. The regions below the neutral axis, c-d and e-f, have positive (tensile) strain and the regions above it, a-b and g-h, have negative (compressive) strain. Similarly, the difference between the strain distribution under Ot and nomast is shown in Fig. 7, and we can see the structural state of the boundary between the hull and the bulkhead loaded by the sidestay tension (30 tons). Both in Fig. 6 and Fig. 7, the measurement results are in good

agreement with the results calculated by FEA. Additionally, we can assume the adhered boundary to be undamaged, because the strain distribution of the hull region a-k) was almost the same as that of the bulkhead region (1-m) in Fig. 7 when the yacht was subjected to the sidestay tension. Therefore, we can say that the manufactured yacht fulfilled the designer's requirements in terms of structural design.

In addition, we can reach the following conclusions from these results. 1. Strain distribution measured periodically by the

longitudinal sensing fiber under the same forestay tension can provide some information about any degradation in the longitudinal stiffness of the yachts.

2. Strain distribution measured periodically by the circular sensing fiber under the same sidestay tension can provide some information about

damage or debonding of the adhered boundary between the hull and the bulkhead.

Fig. 6. The measured and calculated strain

distribution of the longitudinal sensing fiber

Fig. 7. The measured and calculated strain

distribution of the circular sensing fiber

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V. ASSESSMENT OF STRUCTRAL INTEGRITY The structural health monitoring of the IACC

yachts was conducted by assessing the difference between the latest strain distribution and the earlier data measured under the same loading condition. In order to detect any degradation in longitudinal stiffness, we measured the strain distributions of the longitudinal sensing fiber under the same loading condition of 4t and checked the difference between the most recent and earlier data. Additionally, we examined the difference between the strain distributions measured with the circular sensing fiber at different dates to detect debonding the boundary between the hull and the bulkhead.

In the base camp of Nippon Challenge syndicate, the periodical strain measurements and data analysis were implemented before and after a series of races. According to the measurement results before the races, we judged whether we had better reinforce the yacht additionally. After the races, we assessed accumulated structural fatigue during races. The data obtained from the BO R as shown in Figs. 8 and 9

NTT and the results of the analysis were reported to

Nippon Challenge. Based on these reports, they had

planed structural strategies.

Fig. 8. An example of structural assessment

using the longitudinal sensing fiber

Fig. 9. An example of structural assessment

using the circular sensing fiber

were sent to the laboratory of the University and

VI. CONCLUSIONS

A structural health monitoring system with fiber optic distributed strain sensors was applied to IACC

yachts. We successfully measured strain distributions of the yachts using the sensing fibers that were fixed to the yacht's structure and covered with CFRP

sheets. The strain distributions of the sensing fibers

acquired from a BOTDR were used to assess the

structural properties and integrity. Consequently, the crew of Nippon Challenge and we could trust our

yachts during races.

ACKNOWLEDGEMENTS The authors thank Dr. H. Miyata of Tokyo University

who is also the technical director of Nippon Challenge syndicate and Mr. K. Uzawa of GH Craft Ltd. for their kind support and helpful advice. Thanks are due to Mr. M. Katori who is a technical member of Nippon Challenge and also a navigator in the races, Mr. Y. Murata, Mr. G. Hayashi and all other stuff of Nippon Challenge for the measurement operations that were carried out in the base camp.

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Presented at 3rd Japan France Seminar on IMS

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