Environmental stress cracking of PVC: Effects of natural gas with different amounts of benzene

Craze initiation, craze growth and ultimate fracture were studied in PVC (Polyvinylchloride) and PVC-CPE (PVC blend with 10% chlorinated polyethylene) under a constant load in air and in natural gas enriched with benzene. The craze initiation results are similar for PVC and PVC-CPE. The craze initiation stress is decreased dramatically at high benzene concentrations ( 25000 p.p.m.). Preferential and enhanced sorption of benzene molecules near surface inhomogeneities, the craze initiators, are held responsible for this phenomenon. Initial crazegrowth rates in natural gas enriched with benzene increase compared to those in air. However, in all the environments studied, a limited logarithmic craze growth is observed, and growth seems limited to this constant logarithmic interval. The logarithmic craze growth and the termination of craze growth are attributed to a reduction of the stress at the craze tip. The failure mode of PVC and PVC-CPE in air and in gas with 5000–6000 p.p.m. benzene is yielding. A brittle branch appears in the failure curve (failure stress against loading time) in the more concentrated benzene vapours. This branch is attributed to a reduction in the stability of the crazes.     

The temperature dependence of the fracture toughness and acoustic emission of polycrystalline alumina

The technique of acoustic emission has been shown to be suitable for the monitoring of fracture-toughness tests over a range of temperatures. Commercial polycrystalline alumina has been tested at temperatures up to 1000° C to determine the effect of microstructure and impurity content on fracture toughness and acoustic emission. For a given alumina there was no significant variation in acoustic response or fracture toughness up to 650° C. The emissions observed prior to fracture in this temperature range were attributed to subcritical crack growth. The number of emissions depended on the amount of subcritical crack growth, the grain size, and the presence and amount of porosity. Above 650° C the fracture behaviour changed due to the flow of a grain-boundary glassy phase. This was associated with a peak in the temperature dependence of the apparentK _IC and was accompanied by a large number of acoustic events of low amplitude and low pulse width. At these elevated temperatures the extent of grain-boundary glassy flow, and hence the acoustic response, increased with decreasing grain size and increasing impurity content.     

Rate-dependent fracture toughness of pure polycrystalline ice

A series of three-point bend tests using single edge notched testpieces of pure polycrystalline ice have been performed at three different temperatures (–20°C, –30°C and –40°C). The displacement rate was varied from 1 mm/min to 100 mm/min, producing the crack tip strain rates from about 10^–3 to 10^–1 s^–1. The results show that (a) the fracture toughness of pure polycrystalline ice given by the critical stress intensity factor (K _IC) is much lower than that measured from the J—integral under identical conditions; (b) from the determination of K _IC, the fracture toughness of pure polycrystalline ice decreases with increasing strain rate and there is good power law relationship between them; (c) from the measurement of the J—integral, a different tendency was appeared: when the crack tip strain rate exceeds a critical value of 6 × 10^–3 s^–1, the fracture toughness is almost constant but when the crack tip strain rate is less than this value, the fracture toughness increases with decreasing crack tip strain rate. Re-examination of the mechanisms of rate-dependent fracture toughness of pure polycrystalline ice shows that the effect of strain rate is related not only to the blunting of crack tips due to plasticity, creep and stress relaxation but also to the nucleation and growth of microcracks in the specimen.     

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.     

Flaw growth in a polycrystalline lithium-aluminium-silicate glass ceramic

The growth of indentation-produced controlled flaws in a polycrystalline lithium-aluminium-silicate glass ceramic has been studied, over a wide range of temperatures and strain rates. Significant scatter in the fracture stress at elevated temperatures suggests that the extent of slow crack growth is highly sensitive to microstructural details. The initial flaw shape is important inK _IC determination. Up to 1000° C the fracture toughness,K _IC, is essentially strain-rate insensitive. The value ofK _IC decreases with temperature beyond 850° C. Intergranular cavity formation is suggested as the reason. Crack blunting by diffusive crack healing probably occurs at high temperatures. Also, intergranular slow crack growth occurs essentially under Mode I loading.     

