Simulation of the scale factor of a ductile fracture of polymer blends and composites on the basis of the specific interphase concept

The concept of interfacial layers surrounding inclusions in a host polymer is accepted as a basic source of the scale factor in the simulation of the large‐strain deformation and fracture of polymer blends and composites. The essence of the phenomenon is the ductile–brittle transition if the polymer ligament thickness exceeds a critical value, which is determined by the nature of the polymer. Original texture‐sensitive constitutive equations have been applied for the simulation of the polymer large‐strain deformation. Two periodic structural models of composites have been used. The disperse component (elastomeric or rigid inclusion) has been replaced by a geometrically identical system of pores, which coincides with phenomena of elastomeric inclusion rupture in rubber‐toughened plastics and debonding in particulate‐filled composites. The local loss of stability of an elastic deformation is used as a composite fracture criterion. The specific properties of the interphase can be caused by a specific texture and by closeness to a free surface. This specificity is stated in the model by an improved plastic ability compared with that of a bulk polymer. Our simulations show that the percolation of an interfacial polymer provides the brittle–ductile transition. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 2771–2777, 2003     

Structure and properties of biaxial‐oriented crystalline polymers by solid‐state crossrolling

The effect of biaxial orientation by solid‐state crossrolling on the morphology of crystalline polymers including polypropylene (PP), high density polyethylene (HDPE) and Nylon 6/6 was investigated with polarized optical microscopy, atomic force microscopy, wide‐angle X‐ray scattering, and small‐angle X‐ray scattering techniques. It was found that crossrolling gradually changed the initial spherulitic structure into a biaxially oriented crystal texture with chain axis of crystals becoming parallel to the rolling direction for all three polymers. The effect of microstructure change on the macromechanical properties was studied in tension at both ambient temperature and ?40°C. In tension at room temperature, the localized necking deformation of HDPE and PP control changed upon orientation into homogeneous deformation for the entire sample length. This was attributed to that the oriented crystal morphology eliminated the stress concentration, which existed in the original spherulitic structure from lamellae orientation in the polar and equatorial regions. At ambient conditions, the elastic moduli of HDPE and PP were found to decrease slightly with orientation whereas the modulus of Nylon 6/6 increased with increasing orientation. This was due to the fact that the amorphous chains of HDPE and PP are in a rubbery state and orientation increased the shear relaxation in the orientation direction but the amorphous chains of Nylon 6/6 are in the glassy state inhibited the shear relaxation. Both the yield stress and strain hardening exponent increased with increasing orientation for all three polymers. In tension at ?40°C, orientation changed the failure mechanism of all three polymers from brittle fracture into ductile failure, as the original spherulitic structure was changed into an oriented structure with chain axis of crystals becoming parallel to the tension direction, which allowed chain slip deformation of crystals and resulted in oriented samples showing ductile failure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Orientation distribution functions for biaxially oriented polymers

The use of the ‘most probable’ distribution function previously introduced for the prediction of orientation distribution functions for uniaxially oriented polymers is extended to biaxial polymers. Isometric projections of plots of the functions for model distributions are given, together with the application to a set of experimental data.     

Effect of molecular weight on brittle‐to‐ductile transition temperature of polyetherimide

The fracture and yield strength of polyetherimide was evaluated over a temperature range of 23 to 140°C for materials with number‐average (M_n) and weight‐average molecular weight (M_w) ranging from 15.6 to 22.8 and 36.6 to 52.3 kg/mol, respectively. The brittle‐to‐ductile transition temperature, where an equal probability exists that an impact will result in a brittle or ductile failure, was determined by evaluating the temperature at which fracture and yield strength are equal. The transition temperature decreased from 155 to 60°C with increasing molecular weight and provided a measure of relative ductility between material samples. As a case study, the practical impact strength of an injection‐molded food service tray was determined at 20°C and correlated with fracture strength as a function of molecular weight. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 1666–1671, 2004     

Compensation method for zero birefringence in oriented polymers

Optical birefringence is usually linked to molecular orientation in polymers. Birefringence in materials used for optical applications represents non-isotropic optical properties, and is therefore undesired, but cannot always be avoided. Molecular orientation was found to be possible without necessarily resulting in birefringence. This effect is achieved by combining suitable amounts of PMMA and poly(vinylidene fluoride) PVDF in the form of a compatible, amorphous blend. The observed drastic reduction in birefringence is the result of the compensation of positive and negative contributions to the overall birefringence.     

