Crazes and fracture in polymers

A review of crazes in glassy thermoplastic polymers is presented with particular emphasis on those aspects of craze properties that influence and control fracture behavior. Both crazes as they normally occur, and crazes at the top of cracks are covered. The occurrence of crazes, their microstructure, the stress distribution within them and the nature of craze fibrils are discussed. Theoretical treatments of the effect of crazes on polymer fracture are reviewed.     

Morphology of crazes in glassy polycarbonate

The craze morphology of amorphous polycarbonate in bulk samples and thin films has been examined using electron microscopy techniques. Results indicate polycarbonate crazes induced by heat or solvent have a “lamellar” texture oriented nearly perpendicular to the tensile stress direction. However, the craze texture of polycarbonate blended with antiplasticizer is observed to be fibrillar, similar to polystyrene, whereas the craze texture of polycarbonate blended with plasticizers is observed to contain crystallites.

Transmission electron microscopy studies demonstrate that craze initiation does not occur by void formation or cavitation but rather by a local thinning or yielding at the craze tip. The initial stage of craze development appears similar for crazes in both polystyrene and polycarbonate.     

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

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 comparable with the ones from the sorption experiments. In conclusion, the induction time-that is, the time at which crazes appear for a given strain–is controlled by the diffusion of the crazing liquid into the polymer. Thus, in the case of these liquids, which are miscible with the polymer, the crazing mechanism can be explained in terms of facilitated craze formation of a plasticized polymer.     

MICROMECHANICAL BEHAVIOR OF BRANCHED POLYSTYRENE AS REVEALED BY IN SITU TRANSMISSION ELECTRON MICROSCOPY AND MICROHARDNESS*

The internal structure and failure of crazes in linear and long-chain branched poly(styrene) (PS) were investigated by means of in situ transmission electron microscopy (TEM). It was found that long-chain branched PS presents more finely fibrillated up to homogeneous crazes at room temperature instead of the typical fibrillated ones for linear PS. The failure of homogeneous crazes in long-chain branched PS indicates a more ductile behavior than the fibrillated ones. The microhardness of these materials was measured, and it was seen that the hardness value increased with increasing amount of long branches in the PS. In addition, the rate of creep under the indenter (creep constant) for these materials was investigated. The lowest value for the creep constant corresponded to the PS sample with the largest amount of long branches.

     

Regularly arranged superstructures on stretched high polymers

Contrary to previous assumptions, the deformation processes occurring in amorphous and partially crystalline high polymers cannot be considered as locally uniform but is anisotropic, with the formation of regularly arranged crazes. Newly developed methods of preparation allow for the first time the obtaining of direct evidence of these superlattice structures in many high polymers subjected to uniaxial, biaxial, or multiaxial stretching. The investigations have led to new concepts of the supramolecular structure of high polymers; they extend our knowledge of the relationships between the technological properties and molecular structures of polymers.     

Crazing and fracture of glassy plastics

A recently proposed mechanism of crazing in glassy plastics is reviewed and then extended to account for the observed structure of crazes, for the tensile properties of the crazed material, and for the effect of rate of loading upon the crazing process.     

Crazing in semicrystalline thermoplastics

The deformation processes in crystalline polymers have been studie ever since the discovery of chain folding in 1957. Since then, scientists have been intrigued by the different steps of the transformation of the folded-chain lamellar structure of single crystals or of macroscopically isotropic, often spherulitic, polymers into fibrous morphologies (see Refs. 1 and 2 for early reviews). The importance of molecular tilt, of inter- and intralamellar slip, and of micronecking were rapidly recognized [1–4]. In this paper, we discuss the analogies and differences with respect to crazing of glassy amorphous polymers. Obviously, there is an extensive body of literature on the micromechanics of crazing (see the reviews in Refs. 5–9). On the basis of these studies, it has been established that crazes in amorphous polymers are well-defined regions with approximately planar boundaries that extend perpendicular to the direction of maximum principal tensile stress and that contain highly stretched and voided material [7]. However, crazelike features have also been observed in many semicrystalline polymers (polyethylene [PE], isotactic polypropylene [IPP], isotatic polystyrene [IPS], polyoxymethylene [POM], polyamide 6 and 66 [PA6 and PA66], polycarbonate [PC], polyethylene terephthalate [PET], polybutylene terephthalate [PBT], polyvinylidene fluoride [PVDF], and polyether ether ketone [PEEK]). They are designated in the literature [3–10] as micronecks, true crazes, fibrillar deformation zones (DZs), or simply as crazes since they correspond well to the above definition.     

