Introduction
There are a certain number of parameters in materials mechanics that represent very different characteristics but are expressed by the same symbol. Two of them have almost the same name: stress concentration factor (\(K_t\)) and stress intensity factor (\(K_I\) [\(K_{II}\) or \(K_{III}\)]), the third being the impact strength energy per unit surface (\(K_{Cu}\) [\(K_{CV}\)]). Two of them are parameters derived from continuous medium mechanics (\(K_t\) et \(K_{Cu}\) [\(K_{CV}\)]), the third being related to fractured medium mechanics, or fracture mechanics.
The table below summarises a few essential characteristics of these three parameters.
Continuum medium mechanics | Fracture mechanics | |
|---|---|---|
Impact strength energy | stress concentration factor | stress intensity factor |
\(K_{Cu}\) [\(K_{CV}\)] | \(K_t\) | \(K_I\) [\(K_{II}\) ou \(K_{III}\)] |
expressed in \({\rm \, J.cm^{-2}}\) | no unit (this is a stress ratio) | expressed in \({\rm \, MPa.m^{1/2}}\) |
characterizes the energy of an impact fracture | characterizes the state of stress at notch tip | characterizes the state of stress at crack tip |
The impact strength energy per unit surface and the stress concentration factor are both defined in the paragraph above. We are going to focus on the stress intensity factor, which characterizes the state of stress at crack tip within a material. In materials mechanics, a crack is a specific object that does not correspond to the limited case of a notch, which is generally intentionally machined, and treated according to a specific approach.