Deformation and failure in polyethylene: correlation between mechanisms of creep and fatigue

Fatigue and creep tests have been conducted on medium density gas- and water-grade pipe materials using a longitudinally notched pipe geometry with internal pressurization. The failure in both types of test was brittle compared to the failure occurring when the unnotched pipe is exposed to a constant intense pressure. Further to this superficial similarity, fractographic studies revealed creep and fatigue failures to have many features in common. Thus it is perhaps not surprising that fatigue and creep lifetimes have been found to correlate well. The findings of this study give support to the concept of adopting fatigue as a quality control method in the production of plastic pipe.     

Failure analysis of IEU drill pipe wash out

Many 127.0×9.19 mm IEU G105 drill pipe failures of wash out occurred after 2367 h of pure drilling time and 8726 m of penetration footage. This paper gives a detailed investigation on these failures and a systematic analysis is carried out on service and loading conditions of the drill pipes. Measurement and inspection were performed on configuration dimensions, chemical composition, mechanical performance, metallography, macro-fractography, micro-fractography, and corrosion products. Configuration stresses at the crack positions of the drill pipe were calculated by FEA. Crack extending velocity of the drill pipe material under corrosion medium was also measured. It is thought from test and analysis results that the drill pipe wash out or fracture accidents were premature corrosion fatigue failure accident. The failure courses were as the following: corrosion pits occurred first on the internal surface at the stress concentrating area of the drill pipe, and then fatigue cracks initiated in pit bottoms, and washed out or fractured subsequently as cracks penetrated through the wall thickness of the drill pipe. The reasons of drill pipe wash out were related to configuration, material quality, and load condition of the drill pipe string.     

Fatigue testing of uPVC pressure pipe

Many uPVC pressure pipe service failures have been due to fatigue crack propagation from a flaw in the internal wall surface of the pipe, often aggravated by the local soil loading conditions. Such failures can be simulated by machining an internal notch (known flaw) into the wall of the pipe before cyclic internal water pressurization.A constant-displacement compressive loading has been found to reduce significantly the fatigue life of uPVC pressure pipe. The frequency and amplitude of the internal cyclic pressure have both been found to affect the fatigue life of uPVC pipe.     

Plastic limit load of Grade 91 steel pipe containing local wall thinning defect at high temperature

This paper aims to predict the plastic limit load of Grade 91 pipe at high temperature by experimental and numerical analysis methods. The elastic–plastic constitutive relation of P91 considering creep damage at high temperature is proposed and the effectiveness is verified by high-temperature instantaneous tensile test after creep of P91 sample. Then, using the elastic–plastic constitutive relation considering creep damage, the plastic limit loads of P91 pipe containing local wall thinning (LWT) defect at high temperature have been calculated by finite element (FE) method and the accuracy is checked by high-temperature burst experiment after creep of T91 pipe containing LWT. Finally, the fracture analysis and metallographic analysis of Grade 91 sample and pipe containing LWT defect are performed. The results show that the elastic–plastic constitutive relation of P91 considering creep damage proposed by this paper can well describe the elastic–plastic behavior of P91 under creep condition and is useful to calculate the plastic limit load of P91 pipe containing LWT. The failures of P91 tensile samples and T91 pipes containing LWT defect are both plastic fracture. This research would provide an optional method for structure integrity analysis of the pipe containing LWT defect at high temperature.     

Failure analysis and prediction of pipes due to the interaction between multiphase flow and structure

Frequently found in piping, erosion failures may lead to the leakage of pipes and even damage the whole system. Erosion is a form of material degradation that involves electrochemical corrosion and mechanical wear processes encountered on the surface of metal pipes. Research on the erosion–corrosion mechanism indicates that the erosion mainly results from the interactions between the pipe surface and the fluid traveling along the surface. This paper studied multiphase flow induced erosion–corrosion in pipes, mainly focusing on the interactions between the multiphase flow and the structure. In order to evaluate the erosion effects caused by the flow and the coupled dynamics, the multiphase flow regimes, flow dynamics parameters and the distribution of phases on different positions of the boundary layer in REAC pipe inner surface were analyzed. The CFD simulation results proved that the location, rate and the extent of erosion failures on the pipe surfaces can be well predicted, as compared with actual instances. Practically, this method can be used in optimizing the design of the inner sleeves of pipes.     

