Stress-relaxation crazing of polymers — an energy criterion

One of the most important signs of failure, easily observable in transparent and translucent polymers, is the appearance of visible crazes. The observation of first crazes is relatively simple in uniaxial long-term experiments (such as creep and stress-relaxation). An energy-related crazing criterion has allowed good predictions of the appearance of first visible crazes under uniaxial creep loading conditions. The results have shown that in spite of the different applied stresses and different corresponding crazing times, the specific dissipated energy at crazing was the same. The objective of the present work was to investigate if this criterion, based on the constancy of the specific dissipated energy, is valid also for uniaxial stress-relaxation. The good agreement between theoretical predictions and experimental results confirms this assumption.     

An investigation of brittle failure in ductile,notch-sensitive thermoplastics

Brittle failure, a significant design issue for plastic components subject to impact loads, is especially catastrophic when the material is normally ductile. Such behavior is not adequately understood relative to the micromechanisms, controlling parameters, and design consequences in plastics. Previous work has identified the process of crazing as being relevant to these failures in thermoplastics. The relationship between crazes generated through mechanical loading and subsequent brittle failure of amorphous thermoplastics is discussed and the hypothesis that the craze event is a necessary but insufficient condition for brittle failure is employed. Emphasis is focused upon the engineering prediction of craze formation and its use as a conservative brittle failure criteria for defining geometric details to prevent brittle failure. First, a series of experiments using one geometry is applied to study the concept of crazing as a precursor to brittle fracture in the two amorphous polymers polycarbonate and polyetherimide. Second, three-dimensional finite element analyses are used to assess the effects of changes in geometric detail upon the continuum stress state and eventual failure of the specimen for these two materials.     

Crazing limit of polymers in creep and stress relaxation

The appearance of the first visible damage in polymers in the form of crazes or microcracks may be assumed to be a sign of failure. Experimental investigations have shown that in uniaxial creep and stress relaxation experiments, under isothermal conditions, a certain time between the quasi-spontaneous loading and visible crazing is needed. This ‘incubation time’ is very strongly dependent on the magnitude of the quasi-spontaneously applied stress or induced strain. Based on the Reiner-Weissenberg theory of strength, simple relations allowing the prediction of crazing are developed. The agreement between theoretical computation and experiment is very good.     

Deformation zones and entanglements in glassy polymers

Deformation zones are regions of drawn but unfibrillated material which grow from crack tips in thin films of glassy polymers which have a low value of I_e′, the chain contour length between entanglements. The growth of these zones is observed optically and their final structure characterized by transmission electron microscopy. By microdensitometry of the electron image plate the average value of the extension ratio within the deformation zone, λ_DZ, is measured. Such deformation zones have been grown in thin films of four homopolymers and a series of polymer blends. It is found that λ_DZ is approximately 0.6 λ_max, where λ_max is a predicted maximum extension ratio derived from a simple model in which the entanglement points are assumed to act as permanent crosslinks with no chain slippage or scission occurring. This value of λ_DZ is lower than the extension ratios previously measured for crazes grown in the same polymers; typical λ_craze values lie much closer to λ_max. This result can be rationalized by realizing that at least a limited degree of chain scission/slippage must occur during crazing to permit the generation of the void-fibril network. For those polymers where both crazing and deformation zones may form, the latter grow rapidly whereas the formation of crazes requires longer times. This observation also indicates the importance of the kinetic process of chain scission/chain slippage for crazing. Annealing of the polymer films below the glass transition temperature leads to an increased tendency for crazing relative to the growth of deformation zones.     

A criterion for craze formation

A general criterion for craze formation is presented. Crazes are deformation zones that are common to both glassy and semicrystalline polymers. Crazes are composed primarily of fibrils. This paper attempts to describe the process that transforms unoriented glassy and semicrystalline polymeric solids into a fibrous state. The criterion for crazing discussed is a local phase transition. The transition occurs at the draw temperature. Unoriented solid-phase macromolecules, at local high-stress regions, undergo a transition to the elastomeric phase. Rapid extension and accompanying resolidification produce the fibrous morphology of craze fibrils. Cavitation of the deforming rubber phase ocurs because the local length increase is riot compensated for by an overall area decrease. Craze formation in glassy polymers has long been suspected to involve a local solid-to-rubber phase change. To relate crazes in glassy and semicrystalline polymers, one can assume that a solid-to-rubber phase change is required to produce craze fibrils in semicrystalline polymers. The transient melt phase would undergo rapid elongation, causing the formation of extended chain crystallites. These subsequently nucleate the remaining melt, which then crystallizes epitaxially as lamellae. Crystallization during flow would, therefore, be the mechanism of fiber formation.     

