A test method for the determination of the stress cracking effects of liquid environments on thermoplastics by measurement of critical strain

The stress cracking effect of liquids on thermoplastic materials can be quantified by measuring the critical strain to cause cracking or crazing. The critical strain is ideally defined as the value of applied strain, for a given material and liquid combination, below which no cracking or crazing occurs. A method is described for the determination of critical strain using a simple straining jig. A strain gauge extensometer attached directly to the specimen is used to accurately monitor the applied strain while visual observation of the sample is used to record the time to crack or craze.Critical strain values measured by this technique are quoted for various alcohols and ketones in contact with polycarbonate and compared with literature values.     

A kinetic effect in the environmental stress cracking of polyethylene due to liquid viscosity

An Interesting kinetic effect in the environmental stress cracking (E.S.C.) of polyethylene has been observed, in which the liquid viscosity plays an important role. E.S.C. of a low density, high melt index polyethylene due to silicone oils has been studied using constant load creep experiments. For relatively low stresses, it has been found that the time to fracture is independent of the viscosity of the silicone oil, all other factors being approximately equal. However, at high stresses, the time to fracture increases with increasing viscosity for a given stress. This effect has been shown to be due to the relative ease with which the liquid penetrates a growing crack and thus always be at the crack front. Times to fracture for viscous liquids at high stresses are longer since crack propagation continues partially with and partially without liquid contact, fracture rate being much slower when not in the presence of the liquid.     

Crack propagation behavior in rubber materials

Within the linear viscoelastic theory, crack tip fields are calculated at various crack tip velocities. A transition from rubbery to glassy material behavior in the vicinity of the crack tip can be observed. Shear and bulk behavior is analyzed separately. Whereas the increase of tearing energy at higher crack tip velocities can be ascribed to the shear behavior, bulk behavior influences the fracture mechanism. The results support experimental investigations that the instability separating stable from unstable crack propagation is related to a change in the fracture mechanism. At low crack tip velocities, material separation is the result of formation, growth, and coalescence of cavities. At high crack tip velocities, cavitation is suppressed and fracture is driven by a rather brittle mechanism resulting in a decreased amount of energy to propagate the fracture process zone. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 5, pp. 882–891. This article was submitted by the authors in English.     

Correlation between environmental stress cracking and liquid sorption for low-swelling liquids

The environmental stress cracking (ESC) of polyethylene has been studied under conditions of dynamic equilibrium with the liquid for low-swelling liquids. Even among active ESC agents, there is a clear relative order of efficiency. It has been shown that the liquid becomes less efficient with increasing equilibrium swelling. This fact has been attributed to local plasticization of the crack front leading, in turn, to a reduction in the high-stress concentrations associated with a wedge-shaped crack. Some semiquantitative ideas are proposed in an attempt to explain the relation between ESC efficiency and volume/volume sorption.     

Environmental stress cracking of polyethylene: Criteria for liquid efficiency

The environmental stress cracking (ESC) of polyethylene in nonreacting, nonswelling liquids has been studied using uniaxial creep tests. For active liquids, three types of behavior have been recognized. At low stresses, “pure” ESC occurs; at intermediate stresses, time to failure is largely controlled by the ability of the liquid to flow into a growing crack; and at high stresses, ESC is in competition with failure by necking, and the latter prevails. The liquid does not therefore play a significant role in this last case. Nonactive liquids produce results similar to those observed in air. It is believed that this is because these liquids are unable to flow into growing cracks sufficiently quickly even at low stresses and thus the liquid does not influence failure behavior. This criterion for activity of the liquid is largely determined by the viscosity of the liquid and by the spreading coefficient of the liquid on the solid—a parameter defining the ability of the liquid to wet the solid.     

A Test Concept for Lifetime Prediction of Polyethylene Pressure Pipes

Summary. The present paper describes the main elements of a novel concept for lifetime and safety assessment of polyethylene pressure pipes for arbitrary installation conditions based on modern methods of fracture mechanics. At the core of the proposed concept is the accelerated generation of so-called synthetic crack growth curves and corresponding material laws for crack growth initiation and slow crack growth for service-near temperature conditions without the use of stress cracking liquids.     

