Correlation of stepwise fatigue and creep slow crack growth in high density polyethylene

The kinetics and mechanism of slow crack growth in fatigue and creep of high density polyethylene were studied. The relationship between fatigue and creep was examined by varying the R-ratio (the minimum/maximum loads in the fatigue loading cycle) in the tensile mode such that loading ranged from mainly dynamic (R = 0.1) to static (R = 1.0, creep test). The stepwise crack propagation mechanism characteristic of long-term failures in polyethylene was observed for all loading conditions studied. Fatigue fracture kinetics allowed for extrapolation to the case of creep failure, which suggested that short-term fatigue testing can be used to predict long-term creep fracture properties. The size of the craze damage zone ahead of the arrested crack tip was controlled only by the mean stress, however the lifetime of the zone was determined by both the maximum stress and the mean stress. Crack growth rate was related to the maximum stress and the mean stress by a power law relationship, which described crack growth over the entire range of loading conditions studied.     

Correlation of fatigue and creep slow crack growth in a medium density polyethylene pipe material

The relationship between slow crack propagation in creep and fatigue in a medium density polyethylene pipe material was studied by increasing the R-ratio (defined as the ratio of minimum to maximum stress in the fatigue loading cycle) from 0.1 to 1.0 (creep). The study included characterization of the effects of R-ratio and temperature (21 to 80°C) on the mechanism and kinetics of slow crack propagation. With increasing R-ratio and decreasing temperature, the fracture mode changed from stepwise crack propagation, i.e. crack growth by the sequential formation and breakdown of a craze zone, to a quasi-continuous mode of crack growth through the preexisting craze. Despite the change in fracture mode, the damage zone, as characterized by the length of the main craze, shear crazes, and crack tip opening displacement, followed the same dependence on loading parameters, and crack growth rate followed the same kinetics. Crack growth rate (da/dt) was related to the maximum stress intensity factor K_{I, max} and R-ratio by a power law relationship (da/dt) = BK⁴ _{I, max}(1 + R)^–6. Alternatively, crack growth rate was expressed as (da/dt) = BK _I ⁴ (t)_T() with a creep contribution B‹K _I ⁴ (t)›_T, calculated by averaging the known dependence of creep crack growth rate on stress intensity factor K_I over the period T of the sinusoidal loading curve, and a fatigue acceleration factor () that depended on strain rate only. The correlation in crack growth kinetics allowed for extrapolation to creep fracture from short-term fatigue testing. The temperature dependence of crack growth rate was contained in the prefactors B and B. A change in slope of the Arrhenius plot of B at 55°C indicated that at least two mechanisms contributed to crack propagation, each dominating in a different temperature region. This implied that a simple extrapolation to ambient temperature creep fracture from elevated temperature tests might not be reliable.     

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.     

Application of strain energy density factor to fatigue crack growth analysis

The rate of crack propagation is postulated to be a function of the strain energy density factor range, which is used to correlate the subcritical crack growth data of several different materials. Subsequently, this concept is applied to predict crack growth due to spectrum loads.     

The fracture mechanics of slow crack growth in polyethylene

The following theoretical equation was obtained for the rate of initiation fx- of slow crack growth in polyethylene:% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbnL2yY9% 2CVzgDGmvyUnhitvMCPzgarmqr1ngBPrgitLxBI9gBaerbd9wDYLwz% YbItLDharuavP1wzZbItLDhis9wBH5garqqr1ngBPrgifHhDYfgasa% acOqpw0le9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% aabaqaciGacaGaaeqabaWaaqaafaaakeaadaWfGaqaaiabes7aKbWc% beqaaiaac6caaaGccqGH9aqpdaWcaaqaaiabeo8aZnaaBaaaleaaca% WG5baabeaacaWGlbWaaWbaaWqabeaacaaI0aaaaSGaaiikaiaaigda% cqGHsislcqaHZoWzdaahaaadbeqaaiaaikdaaaWccaGGPaWaaWbaaW% qabeaacaaIYaaaaaGcbaGaeq4TdGMaamizaiaadweadaahaaWcbeqa% aiaaikdaaaGccqaHdpWCdaqhaaWcbaGaam4yaaqaaiaaikdaaaaaaa% aa!5AB3!\[\mathop \delta \limits^. = \frac{{\sigma _y K^4 (1 - \gamma ^2 )^2 }}{{\eta dE^2 \sigma _c^2 }}\] where = applied stress, K = stress intensity, = Poisson's ratio, E = Young's modulus, _c = stress to produce a craze, _y = yield point, d = primordial thickness of the craze and = the intrinsic viscosity of the fibrils of the craze. The dependence of % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbnL2yY9% 2CVzgDGmvyUnhitvMCPzgarmqr1ngBPrgitLxBI9gBaerbd9wDYLwz% YbItLDharuavP1wzZbItLDhis9wBH5garqqr1ngBPrgifHhDYfgasa% acOqpw0le9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% aabaqaciGacaGaaeqabaWaaqaafaaakeaadaWfGaqaaiabes7aKbWc% beqaaiaac6caaaaaaa!4668!\[\mathop \delta \limits^. \] on K agrees with the experimental data. The experimental values of % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbnL2yY9% 2CVzgDGmvyUnhitvMCPzgarmqr1ngBPrgitLxBI9gBaerbd9wDYLwz% YbItLDharuavP1wzZbItLDhis9wBH5garqqr1ngBPrgifHhDYfgasa% acOqpw0le9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% aabaqaciGacaGaaeqabaWaaqaafaaakeaadaWfGaqaaiabes7aKbWc% beqaaiaac6caaaaaaa!4668!\[\mathop \delta \limits^. \] vary by a factor of 10⁷ depending on the type of polyethylene. This large variation in % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbnL2yY9% 2CVzgDGmvyUnhitvMCPzgarmqr1ngBPrgitLxBI9gBaerbd9wDYLwz% YbItLDharuavP1wzZbItLDhis9wBH5garqqr1ngBPrgifHhDYfgasa% acOqpw0le9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% aabaqaciGacaGaaeqabaWaaqaafaaakeaadaWfGaqaaiabes7aKbWc% beqaaiaac6caaaaaaa!4668!\[\mathop \delta \limits^. \]% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbnL2yY9% 2CVzgDGmvyUnhitvMCPzgarmqr1ngBPrgitLxBI9gBaerbd9wDYLwz% YbItLDharuavP1wzZbItLDhis9wBH5garqqr1ngBPrgifHhDYfgasa% acOqpw0le9v8qqaqFD0xXdHaVhbbf9v8qqaqFr0xc9pk0xbba9q8Wq% Ffea0-yr0RYxir-Jbba9q8aq0-yq-He9q8qqQ8frFve9Fve9Ff0dme% aabaqaciGacaGaaeqabaWaaqaafaaakeaadaWfGaqaaiabes7aKbWc% beqaaiaac6caaaaaaa!4668!\[\mathop \delta \limits^. \] is directly related to intrinsic viscosity which evolved from the theory.     