The fracture mechanism of polylactic acid resin and the improving mechanism of its toughness by addition of acrylic modifier

The fracture mechanism of polylactic acid (PLA) resin and the improving mechanism of its toughness by addition of an acrylic modifier were examined. Plane strain compression testing of PLA clearly showed strong softening after yielding. Because the stress for craze nucleation of PLA was close to the yield stress, brittle fractures resulted. The addition of an acrylic modifier to the PLA significantly lowered the yield stress and formed many voids. The release of the strain constraint because of the formation of many voids and the decrease of yield stress resulted in the relaxation of stress concentration, and the toughness was improved. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Conductive polymer tape containing highly oriented carbon nanofillers

The effect of the orientation of carbon fillers with different aspect ratios on the resistivity and morphology of conductive polymer composites (CPCs) based on polypropylene was investigated in this study. Multiwall carbon nanotubes (MWNTs) and carbon black (CB) were used as conductive fillers. The CPCs were made by melt compounding, hot pressing, and solid‐state drawing. The alignment of the filler was observed after solid‐state drawing. The resistivity of the composites increased with the draw ratio at relatively low carbon filler loadings (<20 wt %), whereas it remained unchanged at a high filler loading (20 wt % CB). Orientation‐promoted anisotropy of the conductive network was observed in both the morphology and resistivity. MWNTs were found to be better at maintaining a percolating network under large deformations than CB because of their larger aspect ratio and their entangled network structure. The experimentally obtained resistivity was analyzed with percolation theory, and this indicated that the initial three‐dimensional conductive network was deformed into a two‐dimensional network after solid‐state drawing for the composites containing CB. The three‐dimensional network was found in isotropic CPCs containing MWNTs with the same analysis. Theoretical analysis using excluded volume theory was in good agreement with results obtained experimentally. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Tensile strength elevation of brittle polymers by entanglements

Plots of tensile strength (T) versus reciprocal number average molecular weight (M^?1) have been made using previously reported data for linear polymers tested in the glassy state. Over a wide range of molecular weights there is conformity to Flory's empirical equation T = A - BM^?1, in which A and B are constants. Values of M obtained by extrapolation to T = 0 correlate with critical values of molecular weight which are diagnostic of incipient formation of an entangled network. The entanglement thesis is developed further by reference to a model in which brittle strength is attributed to the breaking of covalent backbone bonds. Theoretical values are calculated which exceed experimental values by a factor of only three. Such close agreement is attributed to the insensitivity of glassy linear polymers to flaws.     

Critique of materials-based models of ductile machining in brittle solids

Indentation fracture models of ductile machining in hard and brittle materials are critically appraised. Relations obtained in a seminal study for critical depths of cut below which fracture is suppressed are examined and amended. Limitations inherent in any such materials-based analysis, in addition to uncertainties in empirical measurements of underpinning mechanical properties (modulus, hardness, toughness) and of threshold grinding depths, suggest that caution should be exercised in unconditional usage. Notwithstanding these limitations, the value of the indentation fracture methodology in placing ductile machining on a sound materials science footing is maintained.     

Thermally stimulated shrinkage forces in oriented polymers: 2. Time dependence

The four-state model proposed in a previous paper to describe thermal shrinkage in oriented polymers is solved to describe the time dependences of shrinkage forces under constant temperature conditions. The time dependences of shrinkage forces have been recorded for polycarbonate and polyethylene terephthalate at temperatures below the glass transition. It is shown that experimentally recorded time dependences of shrinkage forces can be satisfactorily fitted by dependences predicted by the model, and the comparison leads to reasonable values of activation energy and activation volume for the shrinkage process.     

Thermally stimulated shrinkage forces in oriented polymers: 1. Temperature dependence

Temperature dependences of shrinkage forces appearing in oriented polymer samples when heated at constant length were recorded for polycarbonate, polyethylene terephthalate, polyethylene and polypropylene. The influence of various processing conditions on thermally stimulated shrinkage forces is demonstrated. A four-state model is proposed which qualitatively describes the temperature dependences of shrinkage forces in amorphous polymers.     

Influence of nucleation on the brittle‐ductile transition temperature of impact‐resistant polypropylene copolymer: From the sight of phase morphology

Influence of α‐ and β‐nucleation on brittle‐ductile transition temperature (BDTT) of impact‐resistant polypropylene copolymers (IPCs) and their phase morphologies were comparatively investigated. Impact test showed that the BDTT of β‐nucleated IPC (β‐IPC) is ～ 24°C lower than that of α‐nucleated one (α‐IPC). Structural characterizations including atomic force and scanning electron microscopic observations, small angle X‐ray scattering examination, and dynamical mechanical analysis revealed that dispersion of the ethylene‐propylene random copolymer‐rich (EPR‐rich) phase was finer in β‐IPC in comparison with that in α‐IPC. For the reason of looser lamellar arrangement, the portion of EPR‐rich components included in the interlamellar region of β‐IPC was higher than those of α‐IPC, which led to improved mobility for the amorphous polypropylene chains. It was proposed that the finer distribution of EPR‐rich phase, which might result from faster growth rate of the β‐crystal and looser lamellar arrangement of β‐spherulite, should be responsible for the improved impact‐resistance and lower BDTT in β‐IPC samples. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Iosipescu shear deformation and fracture in model thermoplastic polyolefins