A Test Concept for Lifetime Prediction of Polyethylene Pressure Pipes

The present paper describes the main elements of a novel concept for lifetime and safety assessment of polyethylene pressure pipes for arbitrary installation conditions based on modern methods of fracture mechanics. At the core of the proposed concept is the accelerated generation of so-called synthetic crack growth curves and corresponding material laws for crack growth initiation and slow crack growth for service-near temperature conditions without the use of stress cracking liquids.     

Flow,high-elastic (recoverable) deformation,and rupture of uncured high molecular weight linear polymers in uniaxial extension

The behavior of narrow molecular weight distribution polymers has been investigated under uniaxial extension at constant deformation rate and at constant stress. It has been established that up to rupture these polymers behave as linear viscoelastic bodies. A detailed investigation of the rupture phenomenon has shown that the rupture of fluid polymers is due to their transition to the rubbery state at critical deformation rates, with the result that they disintegrate like quasi-cured rubbers. The effect of the temperature and the molecular weight on the critical conditions of rupture has been described in terms of viscoelastic relaxation.     

Chemical degradative stress cracking of poly(ethylene terephthalate) fibers

Poly(ethylene terephthalate) (PET), after certain thermal and mechanical histories, exhibits stress cracking when exposed to 40% aqueous methylamine. This reagent has also been used for selective degradation of PET films. Stress cracking is shown to occur during degradation only when a specimen supports an internal or externally applied stress, above a critical level. The cracking density in a filament is shown by the present work to increase as the draw ratio is increased or when the fiber is highly annealed. This increased cracking is associated with an increase in the magnitude of the internal residual stress resulting from molecular orientation developed during these processes. Because of this, crack density and fiber birefringence were found to correlate well. In addition, the geometry of the stress-cracking pattern along a filament is affected by internal residual stress since the propagation of spiral and helical cracks results from the effect of a biaxial stress field remaining at the filament surface after processing.     

Comportement en fatigue-corrosion et corrosion sous contrainte d'alliages de titane sous environnement gazeux a 500 °c

This paper deals with results obtained in fatigue crack propagation at 500 °C in Ti6242 and Ti6246 alloys. Following an analysis of the influence of alloy composition and microstructure, the influence of the gaseous atmosphere is specially studied on the Ti6246 alloy. A corrosion fatigue mechanism is shown to be related to an embrittling effect of water vapour. An additional stress corrosion cracking mechanism is operative for some critical conditions.     

Ultimate toughness of amorphous polymers

The deformation and toughness of amorphous glassy polymers is discussed in terms of both the molecular network structure and the microscopic structure at length scales of 50–300 nm. Two model systems were used: polystyrene-poly(2,6-dimethyl-1,4-phenylene ether) blends (PS-PPE; where PS possesses a low entanglement density and PPE a relatively high entanglement density) and epoxides based on diglycidyl ether of bisphenol A (DGEBA) with crosslink densities comparable with up to values much higher than the thermoplastic model system. The microscopic structure was controlled by the addition of different amounts of non-adhering core-shell-rubber particles. Toughness is mainly determined by the maximum macroscopic draw ratio since the yield stress of most polymers approximately is identical (50–80 MPa). It is shown that the theoretical maximum draw ratio, derived from the maximum (entanglement or crosslink) network deformation, is obtained macroscopically when the characteristic length scale of the microstructure of the material is below a certain dimension; i.e. the critical matrix ligament thickness between added non-adhering rubbery particles (‘holes’). The value of the critical matrix ligament thickness (ID_c) uniquely depends on the molecular structure: at an increasing network density, ID_c increases independent of the nature of the network structure (entanglements or crosslinks). A simple model is presented based on an energy criterion to account for the phenomenon of a critical ligament thickness and to describe its strain-rate and temperature dependency.     