Environmental dynamic fatigue crack propagation in high density polyethylene: an empirical modelling approach

Part of a programme to study environmental dynamic fatigue crack propagation in engineering thermoplastics is presented. High density polyethylene has been studied in terms of its stress cracking properties under dynamic loading conditions in detergent. This work complements previous investigations on stress cracking in detergent under static loading. The stress cracking was found to vary according to loading conditions and in conclusion, a dependence of crack growth rate on test frequency, amplitude and level of stress intensity factor is reported. An emperical model describing environmental fatigue crack propagation is proposed which adequately represents the experimental results of high density polyethylene, Nylon 66 and which, it is suggested, may be suitable for use with other polymers.     

Scatter bands in creep and fatigue crack growth rates in high temperature plant materials data

As a part of the European Commission supported project BE 1702: ‘HIDA’ Creep, creep-fatigue and high temperature fatigue crack growth data for five high temperature plant steels were accessed from a number of published and unpublished sources. These large sets of data were reviewed, and re-analysed where necessary, and plotted in terms of various crack growth rate correlating parameters. Thus limits of scatter bands and mean and upper 95% confidence limit creep and fatigue crack growth correlations are proposed. The present work covers a wide range of variables such as test specimen geometries, sizes, loading conditions and temperatures. Therefore, the correlations proposed are considered universal. However, it is envisaged that these correlations will be refined in future by enlarging the database and exploring the effect of the variables described above. The five materials studied are AISI 316 stainless, 2.25CrlMo, P91, 1CrMoV (forged), and 1CrMoV (cast) steel.     

The crack initiation and growth under high temperature creep, fatigue and creep-fatigue multiplication

This article concerns some of our recent studies on the crack initiation, early stage crack growth and its subsequent crack growth under high temperature creep, fatigue and creep-fatigue multiplication. The criteria for these and some new ideas are proposed. For instance, the relative notch opening displacement (RNOD) criterion for the crack initiation and the Q^* parameter for the crack growth are critically reviewed. Early stage crack growth and its subsequent crack growth as affected by notch tip acuity were studied. The behaviour of the tail part in the log da/dN vs log C^* curve has been attempted to explain in terms of the curve of the creep behaviour and of the crack length against time. Furthermore it was proposed that early stage crack growth, say, the so-called first stage crack growth in terms of log da/dN vs log K curve may be characterized by the parameter different from those for the so-called second stage crack growth.     

High-temperature fatigue crack growth in Inconel 718 subjected to high strain amplitudes

Fatigue crack growth experiments in Inconel 718 subjected to high strain amplitudes at 650 °C have been conducted. In the study the effects of load amplitude, ratio and frequency have been investigated. It was found that crack growth is a mixture of cyclic and time dependent mechanisms, depending on the load frequency. The load frequency was also found to have a strong influence on the crack growth rate. Also, crack closure was found to play an important role. By using an effective J‐integral and including a frequency compensation term it was possible to summarize crack growth data into an empirical life prediction law, which seems to be in reasonable agreement with data from other studies.     

