Constitutive model and failure locus of a polypropylene grade used in offshore intake pipes

BorECO^®™ BA212E is a polypropylene block co-polymer which has become a common material in the manufacturing of large diameter non-pressurized gravity offshore intake pipelines. These lines are used for transportation of sea water for cooling of petrochemical process plants. The pipe sections are joined by butt heat fusion welding to create the pipeline. Recently a few premature failures of such pipelines have been reported in the field. Hence, there is a need to characterize the constitutive behavior of the pipe and weld material in order to properly design these pipes. The aim of this work is to determine the material constitutive behaviors of the pipe material and the welded joint material. Uniaxial tensile tests of both the pipe and weld joint material are conducted at various strain rates. Both the pipe and weld material show a rather high strain rate dependency, with the weld material having about half the yield strength than that of the pipe material. An analytical constitutive material model is developed for both the pipe and weld material, incorporating the effect of strain rate. The failure locus, expressed in terms of the equivalent plastic strain at failure vs. the stress triaxiality, for both materials is also determined as part of the constitutive model using notched dumbbell specimens. The constitutive model and failure loci for the pipe and weld material are implemented in a finite element model (FEM) and are validated by conducting a series of independent four-point bend experiments on both material types. The validation is carried out by comparing the FEM results of the four-point bend model with the experimental results, which show a rather good agreement.     

Special fracture mechanics specimens for multilayer plastic pipes testing

Pipes consisting of layers of different materials (multilayer pipes) are considered. The fracture toughness value of the main pipe is taken into account as a parameter relevant to fracture assessment connected with the resistance of pipe material against slow crack growth. With the aim of simplifying estimation of main pipe material fracture toughness, non-homogeneous test specimens cut directly from multi-layer pipes are suggested and numerically analysed. The values of the corresponding stress intensity factor K_I and biaxiality factors B are calculated for the case of two and three layer test specimens. Based on the results obtained, the transferability of fracture toughness values measured on laboratory specimens to pipe systems is discussed. It is shown that in most cases of multi-layer commercial pipes and routine fracture toughness measurements the values of the stress intensity factor calculated on the basis of homogeneous specimens can be used.     

Untersuchung von Kristallisations- und Fließvorgängen bei Langzeitprüfungen von Polypropylen

The present study deals with changes in the nature of pipe material (isotactic polypropylene unpigmented, pigmented) during burst tests. In practical operation standardized burst tests are commonly used. The observed recrystallization increases proportionally to the temperature and time of storing and is effective even after 2500 h. Increasing crystallinity effects a rise of the determined Ultra Micro Hardness (UMH) of the pipe material. The application of an internal pressure in the pipe, however, increases the UMH to a remarkably higher extent than the simple influence of crystallinity. The pipes which are exposed to burst tests show creep effects of the material; these effects cause a higher hardness and increasing internal stresses of the pipe material. During the burst tests the original internal stresses are reduced by the influence of heat; on the other hand new internal stresses are introduced by the creep of the material caused by the effects of the internal pressure. The influence of recrystallization and the creep during burst tests of pipes on microscopical visible changes of the material will be shown in a following paper [18].     

Using developed creep constitutive model for optimum design of HDPE pipes

Unlike metal pipes, high density polyethylene (HDPE) pipes are not susceptible to erosion and corrosion. However, the most important mechanical feature of the HDPE pipes is that this material creeps even at room temperature. Therefore, it is essential to study the creep behavior of this material in order to develop a model. In this paper, creep behavior of HDPE at different temperature and stress levels has been experimentally studied to obtain the creep constitutive parameters of the material. These parameters are used to predict the creep behavior of different structures such as HDPE pipes. For this purpose, a number of specimens have been machined from industrial manufactured pipe walls. Uniaxial creep tests have been carried out and creep strain curves with time for each test were recorded. Then, a constitutive model is proposed for HDPE based on the experimental data and optimization methods. The results of this model have been compared with the test data and good agreement is observed. The developed constitutive model and reference stress method (RSM) were used to produce graphs which provide optimum creep lifetime and design conditions for HDPE pipes that are subjected to combined internal pressure and rotation. These graphs can facilitate the design process of HDPE pipes.     

