Читать книгу The Rheology Handbook - Thomas Mezger - Страница 89

A certain amount of force must be applied before the toothpaste starts to flow. A sample with a yield point begins to flow not before the external forces Fext acting on the material are larger than the internal structural forces Fint. Below the yield point, the material shows elastic behavior, i. e. it behaves like a rigid solid, exhibiting under load only a very small degree of deformation, which however recovers completely after removing the load. The following applies: If Fext < Fint the material is deformed to such a small degree only that it is hardly perceptible to the human eye. The sample does not begin to flow before Fext > Fint. The yield point is also referred to as yield stress or yield value . Also, other terms were proposed formerly, e. g. such as stiffness-at-rest (see Chapter 14.2: T. Schwedoff in 1880).