Some fundamental questions about R‐curves

With the help of two simple thought experiments it is demonstrated that there exist two physically different types of fracture toughness. The crack‐growth toughness, which is identical to the Griffith crack growth resistance, R , is a measure of the non‐reversible energy which is needed to produce an increment of new crack area. The size of R is reflected by the slopes of the R ‐curves commonly used. So an increasing J ‐Δ a ‐curve does not mean that the crack‐growth resistance increases. The fracture initiation toughness, J i , is a normalized total energy (related to the ligament area) which must be put into the specimen up to fracture initiation. Only for ideally brittle materials R and J i have equal sizes. For small‐scale yielding a relationship exists between R and J i , so a one‐parameter description of fracture processes is applicable. For large‐scale yielding R and J i are not strictly related and both parameters are necessary to describe the fracture process.