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     

Abnormality in using cyclic fatigue for ranking static fatigue induced slow crack growth behavior of polyethylene pipe grade resins

Slow crack growth behavior in polyethylene pipe grade resins were studied using both static fatigue (stress-rupture) and cyclic fatigue tests. This was done to better understand the applicability of cyclic fatigue in the prediction of slow crack growth ranking determined from the static fatigue test. In all polyethylene pipe grade resins tested at 80 °C, reduced crack growth failure times were exhibited when the cyclic fatigue test was employed. However, when applied to rank the resins through their slow crack failure times, the cyclic fatigue results did not always confirm those obtained from the static fatigue test. That is, in some cases, a resin with higher slow crack resistance ranking (longer failure times) than another resin in static fatigue exhibited lower ranking (shorter failure times) in the cyclic fatigue test. This abnormality of reversal in ranking is not a general observation but does occur. Based on the data obtained so far, when resins with smaller differences between static fatigue and cyclic fatigue slow crack growth failure times are compared with those resins having larger differences, the chances of correctly predicting the ranking obtained from static fatigue using cyclic fatigue tend to decrease. Hence, it is suggested that one needs to practice caution when using cyclic fatigue to predict the static fatigue ranking of resins for slow cracking resistance. Some insight into the cause of such abnormality is discussed with reference to creep-fatigue interactions.     

Neck propagation in polyethylene

Intrinsic true stress–true strain response was evaluated at room temperature for three linear polyethylene samples deformed in conventional tensile tests. It was observed that high crystallinity is associated with a low rate of strain hardening that results in a sharp neck and a large drop in nominal stress. The maximum and minimum deformation loads are accounted for by Considère's analysis of neck initiation and stabilization, respectively. Following stabilization, neck propagation occurs at a load or nominal stress that is lower than the yield stress. The jump analysis of Ericksen and Hutchinson/Neale predicts steady state neck propagation stresses that are in very good (ca. 10%) agreement with experiment. Although the jump analysis is done in terms of uniaxial stress, the actual value of the propagation stress is established by the triaxial stress state in the neck shoulders. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2081–2091, 2004     

The critical molecular weight for resisting slow crack growth in a polyethylene

An ethylene-hexene copolymer was fractionated into five fractions and the density of short-chain branches was measured for each fraction. The slow crack growth behavior was measured on each fraction by sandwiching the small amount of fractionated resin of about 0.2 g between polyethylene grips. The resistance to slow crack growth was negligible for the three fractions whose M_w was less than 1.5 × 10⁵. For the fourth fraction with M_w greater than 1.5 × 10⁵, the resistance to slow crack growth was very high, being greater than that for the whole resin even though its density of short-chain branches was less than that of the whole resin. It is concluded that a molecular weight greater than 1.5 × 10⁵ is required to create the number of tie molecules that is necessary to produce a high resistance to slow crack growth in this particular copolymer. © 1996 John Wiley & Sons, Inc.     

The effect of annealing on slow crack growth in an ethylene-hexene copolymer

A quenched ethylene-hexene copolymer was annealed in the temperature range of 86 to 127°C. The morphological changes were monitored by differential scanning calorimetry (DSC) and density. The slow crack growth resistance tested at 80°C was a maximum at an annealing temperature of 113°C and a minimum of 123°C. The lifetimes can be varied by more than a factor of 20 depending on the thermal treatment. The increase in slow crack growth resistance between 86 and 113°C is attributed to an increase in the strength of the crystals by becoming more perfect and to the conversion of loose tie molecules into taut tie molecules. The decrease in strength between 113 and 123°C is attributed to the decrease in tie molecules when a large fraction of the as-quenched crystals begin to melt.     

Effect of γ-irradiation on slow crack growth in a copolymer of polyethylene

The effect of γ-irradiation on slow crack growth (SCG) in a medium density polyethylene (MDPE) was measured and compared with behavior of high density polyethylene (HDPE) and a recrystallized HDPE (RCHDPE). The three materials exhibited the same dependence on dose up to 3 Mrd. The HDPE became brittle above 50 Mrd. The resistance to SCG of MDPE and RCHDPE increased very rapidly with dose above 3 Mrd, until at 50–80 Mrd their resistance to SCG became extraordinarily high. This high resistance to SCG was accompanied by a transition from crazing to shear deformation at the root of a notch. It was found that for the same concentration, crosslinks are more effective than short chain branches for increasing resistance to SCG. © 1998 John Wiley & Sons, Inc. J. Polym. Sci. B Polym. Phys. 36: 2349–2354, 1998     

