Creep

This refers to plastic deformation that occurs over time under a constant stress. It is generally a problem with materials used at high temperatures, but since the temperature should be expressed as the ratio of the target temperature ( T K) to the melting point ( T _m K) (specific temperature, homologous temperature), this property is important even at room temperature for materials with low melting points. It becomes a problem at temperatures of 0.3 to 0.4 T _m or higher, and creep occurs severely at 0.5 T _m . Since the creep rupture stress is lower than the endurance limit at high temperatures, materials used at high temperatures must be designed based on their creep properties. The relationship between creep strain and time occurs in three stages, as shown in the figure. The first and third stages are short-term phenomena (within a few tens of hours), while the second stage lasts for an extremely long time, so the strain rate at this stage (steady state creep rate, minimum creep rate) is sometimes used as the design standard. There is also a method in which the deformation during the process is not measured, but is considered by conducting experiments up to destruction, which is currently more commonly used. If the creep rupture time is t _r (rupture time), the temperature is T , and an Arrhenius-type arrangement is performed, it has been found that if a graph is created with the applied stress σ on the vertical axis and T (log t _r + c ) on the horizontal axis, a large amount of experimental data with different stresses, temperatures, and rupture times can be represented by a single curve. Here, c is a constant that depends on the material and, strictly speaking, the temperature and stress, but it is treated as a constant value. This curve is called a master curve. There are various proposals for the parameters to be placed on the horizontal axis, and the above is called the Larson-Miller parameter. Creep is a property that exists throughout the entire lifespan of a component, so it is necessary to estimate the rupture strength and creep deformation over long periods of time, such as the lifespan of the equipment (one standard is 10 ⁵ h), and in many cases estimates at 10 ⁵ h are made from experiments lasting around 10 ⁴ h.

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

一定応力のもとで時間の経過とともに現れる塑性変形をいう．一般には高温で使用する材料で問題になるが，温度は対象とする温度(T K)と融点(T_m K)との比(比温度，homologous temperature)によって表すべきであるので，融点の低い材料では室温でも，この性質が重要になる．0.3～0.4T_m 以上の温度で問題になり，0.5T_m ではクリープははげしく起こる．高温で使用する耐久限度よりクリープ破断応力のほうが低くなるので，高温で使用する材料はクリープ性質によって設計をしなければならない．クリープひずみと時間の関係は図に示すように3段階で生じる．第一と第三段階は短時間(数十時間以内)の現象であり，第二段階はきわめて長時間にわたるので，この段階のひずみ速度(定常クリープ速度，または最小クリープ速度，steady state creep rate, minimum creep rate)をもって設計の基準にすることがある．また，途中の変形は測定しないで，破壊するまでの実験によって考慮する方法があり，現在はこのほうが多く行われる．クリープ破断時間を t_r(rupture time)，温度をTとし，アレニウス型の整理を行った場合，負荷応力σを縦軸に，T(log t_r ＋ c)を横軸にとった線図をつくると，応力，温度および破断時間の相違した多くの実験データを，1本の曲線で表すことができることがわかっている．ここで，cは材料と，厳密には温度と応力によって決まる定数であるが，一定値として取り扱う．この曲線をマスター曲線という．横軸にとるパラメーターにはいろいろの形式の提案があり，上述のものはLarson-Millerのパラメーターといっている．クリープは，部品の寿命全体にわたって存在する性質であるので，機器の寿命程度の長時間(一つの基準は 10⁵ h)の破断強さ，クリープ変形量を推定する必要があり，多くは 10⁴ h 程度の実験から 10⁵ h におけるものを推定している．

出典　森北出版「化学辞典（第2版）」化学辞典 第2版について　情報