Environmental stress cracking of PVC and PVC-CPE

The failure of unnotched PVC and PVC-CPE was studied in a number of vapour and liquid environments. The failure mechanisms observed are related to craze initiation and logarithmic craze growth. A model to explain the origin of the logarithmic craze growth is presented. The failure mechanism in air and in benzene and toluene vapours is ductile, whereas it is brittle in n-hexane, n-decane and ethanol vapours, n-octane/benzene mixtures and natural gas condensate. Equations to describe the time to failure for both failure mechanisms are derived. The ductile failure is ascribed to plasticization and/or plastic deformation around the craze. Brittle failure is thought to arise from crazes which reach a critical size.     

Fracture phenomenology of a lithium-aluminium-silicate glass-ceramic

Crack propagation mechanisms in a lithium-aluminium-silicate glass-ceramic have been studied as a function of both initial flaw size and temperature. Using controlled surface cracks, the fracture stress at room temperature was found to conform to the Griffith flaw size dependence. Extrapolation suggests that processing flaws of the order of 6 to 8 m are strength-controlling in the material investigated. Two mechanistic regimes were manifest in the temperature dependence of the fracture stress. Up to 900° C, catastrophic transgranular crack propagation occurred from emplaced cracks. At 1000° C and above, subcritical crack growth occurred intergranularly and the extent of slow crack growth prior to catastrophic failure increased with increasing temperature. The influence of loading rate on slow crack growth and fracture stress was explored at 950 and 1100° C. Generally, the extent of slow crack growth decreased with increased loading rate until at a sufficiently high rate, catastrophic fracture occurred directly with no slow crack extension. These results are discussed in terms of the role of plastic accommodation in the crack extension process, a phenomenon which seems mechanistically dependent upon remnant amorphous (uncrystallized) phase at grain boundaries.     

High temperature uniaxial tensile stress rupture strength of sintered alpha SiC

Uniaxial tensile stress rupture testing of sintered-SiC was carried out at 1200 and 1300° C in air at various applied stress levels and the corresponding times-to-failure were measureD. Fractographic evidence from uniaxial tensile stress rupture testing at 1200° C revealed limited presence of subcritical (slow) crack growth associated with surface connected porosity failure sites. The extent of slow crack growth (SCG) increased with increasing temperature and large regions of SCG were observed in tests made at 1300° C. An estimate of the crack velocity exponent,n, in the crack velocity—stress intensity factor relation,V=AK ₁ ⁿ , for SCG has been made. Slow crack growth is characterized primarily by intergranular crack propagation while fast fracture (catastrophic failure) occurs transgranularly.     

Rate and temperature effects on crack blunting mechanisms in pure and modified epoxies

The failure mechanisms of several epoxy polymers (including pure, rubber- and particulatemodified, as well as rubber/particulate hybrid epoxies) were investigated over a wide range of strain rates (10^–6 to 10² sec^–1) and temperatures (–80 to 60° C). A substantial variation in fracture toughness, G_Ic, with rate was observed at both very high and very low strain rates. Under impact testing conditions, G_Ic for both pure and rubber-modified epoxies displayed peaks at about 23 and –80° C which appeared to correlate with the corresponding size of the crack tip plastic zone. In order to explain these rate and temperature-dependent G_Ic results, two separate crack blunting mechanisms were proposed: thermal blunting due to crack tip adiabatic heating and plastic blunting associated with shear yield/flow processes. Thermal blunting was found to occur in the pure- and rubber-modified epoxies under all impact testing conditions and temperatures above 0° C. For temperatures below –20° C under impact conditions, the fracture toughness is dependent on viscoelastic loss processes and not thermal blunting. Plastic blunting was predominant at very slow strain rates less than 10^–2 sec^–1 for the pure- and rubber-modified epoxies and at impact strain rates for the fibre and hybrid epoxies. Microstructural studies of fracture surfaces provided some essential support for the two proposed crack blunting mechanisms.     

Viscoelastic effects in testing of an Al2O3 ceramic containing a glassy phase

An alumina with 3 wt% glassy phase was tested at different loading rates at two temperatures (900 and 1000 °C). It was found that an increase in fracture toughness was accompanied by a decrease of the bending strength at the same loading rates. A model is given, which describes the experimental results by linear viscoelasticity of the second phase. Whereas the bulk properties are mainly due to the alumina grains and, therefore, remain nearly unchanged, the crack growth and the fracture behaviour in the intergranular regions is dominated by the viscosity of the glassy phase. This leads to a non-unique value ofK _Ic, which is dependent on the temperature and the loading rate.     

