J. Capelle

Burst Test and J-Integral Crack Growth Criterion in High Density Poly-Ethylene Pipe Subjected to Internal Pressure

In the present work we are interested on the analysis of the severity of crack defects created by a disc cutter and to study the behavior of a high density polyethylene pipe (HDPE pipe) when subjected to an internal pressure, either in the absence or presence of a pre-crack. In order to do this, experimental tests was performed to measure the toughness and to determine the mechanical behavior of HDPE. These features were used to perform numerical simulations using ABAQUS on pipe solicited by an increase in internal pressure. This allows to compare with burst tests of cracked pipes and to determine the fracture energy that will be compared to the toughness.

Modelling Crack Propagation and Arrest in Gas Pipes Using CTOA Criterion

In this paper, the resistance to ductile crack extension is discussed in terms of Charpy or DWTT energy, R curve and CTOA. Methods used in numerical simulations of ductile crack extension are presented including the cohesive zone model, a critical damage with the Gurson–Tvergaard–Needleman model, critical damage given by SRDD model or a critical crack opening angle (CTOA). Selection of CTOA is based on the reduced number of parameters and the low sensitivity to pipe geometry. Numerical simulations of crack propagation and arrest based on CTOA, use the node release technique, which is described. Results on a pipe made in steel API L X65 are presented. The influence of geometrical and material parameters on crack arrest and velocity using this technique are presented. Finally, an arrest pressure equation similar to the BTCM’s equation but including critical CTOA is introduced. For the same decompression wave pressure, the crack propagation velocity is inversely proportional to the resistance to crack extension of the material, which is the dominant parameter. The crack velocity versus decompression is expressed by a CTOAc function of resistance to crack extension.

The CTOA as a Parameter of Resistance to Crack Extension in Pipes Under Internal Pressure

Different methods of CTOA measurements are described: Optical microscopy coupled with digital image correlation, analytical analysis of experimental load-displacement curves or simulation by Finite Element method. Using a Modified Compact Tension (MCT) specimen at room temperature, tests are performed to measure the value of the CTOA of API 5L X65 pipe steel. The influence of thickness on CTOA has been studied and explained through a “triaxial stress constraint”. Crack extension is modelled by the finite element method using the CTOA criterion coupled with the node release technique. Crack velocity, arrest pressure and crack extension at arrest have been determined. Values of the CTOA are not intrinsic to materials. Like other measures of fracture toughness, it is sensitive to geometry and loading mode. This sensitivity can be described by a constraint parameter. For the thickness effect, the constraint parameter Tz is very appropriate.

Hydrogen Effect on Local Fracture Emanating from Notches in Pipeline Steels

The comparative assessment strength and fracture of three API grade pipeline steels: X52; X70 and X100 has been done for conditions of hydrogenating. The factors of cathodic hydrogen charging, time of exposition, hydrogen concentration in metal and applied load were taken into account. The relationship between hydrogen concentration and critical (failure) loading has been found. Also, the existence of some critical hydrogen concentration, which causes the significant loss of local fracture resistance of material, was shown.

Hydrogen Effect on Fatigue Life of a Pipe Steel

Transport by pipe is the means more used, at the pr esent time, to convey energies of their point of extraction until their field sites f inal. To limit any risk of explosion or of escape and thus to limit the geological problems of pollut ion and the human risks, it is necessary to be able to know the mechanical properties of the st e l used in the manufacture of these pipes. With the reduction in oil stocks, it is necessary t o find a new energy. Hydrogen is this new energy vector, it thus will also be necessary to be a l to transport it. This study makes it possible to emphasize the assignment of the lifespa n of hydrogen on a pipeline steel normally used in the transport of gas. The fatigue tests in 3 points bending are carried out on samples not standards because of dimensions of the tube of origin.