Craze Initiation Research Articles

An approach to yielding and toughness in rubber‐modified thermoplastics

AbstractAn interpretation of yielding and fracture of rubber‐toughened polymers is attempted, considering the fracture mechanics behavior of the matrices, with the rubber particles as stress‐intensification sites. The fit of effective tensile yield stresses of composites vs. particle radii defines a stress‐intensity factor KYc for craze yielding much smaller than the classical fracture factor Kc, values are found for polystyrene and poly(styrene‐co‐acrylonitrile)‐based polymers. These factors are considered characteristic for craze initiation and propagation in the matrices, while Kc, in turn, would include also the craze‐crack transformation contribution. KYc appears independent of the rubbery‐phase volume fraction and characteristics, but two different values are found and discussed for poly(styrene‐co‐acrylonitrile)‐based materials in two different particle‐size ranges. A similar treatment on notched specimens' yield stress indicates the presence of a maximum in different radius ranges for polystyrene and poly(styrene‐co‐acrylonitrile) matrices, with higher values than their breakdown stresses. This stress increment is in relation to the minimum particle size inducing and still stabilizing crazes and preventing crack formation. This maximum seems to control the reinforcing extent of the polymer matrix conditioning the Izod fracture initiation energy.

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. © 1991 Chapman & Hall.

Impact modification of styrene‐acrylonitrile copolymers by methyl methacrylate grafted rubbers

AbstractMonodisperse methyl methacrylate grafted latex rubber (MMA‐g) particles (0.12‐0.13 µm) have been used for the impact toughening of a series of styrene‐acrylonitrile (SAN) copolymers of varied AN content. It was found that the MMA‐g particles are most effective in toughening SANs containing about 25 to 34% AN. Mechanical properties, interphase adhesion, mechanical dilatometry, and phase morphology by transmission electron microscopy were determined on blends containing 20‐30% rubber. Also, the miscibility range of SANs and poly(methyl methacrylate) (PMMA) graft was considered in attempting to analyze the relationships between the mechanical properties and the characteristic features of the blends. Issues relating the impact strength and deformation mechanisms, interphase adhesion, and particle morphology are discussed. It appears that the high impact strength of some of the blends may be caused primarily by certain morphological features such as particle clustering or small interparticle distances. Thus, craze initiation, shear yielding, and significant matrix deformation may all contribute to the overall ductility of the blend.

Formation of crazes in polymer blends

AbstractHigh‐voltage electron microscopy was used to study the micromechanical processes of deformation directly on thin deformed samples of rubber‐modified, high‐impact polymers. In these polymers the microprocesses are closely connected with the initiation and formation of crazes. Craze formation with its effects on the fracture toughness are discussed in dependence on several important morphological factors, particularly on the rubber volume content, particle diameter, and particle diameter distribution.

Size effect of compliant rubbery particles on craze plasticity in polystyrene

Blends of polystyrene with of spherical shell particles composed of alternating shells of PS and lightly crosslinked polybutadiene of averaze sizes of 0.32 and 2.3 μm are prepared at particle weight fractions of 0,05 and 0,15. It is found that the craze flow stress increases systematically with decreasing particle volume fraction at constant particle size and that the blends with the small particles that are less potent craze initiators have higher flow stresses than blends with large particles

Intrinsic crazing and anisotropy of maximum recoverable extension in oriented glassy polycarbonate

AbstractThe critical draw ratio for initiation of intrinsic crazing in polycarbonate below the glass transition temperature has been determined after preorientation in the melt. Even when referred to the isotropic state, this ratio is found to depend not only on the extent of preorientation but also to be distinctly anisotropic. Under the assumption that intrinsic crazing occurs at the maximum extension of the entanglement network, these results are incompatible with a strain dependent but homogeneous breakdown of the entanglement network, but can be interpreted in terms of departure from affine transformation of the radius of gyration of an entangled length.

Deformation of poly(ethylene terephthalate) films of various thickness in liquid adsorption-active media

The effect of initial thickness of PETP films on the process of their deformation in liquid adsorption-active media was investigated by the methods of liquid permeation under a pressure gradient, dynamometry and electron microscopy. It was found that the geometry of the initial sample substantially affects both the process of initiation and growth of crazes, and the process of their immediate coagulation in a liquid under strain.

Dependence of solvent crazing in bisphenol–A polycarbonate on molecular weight of organic liquids

Abstract Solvent crazing has been investigated for bisphenol-A poly-carbonate exposed to the liquids of n-alcohols, di-n-alkylphthalates, and adipic acid polyesters. The critical strain at which crazes appeared was determined with a Bergen elliptical strain device. In the case of the n-alcohol and di-n-alkylphthalate liquids with a small number of carbons in the alkyl chain, crazes spread rapidly to areas of lower strain with time and ceased within 30 min at room temperature. Such a lower limiting value of critical strain can be predicted by using polar-nonpolar solubility parameter plotting representations employing the molar volume term. On the other hand, craze initiation was more delayed for two higher molecular weight liquids, di-n-octylphthalate and adipic acid polyester. Sorption studies were also conducted on polycarbonate immersed in both liquids. The values of the activation energy and diffusion coefficient estimated from the experimental data on craze initiation were found to be nearly comparab...

