An examination of the role of flaw size and material toughness in the brittle fracture of polyethylene pipes

At low temperatures and hoop stresses, polyethylene pipes fail by the time-dependent propagation of a crack. These brittle, fissure-like failures have been observed to initiate from adventitious flaws, and the concepts and methods of fracture mechanics indicate that flaw size should determine stress rupture lifetime. A number of controlled model experiments have therefore been undertaken to assess the influence of flaw size and material toughness on the stress rupture lifetimes of polyethylene pipes. To two different pipe grade polyethylene resins (one shorter, one longer lifetime resin) flaws of varying sizes have been added. For the shorter lifetime resin small flaws were, in addition, purposely excluded by the use of fine melt filtration techniques. Pipes containing added flaws or pipes where flaws were excluded were then stress rupture tested under those conditions designed to induce brittle failure by slow crack growth. The stress rupture lifetimes of the various pipes are then correlated with flaw size. The results of the tests using the shorter lifetime resin show that flaw size does have a significant influence. It is particularly interesting to note that melt filtration, which removes large inherent flaws, substantially improved the stress rupture lifetime. With respect to material toughness, the longer lifetime pipe grade polyethylene resin showed a healthy tolerance to included flaws. In respect of the stress rupture test preferred resins can therefore be identified in terms of their tolerance to included flaws.     

Fracture of hot-pressed alumina and SiC-whisker-reinforced alumina composite

The flexural strength of hot-pressed alumina and SiC-whisker-reinforced alumina composite were evaluated as a function of temperature (20 to 1400° C in air environment), applied stress and time. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. A temperature-independent region of fast fracture (catastrophic crack extension) existed up to 800° C, in which the failure mode was a mixture of transgranular and intergranular crack propagation. In this region, the alumina composite showed significantly higher fracture strength and toughness compared to polycrystalline alumina. Above 800° C, both materials (alumina and alumina composite) displayed a decreasing fracture strength due to the presence of subcritical or slow crack growth which occurred intergranularly. Flexural stress rupture evaluation in the temperature range 600 to 1200° C has identified the stress levels for time-dependent and time-independent failures.     

A fracture mechanics concept for the accelerated characterization of creep crack growth in PE-HD pipe grades

For the lifetime prediction of pressurized polyethylene (PE) pipes based on methods of the linear elastic fracture mechanics the knowledge of the crack resistance and the kinetics of creep crack growth (CCG) is essential. In the present work a rather brittle nonpipe material was used to develop a methodology for an accelerated measurement of crack kinetics in fatigue tests on cracked round bar (CRB) specimens at ambient temperatures of 23 °C. A material and specimen specific compliance calibration curve was generated to detect the crack kinetics with only one single CRB test. Based on an already proposed concept the kinetics at different R-ratios (minimum/maximum load) was measured and extrapolated to the case of CCG. To demonstrate the transferability of this concept to pipe materials a PE 80 pipe grade was used. Although the necessary testing time increased considerably the concept still has the potential to reduce the overall testing time for new pipe materials to be certified significantly. With the presented procedure a characterization of CCG in modern PE pipe grades at room temperature and without the use of stress cracking liquids is possible within a few months.     

Influence of ageing in the failure pressure of a GFRP pipe used in oil industry

This paper is concerned with the influence of ageing in the failure pressure of a glass fibre reinforced polymer (GFRP) pipes used for oil and gas transport. Burst tests were performed on pipes submitted to accelerated ageing combining hydraulic pressure and temperature (1 MPa and 80 °C). An alternative method, which does not require the immersion in a water bath or other fluid bath, was adopted for the ageing of the specimens. The experiments show that the burst pressure can be strongly affected by the ageing period. Tensile tests also have been performed, showing a brittle-elastic behaviour. For this particular composite, the stiffness of the tensile specimen is not significantly affected by the ageing time, but the ultimate tensile stress (UTS) is affected by the ageing time. A methodology to obtain analytic estimates of both UTS and failure pressure for a given ageing time is proposed. In order to obtain a lower bound of the failure pressure at a given operation time, besides the pipe geometry, it is only necessary to know the UTS of the composite obtained in a minimum of three tensile tests performed at different ageing times. The prediction error is less than 0.8% for the UTS and is less than 25% for the failure pressure.     