The performance and causes of failure of polyethylene pipes subjected to constant and fluctuating internal pressure loadings

Three different pipe-grade polyethylenes, in the form of one large and three small diameter pipe systems, have been tested at elevated temperatures, using constant and fluctuating internal pressure loadings that resulted in brittle fractures. The behaviour under fatigue of two of the three types of small diameter polyethylene pipes was substantially described by a cumulative damage model, whilst the third exhibited a fatigue weakness, an observation not previously reported. The performance of the large diameter pipes under fatigue was dominated by the presence of large voids in the pipe wall that arose from incorrect processing and resulted in premature failure. The sites of crack initiation in one material grade of the small diameter systems were examined in detail. In particular the size, position and composition of particles initiating fracture were determined. The maximum particle size on the fracture surface of the pipe was found to correlate reasonably well with a measure of pipe lifetime, as predicted by a fracture mechanics approach, and indicated that the lifetime of this one type of polyethylene pipe was dependent on the size of the inclusions initiating fracture.     

Life extension technology for steam pipe systems—II. Development of life prediction methodology

Part II of this paper addresses the development of a fracture mechanics based life prediction methodology of steam pipes which operate at elevated temperatures but in the sub-creep temperature range. Elastic-plastic fracture mechanics concepts were employed to establish the remaining life prediction methodology and inspection criteria of steam pipes. Leak-before-break analyses were utilized to determine the flaw inspection criteria. Both tension and bending type loading conditions were considered in the life prediction analysis. The life assessment technology is concerned with the fatigue crack growth life of circumferential cracks in a pipe. The material properties of the A106B steam pipe steel reported in Part I of this paper were used to predict the fatigue life of steam pipes. The effects of operating parameters (e.g. stress and temperature), pipe size, and material properties on the remaining life and inspection intervals of steam pipes can be quantitatively evaluated.     

Study of polyethylene pipe resins by a fatigue test that simulates crack propagation in a real pipe

A fatigue test that simulates the step-wise crack propagation found in pipes in the field, and uses a standard compact-tension specimen, was employed to study and rank crack resistance of various pipe resins. The thermal history during compression moulding of the test specimens strongly affected fracture kinetics. It was found that crack-resistant properties of in-service pipe were best reproduced if compression-moulded plaques were fast cooled under load. This procedure was used to prepare specimens from candidate pipe resins for fatigue testing. The resins were compared in terms of discontinuous crack growth kinetics. The ranking based on resistance to fatigue crack propagation correlated with results of a standard PENT creep test. However, fatigue failure times were an order of magnitude less than the standard creep times. After comparing the initiation and failure times of the resins with detailed kinetics of step-wisse crack propagation, a simplified and rapid procedure is proposed which calls for evaluating only the first jump after initiation.     

Some observations on the strength and fatigue properties of samples extracted from cast iron water mains

The strength and fatigue properties of cast iron samples taken from water distribution mains have been investigated. Specimens were sourced from three sections of pipe which had experienced varying amounts of corrosion in service, enabling the variable of pipe condition to be incorporated within the study.
The strengths in four-point flexure of small specimens from the pipes examined were described using Weibull statistics; different characteristic strengths and Weibull moduli were obtained, according to the pipe condition. A further set of samples from each pipe were subjected to flexural fatigue at a range of stress levels (different stress levels were chosen for each pipe based on the short-term strength properties) and residual strength tests were carried out on the surviving samples from one stress level for each pipe. There is evidence of a fatigue effect for all sample sets. There were slight differences in the residual strength behaviour – the residual strength of the survivors was reduced in the samples from the section in best condition while the residual strength of the survivors from the other two pipe sections was relatively unaffected. These trends are discussed with reference to condition and fatigue stress level.
The results suggest that mechanical fatigue may be a factor in the failure of water distribution pipes. The results may have implications for large diameter trunk mains as well as the small diameter water distribution pipes tested here. To assess the effect in more detail, consideration needs to be given to scaling effects in fatigue and the likely levels of any fatigue stress seen in service.     

Investigation of low temperature effects on fatigue behavior of PVDF

Fatigue behavior of polyvinylidiene fluoride (PVDF) pipes is investigated under low temperatures to characterize the temperature effects. The analysis included experimental evaluation of fatigue life for test samples taken directly from the manufactured pipes used for service as opposed to compression molded compact tension samples used in previous works. In this test, short sections from an extruded pipe are used to better represent the material service conditions. A compact test chamber was designed to control the test temperature. The samples were loaded into the test rig and allowed to cool for 30 min ensuring a constant and even temperature distribution. Cooling was done in a sealed test chamber using carbon dioxide gas. Two test temperatures of −20 °C and −10 °C were chosen since they represent typical temperature during which failure occurs during actual pipe service in cold environments. Fractured surfaces were inspected and fatigue data were analyzed using a standard procedure for calculation of fatigue life with a semi-elliptical surface crack assumption was performed; from which parameters of the Paris law for fatigue fracture were obtained. Comparing the results with previous works it is found that they capture the trend of the PVDF material behavior for high temperature.     

