Suppression Of Crack Initiation Research Articles

Elimination of Shrinkage Cavities by Liquid Hot Isostatic Pressing in Aluminum Alloys under Superplastic Conditions

The liquid hot isostatic pressure in superplastic conditions was applied to eliminate macroporosity in a die cast Al-12%Si eutectic alloy. The removing of macroporosity provides the yield stress increase of 56%, the ultimate tensile strength increase of 46%, the total elongation increase of 21%, the fracture toughness increase of 340% and the fatigue strength increase of 58%. Remarkable improvement of mechanical properties is caused by suppression of crack initiation on internal coarse pores.

Fatigue Properties of Radical Nitrided Alloy 718 at Elevated Temperature under Push Pull Loading

Effect of radical nitriding on fatigue strength of Alloy 718 was investigated at room temperature and 500°C under push-pull loading. Nitrided alloy used was with compound layer of 4μm. Fatigue strength was increased at room temperature by nitriding, though there was no or little difference in fatigue strengths between aged alloy and nitrided one at 500°C. All of fracture occurred from specimen surface in both alloys at room temperature. On the other hand, fracture origins were specimen surface at high stress levels and subsurface at low stress levels in both alloys at 500°C. Surface crack in the aged alloy initiated in ductile manner and the one in nitrided alloy was in brittle manner. The increase in fatigue strength at room temperature was mainly caused by the suppression of crack initiation by compound layer. The reasons for the change in fracture origin from specimen surface to subsurface at 500°C were the suppressions of crack initiation by compound layer in the nitrided alloy and the one of propagation of small crack by oxide induced crack closure effect in the aged alloy. In both of aged and nitrided alloys, fatigue strength at 500°C was larger than that at room temperature in high cycle region where subsurface fracture occurred.

Simulation for intergranular stress corrosion cracking based on a three-dimensional polycrystalline model

In stress corrosion cracking of stainless steel, two different schemes of analysis of crack growth should be employed for the crack initiation phase and crack growth phase. However, this distinction is not clear-cut in the crack initiation phase, since the vicinity of the pre-existing crack is a preferential area of crack initiation due to concentration of stress. Therefore, initiation of crack tends to occur at the tip of a pre-existing crack and it can be regarded as crack growth. In this study, the contribution of this type of apparent crack growth, referred to as initiation dominant growth (IDG), to crack growth was evaluated by a Monte Carlo simulation. A three-dimensional polycrystalline body was generated by Voronoi tessellation. The cracks were assumed to grow along grain boundaries. The effect of stress-concentration around pre-existing cracks was taken into account by applying the finite element method. Initiation and propagation of the cracks were modeled based on concepts of damage mechanics. The simulation could reproduce the changes in number of cracks and the sum of crack length obtained experimentally as well as preferential crack initiation at the stress-concentration zones and suppression of crack initiation in stress-shielding zones. It was shown that the contribution of IDG to crack growth was large for small cracks, and that damage by crack initiation accounted for more than 50% of total damage even when the length of a crack was 0.6 mm at the surface.

Effect of Grain Size on Ultrasonic Fatigue Properties of Ni-Base Super Alloy Inconel 718

Ultrasonic fatigue tests of plain specimens with different grain sizes were carried out for Ni-base super alloy, Inconel 718, in ambient air in order to investigate the effect of grain size on fatigue properties. Fatigue strength was increased with decrease in grain size. The increase in fatigue strength by refining grain size was mainly caused by the suppression of crack initiation. That is, the effect of grain size on crack growth rate was hardly recognized, though crack morphology was rougher in the specimen of larger grain, meaning that the crack growth in the large-grained specimens was suppressed by roughness induced crack closure. On the other hand, however, more flat facets caused by twin boundary cracking were observed in the large-grained specimens, which inversely led to crack growth acceleration.

445 高強度 Al 合金の疲労強度に及ぼす繰返し速度の影響

In order to investigate the influence of loading frequency on the fatigue properties of Al alloy, ultrasonic fatigue tests were carried out for an age-hardened Al alloy, 6061-T6. The results were compared with those of rotating bending fatigue in view points of initiation and propagation behavior of a crack and fracture mechanism. Fatigue life was longer in ultrasonic fatigue than in rotating bending fatigue, which was mainly caused by the suppression of crack initiation. In ultrasonic fatigue, many fatigue voids and cleavage cracks were observed on the fracture surface in addition to the shear mode cracks, striations and dimples, which were typical fracture modes in rotating bending.

Ａｇ｀＋´イオン照射したイットリア安定化ＺｒＯ２単結晶の耐亀裂性

400KeV Ag+ ions were irradiated to the (100) surfaces of 9.5m% Y2O3 stabilized ZrO2 with the doses ranging from 5×1015 ions/cm2 to 5×1017 ions /cm2 at room temperature in order to investigate the correlations between the state of implanted Ag and the suppression of crack initiation.1) Ag precipitated as particles in orienting the (200) face of Ag. The sizes of the Ag particles were ranging from 2nm(5×1016ions/cm2)to 11nm (5×1017ions/cm2).2) Knoop hardness of the surface of ZrO2 increased by about 20-30% by Ag+ ion-irradiation at the doses less than 5×1016ions/cm2 and decreased at the doses larger than 1×1017ions/cm2.3) Vickers indentation crack at a loads of 0.25N was completely suppressed in the surface of ZrO2 irradiated to a dose of 5×1017ions/cm2.4) The suppression of the crack initiation may be ascribed to the influence of the nanometer composite layer containing max. 23at% Ag.