中村 俊哉

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.

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.

Japan Aerospace Exploration Agency (JAXA) is conducting concept studies on some experimental vehicles that demonstrate the technologies needed to establish reusable space transportation system. Piggyback Atmospheric Reentry Technology Testbed (PARTT), lifting body reentry experimental vehicle and rocket plane are among them. PARTT is aimed to establish a cheaper and faster reentry testbed that can be used to demonstrate key reentry technologies. The spacecraft for this testbed is composed of an orbiter module and a reentry module. The orbiter module is equipped with systems to control the attitude and the orbit of the spacecraft while in orbit. It is assumed that the spacecraft is injected into a circular orbit of altitude 250km, which is a parking orbit toward geostationary transfer orbit (GTO) of H-IIA launch vehicle1. After that, this spacecraft conducts several maneuvers necessary to deorbit from this low earth orbit (LEO) and the reentry module reenters to the atmosphere. The reentry module consists of a main body in which most of the instruments are installed and a shell to protect the main body from aerodynamic heating in the reentry phase. The shell is connected to the main body with a support structure. The shell and the support are supposed to be made of carbon/carbon (C/C). A brief feasibility study on PARTT was already completed and capability of manufacturing a heat shield for PARTT was confirmed by manufacturing a half size trial production. Antenna pattern data was obtained to confirm the high frequency electric character of C/C material. The objective of the lifting body reentry experimental vehicle presented in this paper is to demonstrate by using a single vehicle the overall re-entry technology that was confirmed in the series of flight tests in the HOPE-X Project2. The vehicle is injected into LEO loaded in H-IIA fairing, deorbits, flies from reentry through just below the sound speed, reduces its speed by parachute and makes a soft landing by using airbags. The lifting body shape is considered to be one of the promising vehicle configuration of the future reusable space transportation system, however we don't have much experience to design a lifting body shape vehicle in Japan and technical background of this shape should be constructed. Therefore lifting body shape was selected for the experimental vehicle. Vehicle size and aerodynamic shape was studied. And the results of weight estimation and flight analysis show the feasibility of this experimental vehicle. The detail of the experiment vehicles will be determined according as the progress of the planning of the research on reusable space transportation system. Related guidance technology using parafoil was studied to estimate the distribution of the landing point. This result enables us to have a landing site plan for the lifting body reentry experiment vehicle. The objective of the rocket plane experiment vehicle is to demonstrate and to accumulate technology for reusable space transportation system under the flight environment. Also this vehicle is expected to be useful to conduct flight experiment of advanced guidance and control technology which contains a function to abort flight and return to the ground safely and to conduct flight tests of air breathing engine and other various experiments using this vehicle in the future. The basic concept of this vehicle is wing-body configuration with a rocket engine, horizontal take-off and landing and flying back to take-off site after completion of its mission. Three types of experimental vehicles that are vehicle scales are different were examined. Woomera was presumed as a landing field, and the examination of the landing field was also conducted. © 2003 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

National Aerospace Laboratory of Japan (NAL) is conducting concept study on some experimental vehicles that demonstrate the technologies needed to establish reusable system. Piggyback Atmospheric Reentry Technology Testbed (PARTT) and lifting body reentry experimental vehicle are among them. PARTT is aimed to establish a cheaper and faster reentry testbed that can be used to demonstrate key reentry technologies. The spacecraft for this testbed is composed of an orbiter module and a reentry module. The orbiter module is equipped with systems to control the attitude and the orbit of the spacecraft while in orbit. It is assumed that the spacecraft is injected into a circular orbit of altitude 250km, which is a parking orbit toward geostationary transfer orbit (GTO) of H-IIA launch vehicle [1]. After that, this spacecraft conducts several maneuvers necessary to deorbit from this low earth orbit (LEO) and the reentry module reenters to the atmosphere. The reentry module consists of a main body in which most of the instruments are installed and a shell to protect the main body from aerodynamic heating in the reentry phase. The shell is connected to the main body with a support structure. The shell and the support are supposed to be made of carbon/carbon (C/C). The objective of the lifting body reentry experimental vehicle presented in this paper is to demonstrate by using a single vehicle the overall re-entry technology that was confirmed in the series of flight tests in the HOPE-X Project [2]. The vehicle is injected into LEO loaded in H-IIA fairing, deorbits, flies from reentry through just below the sound speed, reduces its speed by parachute and makes a soft landing by using airbags. The lifting body shape is considered to be one of the promising vehicle configuration of the future reusable space transportation system, however we don't have much experience to design a lifting body shape vehicle in Japan and technical background of this shape should be constructed. Therefore lifting body shape was selected for the experimental vehicle. Vehicle size and aerodynamic shape was studied. And the results of weight estimation and flight analysis show the feasibility of this experimental vehicle. The detail of the experiment vehicles will be determined according as the progress of the planning of the research on reusable space transportation systems. © 2002 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.