Strength characterization of MgO-partially stabilized zirconia

The flexural strength of MgO-partially stabilized zirconia was evaluated as a function of temperature (20–1300 °C in air environment), applied stress and time. The indentation-induced-flaw technique did not produce well-defined symmetrical cracks of controlled size, whose length (on the tensile surface) or depth (on the fracture face) can be measured unambiguously, and therefore it should not be used for measuring fracture toughness. The sudden decrease in fracture strength at moderately low temperatures (200–800 °C) is believed to be due to stability of the tetragonal phase and relative decrease in the extent of the stress-induced martensitic phase transformation of the tetragonal to monoclinic phase. Flexural stress rupture testing at 500–800 °C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature. Stress rupture testing at 1000 °C and above at low applied stress levels showed bending of specimens, indicating the onset of plasticity or viscous flow of the glassy phase and consequent degradation of material strength.     

Strength characterization of yttria-partially stabilized zirconia/alumina composite

The flexural strength of yttria-partially stabilized zirconia/alumina composite in the sintered and hot isostatically pressed condition (Super PSZ) was evaluated as a function of temperature (20–1300°C in air environment), applied stress and time. Failure was essentially governed by the presence of processing defects such as zirconia or alumina agglomerates. The sudden decrease in fracture strength at relatively low temperatures (400–600°C) is believed to be due to the stability of the tetragonal phase and relative decrease in the extent of the stress-induced martensitic phase transformation of the tetragonal to monoclinic phase. Flexural stress rupture testing at 300–1000°C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature. Stress rupture testing at 1000°C at low applied stress levels showed bending of specimens, indicating the onset of plasticity or viscous flow of the glassy phase and consequent degradation of material strength.     

Fracture mechanics global approach concepts applied to creep slow crack growth in a medium density polyethylene (MDPE)

Creep fracture by slow crack growth is studied in a medium density polyethylene at 60 °C and 80 °C. Whereas elastic-plastic fracture mechanics load parameters fail to provide a unique temperature-independent correlation, that of the fracture mechanics for creeping solids C^∗ is proved to be relevant since this parameter correlates very well with the time to failure. Correlation established on both full notched creep tensile and double edge notched tensile tests was validated on cracked gas-pipe samples tested under hydrostatic pressure, extending the use of time to failure versus C^∗ diagram to predict lifetime of engineering components.     

Influence of welding technique and temperature on fatigue properties of steel deposited with Co-based alloy hardfacing coating

This paper aimed to investigate the influence of welding technique and temperature on fatigue properties of heat-resistant steel with hardfacing coatings. The plasma transferred arc welding (PTAW) and the oxy-acetylene welding (OAW) were employed. The rotating bending fatigue tests were performed at room temperature (RT) and 500 °C. It was found the fatigue strength with 10⁷ cycles of OAW specimens at RT was lower than that of PTAW ones, possibly resulting from the higher amount of carbides in OAW coatings. The fatigue strength with 10⁷ cycles at 500 °C was higher than that at RT, which was mainly due to the interface delamination and the increase in ductility with increasing temperature. Two failure modes, i.e. the coating failure mode at RT and the coating-interface failure mode at 500 °C, were proposed. The fatigue life was predicted with the model considering the characteristic geometry of inclusions, the average hardness of coating, and the effect of external stress.     

Yield behavior and failure response of an aliphatic polyketone terpolymer subjected to multi-axial stress states

The yield behavior of an engineering thermoplastic under biaxial stress states has been investigated. The material considered is an aliphatic polyketone terpolymer. Multiaxial testing was performed on thin-walled hollow cylinders at four different temperatures and three strain rates. Various stress states were applied in order to develop failure envelopes. Within each envelope, the nominal strain rate along the octahedral shear plane, ^oct, was held constant. These tests were performed at 0, 20, 50 and 80°C at ^oct = 0.05 min^–1. At 20°C, samples were also tested at ^oct = 0.005 and 0.5 min^–1. Below the T^g of 12°C, failures in all stress states investigated, except axial compression, were brittle. At temperatures of 50 and 80°C, all failures were ductile. At 20°C, both ductile and brittle failures were observed. Although the rate affected the yield strength of the material, it had little effect on the mode of failure. In contrast, the temperature had a significant effect on the yield strength and mode of failure. While the effect of strain rate on yield strength was greater in the hoop direction than axial, the opposite was true for the effect of temperature. It was also observed that the state of stress played a significant role in the material failure.     