Stress whitening and yielding mechanism of rubber-modified PVC

Structure-property relationships were investigated for blends of grafted rubbery polymers with PVC. Increasing grafting levels as well as higher blending temperatures improved the dispersion of the graft copolymers in PVC, lowered the impact strength, and reduced stress whitening. Presuming a mechanistic connection between impact strength and stress whitening, the causes of whitening due to mechanical deformations were studied by a variety of methods. Electron microscopy of stress-whitened zones revealed a large number of cavities formed by rupture of rubber particles, which correlated with the extent of whitening. Density measurements and quantitative evaluations of the volume increase due to the cavities in the stress-whitened zones were in agreement and proved that crazing did not significantly contribute to either volume dilation or stress-whitening. Light scattering studies indicated the existence of reflecting planes oriented at an angle of 55 to 64° to the direction of the applied stress, depending on the particle size of the modifier in the blends. The orientation of the scattered light could not be attributed to the cavities in the rubber particles because of their smallness (< 0.5 μm). An explanation was finally found by transmission light microscopy at various resolving powers. It was demonstrated that the ruptured rubber particles were accumulated in bands which corresponded to shear bands in the PVC matrix. Thus it was concluded that the rubber particles improved the impact strength of PVC by initiating shear bands and not by generating crazes.     

Generation of Craze-Formation Centers in Polymer Films under the Action of Electric Discharge Plasma

We propose a technology for generating craze-formation centers in polymers under the action of the electric discharge plasma and consider questions associated with a method for varying the geometrical sizes of crazes. The possibility of obtaining nanostructured polymers are also considered.     

Generation of shear bands in subsurface layers of metals in sliding

Nanostructuring of the surface of specimens during friction is investigated under the conditions of shear instability of the subsurface layers of the material due to the strong localization of deformation. It is demonstrated that the localization of deformation in the subsurface layers occurs in three stages. The structure of the localization zone is studied. The formation of a nanocrystalline material on the surface due to the shear instability is considered by analogy with the formation of the shear band. The formation of the nanocrystalline material can be responsible for the crossover from the mild wear to the adhesive wear in the absence of mechanisms providing structural adaptability.     

Magnetic shear damping of dissipative drift wave turbulence

The influence of local and global magnetic field line shear on structure formation and transport in dissipative drift-Alfvén turbulence is explored. It is found that the generation of zonal flow shear is connected to magnetic shear in ways not accounted previously. The concept of a locally sheared slab flux tube model (including toroidicity) is introduced in order to extend previous analyses to general local variations of magnetic field line shear. It is shown that local shear damping is efficient even when flux surface averaged shear is low.     

Sharkskin Mechanism of LDPE and the Dispersion of Inorganic Fillers in Flow

The correlation between the sharkskin formation of polymers and the flow‐induced dispersion of fillers is discussed. The rheological behaviors of low‐density polyethylene (LDPE) and LDPE/TiO₂ composites were investigated and analyzed based on the temperature dependence of critical points of the sharkskin formation. This examination revealed the influence of shear thinning on sharkskin behavior. The analysis indicated that the sharkskin formation of LDPE was governed by the mechanism of molecular absorption and de‐absorption at the flow boundary, or by wall‐slip at the polymeric layer with low viscosity. Sharkskin formation of LDPE occurs at different ranges of shear rates, depending on the mechanism in effect. In a further study, the shear‐induced de‐aggregation and dispersion of the TiO₂ in LDPE was found to occur at a shear‐rate range that was close to the range for sharkskin formations that are controlled by the adsorption mechanism.     

