Review of CTOA as a Measure of Ductile Fracture Toughness

Abstract

The ductile fracture toughness of steel is used to assess the ability of a pipeline to resist long running ductile fractures in a burst event. With the introduction of modern low carbon clean steels with ultra high toughness, conventional measures of ductile fracture toughness (standard Charpy and DWTT energy) are under review, and alternatives are being studied. The crack tip opening angle (CTOA) was investigated to evaluate its appropriateness as a measure of modern pipeline steel ductile fracture toughness. At first, fracture mechanics tests at quasi-static rate were analyzed to examine the constancy of CTOA with crack growth. The results of this initial review are based on four pipeline steels with a range of ductile fracture toughness. The CTOA values are also compared with appropriate parameters from conventional tests to examine potential relationships that may be used to indicate the relative resistance of pipeline steels to ductile fracture propagation. The final objective is to compare CTOA values determined by the simple two specimen method and those developed through a formal fracture mechanics based technique.

Determination of dynamic rotation factor for DWTT specimens by strain analysis and hardness measurement

“…Fortunately, different to the Demofonti two-specimen method 4 and Di Biagio single specimen method, 5 A * and σ f disappear in some regression methods, like S-SSM method, 6 energy-based regression method 7 and load-line displacement method. 8,9 On the other hand, the experimental approach to determine the product (A * σ f ) is available, 7 also ascertaining that the value of A * is nearly 1/3 (i.e. θ 2 equals to 4/3) 10 and σ f could be 1.3 times the average of material yield and ultimate strengths.…”

Comparison of fracture criteria for cracked steel pipelines by finite element method

“…[4][5][6][7]. Наличие ряда подходов (а не одного общепризнанного) связано с наличием нескольких разных специфических черт задачи, причем в разных конструкциях эти черты могут проявляться в разной мере.…”

Effect of fracture tunneling in DWT Tests

“…In the static experiment it is possible to stop the test when the crack lies inside the region of stable propagation [23,24] characterized by a stable CTOA value. From this uncompleted test it is possible to attempt the analysis of the crack conformation.…”

“…Some considerations connected to this phenomenon are discussed by some authors [8,24], who claims that the r à accounted when dealing with high strength steels should be properly increased. At which amount this increase should be attributed to material characteristic if not to crack tunneling remains a subject still under investigation.…”