Alternative accelerated and short-term methods for evaluating slow crack growth in polyethylene resins with high crack resistance

The current market has widely adopted the new polyethylene pipe grade PE 100 RC (resistant to cracks) for pipe applications. However, the main drawback of this material is the long test period (∼10,000 h) required for ranking the resins. This paper proposes a modified Pennsylvania edge-notch tensile (PENT) test with higher load and temperature conditions (2.8 MPa and 90 °C). With the modified PENT test, failure time is six times shorter but slow crack growth is maintained. Additionally, it evaluates and finds an unexpected relationship between the strain hardening modulus and specimen thickness. These results suggest that the 0.30-mm thickness recommended by ISO 18488 is not optimal. Therefore, thicker specimens are proposed for accurate strain hardening modulus determination. Both methods are viable alternatives for evaluating the failure resistance of the new polyethylene pipe grades.     

Creep behavior of in-service flexible flowline polyamide 11

The high cost (material, service, and production loss) involved to substitute a condemned flexible pipe whose pressure sheath has reached its theoretical preconized service life has motivated this study. Therefore, the main objective is to propose a constitutive equation for in-service aged polyamide 11 (PA11) describing the creep behavior as a function of temperature, stress level, and Corrected Inherent Viscosity (CIV), this latter parameter representing the level of material degradation due to hydrolysis. The constitutive equation may be employed for gap spanning analysis and also to subsidize the decision to extend the operational life of flexible pipes that have experienced more severe conditions or have been used for a longer time than designed. The current models to assess the remaining life of the sheath are based only on a single property decay based on corrected intrinsic viscosity (CIV) curves obtained from laboratory tests. To compare the result from the life-prediction model in use and the material mechanical behavior, an experimental campaign was performed using polyamide 11 (PA 11) samples retrieved from a 6″ gas production flexible flowline, which theoretically had reached a full-damaged condition after nearly 3 years operating at higher than specified temperature (80 °C). Dog-bone geometry specimens were machined from the internal, intermediate, and external layers of the flexible flowline pressure sheath. Once polymers are excellent thermal insulators, it was assumed that the material operated under different temperatures within the thickness and, therefore, presents different degradation degrees. CIV, tensile, and creep analyses were performed, confirming that the behavior is different for each region within the thickness of the pressure sheath. Differential scanning calorimetry (DSC), thermogravimetry analyses (TGA), and dynamic thermomechanical analysis (DMA) were performed to comparatively characterize the degree of crystallinity, amount of extractables and morphology of each section. A creep behavior model considering the gradient difference in the material is proposed. It is concluded that aging is different across the liner thickness, and the PA11 creep behavior may be expressed as a function of the CIV, temperature, and stress.     

Stepwise fatigue crack propagation in polyethylene resins of different molecular structure

Stepwise fatigue crack propagation in a range of polyethylene resins, some of which are candidates for use in pipes for natural gas distribution, was studied. Examination of the effect of molding conditions on fatigue crack propagation in a pipe resin indicated that fast cooling under pressure produced specimens with the same crack resistance as specimens taken from a pipe extruded from this resin. The mechanism of stepwise crack propagation in fatigue was the same as reported previously for creep loading. Observations of the region ahead of the arrested crack revealed a complex damage zone that consisted of a thick membrane at the crack tip followed by a main craze with subsidiary shear crazes that emerged from the crack tip at an angle to the main craze. The effects of molecular parameters, such as molecular weight, comonomer content, and branch distribution, on the kinetics of fatigue crack propagation were examined. Correlation of creep and fatigue crack resistance made it possible to relate fatigue fracture toughness to molecular parameters by invoking concepts of craze fibril stability developed for creep. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2355–2369, 1998     

Quantitative assessment of deformation-induced damage in polyethylene pressure pipe

This paper presents results from a study that quantifies the influence of excessive deformation on the damage development in polyethylene (PE) pressure pipe. The experimental investigation is through the application of a novel two-stage approach to the D-split test of notched pipe ring (NPR) specimens. The first test is to introduce damage by subjecting the specimens to different levels of tensile strain at crosshead speeds of 0.01, 1, 10 or 100 mm/min. The second test is to apply monotonic tensile loading at a crosshead speed of 0.01 mm/min to characterize the mechanical properties for specimens that have had damage generated in the first test. Experimental results suggest that elastic modulus and yield stress decrease and yield strain increases with increase of the strain introduced in the first test. Variation of experimentally measured elastic modulus is used to establish influence of crosshead speed on the damage evolution in the PE pressure pipe.     

