中村 俊哉

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.

The ratcheting behavior of the "unsymmetric two bar system" was investigated by numerical experiments. The two bars are restrained to the same length and are subjected to a constant load. One bar sees cyclic temperature variations while the other bar is kept a: constant temperature. The material models employed are rate-independent plasticity (kinematic hardening) and the Viscoplasticity Theory based on Overstress (VBO) matched to represent the cyclic neutral 6061 T6 Aluminum alloy elastic and inelastic deformation behavior. For simplicity temperature independent material properties were assumed. Numerical analyses were performed to investigate the effects of rate of thermal loading and temperature range. Elastic-inelastic shakedown is ultimately achieved due to work hardening. There is a strain range increase until it reaches a steady value. Kinematic hardening and VBO predict almost the same strain range which for the case of VBO is nearly rate independent. The behavior for both material models is very different for the mean strain For VBO. the number of cycles to shakedown is rate-dependent and is considerably larger than for kinematic hardening. Finally, the steady state mean strain and strain range are computed directly for VBO.

A loading history effect on biaxial mechanical ratchetting under the interaction of creep and plasticity has been experimentally investigated with Modified 9Cr-1Mo steel. The experiments were conducted at 873K under the biaxial loading of a steady torsional shear stress superposing upon a cyclic push-pull straining. In the first series of experiments it has been found that the slower cyclic straining and/or the larger steady shear stress result in the larger accumulation of ratchet strain. The second series of experiments includes changes in the cyclic strain rate and the ″steady″ shear stress to investigate the loading history effect. Simple empirical equations were derived. Additional analysis has been done with inelastic constitutive equations of creep-plasticity superposition type and Chaboche type.