[Skip to content]

.

Cohesive zone modelling of ductile crack growth in high strength steels

TWI Industrial Member Report Summary 834/2005

By Y Tkach

Background

The problem of characterising and preventing long ductile fracture propagation and predicting arrest is one of the fundamental challenges of fracture mechanics that is still not resolved. Whilst initiation and small amounts of ductile crack growth may be characterised by measuring the R curve using the parameter J, the validity of this approach is a function of the specimen size and normally only valid for a few millimetres of growth. Long ductile fractures may extend many metres and a different approach is required to describe the relationship between the material properties and the applied loading as the crack propagates.

The initiation and dynamic propagation of a longitudinal crack in a long ductile pipeline is one of the most serious structural failures. The pipeline industry currently uses empirically derived relationships between the Charpy V-notch (CVN) energy of the material and the pipe design stress and geometry as a means to select material with a CVN energy sufficient to arrest a propagating ductile fracture. However, the use of Charpy V-notch energy has proven to be unreliable for steels of higher strength than X70 grade line pipe.

It has been demonstrated that, in the case of high-strength steels, too much of the Charpy V-notch test energy is related to crack initiation, bending and plastic deformation for this to be a good measure of resistance to ductile fracture propagation and arrest. A few companies are now working to develop Grades X100 and X120 pipeline for long-distance pipelines. There is therefore a clear necessity for the development of an improved methodology for the analysis of dynamic ductile crack propagation and arrest in high strength pipeline steels. The cohesive zone model of fracture is one possible methodology that is attracting interest.

Objectives

The specific objectives of the work described in this report are:

  • To review current approaches for selecting material to avoid/arrest large ductile fractures.
  • To examine the cohesive zone model as a potential means to predict both stable and unstable ductile crack growth and arrest in fracture mechanics specimens and components
  • To implement the model in finite element analysis and analyse selected test specimens.
  • To propose a test by which the parameters of the cohesive zone model can be determined.