On criteria for craze initiation in glassy polymers

The previous published data on biaxial critical stresses for craze initiation on the surface of cylindrical specimen of glassy polymers have been reexamined. A new interpretation is presented for the trend of crazing stress near pure shear state in the second quadrant of principal stress space which was observed in air at elevated temperatures. That is, an increase in the tensile stress for crazing with an increase in the magnitude of the compressive stress is interpreted not to be followed by a decrease in the dilatational stress but by development of shear yielding. On the basis of this interpretation, a new empirical criterion for craze initiation is proposed by considering the stress concentration due to surface scratch. The theoretical crazing locus accorded with the previous experimental results, except for the data near the shear yield locus in the second quadrant, which were considered to be affected by shear yielding. It was also indicated in the calculation that the shape and direction of surface scratch exert a considerable influence on the trend of biaxial crazing stress.

Progress and challenge in polymer crazing and fatigue

AbstractIn this review on polymer crazing and fatigue three aspects have been treated more explicitely: the molecular rearrangements preceding and provoking craze initiation, the competition between disentanglement and chain scission during lateral craze growth, and the distinct fatigue failure mechanism occurring in cyclically loaded PET and PA fibers. An overview on other aspects is given including references to work in progress.

Radius of gyration, maximum extensibility and intrinsic crazing in thermoplastics

An expression for the radius of gyration of a deformed chain is derived, assuming affine deformation of end-to-end vectors defining an entangled length, and random chain behaviour on a shorter scale. This expression is compared with small-angle neutron scattering results from the literature on variation with strain of the radius of gyration. The theoretical variation of the radius of gyration with strain is used to derive an expression for the dependence of limiting glassy-state extensibility on rubbery state pre-strain. This expression is found to give an adequate description of experimental observations of limiting extensibility identified with the critical strain for initiation of intrinsic crazing.

Craze initiation in poly(methyl methacrylate) exposed to n-alkanes

The critical stresses for craze initiation were measured on poly(methyl methacrylate) exposed to n-alkanes both under static torsion and under static tension at room temperature. Crazes are observed even under torsion where the dilatational stress is absent, as the authors previously pointed out. The critical stresses for craze initiation under both types of loading increase with increasing liquid molar volume, and decrease with increasing loading time. The ratio of the critical crazing stress under torsion to that under tension is almost constant irrespective of the kind of liquid and the loading time. In contrast with the critical crazing strain measured by the use of the elliptical bending device, the critical crazing stresses do not show a lower limiting value even under long-time (200 min) loading, and are correlated with neither the solubility parameter nor the equilibrium solubility of the liquid predicted from the solution theory.

The Kinetics of Crazing and Shear Yielding in Rubber-Toughened Plastics

Abstract The work presented and reviewed in this paper leads to the following conclusions: (a) The kinetics of creep in HIPS containing 0.2 µm diameter core-shell rubber particles are similar to those in more conventional HIPS containing larger multiple-inclusion particles. Both deform by multiple crazing. (b) Comparisons of mechanical properties should be made on the basis of equal rubber volume fractions ϕ rather than equal elastomer contents. (c) The kinetics of creep in rubber-toughened PMMA, also containing small core-shell particles, are quite different from those of HIPS. Deformation occurs by shear yielding accompanied by a small volume increase which is probably due to cavitation in the rubber particles. (d) The dependence of deformation rates upon applied stress and temperature is well described by the Eyring model for activated flow. (e) Analysis based on the Eyring slopes can be used to compare stress concentration factors γ in toughened plastics. In polymers that deform by shear yielding, absolute values of γ can be obtained by comparison of data from toughened and untoughened plastics. Difficulties remain in applying a similar approach to polymers that deform by multiple crazing. (f) The rate-determining step occurs in regions of maximum stress concentration in HIPS and appears to be identified with initiation of crazing. (g) The rate-determining step in toughened polymers that deform by shear yield is not associated with regions of maximum stress concentration.

Initiation and growth of crazes in glassy polymers. 3rd report Cessation of craze growth.

The initiation and growth of crazes on the surface of PMMA were investigated under various levels of environmental temperature and applied stress. In this report, craze growth equations were derived on the basis of the rate process theory, considering the increase of activation energy for craze growth, and cessation of craze growth was discussed. Average craze length and total sum of craze length per unit area were calculated theoretically and experimentally and the theory agreed with the experimental results for low craze density. Crazing and uncrazing surfaces were observed by means of metallographical and interference microscopes. Slip lines were observed at the craze tip of unyielded specimens. It was found protuberances 0.1-0.2μm in height occurred around the craze due unloading.

