Marco Gonzalez

Fatigue Analysis of PE-100 Pipe Under Axial Loading

In this paper, an experimental analysis for determining the fatigue strength of PE-100, one of the most used High Density Polyethylene (HDPE) materials for pipes, under cyclic axial loadings is presented. HDPE is a thermoplastic material used for piping systems, such as natural gas distribution systems, sewer systems and cold water systems, becoming in a good alternative to metals, as cast iron or carbon steel. One of the causes for failures of HDPE pipes is fatigue, due to pipes are under cyclic loading, such as internal pressure, weight loads or external loadings on buried pipes, which generate stress in different directions: circumferential, longitudinal and radial. HDPE pipes are fabricated using an extrusion process, which generates anisotropic properties. By testing in the Laboratory a series of identical specimens obtained directly from PE-100 HDPE pipes in longitudinal and circumferential directions, the relationships between amplitude stress and number of cycles (S-N curves) for two values of test frequency (2 and 5 Hz.) and stress ratio (R = 0.0 and R = 0.5), are established. For each case, three sets of survival probability data (90%, 50% and 10%) and coefficients of Basquin’s equation for the Ps = 50% curves, were obtained. The results obtained are in good agreement with the literature results, showing that stress direction in the pipe, tests frequency and stress ratio affect the fatigue strength of HDPE grade PE-100 pipes.Copyright

A new displacement-based approach to calculate stress intensity factors with the boundary element method

The analysis of cracked brittle mechanical components considering linear elastic fracture mechanics is usually reduced to the evaluation of stress intensity factors (SIFs). The SIF calculation can be carried out experimentally, theoretically or numerically. Each methodology has its own advantages but the use of numerical methods has become very popular. Several schemes for numerical SIF calculations have been developed, the J-integral method being one of the most widely used because of its energy-like formulation. Additionally, some variations of the J-integral method, such as displacement-based methods, are also becoming popular due to their simplicity. In this work, a simple displacement-based scheme is proposed to calculate SIFs, and its performance is compared with contour integrals. These schemes are all implemented with the Boundary Element Method (BEM) in order to exploit its advantages in crack growth modelling. Some simple examples are solved with the BEM and the calculated SIF values are compared against available solutions, showing good agreement between the different schemes.

Thermo-Mechanical Numerical Analysis in Steel Plates Butt Welded

In this work, 2D and 3D Finite Element models to simulate the temperature distribution and residual stress in butt-welded steel plates with the aid of computer simulation, using the commercial software Abaqus®, are developed. The work is carried out in two stages: 1) An analysis of heat transfer in transient state regardless of the structural part is performed, and 2) Thermal and structural responses are sequentially coupled in a thermo-mechanical process simulation in order to determine the final residual stresses induced during progressive heating and subsequent cooling. The results show that for 2D and 3D models the residual stress distribution for relatively thick plate welding can be characterized by a state of stresses plane, dominated by longitudinal stresses. The main difference between both models occurs for transverse stress σY where the values for 3D are significantly greater than for 2D.Copyright

Numerical Analysis of Fracture Mechanics of SENB Specimens Prepared From HDPE Pipes

High density polyethylene (HDPE) is commonly used in pipe fabrication for water and natural gas systems, due to its versatility, low cost and lightweight. A piping system is subject to service conditions such as impact and cyclic loads as a consequence of internal pressure or external pressure fluctuations, and the existence of discontinuities in the material. These conditions cause material damage, cracking and weakening, and have to be considered in the piping design. The Boundary Element Method (BEM) is a numerical method based on integral equations that consider only the contour of the solid (meaning an easier meshing). Crack modeling is one of the most important applications for the BEM, since it allows an accurate stress analysis around the crack tip. In this work, a computational study based on the BEM in two dimensions whose aim is to determine the stress intensity factors (SIFs) in order to evaluate the mechanical resistance to fracture of HDPE PE100 pipes and its comparison with the results obtained by previous experimental tests, is developed. Numerical simulations of specimens subject to three point bending loads (SENB specimens) using the characteristics of the linear elastic fracture mechanic (LEFM), are developed. As a first attempt, the numerical models of different SENB geometries are validated comparing the numerical solution versus the results given by a reference solution from literature. The results show that the BEM under the LEFM approach is valid for loads within the linear range of HDPE since LEFM gives an upper bound of the fracture load of HDPE specimens; however, an Elastic-Plastic fracture analysis could be required for loads in the plastic range of the material.Copyright

