Craze Growth Rate Research Articles

Understanding the brittle-ductile transition of glass polymer on mesoscopic scale by in-situ small angle X-ray scattering

The structural evolution of poly (methyl methacrylate) (PMMA) under uniaxial stretching has been studied with in-situ synchrotron radiation small angle X-ray scattering (SAXS) to reveal the brittle-ductile transition (BDT) mechanism. The scattering invariant analysis method was applied to semi-quantitatively analyze the evolution of crazing. For the sample without melt pre-stretching, which exhibits almost the same BDT behavior in different directions. The formed crazes are enabled to grow and propagate stably via shear deformation of the matrix. While for samples with melt pre-stretching, crazing is further promoted when stretching along the transverse direction. With a higher craze growth rate and lower craze fibril fraction, the destructive crazing cannot be fully stabilized by shear banding and premature craze fracture occurs. In contrast, shear banding dominates the deformation when stretching along the pre-stretching direction. Due to the low craze concentration and slow craze growth rate, crazes with higher fibril fraction gradually slip with matrix shearing. The premature craze damage is consequently avoided and an enhanced toughness is achieved. The structural evolution of these samples indicates that the kinetic competition between crazing and shear banding is the key to the macroscopic toughness of glass polymers.

Creep life assessment craze damage evolution of polyethylene methacrylate

AbstractTests have been conducted to investigate the influence of temperature, stress, and crazing for polyethylene methacrylate (PMMA) creep life. Creep life decreases with stress in a logarithmic relation, and rupture strains increase with decreasing stress. To assess the creep life of PMMA, the Larson–Miller parameter is first introduced to correlate stress and temperature with rupture time. The important role of crazing in polymer material damage, especially, the craze density and craze growth rate in the stable creep stage, is demonstrated. The craze initiation time is related to the tensile strength and to the stress and temperature at the elastic limit, and the craze initiation time can be predicted by an empirical model subsequently presented in this paper. Based on the connection between craze growth and creep rupture time, thereafter, a modified Monkman–Grant model is presented to predict creep life. The curve of craze density over creep time has the same two distinct stages as the curve of strain over time. In the final sections, a critical assessment of creep damage is also presented to provide a convenient method to predict residual life using only craze density.

結晶性ポリマーの劣化挙動のマルチスケールモデリング

Coarse-grained molecular dynamics simulations are conducted, for the purpose of the study on the microscopic structure change of crystalline polymer due to degradation. Interlamellar structure changes are observed during the degradation, decreasing loop and bridge chains, increasing short tail chains. From the elongation simulation of lamellar structure, craze strength is dropped during the degradation, because of decreasing oriented interlamellar chains and drawing of the molecular chains from the crystal part. Multi-scale modeling is conducted to demonstrate the relationship between microscopic structure changes and macroscopic mechanical properties. As a result, macroscopic fracture strain is reduced by the degradation, because craze growth rate becomes faster with the degradation.

Crazing in Glassy Block Copolymer Thin Films

We investigate the crazing process (craze initiation, growth, and failure) in symmetric polystyrene-b-poly(2-vinylpyridine) (PS-b-P2VP) glassy thin films. PS-b-P2VP is a model material for this study due to its ability to self-assemble into well-aligned lamellar microdomains and the similar mechanical properties for each block (glass transition temperature, crazing stress, entanglement molecular weight, and elastic modulus). These attributes allow us to focus on two primary effects: the effect of an ordered microstructure and the effect of surface terraces on the crazing process in thin glassy polymer films. We find that an ordered lamellar microstructure leads to a higher ratio of craze depth to film thickness in the micronecking process and a lower craze growth rate compared with polystyrene homopolymer. Additionally, surface terracing, or nanoscale steps in the surface topography, impedes craze initiation and significantly decreases the failure strain compared with polystyrene and PS-b-P2VP thin films without surface terraces. These results demonstrate the importance of nanoscale morphologies on the crazing process and point to new directions for understanding the fundamental mechanisms for crazing in advanced materials.

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

AbstractA 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.

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.

A model of environmental craze growth in polymers

AbstractA model of environmental craze growth has been developed based on the customary meniscus (or Rayleigh‐Taylor) instability model of craze propagation but allowing for the possibility that environmental plasticization may cause the active layer of material adjacent to the craze to be of significant thickness with respect to the fibril spacing. Initially, as the active layer thickness increases, the fibril growth rate increases at constant fibril spacing, but eventually the fibril spacing comes to be controlled only by the active layer thickness and not by the surface tension and stress. This model of craze growth has been coupled to a model of stress‐enhanced case II diffusion that is itself based on the Thomas‐Windle model. Two main regimes of craze thickness growth are distinguished. In one the craze growth rate is controlled by the velocity of the diffusion front, the meniscus instability growth rate is assumed to be relatively slow, so that a significant plasticized active layer exists whose thickness assures that the meniscus instability front travels at the same speed as the diffusion front. In the other regime the propagation of the craze front is sufficiently fast that it also forms the diffusion front, so the growth rate is controlled by a combination of the two processes: diffusion and meniscus instability.

