Research

- My Researches -

Be careful that a Japanese character code mixes in this text.

　 Theme Abstract

Study on Fracture of Brittle Solids

1 Study on contact strength estimation of ceramics
Ceramics and glass have already been applied to various structural elements such as parts of automobile, bearing of precision rotating machine and so on. However, it is not avoided that ceramics and glass utilized as structural elements often become combinational structures with other elements. Especially, low toughness of both ceramics and glass are serious faults. Impact stress is relaxed by local deformation in metallic material. On the other hand, ceramic materials are failed to hardly relax. As a similar phenomenon, a combination structure of high rigidity materials is failed by contact stress. Hertzian-type contact stress causes the maximum value on the contact boundary of sphere and plate. But, the stress is dramatically decreased from tensile stress region to compression stress region with going away to thickness direction of the plate. A crack forms around the contact boundary, and the plate is failed. This is one of facts that should be noted most for the reliability of ceramics and glass strength. Therefore, in order to use ceramics, it is necessary to quantitatively evaluate contact fracture strength which is caused due to brittleness and high rigidity.

2 Study on strength estimation of ceramic-metal joints The technology of ceramic-metal joints is highly suited to increasing the number of applications of monolithic ceramics. In particular, the “active-metal” brazing method, which uses Ag-Cu-Ti, is already used in a wide range of applications such as vacuum valve chambers, semiconductor plates and automobile parts, and it is expected that it will also have both structural and functional applications in the future. However, this technology has been used in many joint products without examination of safety and reliability considerations. As a result, accidents have occurred as a result of improper usage. Therefore, it is necessary to develop design standards and to establish the strength of the material with reference to safety and reliability of the joint design.
A major problem in ceramic-metal joints is mismatches in the thermal expansion coefficients between ceramics and metals. This mismatch results in the generation of significant residual stresses at the material interfaces during the joint-cooling process. When this occurs, an interlayer of ductile metal such as copper or nickel is inserted between the two materials in order to relax the stress. It should be remembered that interlayer thickness has a significant influence on joint strength.

3
Fracture behavior of defect-free ceramics
Defect-free ceramics (DFC) with very few existing defects was developed by new process of manufacture. This ceramic material was investigated mechanical properties and fracture behavior such as 4-point bending, static fatigue and indentation test at room temperature.

It was found that bending strength of DFC indicates about double strength than that of regular same ceramics. In addition, mechanical properties (such as Young’s modulus, Poisson’s ratio and Fracture toughness) are also equal to them. However, arranging by relationship between bending strength and fracture probability, Weibull modulus of DFC decreases compared with them. It is attributable to different fracture mechanics. Based on the weakest link model, fracture origin of ceramics is certain one defect. On the other hand, as for DFC, binder exists instead of a potential defect and they are distributed minutely and uniformly. When a specimen is load, a lot of cracks occur in boundary between binder and ceramic grain and strain energy is released. Some clusters are formed by some cracks' uniting. The scatter of fracture strength that depends on the size of the cluster is caused.

As for static fatigue test of DFC, the time dependence fracture wasn't caused. In addition, all experimental values are more than average of bending strength. Fracture model as above can explain this behavior to which crack coalescences stop.

4 Study on fracture property of hardening steel materials (Fracture toughness) Under construction

Study on Advanced Plastic Working Method

1 Development and mechanism of ultrasonic plastic working method In plastic processing, if supersonic vibration is impressed to workpiece materials, power required for processing is decreased. As a result, processing limit is improved. This is called Blaha effect. This effect has been tried as application of plastic processing. However, this mechanism has not been clarified yet. Therefore, since the theoretical law of the impression method of an ultrasonic wave or the mounting instruction of equipment is not established, it has not resulted in utilization except for some examples of use. The purpose of this study is to solve the property and the mechanism of Blaha effect.

2 Development and mechanism of pulse laser-illuminated plastic working method Use of an ultrasonic transducer has problems, such as modification of a horn apical portion, generation of heat of a vibrator, and change (impedance change) of resonance frequency. In order to develop this study, it is necessary to consider the oscillating grant under non-contact conditions. We pay our attention to laser peening. This method generates high-pressure plasma by irradiating with the laser which has the pulse frequency of 10 Hz underwater. Peening processing is performed using the impulse force. Generally this peening processing brings work hardening to a material list side. In this study, high energy is not needed. The energy which gives vibration in material may be sufficient.