Fracture and flexural characterization of SiCw/SiC composites at room and elevated temperatures

The fracture and flexural behaviour of monolithic SiC and SiC-whisker reinforced SiC composites (SiC_w/SiC) has been investigated at room and elevated temperatures. Flexure and fracture tests were conducted in a four-point beam configuration at 23 °C, 800 °C and 1200 °C to study the effects of whisker reinforcements especially in respect of mechanical and thermal stability at high energy environments. Flexural strengths and fracture toughness data within the test temperature range are presented in graphical as well as in Weibull form, and experimental observations are analysed and discussed. Increase in flexural strength as well as in fracture toughness has been observed with the whisker reinforcement. However, it was found that the trend discontinues after a certain range of temperature. Post-failure analyses have been performed with the scanning electron microscope (SEM). Formation of glass phase has been observed at the whisker/matrix interface and the crack growth was found to be shifting from intergranular to transgranular with the rise in temperature. Effects of whisker reinforcement and the degradation of flexural and fracture properties at elevated temperature are investigated. Ultrasonic velocity measurements have been performed through the thickness of the untested as well as fractured specimen, and the variation in the sonic wave velocity is discussed in this paper.     

Effect of microstructure on elevated-temperature fracture behaviour of two-dimensional carbon/carbon composites

The fracture behaviour of two-dimensional carbon/carbon composites has been studied at temperatures upto 1650°C, using both chevron-and straight-notch single-edge notch beam (SENB) specimens. In all cases, the R-curve behaviour and fracture toughness variations with specimen orientation and temperature are characterized and correlated with the specific microstructure and failure micromechanisms. Higher crack growth resistance and fracture toughness of the longer fibre composite are attributed to the enhanced fibre pull-out and fibre bridging in the following wake region. The relative contribution from the frontal and following wake zone is determined experimentally by the use of renotching methods which demonstrate the effectiveness of the traction zone behind the crack tip. The temperature effects on the toughening mechanisms are examined in terms of crystal structure and fibre matrix interfacial characteristics.     

Mechanical behaviour of MgO-partially stabilized ZrO2 ceramics at elevated temperatures

Transformation toughened partially stabilized zirconia ceramics containing magnesia exhibit quite high fracture toughness (K _lc 8 MPa m^1/2) at temperatures of up to 500° C. The observed temperature dependences of the toughness and the fracture strength are consistent with that of the transformation behaviour. The high toughness of these materials results in a significant reduction in the sensitivity of the flexure strength to crack size increases. Exposure of these materials at 1000° C for prolonged periods results in flexure strength changes associated with the generation of the monoclinic phase by tetragonal precipitate destabilization and eutectoid decomposition. However, when exposed at 500° C, neither the phase contents nor the flexure strength are altered for exposures of up to 1000 h.     

Fracture mechanical analysis of self-fatigue in surface compression strengthened glass plates

This paper presents a fracture mechanical analysis of the static fatigue and spontaneous fragmentation of surface compression-strengthened glass plates in the absence of applied load. It is suggested that if an initial surface crack which is sufficiently deep to penetrate into the tensile zone within the plate interior is introduced into the plate, then static fatigue, and eventually spontaneous fracture may follow. The crack problem for glass plates under various internal stress fields is solved and the stress intensity factor is obtained as a function of the crack depth. Using the fracture toughness and the slow crack growth characterization of the material, the conditions for no crack propagation, crack propagation leading to crack arrest, and that leading to catastrophic failure are established and discussed. The general results obtained are illustrated by means of a numerical example based on a 2 mm thick surface compression-strengthened glass plate exposed to water at 25° C.Such a problem was encountered in relation to an eye-lens during a consulting case by one of the authors (DPHH).     

Characterization of fracture toughness (<Emphasis Type="Italic">G</Emphasis><Subscript>c</Subscript>) of PVC and PES foams

The fracture behavior of polyvinyl chloride (PVC) and polyethersulfone (PES) foams has been examined using the single-edge notch bend and the double cantilever beam (DCB) tests. PVC foam densities ranging from 45 to 100 kg/m³ and PES foam densities ranging from 60 to 130 kg/m³ were examined. The PVC foams failed in a linear elastic brittle manner, whereas the PES foams displayed much more ductility and substantially larger toughness at a comparable foam density. The cell wall thickness of the PES foams was almost twice the thickness of the PVC foams which may have contributed to the high fracture toughness here defined as critical energy release rate (G _c). The PES foam, further displayed low initiation toughness, due to the sharp artificial crack tip and large toughness corresponding to propagation from a natural crack. The results show that the ductile PES foams have toughness close to its solid counterpart whereas the toughness of the PVC foams falls substantially below its solid counterpart.     