Torsion fatigue failure of bus drive shafts

The mystery surrounding high failure rates in the drive shafts of a large municipal transit agency's fleet of 40 newly acquired articulated buses is investigated. The drive shafts were fabricated from a low-carbon (0.45%) steel such as AISI 5046. An examination of the drive shafts on all 40 buses is conducted, and 6 different drive shaft designs are identified among the fleet, but all of the failures, 14 in all, are limited to just one of the identified designs. Microscopic examination of the fracture surface of one of the failed drive shafts under a scanning electron microscope is conducted to determine the failure mode. Evidence of high-cycle fatigue is found, and a finite-element analysis is conducted to compare the maximum stress of the design exhibiting failures with the most common of the other designs that exhibits no failures. A fatigue life prediction is performed to determine just how much longer the expected fatigue life of the surviving design is compared to the design that suffered the early failures.     

Dynamic elastic-plastic behaviour of whipping pipes: experiments and theoretical model

This paper is concerned with the problem of pipe whip, the dynamic response of a high pressure piping system subjected to an end force as the result of a pipe break which releases a jet of fluid from the broken section. Both experimental and theoretical results are presented concerning the dynamic elastic-plastic behaviour of cantilever pipes subjected to a transverse force pulse at the free end. Comparisons between experimental data and theoretical predictions are made for mild-steel pipes with outer diameter-to-thickness ratios of 19.5, 28 and 32. It is demonstrated that, for these geometries, the whipping pipes display three characteristically different responses, viz. elastic, plastic hardening behaviour for thick pipes, elastic, plastic hardening-softening behaviour for moderately thick pipes and elastic, plastic hardening-softening-collapse behaviour for thinner pipes. The experimental data taken from a series of high-speed films are compared with the predictions of the instantaneous shapes of the whipping pipes derived from both a rigid, perfectly-plastic, large deflection, dynamic beam model and a more comprehensive model which incorporates the effects of elasticity and plastic hardening and softening, the details of which are presented in the paper.     

Nonlinear fatigue crack propagation in a baffle module of Wendelstein 7‐X under cyclic bending loads

Simulation of the fatigue crack propagation in a Wendelstein 7‐X baffle module is performed in this study using both a finite element method‐based software and the UniGrow nonlinear model for small‐scale yielding (SSY) conditions. Some experimental fatigue tests of several cracked baffle modules have been performed through a servo‐hydraulic machine. One of these experimental tests has been considered to simulate fatigue crack propagation in the baffle module. Before starting the experimental test, a first crack partly contained in the welding seam and partly in the steel pipe is found. Subsequently, owing to the applied load, the crack propagated both into the welding seam and into the steel pipe until the plastic zone in the near field attains SSY conditions. Finally, owing to the increase in the extension of the plastic zone, SSY conditions are not more valid, and the breakage of the steel pipe is produced by plastic collapse.     

Challenges in Procurement of HIC Resistant Steel Pipes

Welded pipes ordered for wet sour service are required to be manufactured from hydrogen-induced cracking (HIC) resistant steel plates. Pipe manufacturers procure steel plates with HIC testing, and other additional requirements with respect to chemistry, mechanical testing, and non-destructive testing (NDT) to meet client requirements. There have been several worldwide instances when the pipes ordered for wet sour service have failed HIC test at the pipe mill resulting in significant project delays. The causes of HIC test failures range from deliberate mistakes to improper testing practices. Also, subtle changes in steel making and rolling practices contributed to the HIC test failure following the pipe fabrication. Saudi Aramco has established several lines of defense to prevent recurrence of similar failures. Saudi Aramco reservation on mandatory HIC test requirement on seamless pipes as per the latest API 5L 44th edition, is discussed.     

Failure characteristics of glass-reinforced plastic pipe and pipe assembly

In the examination of glass-reinforced plastic (GRP) pipes and components after failure the fracture is often so catastrophic that much of the evidence is destroyed or hidden. When a leak failure occurred during a hydraulic pressure test on a GRP pipe bend the opportunity was taken to examine the bend wall in detail at a number of localities. They included, not only the region where the leak was apparent but also at areas where strain and displacement measurements had indicated the possibility of failure arising. The 250 mm diameter pipe specimen tested consisted of a right-angled pipe bend with flanged, straight lengths each attached by a butt joint. The resin matrix was polyester and the reinforcement was E-glass in the form of chopped strand mat of four layers weighing 2·4 kg/m².

