Kanehiro Nagai

Optimum design of weaving structure of 3-D woven fabric composites by using genetic algorithms

This paper discusses the optimum design method of the weaving structure of three-dimensional (3-D) reinforced composites. We propose the design method which combines the genetic algorithms (GA) and the finite element analysis. GA is one of the optimization techniques for the combinatorial optimization problem. In the finite element analysis, we used the original structure model which can express the fiber arrangement state in the 3-D composites faithfully. In this study, the original weaving structure model is constructed by combining the basic structure which has the fiber bundle and the cubic grid of resin. From analysis results, in the small design region, we can obtain the optimum weaving structure. Moreover, we proposed a new genetic operation, to design the weaving structure at the larger design region. These operations aim to prevent the failure of the partial weaving structure in the analytical model as much as possible. From the analysis results, the optimum weaving structure is obtained at the large design region, similar to above results. Consequently, it seems that the proposed method enables the design of the optimum weaving structure in the 3-D composites.

The stress analysis method for three-dimensional composite materials

This study proposes a stress analysis method for three-dimensionally fiber reinforced composite materials. In this method, the rule-of mixture for composites is successfully applied to 3-D space in which material properties would change 3-dimensionally. The fundamental formulas for Youngs modulus, shear modulus, and Poissons ratio are derived. Also, we discuss a strength estimation and an optimum material design technique for 3-D composite materials. The analysis is executed for a triaxial orthogonally woven fabric, and their results are compared to the experimental data in order to verify the accuracy of this method. The present methodology can be easily understood with basic material mechanics and elementary mathematics, so it enables us to write a computer program of this theory without difficulty. Furthermore, this method can be applied to various types of 3-D composites because of its general-purpose characteristics.

Aircraft Structural Health Monitoring using on-board BOCDA system

We developed the on-board BOCDA system for airplane and verified the flight environmental stability and durability through environmental test. The on-board BOCDA system adopted the polarization diversity technique and temporal gating technique to improve robustness of the BOCDA system. We successfully measured distribution of fiber Brillouin gain spectrum over 500m measurement range with 50mm spatial resolution, 60Hz sampling rate and ±13μ strain accuracy. Furthermore, we considered flight test to verify the validity of the BOCDA system. From these results, it was confirmed that BOCDA system has potential to be applied to an aircraft structure health monitoring system.

Aircraft structural-health monitoring using optical fiber distributed BOTDR sensors

We conducted theoretical and practical studies for applying the Brillouin optical timedomain reflectometer (BOTDR) to an aircraft structural-health-monitoring system. First, we measured strain and temperature independently and simultaneously using a combined system of the BOTDR and fiber Bragg grating (FBG) with wavelength division multiplexing (WDM). Second, we used Brillouin spectrum analysis and processing to enhance up to 0.5 m the spatial resolution of the distributed strain measured by the BOTDR. Finally, we verified the proposed improvement of the BOTDR measurement by manufacturing and testing a composite structure.

Strength analysis for three-dimensional fiber reinforced composites

Three-dimensional fiber reinforced composite materials produced by impregnating resin into woven fabric have superior interlaminar and impact strength and are capable of being formed into complex shapes. Consequently it is expected in the future that they will be used for various structural members which have to date been difficult to make with conventional composite materials. With the growth in their fabrication technoloy, the development of a strength analysis method is being demanded. This paper describes a strength analysis method for three-dimensional composite materials on the basis of a micro-mechanical analysis of a unit cell. The unit cell is a small geometrical unit of fiber architecture. A feature of the present analysis method is to represent a unit cell as a rigid frame structure constructed of fiber-beam elements and matrix-beam and matrix-rod elements. Strength analyses are made for orthogonal weave and 5-axial weave three-dimensional carben/epoxy composite materials; the tensile, compressive, and shear moduli and strengths, and Poissons ratio are calculated. The analytical results show fairly good agreement with experimental results; 11%, 21%, and 20% differences between them on the average for elastic moduli, strengths, and Poissons ratios, respectively. It is also understood that the present idealized analysis model cannot accurately predict the characteristics of undulated fiber composites, especially in respect to the compressive strength.