Jamming under tension in polymer crazes

Molecular dynamics simulations are used to study a unique expanded jammed state. Tension transforms many glassy polymers from a dense glass to a network of fibrils and voids called a craze. Entanglements between polymers and interchain friction jam the system after a fixed increase in volume. As in dense jammed systems, the distribution of forces is exponential, but they are tensile rather than compressive. The broad distribution of forces has important implications for fibril breakdown and the ultimate strength of crazes.     

Spontaneous formation of densely packed shear bands of rotating fragments

Appearance of self-similar space-filling ball bearings has been suggested to provide the explanation for seismic gaps, shear weakness, and lack of detectable frictional heat formation in mature tectonic faults (shear zones). As the material in a shear zone fractures and grinds, it could be thought to eventually form a conformation that allows fragments to largely roll against each other without much sliding. This type of space-filling "ball bearing" can be constructed artificially, but so far how such delicate structures may appear spontaneously has remained unexplained. It is demonstrated here that first-principles simulations of granular packing with fragmenting grains indeed display spontaneous formation of shear bands with fragment conformations very similar to those of densely packed ball bearings.     

Surface Morphology of Deformed Amorphous-Nanocrystalline Materials and the Formation of Nanocrystals

The formation of nanocrystals under deformation in Al-, Fe-, and Co-based amorphous alloys are studied using X-ray diffraction and transmission and scanning microscopy methods. It is shown that the presence of shear bands in deformed amorphous alloys is an insufficient condition for nanocrystal formation. The presence of a large number of intersecting shear bands and an increase in the material temperature in the shear-band region in the case of intense plastic deformation contributes to nanocrystal formation.     

Nature of nanocrystallization in shear bands created by megaplastic deformation in amorphous alloys

A theoretical study is made of the process of nanocrystallization upon the formation of shear bands created by megaplastic deformation in amorphous metallic alloys. Such nanocrystallization is shown to be caused by a considerable increase in temperature inside the shear bands, which in turn is associated with the stored energy of megaplastic deformation. The temperature increment depends on the degree of deformation, the rate of propagation of the shear band, and the physical parameters that determine the thermal characteristics of an amorphous matrix in the range of the shear band.     

Cracks and crazes: on calculating the macroscopic fracture energy of glassy polymers from molecular simulations

We combine molecular dynamics simulations of deformation at the submicron scale with a simple continuum fracture mechanics model for the onset of crack propagation to calculate the macroscopic fracture energy of amorphous glassy polymers. Key ingredients in this multiscale approach are the elastic properties of polymer crazes and the stress at which craze fibrils fail through chain pullout or scission. Our results are in quantitative agreement with dimensionless ratios that describe experimental polymers and their variation with temperature, polymer length, and polymer rigidity.     

Current Structures with Magnetic Shear in Space Plasma

Observations of current structures with magnetic shear by the Cluster satellite quartet in the magnetotail and the Wind satellite in the solar wind have been reported. In current structures with magnetic shear, the following structural features have been identified: (i) thickening of a current layer, (ii) plasma density distribution asymmetric with respect to the layer plane, and (iii) formation of an asymmetric current density profile. The kinetic features of the dynamics of ions in current layers with initial shear deformation have been considered. A mechanism has been proposed for the formation of self-consistent current sheets with a nonzero shear magnetic field component.     

Two-pulse excitation for efficient formation of an sp3 nanodomain with frozen shear in a graphite crystal

We propose a two-pulse excitation to efficiently form a novel sp(3)-bonded nanosize domain with frozen shear in a graphite crystal. This sp(3) structure is well stabilized by shear displacement between two neighboring graphite layers. The shearing motion is induced transiently by the first laser pulse, and is frozen by the second pulse before disappearing, resulting in the efficient formation of the sp(3)-bonded domain with frozen shear. We show this dynamical process qualitatively by molecular dynamics calculations.     

Dislocation dynamics of crystallographic slip

The dynamics of dislocation loops involved in the formation of a crystallographic shear zone is considered. As a dislocation moves in the shear zone, the kinetic energy is shown to attain high values. When hindered near barriers bounding the shear zone, the dislocation exhibits damped oscillations in the vicinity of some equilibrium position. Tomsk State University of Architecture and Building. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 71–76, January, 2000.