In Situ tensile tests to analyze the mechanical response,crack initiation,and crack propagation in single polyamide 66 fibers

Single fiber mechanical testing is challenging to perform, especially when the diameter is as small as tens of micrometers. For this reason, real‐time observations of crack propagation mechanisms have been rarely been investigated experimentally. This article presents experimental and numerical investigations of fracture of monofilamentary high performance polyamide 66 fibers. Their engineering stress–strain curves are compared. The mechanisms of failure starting from crack initiation until the final brittle fracture are studied by in situ tests in Scanning Electron and optical microscopes. Finite element modeling at the individual fiber scale has been performed in three‐dimensional (3D), as a reverse engineering method. The compliance method was used to determine the crack depth that triggers the final failure. The fracture toughness was numerically determined using the J‐integral concept, accounting for the geometry of the crack front (3D) together with plastic deformation. 3D meshes were designed especially from postmortem observations. The average value deduced was about 47 ± 7 kJ m^?2, which will be discussed with other estimates using linear elastic fracture mechanics. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 680–690     

Influence of electron-beam irradiation on plasticity-controlled and crack-growth-controlled failure in high-density polyethylene

In the present study, the influence of electron-beam irradiation on plasticity-controlled and crack-growth-controlled failure in high-density polyethylene (HDPE) is investigated and the effect of both molecular weight distribution (MWD) and short chain branching (SCB) content are taken into account. Size exclusion chromatography (SEC) is used to study the evolution of the MWD of the sol fraction as a function of irradiation dose. Here, it is seen that chains shorter than the percolation threshold (5 kDa) are largely unaffected by electron beam radiation, while the fraction of longest chains (M > 300 kDa) is nearly entirely incorporated into the cross-linked network. Both yield stress and Young's modulus increased with irradiation dose, where the magnitude of the increase appears to be connected to the gel fraction. The (fatigue) crack growth kinetics of the grades changed relatively little with irradiation dose, which is unexpected. Furthermore, convergence of the crack growth kinetics parameter to a narrow range of values could be observed for the investigated grades at relatively high gel fractions. This would imply that the crack growth kinetics become increasingly independent of the MWD upon irradiation cross-linking, which could be attributed to a shift in the underlying crack growth mechanism from chain slip to chain scission.     

The dependence of slow crack growth in a linear polyethylene on test temperature and morphology

The slow crack growth behavior of a linear polyethylene with different morphologies was studied by using three point bending with a single edge notched specimen at testing tem-peratures from 30 to 80°C. The morphology was varied by annealing the quenched material at temperatures from 86°C to 135°C. It was found that at test temperatures of 60°C or less, the changes in failure time with annealing temperature are very similar to the change in density with a maximum at 130°C. At testing temperatures above 60°C, the relationship of between failure time and annealing temperature is altered when the test is in the range of the α transition temperature. These results indicate that with respect to slow crack growth in the case of a homopolymer the strength of the crystals is relatively more important than the number of tie molecules. © 1993 John Wiley & Sons, Inc.     

The effect of cold rolling on crack propagation behavior in high-density polyethylene

Crack propagation behavior in HDPE was studied. The preliminary orientation of the polymer, which is deformed in its isotropic state via necking and breaks down at the neck propagation stage, improves the crack resistance and ductility of the material. The critical crack opening in preoriented HDPE samples dramatically increases at relatively low draw ratios of cold rolling while the speed of transverse crack propagation decreases.     

Effects of γ-irradiation on the morphology and slow crack growth in high-density polyethylene

The effects of γ-irradiation were measured in a HDPE and in the resin after it was recrystallized. The fracture mode of the initial material transformed from crazing to complete brittle failure at a critical dose. The failure mode of the recrystallized material transformed from crazing to shear deformation, which produced an extremely long failure time, and finally, at a higher dose, its fracture became brittle. The relationship between morphology and slow crack growth is presented where crosslinking was the major factor. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1211–1218, 1998     

The influence of temperature and frequency on fatigue crack propagation in polymers

Summary Centrally notched plate specimens of PMMA and PC were cycled in tension between constant stress intensity limits and crack growth was monitored against the number of cycles. A range of frequencies between 0.1 IIz and 100 Hz, and of temperatures between – 70 °C and + 40 °C was investigated. It was found that the cyclic crack growth decreased with decreasing temperature and/or increasing frequency. A linear relationship between crack length and number of cycles was observed. Application of the cyclic crack propagation law proposed byArad-Radon-Culver, namelyd(2a)/dN = ⁿ where is (K _max ² -K _min ²) and K_max andK _min are the respective values of maximum and minimum stress intensity factors, indicates that the indexn is unaffected by temperature or frequency variations in the investigated range (II regime of fatigue crack propagation) and the law would be a valid model if the term is suitably determined.