Assessment of notched tubular ferritic oxide-dispersion-strengthened components subjected to multiaxial creep loading at 1100°C

Notched tubular components of a ferritic oxide-dispersion-strengthened material, MA 956, were subjected to multiaxial creep loading at 1100°C by applying a constant internal pressure. The components proved to be extremely insensitive to circumferential notches of up to 80 percent of the wall thickness due to the high axial creep rupture strength of this material. However, the components were sensitive to both externally and internally axially aligned notches, and displayed similar stress rupture behaviour but consistently longer rupture lives than plane components at the same ligament stress level. Failure was found to be due to strain controlled cavitation in the ligament rather than as a consequence of creep crack growth from the notch. A direct current/potential drop method was shown to provide a reasonable indication of the development of cavitation in these tests. It is shown that the low ductility failure of notched MA 956 components is best described by a creep fracture mechanism rather than by fracture mechanics.     

Creep rupture of lead-free Sn-3.5Ag and Sn-3.5Ag-0.5Cu solders

The aim of this study is to investigate the creep rupture behavior of lead-free Sn-3.5Ag and Sn-3.5Ag-0.5Cu solders at three temperatures ranging from room temperature (RT) to 90 °C, under a tensile stress range of σ/E=10⁻⁴ to 10⁻³. The ultimate tensile strength (UTS) and creep resistance were found to be decreased with increasing temperature for each given lead-free solder. Both the binary and ternary Ag-containing alloys exhibited superior UTS and creep strength to the conventional Sn-37Pb solder at a similar temperature. Due to a more uniform distribution of eutectic phases and a larger volume fraction of intermetallic compounds (IMCs), the Sn-3.5Ag-0.5Cu alloy had greater UTS and creep strength than did the eutectic Sn-3.5Ag solder at each testing temperature. The stress exponents (n) of minimum strain rate (˙ε_min) were decreased from 7 and 9 at RT to 5 and 6 at 60 and 90 °C, for the binary and ternary lead-free alloys, respectively. Fractography analyses revealed typical rupture by the nucleation and growth of voids/microcracks at IMCs on the grain boundaries. Both Monkman-Grant and Larson-Miller relationships showed good results in estimating the rupture times under various combinations of applied stress and temperature. A model, using a term of applied stress normalized by Young’s modulus, was proposed to correlate the rupture times at various temperatures and could explain the rupture time data reasonably well for the given two lead-free solders.     

Phenomenology of fracture in sintered alpha silicon carbide

Crack propagation mechanisms in a sintered alpha silicon carbide were studied as a function of initial flaw size, temperature, loading rate and applied stress. Surface cracks of controlled size were introduced using the microhardness indentation-induced-flaw (IIF) technique. At room temperature, the fracture stress was found to depend on initial crack size according to the Griffith relationship and extrapolation of the data indicated that processing flaws of 20 to 40m were strength controlling. The flexural strength was found to be independent of temperature (20 to 1400° C) and the fracture faces did not show the presence of subcritical crack growth (SCG). Preliminary results from stress rate testing also failed to show the presence of SCG in tests made at 1200° C in air. In contrast, flexural stress rupture tests carried out at 1200 and 1300° C in air using precracked specimens indicated the materials susceptibility to time-dependent deformation and showed the presence of SCG. Fractographic evidence for transgranular crack propagation during fast fracture (catastrophic failure) and intergranular crack propagation during SCG is presented.     

Temperature and Rate Effects on GRP Tubes Under Tensile Hoop Loading

A comprehensive study was undertaken to characterise glass fibre reinforced plastic (GRP) tubes at different temperatures and strain rates. The tests were performed on tubes of 25°, 55° and 75° winding angle. The tubes were burst under internal radial pressure with minimum end constraints. Two separate rigs were used, one for the static and the other for the dynamic tests. The tests were carried out at three temperatures; –46°C (low temperature), +20°C (room temperature) and +70°C (high temperature). For each test the internal pressure and the strains in both circumferential and longitudinal directions were recorded on suitable digital processing equipment. For a particular batch of tubes tested at three different temperatures, there is in general a decrease in hoop strength with increasing temperature during quasi-static tests. The use of a non-structural liner during such tests led to an increase in ultimate hoop strain of 55° tubes, especially at high temperature. The corresponding increase in ultimate hoop strain was markedly less in the case of 75° and almost negligible in the case of 25° tubes. Testing the tubes at high strain rates resulted in substantial increases in burst strength and ultimate hoop strain as compared with the quasi-static and low strain rate values. The mode of failure of 75° tube is a catastrophic fibre breakage under all test conditions. The mode of failure of 55° tube is a combination of weeping and fibre failure. The 25° tubes are characterised by matrix failure, which is very severe at high strain rates.     

