Masaki Kotani

A Comprehensive Model for Determining Tensile Strengths of Various Unidirectional Composites

Simultaneous fiber-failure (SFF) model that determines tensile strengths for various systems of unidirectional composites comprehensively is presented. The SFF derives the strength of unidirectional composites as a function of the single-fiber strength distribution, interfacial shear strength, and matrix strength. The point of the SFF is that a fiber group which is considered to be experiencing simultaneous fiber failures triggered by neighboring fiber failures is assumed to fail when the weakest fiber in the fiber group fails. We discuss a method to determine the magnitude of the fiber group for various systems of composites on a basis of whether a crack located near a bi-materials interface penetrates into another material or deflects along the interface. The SFF is established by integrating the magnitude of the simultaneous fiber failures into the conventional Global load sharing model.

Quality evaluation of spaceborne SiC mirrors (II): evaluation technology for mirror accuracy using actual measurement data of samples cut out from a mirror surface

The authors studied the quality evaluation technology of a spaceborne large-scale lightweight mirror that was made of silicon carbide (SiC)-based material. To correlate the material property of a mirror body and the mirror accuracy, the authors evaluated the mirror surface deviation of a prototype mirror by inputting actually measured coefficient of thermal expansion (CTE) data into a finite element analysis model. The CTE data were obtained by thermodilatometry using a commercial grade thermal dilatometer for the samples cut from all over the mirror surface. The computationally simulated contour diagrams well reproduced the mirror accuracy profile that the actual mirror showed in cryogenic testing. Density data were also useful for evaluating the mirror surface deviation because they had a close relationship with the CTE.

Optical testing of lightweight large all-C/SiC optics

We carried out various tests of 800-mm-diameter aperture, lightweight optics that consisted wholly of carbon fiber-reinforced SiC composite, called HB-Cesic. A cryogenic optical test was performed on the primary mirror to examine any CTE irregularity as a surface change, and only small deformations were observed. The primary mirror was assembled with a convex secondary mirror into an optical system and tested under vacuum at the 6-m-diameter radiometer space chamber at Tsukuba Space Center of JAXA, where we have prepared interferometric metrological facilities to establish techniques to test large optical systems in a horizontal light-axis configuration. The wavefront difference between under vacuum and under atmosphere was confirmed to be less than 0.1 λ at λ=633 nm, if we realigned the optical axis of the interferometer and flat mirror under vacuum. We also demonstrated a stitching interferometry using the Φ800-mm optics by rotating a mask wheel of subapertures in front of the optical reference flat. The wavefront stitched from eight individual measurements of Φ275-mm subapertures differs from the full-aperture measurement without the mask by about 0.1 λ nm RMS, which showed the technique could able to be applied to test large telescopes especially for infrared wavelength region.

Quality evaluation of spaceborne SiC mirrors (I): analytical examination of the effects on mirror accuracy by variation in the thermal expansion property of the mirror surface

The Japan Aerospace Exploration Agency has studied a large-scale lightweight mirror constructed of reaction-bonded silicon carbide-based material as a key technology in future astronomical and earth observation missions. The authors selected silicon carbide as the promising candidate due to excellent characteristics of specific stiffness and thermal stability. One of the most important technical issues for large-scale ceramic components is the uniformity of the materials property, depending on part and processing. It might influence mirror accuracy due to uneven thermal deformation. The authors conducted systematic case studies for the conditions of CTE by finite element analysis to know the typical influence of material property nonuniformity on mirror accuracy and consequently derived a comprehensive empirical equation for the series of CTEs main factors. In addition, the authors computationally reproduced the mirror accuracy profile of a small prototype mirror shown in cryogenic testing and hereby verified wide-range practical computational evaluation technology of mirror accuracy.

Performance of lightweight large C/SiC mirror

Very lightweight mirror will be required in the near future for both astronomical and earth science/observation missions. Silicon carbide is becoming one of the major materials applied especially to large and/or light space-borne optics, such as Herschel, GAIA, and SPICA. On the other hand, the technology of highly accurate optical measurement of large telescopes, especially in visible wavelength or cryogenic circumstances is also indispensable to realize such space-borne telescopes and hence the successful missions. We have manufactured a very lightweight Φ=800mm mirror made of carbon reinforced silicon carbide composite that can be used to evaluate the homogeneity of the mirror substrate and to master and establish the ground testing method and techniques by assembling it as the primary mirror into an optical system. All other parts of the optics model are also made of the same material as the primary mirror. The composite material was assumed to be homogeneous from the mechanical tests of samples cut out from the various areas of the 800mm mirror green-body and the cryogenic optical measurement of the mirror surface deformation of a 160mm sample mirror that is also made from the same green-body as the 800mm mirror. The circumstance and condition of the optical testing facility has been confirmed to be capable for the highly precise optical measurements of large optical systems of horizontal light axis configuration. Stitching measurement method and the algorithm for analysis of the measurement is also under study.

