Prevention of Pipeline Dent Failure Under Fatigue Loading Conditions

Pipeline dents fatigue life prediction is a subject of high interest for pipelines operating companies. Empreinte is an in-house developed pre and post processor to ABAQUS Finite Element Calculations dedicated to pipeline integrity assessment. Empreinte was first developed and experimentally validated for dents assessments under static loading conditions. As oil but also gas transmission pipelines are submitted to cyclic loading conditions (internal pressure variations, shutdowns, temperature variations …), it was decided to introduce a fatigue life criterion in Empreinte based on the Dang Van theory assuming that local mesoscopic stresses drive fatigue crack initiation. Full scale tests performed for PRCI projects PR-201-927, PR-201-9324 and MD-4-2 were used to validate the proposed fatigue assessment methodology: - the first full scale fatigue test was performed in 1994 on an X52 pipe. For this test, limited material and test data were available. - the second full scale fatigue test was performed in 2007 on an X52 pipe. For this test, material characterization (in particular tensile tests with full stress strain curves) and test data (strain gages measurements, indenter geometry …) were available. Fatigue life assessments were performed following three main steps: 1. using available data: non linear kinematic hardening constitutive laws were identified for the two pipes materials; 2. finite elements elastic-plastic modeling of the denting processes were carried out; 3. fatigue calculations were performed following a new approach using Dang Van criterion for which the parameters were determined from literature data. The elastic shakedown assumption allowed the determination of the local stress cycle from the macroscopic stress cycle. The fatigue criterion integrating the combined influences of shear and hydrostatic stresses was checked on all points of the pipe. Good agreement between experimental and calculated fatigue lives and fatigue crack initiation points was reached. This opens a promising way to assess pipeline defects fatigue life. Efforts are now focused on the standardization of a testing method to identify the Dang Van criterion of a pipeline material at least in air environment.

Application of Dang Van criterion to rolling contact fatigue in wind turbine roller bearings under elastohydrodynamic lubrication conditions

A 2D plane strain finite element program has been developed to investigate very high cycle fatigue in wind turbine roller bearings due to rolling contact. Focus is on fatigue in the inner ring, where the effect of residual stresses and hardness variation along the depth is accounted for. Both classic Hertzian and elastohydrodynamic lubrication theories have been used to model the pressure distribution acting on the inner raceway and results are compared according to the Dang Van multiaxial fatigue criterion. The contact on the bearing raceway is simulated by substituting the roller with the equivalent contact pressure distribution. The material used for the simulations is taken to be an AISI 52100 bearing steel and linear elastic behavior is here assumed. The effect of different residual stress distributions is also studied, as well as the effect of variable hardness along the depth, relating its values to the fatigue limit parameters for the material. It is found that both for Hertzian and elastohydrodynamic lubrication contacts, the Dang Van criterion predicts that fatigue failure will first occur in the subsurface region and that, regardless of the specific pressure distribution used, the hardness distribution can have a significant influence on the safety against failure for bearings subjected to very high cycle fatigue loading.

The Dang Van Criterion for Fatigue Design

The present work intends to evaluate, using simple, exemplary cases, the importance of a full elasto-plastic analysis in fatigue design. A strain cycling situation (-ε to ε) was modelled with ABAQUS considering two situations: firstly a linear σ vs. ε relationship was assumed, and secondly, the real cyclic σ vs. ε curve was used to model each cycle, which includes a small plastic deformation. The case of change of cross section in a steel shaft subjected to constant torsion and cyclic bending was analysed through finite element modelling using ABAQUS.

The relationship between the Dang Van criterion and the traditional bulk fatigue criteria

A formal relationship is established between the traditional bulk fatigue criteria such as the Goodman rule and the Sines criterion, and the recent crack initiation criterion of Dang Van. The constants implied by the Dang Van procedure may be formally connected to the fatigue limit and mean stress dependence given by the Goodman rule, under conditions of uniaxial loading. The biaxial fatigue criterion of Sines is also compared with the Dang Van procedure. The similarity in the approaches may be further extended to permit finite initiation times to be estimated by the Dang Van criterion when the fatigue limit is exceeded.