Crazing in polypropylene

The subject of crazing in crystalline polymers is reviewed and specific consideration given to crazing in polypropylene (PP). Tensile tests conducted over a wide spectrum of temperatures and strain rates indicate that, for a given test temperature, there exists a critical strain rate above which crazing is the dominant deformation mode of PP. Similarly, for a given strain rate, there exists a critical temperature which demarcates crazing from shear yielding as the characteristic process of deformation. High deformation rates and low temperatures favor crazing, while low rates and high temperatures favor shear yielding. Crazes in crystalline PP were found to be morphologically similar to those in glassy polymers: high reflectivity, large area-to-thickness ratio, and planarity. They have a higher tendency to bifurcate than those in glassy polymers. Two types of craze fibrils could be identified: those parallel to σ₁₁, and the randomly oriented interconnecting fibrils. It is demonstrated that microtome-trimming at low temperature followed by suitable chemical treatment is an effective technique of sample preparation for SEM examination of craze morphology in crystalline polymers. Further evidence has been provided that crazes in spherulitic polymers do not in general follow an interspherulitie path, but propagate through spherulites. The length of a craze in PP is not restricted to one spherulite diameter, nor does it grow radially.     

A new technique for generating regularly spaced crazes to facilitate piece dyeing of polypropylene filaments

The aim of this study was to investigate crazing that generates regular crazes in polymeric fibers. For carrying out this study, we designed and fabricated an experimental apparatus for generating crazes on polypropylene (PP) filaments. By an optical micrograph and a laser scanning micrograph of the surface and cross‐section of the filaments, it was confirmed that the crazes were generated on the surface of the filaments. Optical microscopes and measurements of the craze morphology on the filaments showed that approximately 30–50% of the contact area was crazed. As the crazing tension increased, the interval between the crazes increased, but the width of the crazes did not change significantly. Moreover, it was confirmed that the filaments had a homogenous crazed structure and pores were formed in their structure. The crazing process did not affect the strength of the crazed filaments significantly; the crazing process decreased the light transmittance of the filaments. The acid dyeing was adsorbed onto crazed region of PP filaments. These crazes in the filaments have the potential to lead to new methods for dyeing PP fibers. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Crazing phenomena in PC/SAN microlayer composites

The crazing behavior of coextruded microlayer sheets consisting of alternating layers of polycarbonate (PC) and styrene–acrylonitrile copolymer (SAN) was investigated as a function of PC and SAN layer thicknesses. In this study, the total sheet thickness remained essentially constant and the PC and SAN layer thicknesses were changed by varying both the total number of layers from 49 to 1857 and the PC/SAN volume ratio. Photographs of the deformation processes were obtained when microspecimens were deformed under an optical microscope. Three different types of crazing behavior were identified: single crazes randomly distributed in the SAN layers, doublets consisting of two aligned crazes in neighboring SAN layers, and craze arrays with many aligned crazes in neighboring SAN layers. The transition from single crazes to doublets was observed when the PC layer thickness was decreased to 6 microns. Craze array development was prevalent in composites with PC layer thickness less than 1.3 microns. It was concluded that SAN layer thickness was not a factor in formation of arrays and doublets; formation of craze doublets and craze arrays was dependent only upon PC layer thickness. © 1994 John Wiley & Sons, Inc.     

Residual stress craze and crack formation in poly(methyl methacrylate)

Samples of poly(methyl methacrylate) with a central circular hole are compressed, and crazes form on or after unloading, provided that the strain attains or exceeds a threshold value ?_t. Crazes induced in air are transformed rapidly to cracks, but environmental crazes are more stable. These residual stress crazes form at the diameter of the hole on a plane perpendicular to the applied stress direction. In contrast, during loading, crazes form on the vertical plane containing the hole axis. Unloading crazes are relatively insensitive to changes in strain rate, whereas loading erazes have a pronounced rate dependence. Environmental residual stress crazing exhibits an apparent rate sensitivity at constant time, but the critical applied strain ?_t is essentially constant, irrespective of rate, if the sample is in contact with the environment for a sufficiently long time to ensure that the minimum ?_t is obtained. Residual stress crazes appear to initiate at the equator of the hole, and the maximum tensile residual strain, indicated by a strain gauge, occurs in this position.     