Prediction of fatigue properties of natural rubber based on the descriptions of the cracks population and of the dissipated energy

The goal of this paper is to relate the fatigue lifetime to the energy dissipation and the crack population for a Natural Rubber (NR) compound filled with carbon black. First, the dissipated energy is measured by thermal measurements and its evolution with the local strain is described. Then, the crack population under fatigue loading is investigated thanks to interrupted fatigue tests and SEM measurements. The dependency of the evolution of the crack surface density on the local strain and number of cycles is described. Finally, a fatigue criterion is suggested, starting from the basic assumption of accumulation of dissipated energy along the fatigue cycles. Combining the evolution of the dissipated energy and the crack surface density, the energetic criterion can be written as a simple expression using a single parameter. The predictions obtained with the identified criterion are compared with the results from classic fatigue tests and very close agreement is found.     

Toughness Properties of Lightly Crosslinked Epoxies Using Block Copolymers

Summary: In this work is discussed an alternative approach to the toughening of epoxy networks by the addition of acrylic block copolymers, composed of rigid and rubbery blocks. Once the reaction is completed, the initial self-assembly of block copolymers in epoxy thermoset precursors produces rubbery domains: depending on the block copolymer structure and composition, these domains are of the micrometer or the nanometer size. Nanostructures are obtained when the rigid block is a random copolymer of methylmethacrylate and N,N-dimethylacrylamide. The rubbery domains prevent rapid crack propagation and the highest toughness is obtained with filament-like microparticles or wormlike micelles.     

Influence of wetting properties of the liquid on environmental stress cracking of polyethylene at high stress

It has previously been shown that, under high stress and consequently at short times to failure, a major factor governing the environmental stress cracking (ESC) of polyethylene is the ability of a liquid environment to penetrate a growing fissure at a sufficiently high speed to maintain contact with the fracture front. In this earlier study, viscosity was shown to play a significant role in this kinetic effect. The purpose of the present paper is to demonstrate that another property of the solid–liquid system influencing ESC under these high stress conditions is the spreading coefficient of the liquid on the polymer—the parameter defining the tendency of the liquid to wet the polyethylene. It has been shown that the spreading coefficient can be considered as a force and this force in conjunction with atmospheric pressure constitutes the force necessary to drive the liquid into the growing crack.     

Effects of dissolution on plastic deformation and cracking of metals

The effects of surface dissolution on plastic deformation of metals, and the operating mechanisms, depend on many variables: stress; frequency of stress fluctuations; strain rate; temperature; metallographic constituents and their distribution; surface films; adsorbed species; surface energy; electrochemical potential and current; dissolution morphology; crack tip geometry; dislocation mobility; and the chemistry at the crack tip in the environment as well as in the metal. The manifestation of these variables in different relative proportions in different systems causes a variety of metal/environment interactions, the most extreme of which are stress-corrosion cracking and corrosion fatigue. During simultaneous action of stress and dissolution, stress-corrosion cracking occurs only with specific metal/environment combinations within critical electrochemical-potential ranges, whereas corrosion fatigue develops at dissolution rates greater than a critical value. In addition to these forms of degradation, the dissolution process may inject defects — such as vacancies, divacancies, or dislocations — into the metal lattice, with resulting alteration in dislocation behaviour and plastic-deformation characteristics. Dissolution may also influence the films that normally exist on metal surfaces and, consequently, the dislocation arrangements which depend on these films. This review is a systematic presentation and discussion of those critical — and sometimes controversial — experiments that elucidate the effects at an atomic level of surface dissolution on plastic deformation of metals.     

Environmental stress cracking of polyethylene

Modification of the Griffith theory for the presence of liquids has been shown to explain some facets of the environmental stress cracking of polyethylene. In the absence of intercrystalline links and tie molecules, we find that one important factor is the interfacial tension generated between the spherulite boundary and the liquid environment. Judicious incorporation of silanes into polyethylene appear to reduce the tendency to stress cracking by modifying the interfacial tensions between the environment and the polymer.     

