a – The concept behind leak –before-burst
In a piping or pressure vessel system, with a crack going in the wall, there can be two possibilities
1. Crack may gradually extend and penetrate the wall, causing a leak before sudden brittle fracture can occur
2. Sudden brittle fracture can occur prior to the pipe leaking.
Since a brittle fracture in a pipe may involve the explosive release of the pipe contents, a leak is by far preferable. Also, a leak is easily detected from pressure drop or from the escape of pipe contents.
A crack in a piping may grow in size due to the influence of the cyclic loading associated with pressure changes, or due to hostile chemical attack on the material. A crack usually starts from a surface flaw and extends in a plane normal to the maximum stress in the pipe wall as shown in Figure 1. Early in its progress, the crack will often grow with the surface length 2c continuing to be approximately twice the depth a, so that c~a .
If no brittle fracture occurs, the growth will proceed in a pattern similar to that as shown in Figure 1, resulting in a through-wall crack with surface length 2c that is approximately equal to twice the, thickness, 2t as in the figure. However, the sudden brittle fracture will occur before the crack penetrates the wall unless the material has sufficient fracture toughness to support a through-wall crack of at least this size, i.e.