---

Zusammenfassung Ermüdungsversuche im Rahmen der linearelastischen Bruchtheorien wurden ausgeführt auf zentralgekerbten Platten von PMMA und PC, sowie Rißwachstum als Funktion der Belastungszahl gemessen. Frequenzen von 0.1 Hz bis 100 Hz und Temperaturen von – 70 °C bis 40 °C zeigten, daß sich das Rißwachs-tum mit verringerter Temperatur und/oder erhöhter Frequenz verringerte. Eine lineare Abhängigkeit der Rißlänge von Belastungszahl wurde festgestellt. Die Benutzung der Rißausbreitung-Analyse von Arad-Radon-Culverd(2a)/dN =ⁿ, wo ist (K _max ² -K _min ²), zeigt, daß der Indexn nicht beeinflußt ist von Temperatur oder Frequenzwechsel in dem untersuchten Bereich (das sogenannte zweite Regime des Ermüdungswachstums). Wenn man die Konstante festgestellt hat, dann ist die Arad-Radon-Culver-Methode direkt benutzbar.

---

---

With 12 figures     

The mechanism of fatigue failure in a polyethylene copolymer

The fatigue behavior of an ethylene-hexene copolymer was investigated. The effects of R, frequency, relative times under the maximum and minimum stress, and waveform were measured. The phenomenological aspects were related to the microscopic aspects of the failure process. The maximum stress produces damage by disentangling the molecules in the fibrils of the craze and the minimum stress produces damage by bending the fibrils. The net damage, which is a product of these two damage processes, has been represented by a simple equation which accounts for the phenomenological observations.     

Fatigue crack propagation and related failure in modified,andhydride-cured epoxy resins

The fatigue crack propagation (FCP) of neat and modified, anhydridecured epoxy resin (EP) was studied in tensile-tensile mode at ambient temperature. As modifiers, liquid carboxyl-terminated acrylonitrile-butadiene (CTBN) and silicon rubber (SI) dispersions were used. The latter modifier in a defined particle size distribution was produced by a special latex technology, whereas the former developed in situ in the EP by phase separation during curing. The dispersion-type morphologies of the EPs were characterized by using polished sections and viewing them in a scanning electron microscope (SEM). The resulting frequency distribution curves were compared with those analyzed from fatigue fracture surfaces. Probable failure mechanisms were also studied by SEM-fractography.Both modifiers improved the resistance to FCP by shifting the curves to higher stress intensity factor ranges (K) in relation to the reference curve determined for the neat EP-matrix. The failure mechanisms, summarized also schematically, differed basically for the various modifiers. According to this, rubber-induced cavitation and shear yielding of the matrix seemed to be dominant for CTBN, which did not affect the principal crack plane. In contrast to this, crack bifurcation, branching and, hence, a forced deviation in the fatigue crack path induced by debonded SI-particles in the EP-matrix were concluded for the SI modifier. The common use of both modifiers yielded a positive synergistic effect due to the superposition of the above failure mechanisms.Dedicated to the 60th birthday of Prof. H. H. Kausch     

Micromechanics of fatigue crack propagation in glassy thermoplastics

By the aid of the optical interference method the size of the craze zone at the crack tip has been measured during fatigue crack propagation (FCP) in two glassy thermoplastics thus giving a basis to re-examine proposed models. In contrast to previous assumptions it has been found, that in PMMA of high molecular weight crack propagation occurs only during a short interval of the loading cycle when the fibrils are stretched most severely and it is not limited by crack tip blunting; between the dimensions of the craze zone and the crack advance per cycle which is also reflected by markings on the fracture surface no simple correlation has been found. In PVC first the craze grows continuously during many loading cycles up to its final size and then the crack propagates by a jump separating the craze zone only partly. Thus at all stress intensity levels investigated the length of the final craze zone has been found to be distinctly larger than the jump spacing on the fracture surface. By aid of SEM-photography it is shown that in PVC during FCP cracking occurs by separation of fibrils instead of void coalescence.     