Physical ageing and toughness of diblock copolymers

The effects of physical ageing on craze growth behaviour and toughness in a series of polystyrene/polybutadiene (PS/PB) diblock copolymers and diblock/polystyrene blends were studied. Samples with spherical rubber morphology were subjected to one of three thermal histories: quenching, slow cooling or isothermal ageing. Dynamic mechanical spectroscopy revealed significant ageing in the slow cooled and isothermally aged samples. The growth rates of crazes were measured in samples containing volume fractions of polybutadiene ranging from 6% to 18%. No change in craze growth rates occurred following ageing. The rates were successfully modelled using the meniscus instability mechanism with no adjustable parameters for the case of 6% PB. A quantitative analysis was made of the small effect of physical ageing on the thermal stresses that facilitate rubber cavitation and craze growth in the materials with high rubber content. Craze growth measurements on materials with 18% PB supported the analysis in that no change in craze growth rates due to physical ageing was observed. The measured rates for this sample were well described by a cavitation model for craze growth assuming a cavitation stress of 47 MPa. Tensile testing of the aged samples revealed no effect of ageing on the macroscopic toughness of the materials studied here.

The effect of environmental water on the craze growth rate of atactic polystyrene

The effect of water at standard temperature and pressure on the rate of craze growth in polystyrene was studied with an elapsed-time photography method. Water was found to be a mild crazing agent for polystyrene, increasing the craze velocity by somewhat more than an order of magnitude at room temperature. The craze length was found to increase proportional to the square root of time, as is expected for transport-limited solvent crazing. The interface convolution model for craze growth and thickening was found to account well for the stress dependence of the craze front velocity in both the “dry” and the “wet” states, and for the total effect of water on the craze velocity. The stress dependence of the craze front velocity implies that craze fibrils grow by surface drawing and that water molecules interact only with the plastically deforming fringing layer of polymer at the craze borders.

Method of describing kinetic curves for the accumulation of crazes in the deformation of amorphous polymers under creep conditions in adsorption active media

An equation describing the kinetics of accumulation of crazes as a function of time or deformation under creep conditions is proposed. The equation is based on the assumption of the threshold character of craze appearance and the presence of a craze growth rate function. The expression obtained is a good description of the kinetic curves for the accumulation of crazes and the limiting number of crazes as a function of the stress.

Craze plasticity in a series of polystyrene/polybutadiene di-block copolymers with spherical morphology

Abstract The plastic behaviour by crazing of a series of polystyrene/polybutadiene di-block copolymers with a morphology of randomly dispersed PB spheres has been investigated at both +20 and −20°C. Materials with volume fractions of PB exceeding 0.11 were found to be quite tough in tension, with typical craze flow stresses of around 30 MPa and strains to fracture of around 0.4. Growth rates of individual crazes were studied in detail in all blends. Electron microscopy established that in the material exhibiting good toughness, crazes grew by cavitation of the spherical PB domains, followed by plastic drawing of the remaining PS carcass. The growth rates of such crazes could be modelled quite well by the advance of a cavitation front in a planar layer of a thickness equal to the PB domain diameter. When volume fractions of PB were much lower than 0.1, the craze growth rates became quite low and exhibited kinetics that fitted the interface convolution mechanism of craze growth. The good agreement between t...

Craze failure by midrib creep

AbstractIt has been known for some time that crazes thicken during growth mainly by drawing in fresh material from the craze‐bulk interface, keeping the average craze fibril draw ratio approximately constant. Since Creep effects contribute only negligibly to craze growth rates these effects have generally been considered unimportant regarding craze breakdown. However, it is also known that the first stage of fracture is failure of the craze midrib, which is a highly drawn, very thin region down the middle of a craze. Because of the very low thickness of the midrib it has little influence on craze behavior, and information on midrib behavior is difficult to obtain. It is the purpose of this paper to attempt to rationalize what information is available.

Propagation of crazing in viscoelastic media

Up to now, the growth rate of a craze has been considered either constant or inversely proportional to time. By taking into consideration the effect of the surrounding population of crazes, it is found that the craze growth rate is affected by the local effective stress acting in the vicinity of the craze. Measured data of craze length as a function of time are found to be greatly affected by local interactions.

The kinetics of surface craze growth in polycarbonate exposed to normal heptane

A variable radius of curvature strain-bending apparatus has been used to investigate the propagation of surface crazes in polycarbonate exposed to n-heptane. It was found that for crazes growing in a zone free of other crazes, the growth rate was linear at constant temperature and exponentially dependent upon temperature. An activated-state theory was developed to describe the kinetics of growth in terms of two parameters,B−1, a characteristic strain constant that was independent of test variables, andA′, a characteristic growth-rate parameter that varied exponentially with temperature. A strain-temperature equivalence principle was adopted to predict the crazing behaviour over a range of temperatures and strains. A critical strain was defined in terms of a maximum allowable craze growth rate.