　　　

	Theme	Abstract
Study on Fracture of Brittle Solids
1	Study on contact strength estimation of ceramics	Ceramics and glass have already been applied to various structural elements such as parts of automobile, bearing of precision rotating machine and so on. However, it is not avoided that ceramics and glass utilized as structural elements often become combinational structures with other elements. Especially, low toughness of both ceramics and glass are serious faults. Impact stress is relaxed by local deformation in metallic material. On the other hand, ceramic materials are failed to hardly relax. As a similar phenomenon, a combination structure of high rigidity materials is failed by contact stress. Hertzian-type contact stress causes the maximum value on the contact boundary of sphere and plate. But, the stress is dramatically decreased from tensile stress region to compression stress region with going away to thickness direction of the plate. A crack forms around the contact boundary, and the plate is failed. This is one of facts that should be noted most for the reliability of ceramics and glass strength. Therefore, in order to use ceramics, it is necessary to quantitatively evaluate contact fracture strength which is caused due to brittleness and high rigidity.
2	Study on strength estimation of ceramic-metal joints	The technology of ceramic-metal joints is highly suited to increasing the number of applications of monolithic ceramics. In particular, the “active-metal” brazing method, which uses Ag-Cu-Ti, is already used in a wide range of applications such as vacuum valve chambers, semiconductor plates and automobile parts, and it is expected that it will also have both structural and functional applications in the future. However, this technology has been used in many joint products without examination of safety and reliability considerations. As a result, accidents have occurred as a result of improper usage. Therefore, it is necessary to develop design standards and to establish the strength of the material with reference to safety and reliability of the joint design. A major problem in ceramic-metal joints is mismatches in the thermal expansion coefficients between ceramics and metals. This mismatch results in the generation of significant residual stresses at the material interfaces during the joint-cooling process. When this occurs, an interlayer of ductile metal such as copper or nickel is inserted between the two materials in order to relax the stress. It should be remembered that interlayer thickness has a significant influence on joint strength.
3	Fracture behavior of defect-free ceramics	Defect-free ceramics (DFC) with very few existing defects was developed by new process of manufacture. This ceramic material was investigated mechanical properties and fracture behavior such as 4-point bending, static fatigue and indentation test at room temperature. It was found that bending strength of DFC indicates about double strength than that of regular same ceramics. In addition, mechanical properties (such as Young’s modulus, Poisson’s ratio and Fracture toughness) are also equal to them. However, arranging by relationship between bending strength and fracture probability, Weibull modulus of DFC decreases compared with them. It is attributable to different fracture mechanics. Based on the weakest link model, fracture origin of ceramics is certain one defect. On the other hand, as for DFC, binder exists instead of a potential defect and they are distributed minutely and uniformly. When a specimen is load, a lot of cracks occur in boundary between binder and ceramic grain and strain energy is released. Some clusters are formed by some cracks' uniting. The scatter of fracture strength that depends on the size of the cluster is caused. As for static fatigue test of DFC, the time dependence fracture wasn't caused. In addition, all experimental values are more than average of bending strength. Fracture model as above can explain this behavior to which crack coalescences stop.
4	Study on fracture property of hardening steel materials (Fracture toughness)	Under construction
Study on Advanced Plastic Working Method
1	Development and mechanism of ultrasonic plastic working method	In plastic processing, if supersonic vibration is impressed to workpiece materials, power required for processing is decreased. As a result, processing limit is improved. This is called Blaha effect. This effect has been tried as application of plastic processing. However, this mechanism has not been clarified yet. Therefore, since the theoretical law of the impression method of an ultrasonic wave or the mounting instruction of equipment is not established, it has not resulted in utilization except for some examples of use. The purpose of this study is to solve the property and the mechanism of Blaha effect.
2	Development and mechanism of pulse laser-illuminated plastic working method	Use of an ultrasonic transducer has problems, such as modification of a horn apical portion, generation of heat of a vibrator, and change (impedance change) of resonance frequency. In order to develop this study, it is necessary to consider the oscillating grant under non-contact conditions. We pay our attention to laser peening. This method generates high-pressure plasma by irradiating with the laser which has the pulse frequency of 10 Hz underwater. Peening processing is performed using the impulse force. Generally this peening processing brings work hardening to a material list side. In this study, high energy is not needed. The energy which gives vibration in material may be sufficient.

Under Construction

BACK