‘Tail’ phenomenon and fatigue crack propagation of PC/ABS alloy

The fatigue crack propagation (FCP) behavior of the alloy of polycarbonate (PC) and acrylonitrile-butadiene-styrene (PC/ABS) is experimentally investigated in this paper. An improved compliance method is employed to measure the fatigue crack length and optical and scanning electron microscopes (SEM) are used to observe the features of crack tip deformation in situ. ‘Tail’ phenomenon has been observed at the initial stage of fatigue for each specimen, which is regarded as a reflection of the transition process of accumulation of damage and plastic deformation during FCP. The law of FCP from low to high crack growth rate (10⁻⁶-10⁻³ mm/cycle) is obtained and described with Paris law. Porous or dimple features govern the fatigue crack surfaces and coarse features have been seen on the crack surfaces with higher crack growth rate, while smooth features have been observed on the crack surfaces with lower crack growth rate. A stretched band appears when the crack growth transforms from lower to higher region of FCP rate.     

Investigations of morphological changes during annealing of polyethylene single crystals

The morphological evolution of isolated individual single crystals deposited on solid substrates was investigated during annealing experiments using in situ and ex situ atomic force microscopy techniques. The crystal morphology changed during annealing at temperatures slightly above the original crystallization temperature of the crystals, far below their melting temperature. Evenly distributed cavities penetrated the crystals, and the number of cavities increased with a rising annealing temperature until the adjacent cavities coalesced. The thickness of the crystals increased during annealing at temperatures slightly above the crystallization temperature. Annealing experiments at fixed temperatures showed that the reorganization process (cavity formation and single‐crystal thickening) was fast. Depending on the annealing temperature, the final morphology was formed in seconds. This behavior suggests high chain mobility as well as a homogeneous solid‐state reorganization of the entire single crystal at low annealing temperatures. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 763–770, 2001     

Experimental and theoretical investigation of stress corrosion crack (SCC) growth of polyethylene pipes

Degradation of polymers is usually manifested in a reduction of molecular weight, increase of crystallinity in semicrystalline polymers, increase of material density, a subtle increase in yield strength, and a dramatic reduction in toughness. Stress corrosion cracking (SCC) results from strongly coupled thermo-mechano-chemical processes, and is sensitive to material composition and morphology. The individual crack propagation stage is critical in determining the lifetime of pipe. Based on author's previous works, crack layer (CL) theory model is adopted in this study to describe the individual stress corrosion (SC) crack propagation kinetics and the time interval from crack initiation to instability and break through. The effect of localized chemical degradation at the crack tip on SC crack growth kinetics is addressed. Typical SC crack growth is presented and discussed as a step-wise manner based on the proposed model. In addition, scanning electron microscopy (SEM) observation and Fourier transform Infrared spectroscopy (FTIR) analysis of failed samples obtained by accelerated SCC tests are applied to validate the proposed model. SEM is useful to identify the change of fracture mechanisms from chemically driven crack to mechanically driven crack by the formation of visible striations. FTIR analysis enables tracking of the accumulation of chemical degradation by detecting the amount of carbonyls on the crack surface. Carbonyl index is defined to compare the amount of chemical degradation quantitatively. The purpose of this paper is to continue to develop the technical theory and understanding behind SCC phenomena to facilitate all polymer pipe industries and in particular the polyethylene pipe industry to design better resins and piping systems.     

The effect of γ-irradiation on slow crack growth in polyethylene

HDPE was γ-irradiated at room temperature. The resistance to slow crack growth (SCG) was measured in single edge notched tensile specimens under constant load as a function of the dose. The resistance to SCG initially decreased to a minimum value at a dose between 0.05 and 0.10 Mrd. The minimum value was 45% less than for the undosed state. For doses greater than 0.10 Mrd, the resistance to SCG increased up to a dose of 50 Mrd, where its value had increased by a factor of 10². The gel point occurred at 1–3 Mrd. MI and the crack opening displacement exhibited maximum values at a dose of 0.1 Mrd. The behaviors of SCG, MI and crack opening displacement were consistent with the explanation that chain scission dominated for doses less than 0.1 Mrd, and cross-linking dominated at the higher doses. For doses beyond 50 Mrd, the resin became so brittle that it cracked during the loading of the specimen. Beyond the gel point the density increased from 0.9694 to 0.9716 g/cm³ at a dose of 160 Mrd. ©1995 John Wiley & Sons, Inc.