Near the static fatigue limit in glass

Fracture surfaces formed near the crack growth threshold in soda-lime-silicate glass are examined by atomic force microscopy. Cracks held below the apparent crack growth threshold for 16 h alter their mode of growth. The fracture plane changes from a flat surface to one that exhibits substantial out of plane growth. The direction of crack growth changes from 3° to 5° to the original growth direction. However, the change in growth direction is not uniform along the crack front; some portions of the front propagate at +3° to +5°, while adjacent portions propagating at –3° to –5° to the original growth direction. Thus, the crack is no longer flat, but becomes wavy after the 16 h hold period. This out of plane growth may be partially explained in terms of a crack growth model developed by Chuang and Fuller, which predicts an enhancement in the corrosion rate on the flank of a crack at stresses below the stress corrosion threshold. Alternatively, the unevenness of the crack plane after the hold period may be a consequence of a modification of the fracture toughness of the glass as a consequence of leaching.     

Subcritical crack growth in partially stabilized ZrO2(MgO)

The room temperature slow crack growth resistance in air ( 50% relative humidity) and in water for large cracks in MgO-partially stabilized zircoba (PSZ) improves with increase in critical fracture toughness,K _lc. Ageing the as-fired PSZ at 1400° C for 8 h results in decreasing K_lc from 8.5 MPa m^1/2 to 6 M Pa M^1/2. The ageing treatment also promotes the growth of eutectoid decomposition products on grain boundaries that is accompanied by a decrease in the dependence (A the change in Region I crack velocity with a change in the applied stress intensity. Calculated times to failures are markedly decreased in the aged as compared to the as-fired PSZ ceramic.     

Internal hydrogen effects on thresholds for crack growth in the iron-based superalloy IN903

This study of internal hydrogen-induced crack growth in the iron-based superalloy IN903 shows that slow crack growth thresholds are significantly lower than fracture toughness values at the same prechargsd hydrogen concentrations. However, failure in all precharged samples occurred by slip band fracture which differed only in the extent of local surface plasticity. Quantitative fractography of these surface fracture features indicates that the crack tip hydrogen concentrations at threshold were higher than in fracture toughness samples. These higher concentrations are due to crack tip stress enhancement when sufficient time exists for hydrogen redistribution. In addition, continuum models based on mechanisms of failure demonstrate that the matrix carbides control crack growth susceptibility in slow crack growth and fracture toughness samples by establishing the characteristic distance that the crack tip stresses and strains must span to initiate fracture.     

Strength,toughness and R-curve behaviour of SiC whisker-reinforced composite Si3N4 with reference to monolithic Si3N4

The flexural strength and fracture toughness of 30 vol% SiC whisker-reinforced Si₃N₄ material were determined as a function of temperature from 25 to 1400°C in an air environment. It was found that both strength and toughness of the composite material were almost the same as those of the monolithic counterpart. The room-temperature strength was retained up to 1100°C; however, appreciable strength degradation started at 1200°C and reached a maximum at 1400°C due to stable crack growth. In contrast, the fracture toughness of the two materials was independent of temperature with an average value of 5.66 MPam^1/2. It was also observed that the composite material exhibited no rising R-curve behaviour at room temperature, as was the case for the monolithic material. These results indicate that SiC whisker addition to the Si₃N₄ matrix did not provide any favourable effects on strength, toughness and R-curve behaviour.     

Nicalon-fibre-reinforced silicon-carbide composites via polymer solution infiltration and chemical vapour infiltration

A new, faster process was developed for the fabrication of Nicalon-fibre-reinforced SiC composites by combining polymer solution infiltration (PSI) and chemical vapour infiltration (CVI). The process led to the near-net-shape fabrication of fibre-reinforced ceramic-matrix composites and reduced infiltration time. Typical flexural strength and fracture toughness of these composites were 296 MPa and 10.9 MPa m^1/2 at room temperature (RT) and 252 MPa and 9.6 MPa m^1/2 at 1000 °C, respectively. The composites exhibited load-carrying capability after crack initiation.