In order to secure the continuous use of CFRP, we aim to predict residual strength and lifetime quantitatively, assuming the thermodynamic entropy as the measure of microscopic damage and fatigue. Though CFRP becomes common material for transport vehicle, it has been difficult to estimate the lifetime because of complex response of composite materials and structure. We apply “Entropy-damage”-involved viscoelastic-viscoplastic constitutive relationship to resin properties in CFRP, and then understand various time-dependent and multi-scale failures of CFRP through the comparison of experimental and numerical results. Such microscopic damages and degradation are validated by molecular simulation linked to the entropy damage model. This study will enable the simulation of failure under multiple loadings and fatigue for composite material.

A spacecraft departing from the Earth has to reduce its kinetic energy when it returns to or reaches its destination planet; otherwise, it may pass over without orbiting the planet or it may collide with the planet. This is because a certain amount of kinetic energy is necessary to leave the gravisphere of the earth. On planets with an atmosphere, thermal energy is generated by friction between the atmosphere and the spacecraft; in other words, kinetic energy is converted into thermal energy and the spacecraft slows down allowing it to arrive safely at its destination. Of course, the spacecraft may also burn up when leaving or reentering the atmosphere, a basic principle in the field of aerospace. Koyanagi Laboratory performs studies on protection from heat generated when spacecraft enter the atmosphere of Mars and Earth. We design the heat shield for re-entering spacecraft, the evaluation of its heat resistance, its thermal analysis, and the evaluation of the insulation material. We perform collaborative investigations with Japan Aerospace Exploration Agency (JAXA), so that the student in charge of this theme in a doctoral or master's thesis will continue to work on such research as a trainee of JAXA.

In order to design materials from an atomistic view point, we study molecular dynamics. Our studies focus on interface bonding, graphene composites, liquid crystals, drag delivery, biomimetics, and so on. Koyanagi Laboratory is a key member of the molecular dynamics research group that is affiliated to the Japan Society for Composite Materials. The research group holds regular meetings every 3 months and we broadcast each meeting through the WebEx system. If you are interested in knowing more about our group, please contact us anytime.

An energy director (ED), a sharp, flat- or triangular-shaped resin bead, is usually inserted between CFRPs during ultrasonic welding. In this study, a two-dimensional model was used to conduct finite element analyses for CFRP ultrasonic welding. The effects of the ED shape on the temperature increase, deformation history, and dissipated energy behavior are discussed. The results of the numerical simulations show that the triangular ED more easily increases the temperature than the flat ED does, and thus its use consumes less energy and time than using the flat ED. However, this study indicates that a triangular ED is not always better than a flat ED. In the ED, the temperature is distributed significantly; that is, the temperature range between points is vast. There is the possibility that unexpected chemical reactions such as oxidation occur. It is found that an abrupt temperature increase is caused by a synergic effect. That is, the increase in temperature causes the viscoelastic and frictional dissipated energy to be remarkable, and an increase in dissipated energy increases the temperature. Consequently, it is difficult to optimize the parameters, such as overall pressure, frequency, and welding time, for ultrasonic welding.