Features of the development of forced elastic deformation during the stretching of glasslike polyethyleneterephthalate in an adsorption-active medium

A study has been made of the cold drawing of amorphous polyethyleneterephthalate under strain in an adsorption-active medium as a function of the rate of deformation. A relationship has been found between mechanical behaviour of the polymer under a constant rate of deformation in an adsorption-active medium, and processes of formation and development of specific microcracks. The existence of distributions according to microcrack (craze) growth rates revealed the importance of the role played by microdefects and microheterogeneity in the formation and growth of microcracks.

Environmental craze growth in polycarbonate

AbstractEnvironmental craze growth in polycarbonate was studied under plane stress and plane strain conditions. In plane stress, the craze growth rate could not be described in terms of either initial stress intensity factor, Ko, net or gross stress. In plane strain, Ko was found to be the governing factor for craze growth, as has been previously reported for poly(methyl methacrylate) (PMMA). For shallow cracks (<1 mm), there is evidence of an inherent flaw effect.

Crazing and acoustic emission in poly(methyl methacrylate)/acetone and polystyrene/methanol systems

AbstractCraze initiation and growth in the poly(methyl methacrylate)/acetone and polystyrene/methanol systems was studied using photographic and acoustic emission techniques. The poly(methyl methacrylate) system was the only one that produced detectable acoustic emission; however, it was found that craze initiation occurs predominantly in the first half of a sample's lifetime while acoustic emission is detected predominantly in the second half. The detected acoustic emission in the poly(methyl methacrylate) system is believed to be due to the rupture of craze fibrils. Craze initiation in both polymer systems was found to be dependent on stress and time with higher stresses initiating more crazes at earlier times. Although the craze growth rate in both systems exhibited extreme variability, the average growth rate tended to increase with craze size and applied stress. In the poly(methyl methacrylate) system, craze velocity was independent of time but in the polystyrene system it decreased with time. None of the existing craze growth theories were capable of explaining these results.

The effects of stress cracking on the fracture toughness of polycarbonate

AbstractThe growth of crazes from a sharp crack in extruded polycarbonate sheets immersed in ethanol was measured. Below a critical level of the stress intensity factor craze growth was controlled by solvent diffusion through the end of the notch and fracture was prevented by craze arrest. Above a critical level, growth was controlled by either end diffusion or a combination of end diffusion and diffusion through the faces of the extruded sheet, and in both cases the final result was brittle fracture. The effects of annealing and quenching was studied at various sheet thicknesses. In thin specimens annealing and/or quenching had a significant effect on crack growth rate, which was predictable in terms of the state of stress. As the specimen thickness increased, causing a transition from plane stress to plane strain conditions, the previous thermal history had a diminishing effect on craze growth rate. The effects of thermal history and thickness on the fracture toughness of polycarbonate was also investigated. It was found that thickness was the more important variable and that at a ½ in. thickness the effects of thermal history were statistically insignificant. The effect of ethanol exposure on fracture toughness was studied. It was found that exposure to solvent initially caused an increase in kIC with time to a maximum value, followed by a substantial decrease with time which eventually led to brittle fracture. This behavior was explained as a competition between plasticization of the crack tip and coalescence of crazes to form microcracks.

Neck propagation in polycarbonate and its connection with the mechanism for dry craze growth

An investigation of neck propagation in polycarbonate (PC) as a function of stress and temperature is presented. Neck propagation does not occur at a constant rate in this material: on the contrary, the propagation rate decreases continuously with time. A possible explanation for this behaviour is suggested. Also, a connection is sought between bulk neck propagation and craze growth rates.

クレイズ発生と成長の統計的研究

In spite of many works on the initiation and growth of crazes, few studies on plastic deformation caused by crazing inherent in high polymer solids have been performed. In this paper, as the first step to describe plastic deformation in high polymer quantitatively, the craze density, i.e., the number of crazes per unit surface area, and the craze growth rate were investigated, and their statistical nature were examined.The experiments were performed under the action of a crazing agent, kerosene, at room temperature (20±1°C) with a constant load type testing machine on the specimens of polycarbonate and polyvinylchloride. The dependences of craze initiation and growth on applied stress and testing time were determined by photographs taken through an optical microscope without stopping the test.The main results were: (1) the variation of craze density with time can be described successfully by a simple rate theory, (2) the distribution of the craze growth rate, which has essentially a statistical nature, is approximated by a normal distribution function and (3) the dispersion of craze length at a certain time can be explained by a statistical theory which takes into account both the distribution function of growth rate and the density variation of crazes with time.