An application test using Brillouin optical correlation base analysis method for aircraft structural health monitoring

The authors developed a prototype Brillouin measurement system and carried out some application tests to verify the capability for aircraft structural health monitoring (SHM). The prototype Brillouin measurement system is adopted the Brillouin optical correlation domain analysis (BOCDA) method. This system is able to measure the distribute strain of full-length optical fiber sensor with 50mm of spatial resolution and 2.7Hz sampling of high-speed arbitrary point strain. Moreover, we conducted three application tests to evaluate the effectiveness of SHM using BOCDA system, such as the panel buckling test, the dynamic strain measurement test, and the demonstration flight test using the prototype BOCDA system. We verify the effectiveness of the BOCDA system for the aircraft SHM, and clarify the necessary development subject for the actual application.

Strain measurement using a Brillouin optical time domain reflectometer for development of aircraft structure health monitoring system

We conducted theoretical and experimental approaches for applying Brillouin optical time domain reflectometer (BOTDR) to aircraft and spacecraft structure health monitoring system. Firstly, distributed strain was measured by BOTDR under 3-point bending test and a spatial resolution was enhanced up to 0.5m using Brillouin spectrum analysis and processing though the device used in this experiment had a spatial resolution of 2m normally. Secondly, dynamic strain measurement was executed under cyclic loading conditions. Brillouin spectrum measured under dynamic conditions is equivalent to superposed spectrum using many spectra measured under static loading conditions. As the measured spectrum was decomposed into many spectra in static loading state, the strain amplitude and its ratio could be estimated. Thirdly, strain and temperature could be measured independently using combined system of BOTDR and fiber Bragg grating (FBG) with wavelength division multiplexing (WDM). Additionally, the application of BOTDR sensing system was shown for a prototype carbon fiber reinforced plastic (CFRP) liquid hydrogen (LH2) tank under cryogenic condition.

Delamination detection using embedded BOCDA optical fiber sensor

We conducted the delamination detect test for composites plate using the Brillouin optical correlation domain analysis (BOCDA) method. Firstly, a hole-assisted-fiber was chosen for embedding optical fiber sensor in order to avoid the increase of optical transmission loss induced by embedded into composite plate. The hole-assisted-fiber is functional better than comparing with the telecommunication optical fibers when optical fiber sensor was embedded into composite. Secondly, a delamination propagating was detected by the BOCDA from Brillouin peak frequency distribution and Brillouin gain spectrum shape changes.

Overview of damage detection and damage suppression demonstrator and strain distribution measurement using distributed BOTDR sensors

Structural health monitoring using optical fiber sensor is very attractive for aerospace structures, because of lightweight, durability and capability to be embedded in composites. Especially, distributed optical fiber sensing system, such as Brillouin Optical Time Domain Reflectometer (BOTDR), is hopeful method for large-scale composite structures. However, it is necessary to solve some problems for applying to aerospace structures. Low spatial resolution, strain/temperature effect, and long measuring period are the capital problems to be solved. For solutions of these problems, we have already reported these solutions. In this paper, we present practical application of the proposed techniques through the demonstrator test. Firstly, we measured mechanical strain and temperature simultaneously during CFRP panel curing process using a combined system of BOTDR and fiber Bragg grating (FBG) sensors with wavelength division multiplexing (WDM). Secondly, we measured the distributed strain in the whole structure and applied differential spectra method for improvement of a spatial resolution.

Development of the Damage Detection Method for CFRP Structures Using Distributed BOCDA Optical Fiber Sensor

The Brillouin optical correlation domain analysis (BOCDA) technology is one of the distributed optical sensing technologies utilizing the Brillouin scattering phenomena. The authors are developing, verifying and validating this technology. They are also developing the installation technique of optical fiber sensors, evaluating the durability of the BOCDA system and manufacturing the new device compatible to avionics bays for size. Furthermore, the authors are also improving the monitoring ability for composite damages. This paper reports the developmental status of the BOCDA monitoring ability for composite damages such as debonding at adhesive joints and micro-damages at bolted joints. First, debonding detection tests were conducted using carbon fiber reinforced plastics (CFRP) stiffened or repaired panels. In this test, test plates with a sheet stringer or a repair patch were applied tension load, and expanding of debonding areas was monitored by measuring strain distribution changes near these areas. Second, bearing damage detection tests were conducted using CFRP bolted joint specimens. In this test, multi-fastener single-lap CFRP specimens were applied tension load, and occurrences of micro-damages near bolt holes was detected. Micro-damages were detected by monitoring shapes changes of the Brillouin Gain Spectrum, which is one of the BOCDA measurement results.