Masami Kume

Microscopic surface structure of C/SiC composite mirrors for space cryogenic telescopes

We report on the microscopic surface structure of carbon-fiber-reinforced silicon carbide (C/SiC) composite mirrors that have been improved for the Space Infrared Telescope for Cosmology and Astrophysics (SPICA) and other cooled telescopes. The C/SiC composite consists of carbon fiber, silicon carbide, and residual silicon. Specific microscopic structures are found on the surface of the bare C/SiC mirrors after polishing. These structures are considered to be caused by the different hardness of those materials. The roughness obtained for the bare mirrors is 20 nm rms for flat surfaces and 100 nm rms for curved surfaces. It was confirmed that a SiSiC slurry coating is effective in reducing the roughness to 2 nm rms. The scattering properties of the mirrors were measured at room temperature and also at 95 K. No significant change was found in the scattering properties through cooling, which suggests that the microscopic surface structure is stable with changes in temperature down to cryogenic values. The C/SiC mirror with the SiSiC slurry coating is a promising candidate for the SPICA telescope.

Cryogenic optical measurements of 12-segment-bonded carbon-fiber-reinforced silicon carbide composite mirror with support mechanism

A 720 mm diameter 12-segment-bonded carbon-fiber-reinforced silicon carbide (C/SiC) composite mirror has been fabricated and tested at cryogenic temperatures. Interferometric measurements show significant cryogenic deformation of the C/SiC composite mirror, which is well reproduced by a model analysis with measured properties of the bonded segments. It is concluded that the deformation is due mostly to variation in coefficients of thermal expansion among segments. In parallel, a 4-degree-of-freedom ball-bearing support mechanism has been developed for cryogenic applications. The C/SiC composite mirror was mounted on an aluminum base plate with the support mechanism and tested again. Cryogenic deformation of the mirror attributed to thermal contraction of the aluminum base plate via the support mechanism is highly reduced by the support, confirming that the newly developed support mechanism is promising for its future application to large-aperture cooled space telescopes.

Mechanical and thermal performance of C/SiC composites for SPICA mirror

One of the key technologies for next generation space telescope with a large-scale reflector is a material having high specific strength, high specific stiffness, low coefficient of thermal expansion and high coefficient of thermal conductivity. Several candidates such as fused silica, beryllium, silicon carbide and carbon fiber reinforced composites have been evaluated. Pitch-based carbon fiber reinforced SiC composites were developed for the SPICA space telescope mirror to comply with such requirements. Mechanical performance such as bending stiffness, bending strength and fracture toughness was significantly improved. Evaluation procedures of thermal expansion and thermal conductivity behavior at cryogenic temperatures (as low as 4.5K) were established and excellent performance for the SPICA mirror was demonstrated.

Structural health monitoring of CFRP airframe structures using fiber-optic-based strain mapping

This paper proposes structural health monitoring technology based on the strain mapping of composite airframe structures through their life cycles by FBG sensors. We carried out operational load tests of small-sized mockup specimens of CFRP pressure bulkhead and measured the strain by FBG sensors. In addition, we confirmed strain change due to stiffener debondings. Moreover, debonding detectability of FBG sensors were investigated through the strain monitoring test of CFRP skin-stiffener panel specimens. As a result, the strain distribution varied with damage configurations. Moreover, the change in strain distribution measured by FBG sensors agrees well with numerical simulation. These results demonstrate that FBG sensors can detect stiffener debondings with the dimension of 5mm in composite airframe structures.

Life cycle strain monitoring of composite airframe structures by FBG sensors

Life cycle health monitoring technology for composite airframe structures based on strain mapping is proposed. It detects damages and deformation harmful to the structures by strain mapping using fiber Bragg grating (FBG) sensors through their life cycles including the stages of molding, machining, assembling, operation, and maintenance. In this paper, we firstly carried out a strain monitoring test of CFRP mock-up structure through the life cycle including the stage of molding, machining, assembling, and operation. The experimental result confirms that the strain which arises in each life cycle stage can be measured by FBG sensors embedded in molding stage and demonstrates the feasibility of life cycle structural health monitoring by using FBG sensors. Secondly, we conducted the strain monitoring test of CFRP scarf-repaired specimen subject to fatigue load. FBG sensors were embedded in the scarf-repaired part of the specimen and their reflection spectra were measured in uni-axial cyclic load test. Strain changes were compared with the pulse thermographic inspection. As a result, strain measured by FBG sensors changed sensitively with debonded area of repair patch, which demonstrates that the debondings of repair patches in scarf-repaired composites due to fatigue load can be detected by FBG sensors.