Thermoanalytical study of an aged XLPE-pipe

A high quality pipe made of XLPE (Engel method) for industrial applications was used for the study of the material deterioration process upon accelerated aging. A sample aged at 110°C while submitted to an hydrostatic pressure of 0.53 MPa, giving a hoop stress of 2.66 MPa, was studied by means of Thermal Analysis, Density, and Gel Content measurements. The effects of aging are compared to a reference unexposed sample and the experimental results are discussed in terms of annealing effects, free volume and structural relaxation processes.     

Adequacy in the metrology of chemical analysis and other complicated measurements

The adequacy of specimens to each other, and of reference material to routine specimens, is an important idea in the metrology of chemical analysis. However, the literature gives no strict definition for this notion. In this paper, a determination of adequacy based on the measure of adequacy is proposed. For the case of two specimens, the measure of adequacy is the absolute value of the difference between the systematic errors in the results from analysis of the specimens; it depends on the differences in the composition and properties of the specimens. The measure of adequacy of a certified reference material to a population of routine specimens is the maximum distance between the systematic error of the reference material and one of the quantiles of the systematic error distribution of the routine specimens Q(0.025); Q(0.975); the errors in this case are due to the composition and the properties of the routine specimens and other complicated measurements. When the measure of adequacy can be neglected compared to the permissible error of the analysis, the specimens are considered adequate. Using the determination of total cholesterol in the blood serum of human beings as an example, it was shown that the certified reference material used in the analysis of specimens of complex composition is often inadequate when compared to routine specimens.

Stochastic fatigue limits and fatigue life variability in oriented-PVC pipe

Fatigue data has been collected on oriented polyvinyl chloride (PVC-O) pipe samples up to fatigue lifetimes of 10⁷ cycles. A random fatigue limit model based on the work of Pascual and Meeker has been used to establish the fatigue limit in this material. This model assumes that the fatigue limit is a stochastic quantity and that the fatigue life is conditional on the fatigue limit. Moreover, it allows the use of censored data in establishing the fatigue limit magnitude. Although the most appropriate distributions for the fatigue life and the fatigue limit were difficult to determine unambiguously, the presence of a fatigue limit in PVC-O was established. This statistical analysis of the fatigue data allowed failure probability quantiles to be calculated for PVC-O pipe materials.     

Determination of Pd, Pt and Rh in vehicles escape fumes by GF-AAS and ICP-OES

Automotive exhaust gases from vehicles using catalytic converters were filtered through cellulose filter papers to collect suspended particles expulsed along with the engine's escape fumes. A specially designed sample collector was used for supporting the filter papers during collection. The collector was manufactured from a new car's exhaust pipe. A cellulose circular paper filter, 11 cm diameter, was attached to one end of the pipe and kept centered by pressing it against the borders of the pipe by means of a perforated aluminum cap, slightly wider than the pipe, used to cover this end of the collector. Filter papers loaded with the solid particles were acid-digested using a modified domestic microwave oven to bring the solid material into solution. The resulting solutions were analyzed for Pt by graphite furnace atomic absorption spectrometry (GF-AAS) and for Pd and Rh by inductively coupled plasma (ICP-OES). Results indicate that concentration of these analytes in the particulate is higher for new vehicles, having new catalytic converters, than for old ones. Maximum Pd, Pt and Rh in the samples analyzed were found to be 5.36, 12.60 and 1.03 microg g(-1), respectively.     