ポリ塩化ビニルのクレイズ塑性変形に及ぼす温度と試料寸法の影響

The effects of temperature and specimen size on the plastic deformation by crazing were investigated under constant rate tension at different temperatures in kerosene using poly (vinyl chloride) plates with different thickness. An increase in temperature facilitated the initiation and growth of crazes, and as a result reduced the craze yield point. As the specimen thickness increased, the craze yield point rose and the shape of deformation curve became similar to that of shear yielding. The temperature dependence of craze yielding was favourably explained by the theory which was previously developed on the conception that the plastic strain produced by crazes is given by the total volume of crazes per unit volume of the polymer. In order to account for the size effect on craze yielding, a simple model consisting of two crazed surface layers and non-crazed interior was proposed. By applying the above theory on craze yielding to describe the deformation of the surface layers, the calculated results accorded with the experimental results mentioned above.

Crazing in thin films of styrene–butadiene–styrene block copolymers

AbstractThe mechanism of craze initiation and growth and its relationship to mechanical properties has been studied in thin films of styrene–butadiene–styrene (SBS) block copolymers. Optical microscopy and transmission electron microscopy were used to examine three copolymers which has a spherical rubber domain morphology but varied in rubber content from 20 to 50%. With increasing rubber content, the crazes became longer and less numerous. Widening of the crazes was at least partially responsible for the higher strains achieved in the copolymers, especially for the composition with the highest rubber content where the crazes widened to form micronecks. Transmission electron microscopy revealed that craze initiation and growth at the craze tip occurred by cavitation in the polystyrene phase. Cavitation of the continuous phase rather than the rubber domains was attributed to the concentration of chain‐end flaws in the polystyrene. Crazes in the block copolymers followed a meandering pathway and the boundaries between crazed and uncrazed material were indistinct. Incorporation of fibrillated rubber particles into the craze fibrils strengthened the craze. At higher rubber content, the craze widened in the stress direction by voiding and fibrillation, which produced a cellular morphology.

Crystallizing crazes: The probable source of solvent stress cracking resistance in a polyester/polycarbonate blend

AbstractThe addition of a miscible phthalate polyester to BPA polycarbonate results in a remarkable improvement in stress cracking resistance in strong swelling agents. Comparative measurements of solvent craze initiation resistance, craze strengths, degrees of swelling, and crystallization behavior have been made. The results indicate that the improvement in cracking resistance probably results from stabilization of the craze fibrils by crystallization of the polyester before and/or during fibril formation.

Heterogeneous nucleation of crazes below notches in glassy polymers

A slip-line field study of craze initiation at plastic zone tips below notches in polycarbonate has been carried out over a wide temperature range. Combination of the critical values of hydrostatic tension with Yee's temperature data for bulk modulus leads to a calculated critical elastic dilation in the region beyond the zone tip that is nearly temperature-independent. Microscopic investigation, however, shows strong evidence that crazes in polycarbonate and other glassy resins initiate at micrometre-size foreign particles. Assuming that the foreign particle is a rigid adherent sphere, a stress analysis for the particle situated on the elastic side of the elastoplastic boundary has been effected; the particle is found to raise the hydrostatic tension over the value in the homogeneous field by 104%. In view of this analysis the polycarbonate experimental results suggest a temperature-independent, critical local elastic dilation of greater than 2% as a crazing criterion. Based on these results it may be suspected that all crazes — surface or internal — in glassy polymers, except at crack tips, are heterogeneously nucleated at impurity sites.

Electron microscopic investigations of initiation and growth of crazes in polystyrene

The crazes in polystyrene (PS) were investigated by using a high voltage electron microscope (HVEM, accelerating voltage of 1000 kV). The early stages of the formation and the growth of the crazes were studied in detail.

Instrumented impact testing of polyethersulphone

The impact behaviour of polyethersulphone has been studied using a specially constructed instrumented impact testing machine. This machine is of the pendulum type and the samples are fractured in three-point bend loading. It is shown that accurate force/deformation curves can be obtained, in spite of complications due to flexural vibrations of the test sample. Measurements were made on both sharp-notched and blunt-notched specimens over a range of crack lengths. It was found that the sharp-notched samples could be analysed in terms of fracture toughness, G C, whereas the blunt-notched samples corresponded to a constant critical stress at the root of the notch. The importance of multiple crazes at the crack tip in bluntnotched specimens is emphasized. It is also shown that ageing reduces the fracture toughness, while on the other hand, the critical stress observed in blunt-notched specimens, which has been associated with the craze initiation stress, is not affected by ageing.