Experimental Study of Crack Growth in HDPE PE100 Pipes

The operation conditions of a piping system such as impact loads, cyclic loadings and discontinuities cause damages, cracking or weakening in the material. The High Density Polyethylene (HDPE) is amply used in the fabrication of pipes due to its versatility, low cost and lightweight. In this study, an experimental study of fracture mechanics of HDPE PE100 specimens obtained directly from extruded pipes is developed. The research is aimed at characterizing the pipe mechanical behaviour under operation loadings, for which elastic-plastic mechanical tests under the ASTM D-5045, E-1820, E-399 and E-813 standards and ESIS protocol are carried out. The influence of the orientation induced by the extrusion process (circumferential or longitudinal direction) on pipes fracture resistance is established. SENB type specimens (three point bending) are used for the fracture characterization and J-R Curves (J vs. Δa) for elastic-plastic analysis are generated according to ESIS protocol. The PE100 fracture characterization throughout the J vs. Δa curves indicates that in both circumferential and longitudinal directions require similar quantities of energy to generate new fracture surfaces on HDPE pipes. Indeed, the orientation of the polymer chains as a result of pipe extrusion process could be not so relevant for predicting the direction of crack growth in HDPE pipes.Copyright

Effects of Soil-Pipe Interaction on the Global Buckling Response of Submarine Pipelines

At the moment, in eastern Venezuela several projects involving design, fabrication and installation of submarine pipelines from offshore platforms to onshore plants are being developed. These pipelines will be subjected to high pressure–high temperature conditions, which will cause relevant compressive forces in the pipeline as a consequence of restricted thermal expansion, generating that pipelines can suffer a global buckling. One of the most important factors in this buckling situation is the soil–pipe interaction. In this work, a numerical study of soil–pipe interaction and its effect on the global buckling response for a specific Venezuelan submarine pipeline using the Finite Element Method is presented. A 3D pipe–soil model using shell elements with contact elements for pipe-soil interface for three different pipelines cases is applied. This model has considered nonlinear behaviour for pipe material, loads and contact zones. The main results indicate that global buckling response of a pipeline under axial compressive load depends mainly of pipe-soil interaction, being the main variables: friction factor and submerged weight of pipe.Copyright

Bend Flexibility Factors of Piping Elbows and Piping Elbows With Trunnion Attachments Using the Boundary Element Method

Flexibility Factor is an important parameter for the design of piping system related to oil, gas and power industry. Elbows give a great flexibility to piping system, but where a trunnion is attached to an elbow in order to support vertical pipe sections, the piping flexibility is affected. Generally, determination of elbow flexibility factors has been performed by engineering codes such as ASME B31.3 or ASME B31.8, or using the Finite Element Method (FEM) and Finite Difference Method (FDM). In this work, bend flexibility factors for 3D models of piping elbows and piping elbows with trunnion attachments using the Boundary Element Method (BEM) are calculated. The BEM is a relatively new numerical method for this kind of analysis, for which only the surface of the problem needs to be discretized into elements reducing the dimensionality of the problem. This paper shows the simulation of 9 elbows with commercially available geometries and 29 geometries of elbows with trunnion attachments, 10 of them using commercial elbow dimensions, with applied in-plane and out-of-plane bending moments. Structured meshes are used for all surfaces, except the contact surface of elbow-trunnion joints, and no welded joints are simulated. The results show smaller values of flexibility factors of elbow and elbow–trunnion attachments in all loading cases if are compared to ASME B31.3 or correlations obtained from other works. The results also indicate that flexibility factor for elbow-trunnion attachment subjected to in-plane bending moment is greater than flexibility factor for out-of plane bending moment. Accuracy of BEM’s results were not good when flexibility characteristic values are lesser than 0.300, which confirm the problems of this numerical method with very thin-walled structures. The method of limit element could be used as tool of alternative analysis for the design of made high-pitched system, when the problem with very thin-walled structures is fixed.© 2009 ASME