Although the failure pressure was entirely satisfactory from a design point of view, three important areas of weakness were discovered. They were associated with procedures for butt jointing and for moulding of the bend unit. The resulting micrographs were of sufficient quality to act as comparators for examination of possible service failures.     

Pipeline Failure by Transit Fatigue

A spiral welded pipeline failure that occurred during hydrotest was attributed to transit fatigue, caused during highway transportation. Transit fatigue during rail, road and marine transportation is a well-known failure mechanism. American Petroleum Institute (API) standards for prevention of transit fatigue during railroad and marine transportation are already established. Transit fatigue during highway transportation is rarely reported, and there is no industry standard addressing controls for line pipe transportation by highway.     

Fatigue behavior of surface cracked filament wound pipes with high tangential strength in corrosive environment

The aim of this study is to examine the corrosion fatigue behavior of filament wound composite pipes with a surface crack under alternating internal pressure. The filament wound pipes are composed of multi-layered E-glass/epoxy composites with a [±75°]₃ lay-up. The surface notches were formed on the outer surface of the pipe along the pipe axis. Dilute (0.6 M) HCl acid was applied to the surface crack region by a corrosion cell mounted on the outer surface of the pipe. The results of an experimental investigation into the corrosion fatigue tests are conducted to observe the oil leakage failure and the crack propagation of the composite pipe subjected internal pressure loading with an open ended condition in which the pipe can be deformed freely in the axial direction. The internal pressure was generated by conventional hydraulic oil for fatigue loading. The fatigue tests are performed at 0.42 Hz frequency and a stress ratio of R = 0.05 in accordance with ASTM D-2992 standard. The oil leakage from the crack tip was observed after the crack propagation reached to the critical stress intensity level. The fatigue crack propagation behavior with the environment exposure was strongly dependent on the crack parameters such as crack-depth ratio and crack-aspect ratio. The micro structure of the fracture surface with the effect of environment and the fatigue loading were also observed.     

Failure analysis of anticorrosion plastic alloy composite pipe used for oilfield gathering and transportation

Incident involving failures of anticorrosion plastic alloy composite pipe which was used for oilfield gathering and transportation had caused serious burst after several years of usage. Investigations were performed to identify the most probable causes of the pipe failures. The study was conducted by reviewing the background of the incident, inspecting visual physical morphology of the failed pipe, analyzing FTIR spectra and TG–DSC curves of inner plastic alloy and outer GRP resin. Results revealed that additives in damaged plastic alloy may not be stirred enough before extrusion or the content of additives may exceed their normal range. The lower degree of cure for resin of GRP layer leads to decrease of glass transition temperature as well as its thermal stability, which will make the outer GRP structural layer have a relatively lower mechanical property, thus giving rise to final failure of the composite pipe under the long time service conditions.     

Fatigue analysis of damaged steel pipelines under cyclic internal pressure

A methodology for fatigue analysis of damaged steel pipelines under cyclic internal pressure is proposed. This methodology employs stress concentration factors, which are commonly used to modify standard S–N curves of metallic structures under high cycle fatigue loadings. Experiments are accomplished to evaluate the strain behavior of small-scale steel pipes during denting and cyclic internal pressure. A nonlinear finite element model is developed to obtain stress concentration factors induced by plain dents on steel pipes under internal pressure. Afterwards, analytical expressions are developed to estimate stress concentration factors as function of the damaged pipe geometric parameters. Finally, fatigue tests are conducted to evaluate the finite life behavior of small-scale damaged pipes under cyclic internal pressure and to validate the proposed methodology of fatigue analysis.     

Pipe-defect assessment based on the limit analysis, failure-assessment diagram, and subcritical crack growth

We analyze the serviceability of a pipe containing a defect by the following three methods: limit analysis, failure-assessment diagram, and subcritical crack growth. The use of the proposed complex approach is explained by the fact that, despite the plasticity of pipe steels, they can suffer quasibrittle fracture (under the conditions of plane deformation) or fail as a result of plastic collapse. Moreover, the final fracture of a pipe with defect is preceded by the stage of stable crack growth. Therefore, in analyzing the serviceability of the pipe, one should also take into account the “leak-before-break” criterion. To take into account all these circumstances, it is necessary to use the indicated three methods, as in the case of evaluation of the serviceability of a pipe of API X52 steel containing a surface semielliptic notch. Defects of this sort reflect the influence of the surface gouges and dents responsible for the failures of pipes under the action of internal pressure. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 1, pp. 119–127, January–February, 2006.