Prediction of the remaining lifetime of polyethylene pipes after up to 30 years in use

Plastics pipes made of polyethylene (PE) play an outstanding role in gas and water supply. While for modern pipe grades typical lifetimes of 50 years are taken for granted and service times of 100 years are discussed, pipes made of PE with a lower performance have been used for decades. As the repair and rehabilitation of existing pipe systems involve immense costs, the question of their qualitative condition has to be considered. In this paper, four different pipes used in the gas and water distribution in Austria with an age up to 30 years have been investigated. After a morphological and mechanical study, particular attention was paid to material stabilization, which is essential for long-term applications. Fracture mechanics tools have been used to gain information on the resistance to crack initiation and slow crack growth. Furthermore, a fracture mechanics extrapolation procedure has been applied to predict the remaining lifetime of the pipes. The results have indicated that all the pipes investigated are still in a very good condition and are likely to be sufficiently safe to remain in use.     

Matrix effects in biological reference materials used in the standardization of cholesterol measurements

Summary The precise and accurate laboratory measurement of blood cholesterol has become a national public health priority in the diagnosis and management of patients at risk for coronary heart disease. Unreliable measurements can hamper the national program to control heart disease. The National Cholesterol Education Program (NCEP) recommends that manufacturers and clinical laboratories achieve traceability to the National Reference System for Cholesterol in order to reliably classify patients according to the NCEP guidelines for assessing coronary heart disease risk. Documenting the accuracy of certain clinical analytical systems for the measurement of cholesterol has been complicated by the occurrence of fluid matrix effects. The major difficulty is that some processed materials (including calibrators, controls, proficiency survey samples, and reference materials) may not be reliable and may result in inaccurate measurement of patient specimens. Similarly, use of such material in accuracy assessment or proficiency testing may yield erroneous conclusions regarding system accuracy. Here, we will review the problem of matrix effects in cholesterol materials and present the results of several studies evaluating sources of matrix error. An alternate approach to documenting accuracy, to be used until commutable materials free of matrix effects are available, will also be discussed.     

Oriented crosslinked polyethylene pipes by a novel extrusion method

Crosslinking and stretching (2.5 times along the circumferential direction) of the molten polymer during extrusion produced pipes with dominantly circumferential orientation and a lower degree of axial chain orientation. Differential scanning calorimetry (crystallinity and crystal thickness), density measurements (crystallinity), X-ray diffraction (c-axis orientation), infrared dichroism measurements (crystalline and amorphous chain orientation) and contraction measurements (molecular draw ratio) assessed the microstructure of the pipe material. The mechanical properties of the oriented material were assessed by uniaxial tensile tests. The orientation was biaxial with the main orientation in the circumferential direction and a lesser orientation in the axial direction. The maximum degree of circumferential orientation was obtained at the inner wall of the pipe. The lower degree of crosslinking of the core material allowed slippage of chains during the stretching of the molten polymer and it is suggested that this is the cause of the lower degree of orientation of the core material. The oriented pipe material exhibited a 5-10% higher degree of crystallinity and higher crystal thickness than conventionally crosslinked material. The tensile modulus and the tensile strength of the oriented, cross-linked material was greater along the axial direction than along the circumferential direction. The circumferential and axial moduli for the oriented, crosslinked pipe were greater than the corresponding moduli of the non-oriented cross-linked pipe material. Another pipe based on crosslinked PE that were first circumferentially stretched 2.5 times and later axially stretched 10 times (in the molten state) showed, despite the fact that it exhibited pronounced axial orientation almost a balanced tensile modulus (4.3±0.2 GPa) in the axial-circumferential plane. Atomistic modelling showed that the orientational dependence of the density of the amorphous phase is small.     

Error amplification in capillary viscometry of power law fluids with slip

Rheological characterization through capillary viscometry of molten polymers that slip at solid walls is quite a challenging task. In fact, it is based on an indirect measurement that introduces remarkable error amplifications, mainly because of the Mooney procedure. Sometimes these are so large that unphysical results are obtained. In this paper we study these issues analytically, using a particular power law fluid as a test model. Interestingly, it is found that there is dependence on the fluid characteristics, such as the shear thinning behavior. Two software programs are provided in the Mendeley Data Repository. One quantifies the error amplification, the other one can be used by interested researchers for properly designing the testing setup (e.g. the most convenient choice for the capillary diameters). In order to use the programs, though, the material constants of the fluid that is intended to be characterized must be known at least approximately.     