On the damage energy of polymers in creep

Creep experiments carried out on transparent and translucent polymers permit the observation of irreversible material damage in the form of crazes or microcracks. The formation of such damage is strongly dependent on the applied stress, temperature and environmental conditions. The first appearance of observable material damage seems to be explicable by an energy criterion. The energy supplied by the external load can be divided into conserved and dissipated parts, each of them causing volume (isotropic) and shape (deviatoric) changes. The different parts of the energy can be computed if the creep-compliance is approximated by a Prony-Dirichlet series with a finite number of terms. Computations carried out for experiments with air as the environmental medium under isothermal conditions show the dependence between the appearance of first visible material damage (crazes or microcracks) and the conserved energy.     

Crazing studies of polystyrene. II. Application of fluctuation theory

The present investigation deals with amorphous polystyrene crazing behavior at both the molecular and macroscopic levels. The nature of the amorphous state is considered from the perspective of statistical fluctuation theory, especially when a mechanical force field is acting on the polymer. Past crazing studies have rather fully described the phenomenological behavior. However, these studies did not generate a fundamental molecular explanation of crazing. We now suggest a molecular approach based on the density and density distribution of physical entanglements between polymer chains as a function of molecular weight. This approach permits the molecular entanglement concept and the phenomenological parameters such as stresses and temperatures associated with crazing to be related through the use of fluctuation theory. Two fundamental results are obtained and specifically demonstrated for polystyrene. First, an accurate theoretical prediction of the volume associated with microvoid formation is explicitly determined. Second, the dependence of the number of crazes on molecular weight is also shown.     

The rubber particle size dependence of crazing in polypropylene

The tensile crazing and Charpy impact behavior of polypropylene modified with styrene-butadiene copolymer (SBR) and ethylene-propylene-diene monomer (EPDM) was studied. Various rubber particle size distributions were obtained by varying the relative viscosities between rubbery phase and PP matrix. Transmission electron microscopy and computer-aided image analysis were used to provide particle size information. In general, PP blends with smaller rubber particles are tougher and more ductile than those with larger particles, probably because the former represents a more efficient use of rubbery phase in promoting crazing and/or shear yielding. Samples with average particle diameter D? ≥ 0.5 μm were found to exhibit pronounced crazing. Within a given sample, no crazes appeared to develop around individual rubber particles with D < 0.5 μm. The higher the D, the greater the propensity to form crazes. The behavior of samples with D? ? 0.5 μm appeared to be dominated by shear yielding; very few crazes could be found. That there exists a critical rubber particle size is explained by the requirement that sufficient stress concentration be maintained to a finite radial distance to permit the initiation and growth of a craze, which requires a finite volume. Small particles, inducing smaller stress-enhanced zones, are therefore not effective in initiating crazes.     

The effect of water on the tensile yield of polystyrene

When PS is tested in tension but immersed in water the yield stress is unchanged and the crazing stress is increased, as compared with tests conducted in air. The crazing pattern is otherwise unchanged. If the PS is first equilibrated with water and then tested under water, both crazing stress and yield stress are reduced. A very large number of small crazes are generated which result in a 23% increase in the fracture strain.     

A new approach to the preparation of nanocomposites based on a polymer matrix

A new approach to the preparation of nanocomposites is advanced. This approach includes preliminary formation of a nanoporous matrix and subsequent loading of the formed pores by the second component. These advantageous opportunities are provided by one of the most fundamental phenomena of the physical chemistry of polymers: solvent crazing of polymers in the presence of the liquid media. Several examples illustrate that solvent crazing not only provides a universal means of self-induced dispersion of a polymer material into nanoscale aggregates but also offers a universal route for the delivery of diverse low-molecular-mass compounds to the nanoporous structure of the solvent-crazed polymer. The results on the preparation of new types of nanomaterials, such as porous polymeric sorbents, polymeric separation membranes, new types of polymer-polymer nanoblends, fireproof and conducting polymer nanocomposites, and metal-containing polymers, are reviewed. Some aspects of the practical application and technological design of solvent crazing of polymers as a means for the preparation of diverse nanocomposites are discussed.     

Measurement of the residual mechanical properties of crazed polycarbonate. I: Qualitative analysis

A new technique to quantify the bulk craze density of transparent plates was used to characterize the craze growth behavior of polycarbonate at various stress levels. The craze growth rates were found to exponentially increase with an increase in stress, obeying the Eyring equation for thermally activated processes in the presence of an applied stress. The residual mechanical properties of crazed polycarbonate were then correlated to the crazing stress, relative craze density and strain rate. The results show that increasing the bulk craze density does not affect the yield stress but decreases both the failure stress and ductility of polycarbonate. Also, a crazing stress of 40 MPa was found to cause a much larger degree of degradation of failure properties than a crazing stress of 45 MPa. Correlating the crazing stress to the craze microstructure revealed that fewer, larger crazes form at the lower crazing stress. Therefore, flaw size has a greater effect on the failure properties of polycarbonate than flaw quantity.     