Life cycle strain mapping of composite airframe structures by using FBG sensors

The objective of this work is to develop a system for monitoring the structural integrity of composite airframe structures by strain mapping over the entire lifecycle of the structure. Specifically, we use fiber Bragg grating sensors to measure strain in a pressure bulkhead made of carbon fiber reinforced plastics (CFRPs) through a sequence of lifecycle stages (molding, machining, assembly, operation and maintenance) and detect the damage, defects, and deformation that occurs at each stage from the obtained strain distributions. In previous work, we have evaluated strain monitoring at each step in the FRP molding and machining stages of the lifecycle. In the work reported here, we evaluate the monitoring of the changes in strain that occur at the time of bolt fastening during assembly. The results show that the FBG sensors can detect the changes in strain that occur when a load is applied to the structure during correction of thermal deformation or when there is an offset in the hole position when structures are bolted together. We also conducted experiments to evaluate the detection of damage and deformation modes that occur in the pressure bulkhead during operation. Those results show that the FBG sensors detect the characteristic changes in strain for each mode.

Development of highly reliable advanced grid structure (HRAGS) demonstrator using FBG sensors

There is a growing demand in recent years for lightweight structures in aircraft systems from the viewpoints of energy and cost savings. The authors have continued development of the Highly Reliable Advanced Grid Structure (HRAGS) for aircraft structure. HRAGS is provided with health monitoring functions that make use of Fiber Bragg Grating (FBG) sensors in advanced grid structures. To apply HRAGS technology to aircraft structures, a full-scale demonstrator visualizing the actual aircraft structure needs to be built and evaluated so that the effectiveness of the technology can be validated. So the authors selected the wing tip as the candidate structural member and proceeded to design and build a demonstrator. A box-structure was adopted as the structure for the wing-tip demonstrator, and HRAGS panels were used as the skin panels on the upper and lower surfaces of the structure. The demonstrator was designed using about 600 FBG sensors using a panel size of 1 x 2 m. By using the demonstrator, damage detection functions of HRAGS system were verified analytically. The results of the design and evaluation of the demonstrator are reported here.

Highly reliable advanced grid structures (HRAGS) for aircraft structures using multi-point FBG sensor

There is a growing demand for lightweight structures in aircraft systems for energy and cost savings. The authors have therefore continued development of the Highly Reliable Advanced Grid Structure (HRAGS) with the aim of application of the same to aircraft. HRAGS is provided with health monitoring functions that make use of Fiber Bragg Grating (FBG) sensors in advanced grid structures, which have been the focus of attention in recent years as lightweight structures. It is a new lightweight structural concept that enables lighter weight to be obtained while maintaining high reliability. This report describes the tests and evaluation of the Proto System conducted to verify experimentally the concept of the highly reliable advanced grid structure. The Proto System consists of a skin panel embedded with 29 FBG sensors and a wavelength detection system. The artificial damage to the skin panel of the specimen was successfully detected by comparing the strain distributions before and after the introduction of the damage measured by FBG sensors. Next, the application of HRAGS to the wing tip was studied. The results of the studies above are reported here.

Pitch-based carbon fiber reinforced SiC composites for space optics

One of the key technologies for the next generation space telescope with a large-scale reflector is a material having high strength-to-weight ratio, high stiffness-to-weight ratio, and low coefficient of thermal expansion. Pitch-based carbon fiber reinforced SiC composites were developed to comply with such requirements. Two types of C/C substrates were prepared and carbonization and graphitization conditions were evaluated. Siliconization conditions were also studied to find better conditions to obtain favorable phase composition of the composites. Mechanical performance as measured in bending strength and fracture toughness tests was significantly improved. Thermal expansion behavior and roughness of the polished surface of these composites were also determined to be sufficient for space infrared mirror applications.