Modélisation théorique d'écoulements pulses de fluides non newtoniens en conduites viscoélastiques poreuses et anisotropes

A theoretical study concerning the pulsatile flow of an inelastic fluid through anisotropic porous viscoelastic pipes is presented. The objective is to investigate the effects of porosity, conicity in pause, anisotropy and viscoelasticity of pipe wall material for a generalized Casson fluid. An implicit difference method is used to solve the equations, and to determine the axial and radial velocity profiles, the pressure, the flow rate and the flow rate filtration distributions, the transverse rotation and the wall axial and radial displacement. This study, considered as a step in the modelling of flow in blood vessels, may also contribute to other important fields such as water desalination or gel filtration.     

Monitoring the vulcanization of rubber with ultrasound: Influence of material thickness and temperature

In consideration of the influence of the curing reaction on the material properties of elastomers, it is very important to control the vulcanization process properly.As shown previously [1], the effects of the vulcanization reaction of an elastomer can be monitored by use of ultrasonic sound waves. This technique has to a certain degree a high similarity to the standard curemeter test according to ISO 6502 but can be applied also inside a production tool.So far, the method was only applied to the vulcanization of one compound at a given temperature and sample thickness. This is now complemented with measurements on another compound system at different temperatures and sample thicknesses.The expected effects of temperature and thickness on the curing behaviour were found by use of the ultrasound online control.     

Methodology considering surface roughness in UV water disinfection reactors

Water disinfection making use of an ultraviolet (UV) reactor is an attractive procedure because it does not produce any by-products. In this work, the effects of pipe roughness on the performance of a closed-conduit water disinfection UV reactor were investigated. In order to incorporate the surface roughness effects, a simple, stable, highly accurate model, better than any iterative approximation, was adopted in the numerical simulations. The analysis was carried out on the basis of two performance indicators: reduction equivalent dose (RED) and system dose distribution. The analysis was performed using a commercial computational fluid dynamics (CFD) tool (ANSYS Fluent). The fluence rate within the UV reactor was calculated using UVCalc3D. The pipe surface roughness resulted in longer pathogen residence times and higher dose distribution among the pathogens. The effect of pipe surface roughness on RED depends on the Reynolds number and relative roughness. Pipe surface roughness plays an important role because UV reactors for water disinfection operate at moderate Reynolds numbers. In addition, the positioning of the UV lamp in the reactor plays an important role in determining the RED of the reactor. Search criteria for lamp-positioning are also proposed in the current work. The proposed CFD methodology can be used to analyse the performance of closed-conduit reactors for water disinfection by UV.     

Dimensional considerations on the mechanical properties of 3D printed polymer parts

Additive manufacturing offers a useful and accessible tool for prototyping and manufacturing small volume functional parts. Polylactic acid (PLA) and thermoplastic polyurethane (TPU) are amongst the most commonly used materials. Characterising 3D printed PLA and TPU is potentially important for both designing and finite element modelling of functional parts. This work explores the mechanical properties of additively manufactured PLA/TPU specimens with consideration to design parameters including size, and infill percentage. PLA/TPU specimens are 3D-printed in selected ISO standard geometries with 20%, 60%, 100% infill percentage. Tensile and compression test results suggest that traditional ISO testing standards might be insufficient in characterising 3D printed materials for finite element modelling or application purposes. Infill percentage in combination to design size, may significantly affect the mechanical performance of 3D printed parts. Dimensional variation may cause inhomogeneity in mechanical properties between large and small cross section areas of the same part. The effect was reduced in small cross section parts where reducing the nominal infill had less effect on the resulting specimens. The results suggest that for 3D printed functional parts with significant dimensional differences between sections, the material properties are not necessarily homogeneous. This consideration may be significant for designers using 3D printing for applications, which include mechanical loading.