中村 俊哉

Fiber Bragg Grating (FBG) sensors have widely been used to monitor temperature and strain distributions as a part of the structural health monitoring system. Since FBG has the sensitivity to the variations in both temperature and strain, a compensation is required to separate the strain or temperature data from the sensor output which is the shift of the grating's Bragg wavelength. The present study develops a computational inverse thermoelastic analysis method to separately identify the thermal and mechanical boundary conditions (loads) from the output of the FBG sensor. Numerical study has been made for a corrugate-core sandwich integral thermal protection system (TPS) to examine the method. The discussion is focused on the computational stability. The results reveal that the identification of the mechanical load is less stable than that of the heat flux. It is also shown that the condition number of a coefficient matrix serves as the index of the stability of the inverse analysis.

In this study, crack propagation tests were conducted to clarify property for the fatigue crack propagation of Friction Stir Welded (FSW) 2024-T3 aluminum alloy. FSW panel has residual stress around weld line and the longitude residual stress is higher around it. The peak tensile residual stress is about 180 MPa in this case. To understand fatigue crack growth property on FSW panel, crack opening stress measurement of the base material and FSW joint using an extensometer with the modified tool is also conducted during the crack growth test. The modified tool is jig to mount the extensometer to the specimen with magnets. The test results indicate that the accuracy of crack opening stress measurement is improved. In addition, the crack growth acceleration and decelerate around the FSW line under the low applied stress range (25 MPa) is bigger than that under high applied stress range (50 MPa). This means the effect of residual stress under low stress range is relatively larger than that under high stress range.

We equipped a composite wing structure fiber Bragg grating (FBG) arrays including 246 FBGs with 10 mm gauge length, eight and six long-length FBGs with 300 mm and 500 mm, respectively. The length of the wing was 6 m and it was made of carbon fiber reinforced plastics (CFRP). The sensing system based on optical frequency domain reflectometry (OFDR) was used in a series of load tests. The measured results by FBG arrays showed the overall deformation of the wing and good agreement with analysis results. Additionally, strain distributions of stress concentration zones were successfully measured by long-length FBGs. © 2012 SPIE.

Thermal analysis is required in the design of the structure of atmospheric reentry vehicles, which are subjected to severe aerodynamic heating. However, prediction of aerodynamic heating is difficult owing to highly complex physical phenomena. Inverse heat conduction analysis, the methodology to estimate heat flux on boundaries and entire temperature distribution from limited number of temperature measurement, is expected to solve this problem and to contribute to improving structural integrity. Present study develops computational method of transient inverse heat conduction analysis using finite element method and pseudo-inverse matrix. The developed inverse analysis code is applied to a reentry vehicle in order to examine the present method and to discuss the computational stability and regularization methods. Sequential function specification (SFS) method and rank reduction are employed to improve the accuracy and stability of the inverse analysis. The results of the numerical simulation reveal that the present method works effectively in solving inverse problem stably by using the combination of SFS method and rank reduction. Especially, the rank reduction is quite efficient at regularization. SFS method requires proper number of future time steps. © 2012 American Institute of Physics.

In this study, cyclic loading experiments and metallographical observation with scanning electron microscopy were conducted to clarify the fatigue life and the origin of fatigue failure of friction-stir-welded (FSW) 2024-T3 aluminum alloy sheet (t = 2 mm) and base material. The observation revealed that FSW specimens were fractured by particles as well as the base metal. The fracture occurred away from the weld area, and the FSW specimen had a fatigue life that was comparable to that of the base metal. The metallographical observation revealed that bigger particles in the stirred area were observed much less frequently compared to the rest of the area. Particle distribution in other areas was similar, regardless of the distance from the center of the weld. On the basis of these findings, we conclude that polished FSW joints have a fatigue life similar to polished base metal. © 2011 IBF (RWTH Aachen) & IUL (TU Dortmund).

We have implemented strain measurements along three sensing lines of long-length fiber Bragg grating (FBG) sensors in a wing model. The sensing system based on optical frequency domain reflectometry (OFDR) can measure strain at an arbitrary position along long-length FBG sensors with the high spatial resolution. We estimated displacement from the measured strain data by applying the beam theory. In addition, in order to investigate the applicability of the load identification method based on the inverse analysis, we applied it to a simply supported beam. In this experiment, the strain distribution along the beam was measured by the long-length FBG and the displacement distribution was also estimated from the measured strain data. Then the applied load distribution was identified by the inverse analysis using the strain or the displacement data.

Elasto-plastic FEM is used to examine the crack opening stress of a 2024-T3 Aluminum alloy sheet and a friction stir welded (FSW) panel. To investigate the effect of plastic deformation around the crack on the crack opening stress, the effects of the following two parameters, the distance between the weld line and the center of the crack starter, and the magnitude of the tensile residual stress on the crack opening stress, are evaluated. The da/dN-ΔK curves and a-N curves for the FSW plate are obtained numerically using an experimental da/dN-ΔK curve for the base material and the calculated crack opening stress. In addition, the da/dN-ΔK curves where ΔK is evaluated by correction factor and its a-N curves are obtained analytically. Comparison of these numerical results with the results of empirical tests shows while that the FEM result conforms to experimental data, the calculations based on the correction factor show poorer correspondence. These results demonstrate that FEM can reasonably predict the da/dN-ΔK curves and a-N curves for an FSW panel.