Sorption and diffusion of organic vapors in poly[bis(trifluoroethoxy) phosphazene] and poly[bis(phenoxy)phosphazene] membranes

The sorption isotherms and diffusivities for vapors of some selected simple alcohols (methanol, ethanol, isopropanol, and 2-butanol), ketones (acetone, methyl ethyl ketone, and methyl isobutyl ketone), and aromatic compounds (benzene, toluene, and xylene) in poly[bis(trifluoroethoxy)phosphazene] (PTFEP) and poly[bis(phenoxy)phosphazene] (PPOP) were determined by integral sorption-desorption experiments at 35°C. The sorption isotherms for these compounds evaluated were almost linear to obey Henry's law for the determination of constant solubility of each solvent vapor species, and the corresponding permeabilities for them can be estimated according to the solution-diffusion model. The diffusivities for these vapors in PPOP (10⁻⁹∼10⁻⁸ cm²/s) were about one order smaller than those in PTFEP (10⁻⁸∼10⁻⁷ cm²/s) because the more rigid phenoxy groups and the higher crystallinity in PPOP may hinder the diffusion of sorbed molecules. Relatively weak dependence of diffusivity or permeability on the vapor activity (or concentration) was found, to be in contrast to the exponential dependence for many organic vapors in rubbery organic polymers, probably due to the limited increase of the free volume in sorption for these vapors in PTFEP and PPOP membranes.     

Improved resistance to crack growth of natural rubber by the inclusion of nanoclay

The effect of nanoclay on the fatigue crack growth behavior was investigated. Fatigue tests were carried out on edge notched specimens under cyclic tension loadings. A power–law dependency between crack growth rate and tearing energy was obtained. Natural rubber (NR) filled with 5 phr organically modified montmorillonite (OMMT) possessed the lowest value of the exponent, b, and the smallest crack growth rate at a given tearing energy, denoting the strongest resistance to crack growth. Strain‐induced crystallization was probed by synchrotron WAXD experiments, showing earliest occurrence and strongest ability of crystallization in NR with 5 phr OMMT due to the better exfoliation and orientation of clay layers. The study on the viscoelastic property by dynamic mechanical analysis indicated that NR filled with 10 phr OMMT had the largest contribution to tearing energy attributed to the viscoelastic dissipation in the viscoelastic region in front of the crack tip. This revealed that the strain‐induced crystallization played a more important role in the crack growth resistance than the viscoelastic dissipation for clay filled rubber. Copyright © 2010 John Wiley & Sons, Ltd.     

Notched Ring Test for Measuring Slow Cracking Resistance in Plastics Pipes and Fittings

It is known, that the lifetime of polyethylene pipes is essentially limited by slow crack growth (SCG). For state of the art PE materials common SCG testing methods have reached their limits with respect to extension of testing times. A comparatively new method is the Notched Ring Test (NRT) as developed by Choi et al.^[1] Pipe rings notched at the inner wall are used. The test is carried out in 80 °C water under constant bending load. The arrangement of the notch at the inner wall reduces testing times using the residual stress of extruded pipes. A disadvantage of this method is that there is no clearly defined failure time because SCG takes place between two phases of creeping. The output of this test is an “on-set slow cracking time” (crack initiation), obtained by analysis of the displacement curve. In this work it has been shown that the NRT method yields to brittle fracture within acceptable time frames.^[2] Methods for data analysis are presented. This test could be very useful applied in research and development for resin evaluation and as a tool in quality control in pipe production for evaluating the process conditions.     

The effect of molecular composition and heterogeneity on the environmental stress cracking resistance (ESCR) of propylene impact copolymers

The ESCR of three propylene impact copolymers in the presence of isopropanol was investigated and the variation in stress crack resistance was evaluated in terms of polymer characteristics. The effect of removing both soluble and crystalline material from the copolymers on the ESCR was evaluated. The stress crack resistance appears to be dependent on the crystallinity of the materials, but not solely so. The amount and distribution of the rubbery copolymer in these materials appears to play a role as well.