Shota Harada

Fatigue strength of notched specimens having nearly equal sizes of ferrite

Abstract In this study, rotating bending fatigue tests were conducted on circumferential notched large and small specimens of three kinds of carbon steel with nearly equal (20 μm) ferrite grain sizes but different volume fractions of pearlite. The effect of microstructures on the fatigue strengths σws (the limit stress for slip bands), σw1 (the limit stress for macrocrack initiation), and σw2 (the limit stress for fracture) is discussed by comparing these values in three kinds of carbon steel. The main results newly obtained are as follows. (1) Since σws is localized to a highly specified region, variation of Ktσws (maximum stress repeated at the notch root in the limit stress for slip bands) is small, irrespective of notch root radius. (2) If the grain size of ferrite is nearly equal, variation of σw1 is small irrespective of pearlite under the threshold amount of pearlite. The threshold volume fraction of pearlite is about 50%. (3) In medium-carbon steel, σw2 increases with increasing grain size of ferrite. (4) Notch root radius at the branch point, ϱ0, varies depending on the grain size of ferrite. (5) The values of Ktσw1 (maximum stress repeated at the notch root in the limit stress for macrocrack initiation) and Ktσw2 (maximum stress repeated at the notch root in the limit stress for fracture) can be determined by the notch root radius ϱ alone, independent of geometrical conditions.

Preparation of fullerene/epoxy resin composite with fine dispersion and its breakdown strength

Fullerene is likely to form aggregates so that it is diificult to disperse it well in a polymer matrix, and few reports have been made on electrical insulation properties of fullerene filled composite as electrical insulating material. This paper deals with fullerene dispersion method in conventional epoxy resin (EP) and breakdown strength (BDS) of the composite with different filler loading of fullerene. Composite materials added with fullerenes of 0.1, 0.3, and 0.6 weight% were prepared in a plate shape with a thickness of 0.5 mm using two different dispersion methods, i.e. fullerene dissolved and fullerene direct mixing methods, and neat specimen was also prepared for comparison so as to investigate BDS. As a result, it was found that BDS of fullerene/EP composite with good dispersion greatly increased by 30 %. The increase in BDS of the composite is discussed in terms of the inter-particle distance as an index of the dispersion degree.

Simulation of Coming out of the Shaft from the Ceramics Sleeve During Rotation of the Rollers Connected under Small Shrink Fitting Ratio

自動車用鋼板をはじめとする高級鋼材を生産するための 熱処理炉(Fig.1参照)では,ローラーを用いて鋼材の搬送 が行われている。現在,Fig.2(a)のようなステンレス耐熱 鋼にセラミック溶射でコーティングされた胴部と,ステン レス耐熱鋼の軸部を溶接,焼嵌め等で接合し,中空部を水 冷する構造のローラー(ハースロールと呼ばれる)が,主 として用いられている。このローラーは,炉内の高温環境 下において使用中に,耐熱鋼とセラミックコーティング層 の線膨張係数の違いや,コーティング層の微小欠陥や密着 強度の限界等に起因し,ローラー表面にき裂や剥離 ,そ れに伴う肌あれ,摩耗等の発生を招き,ローラーの寿命を 短くしている。また,ローラー内部を水冷するために熱損 失が無視できない。 そこで,Fig.2(b)のように胴部材料全体をセラミックと し,その両端に鋼製軸を用い,中空部を空冷するローラー 構造を検討している。胴部材料を耐熱性・耐摩耗性 ・耐 腐食性 に優れたセラミックにすることで,現状のコー ティングに起因する欠陥を防止でき,ローラー寿命を飛躍 高温炉中で使われる セラミックローラー稼動中におけるシャフトの抜け