The purpose of thisstudy is to evaluate the crack closure phenomenon of 2024-T3 aluminum alloy panel and the FSW panel joined by the alloy. Focus attention on crack opening stress, its calculation using elasto-plastic FEM is conducted to clarify the influence of residual stress on crack opening behavior. The calculated crack opening stress of base material and the FSW panel are in close agreement with the measured results. And the FEM results show that the part of crack faces around FSW weld line is opened by tensile residual stress despite no external load. This behavior conforms qualitatively to that of test result. The da/dN - aKeff curves for FSW panel are obtained using the crack growth test results of base material and the calculated crack opening stress for base material and the FSW panel. The obtained curve conformed to the experimental result, indicating the crack growth rate of FSW panel could be predicted quantitatively by evaluating crack opening stress in elasto-plastic FEM.

We have developed the technique by which the distributed loads on a plate structure can be predicted based on an inverse analysis using strain distributions measured by strain gauges or fiber-optic sensors on the surface. In this technique, we are using finite element method (FEM) in order to enable integrated and organized operation of design and in-flight distributed load prediction. In this paper, we describe the prediction technique, the result of the load prediction of a plate which is subjected to a distributed load. In addition, the applicability of the technique to an aircraft wing structure was investigated.

Fatigue crack growth rates in an FSW butt joint panel were obtained in order to evaluate the effect of residual stress on crack growth behavior. Two angles between the weld line and loading direction, and two distances between the weld line and the center of the starter notch were employed to investigate the effect of the residual stress field on the crack growth rate and path, and the effect of the initial distance on crack growth acceleration in tensile residual stress field. The test results indicate that the crack growth rate and path on the weld line are merely affected by the angle between the weld line and the loading direction. Furthermore, the acceleration of crack growth was not affected by the distance between the weld line and the initial notch. Under this test condition, the maximum principle stress is not the primary factor determining the direction of crack growth; rather, the direction of the maximum stress range seems to be the primary factor. © Springer Science+Business Media B.V. 2009.

High spatial resolution and sensitivity are required in distributed strain measurements for structural health monitoring. We have developed a distributed strain sensing technique with long gauge FBG sensors, which enables to measure strain at an arbitrary position along the FBG sensors with the high spatial resolution less than 1 mm based on optical frequency domain reflectometry (OFDR). In this paper this technique with a 1500 mm gauge length FBG was applied to monitoring strain distributions of a simply supported beam subjected to bending loads. The agreement between the measured strain and the theoretical one is excellent. Also we succeeded to identify the applied load by the inverse analysis from the measured strain distribution data, and confirmed the validity of these methods. © 2009 SPIE.

Fatigue crack propagation tests on friction stir welded aluminum alloy sheets were carried out to investigate the effect of residual stress on the crack propagation rate. In these tests, the effect of the inclination angle of the weld line on the direction of crack propagation was also investigated. The test results show that crack propagation rate is accelerated by high tensile residual stress around the weld line. The angle of the weld line does not affect the crack propagation rate and direction. This behavior of crack propagation cannot be explained by the consideration of residual stress value alone. The estimation of effective stress range from the crack opening stress is required to satisfactorily evaluate the crack propagation rate of friction stir welded specimens. Copyright © 2009 by the American Institute of Aeronautics and Astronautics, Inc.

This study investigated the damage evolution mechanism in bearing failure of CFRP bolted joints, using both experiments and three-dimensional finite element analysis. First, we conducted the experiment of the bearing failure in bolted joints of CFRP laminates, and observed the damage evolution to the fracture. Kink bands spread in each 0-deg ply, and shear cracks also spread over all plies. Moreover, the bearing strength of bolted joints depends on lateral clamping force, which suppresses the out-of-plane deformation of the laminates. Therefore the bearing strength of bolted joints is closely related to the fiber kinking damage, which is related to the out-ofplane deformation of the laminates. To understand the underlying mechanism of these experimental results, a finite element model was constructed for the simulation of the damage evolution in bolted joints of CFRP laminates. In this model, the fiber kinking damage is addressed by the criterion including the lateral normal stress, in addition to the other in-plane damage. The simulated results well reproduced the damage areas observed in experiments. According to the results, a load drop is caused by kinking onset, and while the load increases slowly the kinking grows gradually. Repeated load drops in the experimental results are caused by repeated kinking spread. As a result, the bearing strength of bolted joints is closely related to both the initiation and the propagation of the fiber kinking damage. In addition, the relationship between the lateral clamping force and the bearing strength of bolted joints was discussed, based on the simulated results. The lateral clamping force suppresses the kinking onset, and lifts the bearing strength of bolted joints.

Metallographic observation, hardness distribution and fatigue tests are conducted to investigate the crack nucleation site in FSW butt joint in 2024-T3 Aluminum alloy. Three types of surface condition are used to evaluate the effect of surface condition on the crack nucleation site and the fatigue life. The results confirm that there are several types of crack nucleation sites for FSW joint depending on the surface finish and that the feature of the fracture surface is also different depending on the site.