Yield Point - Koufukuten

The point where deformation begins when stress exceeds the elastic limit of the material. The stress at this point is called the yield stress. When an external force is applied to a metal material, the distance between the crystal atoms changes, causing it to deform elastically. When the external force exceeds the material's inherent value, a relative movement occurs between the blocks above and below a specific crystal plane (slip plane). This deformation is larger than the elastic deformation, and is recognized as a point on the load-deformation curve where there is no increase in load and deformation progresses rapidly. The yield point in steel is particularly noticeable, and a decrease in load is observed at the yield point. This is thought to be the result of the locking and release of dislocations due to solute carbon, etc. In carbon steel, the stress at which yielding begins is called the upper yield point. If deformation continues further, the stress decreases and shows a stable value. This is called the lower yield point. Unless otherwise specified industrially, the upper yield point is called the yield point. Since there is no decrease in load at the yield point for materials other than carbon steel, the value at which there is a permanent deformation of 0.2% is usually used as the yield point. This is also called the yield strength (0.2% yield strength).

Source: Morikita Publishing "Chemical Dictionary (2nd Edition)" Information about the Chemical Dictionary 2nd Edition

応力が材料の弾性限界を超えて変形がはじまる点．このときの応力を降伏応力という．金属材料に外力が加えられると，まず結晶原子間の距離に変化が生じ，弾性的に変形する．外力がその材料の固有の値を超えると，特定結晶面(すべり面)の上下のブロックの間に相対的な移動が起こる．この変形は，弾性変形に比較して大きく，荷重変形曲線上で荷重の上昇がなく，急激に変形の進む点として認められる．鋼における降伏点はとくに顕著で，降伏点において荷重の低下が認められる．これは固溶炭素などによる転位の固着と解放の結果と考えられている．炭素鋼の場合，降伏を開始した応力を上部降伏点という．さらに変形を続けると応力は低下し，安定した値を示す．これを下部降伏点という．工業的にとくに指定のない場合，上部降伏点を降伏点という．炭素鋼以外の材料には降伏点における荷重低下がないため，通常，0.2％ の永久変形をする値を降伏点として用いる．これは耐力(0.2％ 耐力)ともよばれる．

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