Multiple crazing at crack tips in PVC under plane strain conditions

The nature of the yield zone at the crack tip of poly(vinyl chloride) (PVC) pipe materials has been investigated. Microscopy studies employing a plasma etching technique reveal the presence of multiple crazes ahead of the crack tip in the interior of specimens of pure PVC, CaCO₃ filled PVC, and PVC pipe compound. The craze zone and the fracture toughness of blade-notched specimens are compared with those of fatigue pre-cracked specimens. Both types of specimens have similar fracture toughness values and form multiple crazes upon loading, suggesting that multiple crazing Is an intrinsic property of the material. The kinetics of craze initiation and the development of the multiple craze zones have also been explored.     

Mechanical behaviour and permeability of ABS/glass bead composites

The stress-strain behaviour of acrylonitrile-butadiene-styrene (ABS) containing up to 33.5vol% of 10–40μm glass beads as filler was measured at one strain rate at room temperature. The beads eliminate the yielding and greatly enhance the ultimate elongation and work-to-break. The change in stress-strain response is associated with the dewetting and vacuole formation around the beads and with an increase in the amount of crazing. Initial experiments using water permeability to investigate the crazes are reported.     

Microstructure and its relationship to deformation processes in amorphous polymer glasses

The microdeformation morphology of a number of vinyl polymers with bulky side chains (type I) and arylene polymers with flexible oxygen linkages (type II) was studied by electron microscopy. The polyarylenes crazed only near the glass transition while the polyvinyls exhibited a crazing regime that extended to liquid nitrogen temperatures. In addition significantly less plastic strain was localized in type II glass crazes relative to those in type I glasses. In compatible blends of polystyrene (PS) and 2,6-dimethyl poly(phenylene oxide) (2MPPO), ca. 30% 2MPPO was sufficient to induce a transition from type I to type II crazing behavior. Small amounts of PS suppressed the low-temperature 2MPPO β relaxation but enhanced the intermediate transition of 2MPPO at higher temperatures. Blending increased the conformational energy of the 2MPPO chain and improved interchain packing. The propensity for the polymer glass to form sharp shear bands at the expense of diffuse bands was increased by a decrease in the conformation energy of the polymer chain and an improvement in the glassy state packing.     

Surface modification of poly(tetrafluoroethylene) and poly(ethylene terephthalate) films via environmental crazing

This work addresses the phenomenon of the development of a patterned surface relief on polymer films via different modes of environmental crazing. Commercial films of semicrystalline poly(tetrafluoroethylene) (PTFE) and amorphous glassy poly(ethylene terephthalate) (PET) were subjected to tensile drawing in the presence of physically active liquid environments (carbon tetrachloride or aliphatic alcohols). The structure parameters and wettability of the modified films were studied by AFM, SEM, profilometer measurements and contact angle measurements. Environmental intercrystallite crazing of PTFE is accompanied by the development of an unstable structure with a high free surface, which experiences marked strain recovery upon unloading. As a result of the relief formation, PTFE hydrophobicity is enhanced (the water contact angle increases by 25°). Classical environmental crazing of PET films is accompanied by the formation of an anisotropic surface relief which is an assembly of crazes oriented perpendicular to the direction of tensile drawing, thus leading to the phenomenon of anisotropic wetting. The proposed approach for structural surface modification makes it possible to use the advantages of surface instability and spontaneous self‐organization of the polymer towards the development of a unique surface microrelief. © 2020 Society of Chemical Industry     

An interpretation of the critical strain measure for polymers

During creep deformation many polymeric materials exhibit small cracklike zones, crazes. In the literature it is reported that crazing will not occur if the strain is kept under a critical value, specific for each material. This fact has importance in avoiding rupture. In this work a uniaxial theory is put forward that is based on a creep law describing the buildup of internal stress and on the Kachanov damage law. Crazing is here regarded to be a stage in the damage accumulation process, the final stage of which is rupture. Relations among the critical strain, the corresponding critical stress, and the minimum stress leading to creep rupture are derived. Some conditions that are necessary for the critical strain concept to work are formulated.