Keiji Houjou

Crack‐healing behaviour of ZrO2/SiC composite ceramics

Purpose – Zirconia ceramics exhibit high strength and fracture toughness. The purpose of this paper is to research a possibility of crack healing in zirconia ceramics.Design/methodology/approach – ZrO2/SiC composite ceramics are sintered and subjected to three‐point bending. A surface crack of 100 μm in diameter is formed on each specimen. The cracks are healed and the specimens are tested under bending.Findings – The paper finds that ZrO2/SiC composite ceramic material had a high crack‐healing ability at a considerably low temperature. For example, a crack of 100 μm in diameter is healed even at 600°C.Research limitations/implications – The paper provides a low temperature healing and a new mechanism of crack healing.Originality/value – The paper shows the healing temperature and the minimum time required to heal showed a good proportional relation on the Arrhenius plot at temperatures of 600‐800°C. Moreover, the crack healing is caused by SiO2 cristobalite produced during the healing.

Crack‐healing behavior of ZrO2/SiC composite ceramics and strength properties of crack‐healing specimens

Purpose – The purpose of this paper is to study the crack‐healing mechanism of ZrO2/SiC composite ceramics which have a high crack‐healing ability at low temperature.Design/methodology/approach – The effects of dispersed SiC and the environment on crack‐healing behaviour were investigated. The fatigue strength of crack‐healed specimens was also investigated.Findings – The main conclusions are that for crack‐healing of ZrO2 ceramics, it is necessary to have both a SiC composite and an oxidative environment; and when ZrO2/SiC composite ceramics are heat‐treated in air, a phase transformation attributable to the SiC composite results in crack‐healing and improvement of fracture toughness and bending strength.Originality/value – An appropriate heat treatment for ZrO2/SiC composite caused not only crack‐healing but also the improvement of fracture toughness, and created a multiplier effect on crack‐healing, bending strength and fracture toughness.

Improvement of fatigue limit by overload for high-tensile strength steel containing a crack in the stress concentration zone

Purpose – The purpose of this study was to investigate the effect of overload (bending moment with plastic deformation: Mp) on three point bending specimen at the fatigue limit of high-tensile-strength steel containing a crack in the stress concentration zone. Design/methodology/approach – An artificial semi-circular slit was introduced and Mp was applied after which bending fatigue tests were carried out. Findings – The relationship between the level of Mp and the fatigue limit (σw) was proportional; the fatigue limits of specimens containing 0.2- and 0.3-mm-deep slits are improved by the Mp process as much as twice the original values; the slit size that can be rendered harmless by the Mp process is a=0.05 mm in depth; and all non-propagating cracks appeared around the artificial slit. Originality/value – Very few studies have been conducted on the fatigue limit of materials containing crack-like surface defects after overload in the stress concentration zone. This study elucidated the effect of Mp on t...

Effect of peening on the fatigue limit of welded structural steel with surface crack, and rendering the crack harmless

Purpose – The purpose of this paper is to investigate the effect of peening on the fatigue limit of steels for welded structure with a crack in the weld toe zone. Design/methodology/approach – An artificial semi-circular slit was created in the weld toe, and peening was conducted. Then, bending fatigue tests were carried out. Findings – First, owing to the shot peening, the maximum slit depths that can be rendered harmless were 1.0 and 1.2 mm in SUS316 and SM490, respectively. Second, during the fatigue test, the fracture of a peened specimen originated outside the slit, which indicated that peening eliminated the effect of the slit on the fatigue limit. Third, the fatigue limit of a slit specimen was improved by the enhanced residual stress distribution and the decreased stress concentration due to plastic deformation at the weld toe. Originality/value – There are very few studies about which a fatigue crack is rendered harmless by residual compressive stress, as a result the structures could be continue...

Analytical investigation of effect of stress ratio on threshold stress intensity factor range improved by overload

Purpose – The purpose of this paper is to describe the effects of stress ratio (R) on the threshold stress intensity factor range (ΔKth) by applied overload and to conduct an analytical investigation of the effect of the stress ratio.Design/methodology/approach – Tensile overload was applied to a compact tension specimen, and fatigue tests were performed at R=0.1 or 0.5.Findings – The value of ΔKth increased as the tensile overload was increased, and the nominal threshold values were given by the equation ΔNKth,R = C+ DKov, where C represents ΔKth, and D is a proportional constant. Experimental results showed that the value of D showed good agreement with theoretical value.Originality/value – The paper proposes a new model that arrests crack growth or makes cracks harmless by utilizing the overload effect.

Overloading effect on the fatigue threshold stress intensity factor range (ΔKth) as a function of crack length in SUS316

Purpose – The purpose of this paper is to study the effects of overload on the threshold stress intensity factor range (ΔKth) in SUS316.Design/methodology/approach – The fatigue tests are carried out to determine the resultant threshold stress intensity factor range (ΔKth). The mechanism of the improvement of ΔKth by the tensile overloading is analyzed using the Dugdale model.Findings – It is clarified that the value of ΔKth increases as increasing the overloading.Research limitations/implications – The apparent value of ΔKth of stainless steel can be improved by a tensile overload, the fatigue strength of structural members that have a surface crack can be increased by a tensile overload.Originality/value – As a result, the reliability and safety of structures, such as energy plants, can also be improved.