Structure Evaluation Laboratory

Structure Evaluation Laboratory

(Japanese is here.)

- Introduction -

In the "Structure Evaluation Laboratory", research work is focused on two subjects. One is fracture mechanism of structural ceramics and ceramic composites, and the other is deformation process of wood.

(1)Study on fracture mechanism of ceramics
High performance ceramics are a promising structural material due to their high strength at high temperatures and high corrosion resistance. A prerequiste for reliable use of ceramics in structural applications is the understanding of their fracture mechanism. An analysis of fracture mechanism of ceramics is also important to improve its mechanical properties. In this laboratory, estimations and analysis of mechanical behaviours of structural ceramics are carried out to clarify their fracture mechanics.

(2)Study on deformation process of wood
Fundamental studies of the deformation process of wood are made to utilize forest products effectively. To use wood as industrial material it is necessary to remove water filled in cell lumens and cell walls, because demensions are sensitive to moisture content in wood. Compression in radial direction of wood is successful for removal of water. Influences of moisture content, temperature of wood and strain rate on required flow stress and deformability are subjects of interest.

- Research Projects -

    1. Ceramic Gas Turbine Project
    2. Synergy Ceramics Project
    3. Deformation process of cellular solids with irregular microstructure
       including inorganic compounds in intercellular space

- Staff -

Chief	Yukihiko Yamauchi	yamauchi@nirin.go.jp
	Kouzou Kanayama	kanayama@nirin.go.jp
	Wataru Kanematsu	kanemat@nirin.go.jp
	Tatsuya Miyajima	miyajima@nirin.go.jp
	Yuzou Furuta	furuta@nirin.go.jp

JSTC Domestic Research Fellow	Hiroshi Imanishi
	Naho Soma
	Yae Kondo
	Kazutoshi Takeuchi
Part-time staff	Seisuke Sakai
Part-time staff	Masaru Ito
Part-time staff	Yukio Hirai

- Recent research activities -

1.Ceramic Gas Turbine Project
Fatigue and creep tests under various conditions were carried out to clarify the factors which affect lifetime of ceramic components. The effect of machining damage on strength and fatigue behaviour was also estimated.

(Research Topic)
4-point bending static and cyclic fatigue tests are carried out on several structural ceramics both at room and elevated temperatures. Measured fatigue behaviours are compared with the lifetime predicted from the dynamic fatigue properties and cyclic loading effects on delayed failure are discussed. Experimental results indicate that a delayed failure of ceramics under a cyclic loading is caused mainly by extension of a pre-existing flaw and the lifetime is substantially predictable from a subcritical crack growth behaviour estimated under a static or quasi-static loading condition. It is also pointed out that crack growth is accelerated by compressive stress if grain bridgings and/or crack interlockings are formed on a propagated crack surface. This degradation of lifetime is due to the destruction of grain bridgings by compressive stress. Humidity controlled stress corrosion of glassy phase and the mechanical fracture of grain boundaries due to softening of glassy phase are considered as a subcritical crack growth mechanisms at room and elevated temperatures, respectively. Thus the properties of grain boundaries are essential for the fatigue behaviour of ceramics both at room and elevated temperatures.

2.Synergy Ceramics Project
Research work is focused on improvement of strength and fracture resistance of ceramic materials, and on the analysis and modeling of toughening mechanisms. The point of this work pertains to the analysis of toughening by grain or particle bridging in a propagating crack interface.

(Research Topic)
In-situ SEM observations of crack opening displacement (COD) in loaded flexural bar specimens (3mm x 4mm, span of 16mm) were made along the bridging zone in order to determine the toughening parameters. Test materials studied here are a silicon nitride based hyper thougened synergy ceramic (HT-SN) with a highly anisotropic microstructure, and a commercially available in-situ toughened silicon nitride ceramic (IST-SN). Although the size of the rod-like grains in IST-SN (D=6x10^-6m, L=100x10^-6m) is larger than that of HT-SN (D=2x10^-6m, L=20x10^-6m), the advantages of HT-SN in terms of strengthening and toughening mechanisms are that the unidirectionally aligned rod-like grains work effectively in the bridging mechanism for cracks normal to the long axis of these grains, and reduce the size of fracture origin. The elastic bridging and/or frictional bridging models are applied to analyze the bridging contribution on COD, defined as the difference between the theoretical COD and the measured one. The result reveals that HT-SN is characterized with an extremely short bridging zone (105x10^-6m) just behind the crack tip with a maximum bridging stress of 400MPa, which leads to the steeply rising R-curve behaviour (6.7 to 11MPam^1/2) and high strength (1100MPa). On the other hand, IST-SN has a long critical bridging zone length of 750x10^-6m with bridging stress of 110 MPa and shows gently rising R-curve behaviour.

3.Deformation process of cellular solids with irregular microstructure including inorganic compounds in intercellular space

A laser incising method was applied to increase the removal rate of water which causes the breakage of wood in the deformation process. It is shown that axial tracheids are opened into laser-incised holes, and water contained in wood can be removed through holes with less resistance. The increase in the removal rate of water leads to the decrease in flow stress required for the compressive deformation of wood. Repetition of procedures of loading and unloading to wood results in the increase in the removal rate of water and, therefore, in the decrease of flow stress. High strain rate causes the breakage of wood. When a strain rate is small enough to deform wood without breakage, flow stress, s, can be expressed by the following equation.
s=C(de/dt)^m
whereC is constant, (de/dt) is strain rate, and m is strain-rate sensitivity exponent.

Last Modified: 2000/4/24