Shear deformation and fracture behaviors in polypropylene (PP)‐based model thermoplastic polyolefins (TPOs) were investigated with the Iosipescu shear test. The shear deformation process was monitored in situ via video camera to obtain experimental shear stress–strain curves of model TPOs. Shear fracture mechanisms were studied with optical microscopy and scanning electron microscopy. Macroscopically, the cracks in neat PP propagated along the maximum shear plane, which indicated that mode‐II shear failure existed in neat PP. Microscopically, it was shown that shear fracture initiated in the form of partial, discontinuous inclined microcracks that later coalesced and formed the final continuous crack. The incorporation of rubber in PP could transform the shear fracture process into a stretching process in the shear damage zone. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3201–3214, 2001     

在玻璃上合成有取向的silicalite-1分子筛膜及其生长机理

引言 分子筛膜具有规则的微孔结构、良好的化学和热稳定性,在气体分离、催化膜反应器、化学传感器以及分子筛改性电极等方面有广泛的应用前景[1-2],是催化和分离新材料领域的研究热点之一.     

3-D structure-property relationships in rolltruded polymers. Part II: Anisotropic yielding and deformation in triaxially oriented polymers

Rolltrusion, a single step solid state deformation process, has been successfully applied to isotactic poly(propylene), iPP, to produce unique 3-D properties. Mechanical property enhancement not only occurs in the principal draw direction, but also in the transverse directions. A systematic series of standard (ASTM) tensile and compressive experiments were made on well oriented materials supporting this claim. Tensile testing was also performed on dumb-bell shaped specimens cut at various angles to the main deformation direction. A variety of observations were made during testing. Deformation bands, anisotropic yielding, and ductile-to-brittle behavior were observed. These results are presented along with a morphological model that has been developed to account for the behavior of these unique triaxially oriented polymers.     

Indentation cracking and deformation mechanism of sodium aluminoborosilicate glasses

Developing less brittle oxide glasses is a grand challenge in the field of glass science and technology, as it would pave the way toward new glass applications and limit the overall raw material usage and energy consumption. However, in order to achieve this goal, more insight into the correlation between the chemical composition and material properties is required. In this work, we focus on the mechanical properties of quaternary sodium aluminoborosilicate glasses, wherein systematic changes in glass chemistry yield different resistances to indentation crack initiation. We discuss the origin of the composition dependence of indentation cracking based on an evaluation of the deformation mechanism taking place during the indentation event. To this end, we use a simple metric, the extent of indent side length recovery upon annealing, to quantify the extent of reversible volume deformation. Finally, we also compare the compositional trend in crack initiation resistance to that in crack growth resistance (fracture toughness), showing no simple correlation among the two.     

PP/HDPE/SBS三元共混物的研究——基体韧性和弹性体分散相对PP三元共混物脆—韧转变的影响

研究了固定PP/HDPE/SBS三元共混物配比,采用不同共混工艺条件下的脆-韧转变规律。研究表明:PP三元共混物的冲击强度与SBS分散相粒径有密切关系。当SBS分散相粒间距T等于临界值T_c时,PP三元共混物将发生脆-韧转变。研究还表明基体韧性与T_c有密切关系,当基体韧性增高时,T_c值将增大。     

The importance of real-fluid behavior and nonisentropic effects in modeling decompression characteristics of pipeline fluids for application in ductile fracture propagation analysis

The importance of real-fluid behavior during the rapid decompression of dense natural gas mixtures has been investigated using existing one-dimensional models for real-fluid isentropic decompression (RID) and perfect-gas isentropic decompression (PID). The results show that the assumption of perfect-gas behavior may result in significant errors. In the case of ductile fracture propagation (DFP) analyses, the fracture-tip pressure level may be underestimated by more than 20 percent. A real-fluid nonisentropic decompression (RND) model has been developed in order to investigate the importance of nonisentropic effects in DFP problems. The results indicate that nonisentropic effects may be neglected for pipe sizes above approximately 508 mm O.D.     

Effect of high loading rate and low temperature on mode I fracture toughness of ductile polyurethane adhesive

The mode I fracture toughness of an adhesive at low temperatures under high loading rates are studied experimentally. Typical R-curves of the polyurethane adhesive under different loading rates (0.5?mm/min, 50?mm/min, 500?mm/min) at different temperatures (room temperature, ?20?°C, ?40?°C) respectively are obtained. From the experimental results, the mode I fracture toughness of this adhesive is extremely sensitive to the high loading rates and low temperatures. With the increase of the loading rate and decrease of temperature, the mode I fracture toughness of this adhesive decreases significantly. Under the loading rate of 500?mm/min at ?40?°C, the mode I fracture toughness of adhesive is 15% of the value at room temperature (RT) under quasi-static conditions. Through the experiment, the relationship between mode I fracture toughness of this adhesive, nominal strain rate and temperature is obtained.