Ductile fracture

A type of fracture of a material caused by an external force. When a load is applied to a material, it first undergoes elastic deformation, and when the load is further increased, it generally undergoes slippage on a certain surface, which is accompanied by plastic deformation that remains even after the load is removed, and finally fractures. The amount of plastic deformation leading to fracture is greatly affected by the shape of the material and test piece, the stress conditions, and the environmental temperature. When the amount of plastic deformation leading to fracture is large, it is called ductile fracture, and materials that undergo ductile fracture under simple stress conditions are called ductile materials. On the other hand, when fracture occurs with almost no plastic deformation, it is called brittle fracture, and even steel that is ductile at room temperature undergoes brittle fracture at low temperatures. When a round bar of mild steel is pulled, after elastic deformation, it undergoes large plastic deformation, and part of the sample is constricted, first breaking apart at the center, followed by slippage and fracture at the periphery, resulting in the so-called cup-and-cone fracture.

[Kunio Hayashi]

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

材料が外力によって破壊する様式の一つ。材料に荷重を加えると、まず弾性変形を生じ、さらに荷重を大きくすると一般にある面ですべりを生じ、除荷しても変形が残る塑性変形を伴い、ついには破壊する。破壊までの塑性変形量は、材料や試験片の形状、応力の条件や環境温度によって大きく影響される。破壊に至るまでの塑性変形量の大きい場合を延性破壊といい、単純な応力条件のもとで延性破壊する材料を延性材料という。一方、ほとんど塑性変形することなく破壊する場合を脆性(ぜいせい)破壊といい、常温では延性を示す鋼も低温では脆性破壊をする。軟鋼の丸棒を引っ張ると弾性変形ののち、大きな塑性変形を伴い、試料の一部がくびれて、まず中心部で分離破壊し、引き続いて周辺部がすべり破壊して、いわゆるカップ・アンド・コーン型となる。

［林　邦夫］

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