Fatigue crack growth of filament wound GRP pipes with a surface crack under cyclic internal pressure

In this study, fatigue damage behavior of (±75₃) filament wound composite pipes with a surface crack under alternating internal pressure was investigated. The specimens were tested at room temperature and exposed to open ended fatigue tests in which the pipe can be deformed freely in the axial direction. The tests were carried out in accordance with the ASTM D-2992 standard. The alternating internal pressure was generated by conventional hydraulic oil. The low cycle tests were performed with 0.42 Hz frequency and R = 0.05 stress ratio. Glass reinforced polymer pipes (GRP) are made of E-glass/epoxy and have (±75₃) configuration. Surface cracks were machined in the axial direction of the pipes which have depth-to-thickness ratios a/t = 0.25–0.38–0.50 and depth to length ratio of a/c = 0.2. Tests were performed at three different loads of 50%, 40%, and 30% of ultimate hoop stress strength of unnotched pipes. The failure behavior of GRP pipes during the test was observed and fatigue test results were presented by means of (S–N) curves and delamination damage zone area-cycle (A–N) curves.     

Progressive fatigue failure behavior of glass/epoxy (±75)2 filament-wound pipes under pure internal pressure

Fatigue failure behaviors of filament-wound composite pipes under pure internal pressure were investigated. The filament-wound pipes are made of E-glass/epoxy and have four layers which have ±75° winding angle. The fatigue tests have been done in accordance with ASTM D-2992, which stipulate 0.42 Hz frequency and R = 0.05 stress ratio. Tests have been performed at different load levels from 30% to 70% of ultimate tangential strength of the pipe, the damage progression such as whitening, leakage and final failure have been observed, and S–N curves of these damages were obtained.     

Assessment of the Critical State of the Polyethylene Pipe in the Area of the Mechanical Metal–Polymer Joint at Low Climatic Temperatures

This paper analyzes the distributions of shearing forces arising in the area of the mechanical joint of polyethylene and metal pipes. The value of these forces at low climatic temperatures (down to –60°C) is determined. The results of the theoretical analysis for isothermal conditions permit estimating the working capacity of plastic under different conditions of joints with metal pipes.     

Einfluß des Reinheitsgrades,der Kaltverformung und des Schweißens auf das Zeitstandverhalten von X20 CrMoV 12 1-Rohrstahl bei 550 °C

Influence of purity, cold work and welding on creep rupture behaviour of X20 CrMoV 12 1 tubes at 550 °C In the water steam circuit of the German Thorium High Temperature Reactor (THTR 300) a high tempered martensitic X20 CrMoV 12 1 steel with low P and S contents and improved impact toughness was used at the first time for live steam and reheater pipes. There were similar and dissimilar weldings (with austenitic steel alloy 800). Small pipes were cold bended. Therefore ABB started a material testing programm supported by BMFT. Creep rupture tests at 550 °C til 60 000 h led to following results:

• Creep rupture strength and 1 % creep strain limit of the tougher grade lie in the lower half of the commercial steel scatterband.
• Under service creep loadings tertiary creep begins after a life-time of about 60% resp. a plastic elongation of about 1 %.
• The creep damage can be described by microstructural classes.
• Only for small stress ranges there are nearly constant creep stress exponents which are independent from the melts.
• Low loading speeds reduce static elastic moduls.
• Cold work reduces creep rupture strength and deformation.
• Under stress transverse to welding joints the design according to creep rupture strength of the base material must include a reduction factor. The fine grained part of the HAZ is the weakest area.

With the results of long-time examinations the design of future HTR-plants, fossil-fired power stations, and chemical plants may furtherly be optimized.     

Notchology-the effect of the notching method on the slow crack growth failure in a tough polyethylene

The effect of various notching methods on the life time of slow crack failure at 80 °C for a tough gas pipe grade of polyethylene was investigated. The standard notching procedure involved pressing an ordinary razor blade into the single-edge notched tensile specimen at a rate of 50 m min^–1 at room temperature. The other notching methods involved using a sharper razor blade, cooling in liquid nitrogen, pre-cracking by fatigue, slicing with a scalpel, or using a rotary cutter. The standard procedure gave a life time of 28000 min under a stress of 2.4 MPa; the sharper blade, cooling in liquid nitrogen, and pre-cracking by fatigue gave an equivalent life time. Slicing with a scalpel or a rotary cutter provided a much longer life time.     

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.     

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.