Toshiya Nakamura

Measurement of distributed strain and load identification using 1500 mm gauge length FBG and optical frequency domain reflectometry

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.

Fatigue crack propagation property of friction stir welded 2024-T3 aluminum alloy

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.

Distributed strain and load monitoring of 6 m composite wing structure by FBG arrays and long-length FBGs

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.

Fatigue Crack Growth of Friction-Stir-Welded Aluminum Alloy

Crack propagation tests were conducted to clarify the effect of stress ratio on crack growth rate in friction-stir-welded 2024-T3 aluminum alloy. The effect of the distance between the weld line center and the center of the specimen was also evaluated. The result indicates that the peak of the acceleration is not at the area of maximum tensile residual stress. The modified stress ratio that considers the residual stress identifies that the location of the peak modified stress ratio corresponds to that of the peak maximum acceleration. The relationship between stress ratio and crack growth acceleration in friction-stir-welded panels is also evaluated by an analysis using the stress intensity factor range with and without residual stress and the crack growth rate for the master curve obtained by the base material with a different stress range. The maximum acceleration decreases with the increase of the distance between the weld line center and the center of the specimen and with the increase of the stress r...

Transient inverse heat conduction analysis of atmospheric reentry vehicle using FEM

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 th...

Inverse thermoelastic analysis for thermal and mechanical loads identification using FBG data

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.

Evaluation of Fatigue Crack Growth Behavior in FSW Joint by Experiment, Analysis and Elasto-Plastic FEM

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.

Fatigue Crack Modeling and Simulation Based on Continuum Damage Mechanics

Numerical simulation of fatigue crack propagation based on fracture mechanics and conventional finite element method requires huge amount of computational resources when cracked structure is subjected to complicated condition such as the cases of multiple site damage or thermal fatigue. The objective of the present study is to resolve this difficulty by employing the continuum damage mechanics (CDM). An anisotropic damage variable is defined to model a macroscopic fatigue crack and its validity is examined by comparing the stress distributions around the crack with those obtained by an ordinary fracture mechanics method. Together with the assumptions on crack opening/closing and damage evolution, numerical simulations are conducted for low cycle fatigue crack propagation behaviors in a plate with single and two cracks. The results show good agreement with the experiments. Finally, propagations of multiply distributed cracks under low cycle fatigue loading are simulated to demonstrate the potential applicability of the present method.Copyright