Effect of liquid oilfield-related media on slow crack growth behavior in polyethylene pipe grade materials

Polyethylene, as non-polar material, shows a high affinity especially to liquid non-polar aromatic and aliphatic hydrocarbons, and liquid hydrocarbons (LHC) to a certain extent migrate into the bulk material by sorption, leading to material plasticization (i.e., drop in modulus and yield stress). This paper aims to study the crack growth mechanism and failure behavior of commercial pipe grade materials when exposed to deionized water or LHC (90/10 wt% i-octane/toluene) under the simultaneous application of cyclic loads. The results of the cyclic crack growth experiments with three PE 100 pipe grades, using cracked round bar (CRB) specimens and performed at two different temperatures (35 °C and 60 °C), are compared in terms of the specimen lifetimes, and the micro-modes and kinetics of failure by referring to concepts of fracture mechanics. Most importantly, while crack advance is preceded by crack-tip crazing in water, shear yielding takes place at crack-tips in the LHC environment.     

The initiation of slow crack growth in linear polyethylene under single edge notch tension and plane strain

The kinetics and microstructural changes associated with the initiation of slow crack growth in PE were measured. The initiation process consists of an instantaneous deformation zone which grows at a constant velocity until the beginning of fracture. The velocity of the damaged zone accelerates when the fibril fracture begins at the root of the initial notch. It was found that the initial velocity of the deformation zone depended on stress to about the 4th power and had an activation energy of about 100 kJ mol^–1; these results are about the same as those found by Chan and Williams for the crack growth velocity. It is concluded that both crack initiation and crack growth are governed by the same fundamental process, notably fibril thinning.     

Effect of crystalline morphology on fatigue crack propagation in polyethylene

An investigation of the influence of crystalline morphology on fatigue crack propagation (FCP) resistance in a slightly branched polyethylene is presented. Various thermal histories have been utilized to generate samples with different crystalline microstructures and the samples were characterized thoroughly using standard methods. Estimation of tie molecule densities was obtained from measurements of brittle fracture stress. Differences in FCP behaviour for the quenched and annealed samples were shown to be dictated by a competing effect between the degree of crystallinity and tie molecule density. Further, larger spherulite size and distribution appeared to have a deleterious effect on fatigue properties. In general, crystalline microstructure is shown to have a significant influence on fatigue crack propagation behaviour.     

The relationship of the initiation stage to the rate of slow crack growth in linear polyethylene

The rate of notch opening was observed in single-edge notched tension specimens of linear high-density polyethylene under plane strain conditions as a function of applied stress and notch depth. The initial rate of notch opening was 22K ^4.3 m min^–1 (where units ofK are MN m^–3/2). The initial rate of notch opening was constant until the opening at its root was 25 to 30m for all stresses and notch depths. The accelerating part of the notch opening against time curve and the time to failure could be predicted from the initial rate of notch opening. It is concluded that the same mechanism governs the initiation stage and the subsequent crack growth rate.Visiting Scholar from the University of Science and Technology of China, Hefei, China.     

Effect of specimen size on fatigue crack growth rate in AISI 4340 steel

An investigation has been carried out to study the influence of specimen size parameters (thickness, with and aspect ratio) on fatigue crack growth rate. Compact tension specimens with a TL orientation, prepared from aircraft quality AISI 4340 steel and heat treated to a yield strength level of 1000 MPa, were used. All testing was done at a constant δK level. The investigation demonstrates that specimen thickness and width have no significant influence on fatigue crack growth rate for AISI 4340 steel. On the other hand, fatigue crack growth rate was found to increase marginally at high aspect ratios (a/W0.55). Paris constants C and m were also evaluated.     

A new parameter for prediction of creep crack growth rate at high temperature

By analysis based on a series of experimental data obtained by continuous observations using high temperature microscope during the creep test without interruption in vacuum of 10^?5 mm Hg for the purpose of the crack length measurements, a new mathematical equation for prediction of high temperature creep crack growth rate has been proposed in terms of disposable parameters, that is $\text{α}\text{a}{}_{\mathrm{<mtext>eff</mtext>}}\text{σ}{}_{\mathrm{g}}\text{,σ}{}_{\mathrm{g}}$ and temperature for 304 stainless steel within the range of $\text{α}{}_{\mathrm{g}}$ and temperature concerned. It can be seen that it is the best one to fit the experimental data among any other formula proposed hitherto.The new parameter proposed herein

$\text{8.48 × 10}{}^3\text{t}\text{log}{}_{10}\text{α}\text{α}{}_{\mathrm{<mtext>eff</mtext>}}\text{α}{}_{\mathrm{<mtext>g</mtext>}}\text{4.66 × 10}{}^2\text{+ 5.46}\text{log}{}_{10}\text{α}{}_{\mathrm{<mtext>g</mtext>}}$

where

$\text{α = 1.98 +0.36}\text{a}\text{w}\text{? 2.12}\text{a}\text{w}{}^2\text{+ 3.42}\text{a}\text{w}{}^3\text{, a≦0.7w}$

may be used for characterizing the creep crack growth rate just similar as Larson-Miller parameter for the creep life.