Interface Formation of Unidirectional SiCf/SiC Composites by Electrophoretic Deposition Method

C-and BN-interphases on SiC fibers for unidirectional SiCf/SiC composites were formed by EPD process, and their microstructure and mechanical properties were investigated. Whereas the C-SiCf/SiC composites showed a pseudo-ductile fracture behavior with large amount of fiber pullout, the BN-SiCf/SiC composites fractured in a brittle manner without fiber pullout in spite of sufficient thickness of BN interphase. It is inferred from the results of EDS that sintering additives would react with h-BN-interphase, and the interphase did not act effectively for toughening the SiCf/SiC composites.

Evaluation of Spaceborne SiC Mirror Materials Using Samples Cut from the Periphery of a Mirror Body

The Japan Aerospace Exploration Agency has studied large-scale, lightweight mirrors constructed of SiC-based materials as a key technology for future earth observations and astronomical missions. One of the most important technical issues for large-scale ceramic components is their quality stability (viz., differences in material properties depending on the part and the processing), which might influence the structural and/or thermal reliability through unforeseen deformation and breakage. In this study, the authors used a simple, low-cost method for evaluating the properties of SiC mirror materials. Using mechanical testing, thermodilatometry, and microstructural analysis on samples cut from the periphery of a prototype 800-mm-diameter mirror body, the overall quality of the mirror body material was determined.

Quality evaluation of spaceborne SiC mirrors: the effects on mirror accuracy by variation in the thermal expansion property of the mirror surface

A large-scale lightweight mirror that is made of silicon carbide-based material is required for the coming astronomical and earth observation missions. The influence of the inhomogeneity of the coefficient of thermal expansion (CTE) on specular surface accuracy was studied as an important technological issue for such a large optical component. At first, a systematic case study for the series of CTE’s main factors was conducted using the finite element method, and consequently a comprehensive equation to calculate the amount of surface deviation was derived. Based on that technology, finite element analysis to simulate the surface accuracy profile that a test mirror sample showed during cryogenic measurement was carried out using experimentally obtained CTE data from cutout test pieces, and the profile was successfully reproduced.

Manufacturing and optical testing of 800 mm lightweight all C/SiC optics

Owing to its high specific stiffness and high thermal stability, silicon carbide is one of the materials most suitable for large space-borne optics. Technologies for accurate optical measurements of large optics in the vacuum or cryogenic conditions are also indispensable. Within the framework of the large SiC mirror study program led by JAXA, we manufactured an 800-mm-diameter lightweight telescope, all of which is made of HB-Cesic, a new type of carbon-fiber-reinforced silicon carbide (C/SiC) material developed jointly by ECM, Germany and MELCO, Japan. We first fabricated an 800-mm HB-Cesic primary mirror, and measured the cryogenic deformation of the mirror mounted on an HB-Cesic optical bench in a liquid-helium chamber. We observed the cryo-deformation of 110 nm RMS at 18 K with neither appreciable distortion associated with the mirror support nor significant residual deformation after cooling. We then integrated the primary mirror and a high-order aspheric secondary mirror into a telescope. To evaluate its optical performance, we established a measurement system, which consists of an interferometer in a pressure vessel mounted on a 5-axis adjustable stage, a 900-mm auto-collimating flat mirror, and a flat mirror stand with mechanisms of 2-axis tilt adjustment and rotation with respect to the telescope optical axis. We installed the telescope with the measurement system into the JAXA 6-m chamber and tested them at a vacuum pressure to verify that the system has a sufficiently high tolerance against vibrations in the chamber environment. Finally we conducted a preliminary study of sub-aperture stitching interferometry, which is needed for telescopes of our target missions in this study, by replacing the 900-mm flat mirror with a rotating 300-mm flat mirror.

Monitoring Sensor for the erosion and surface temperature of TPS materials

As a technical option to heighten the structural reliability of a reusable space transportation system, a sensor to monitor the erosion and surface temperature of TPS materials in flight has been studied. It was designed to be a bolt-shaped and light device to be put in an airframe structure and was expected to detect erosion and temperature from the corresponding electrical resistance of thin conductive multiple layers coated on a substrate. The design of a sensor was firstly conducted. Then, important elemental technologies such as multiple layer formation and electrode figuration were examined, and sensor samples were experimentally fabricated and tested using an arc jet facility for functional verification. Numerical estimation of electrical resistance are to be conducted for making a correlation with the test result.