Fracture toughness of polymers in the ductile‐to‐brittle transition region: Statistical approach and lower bound determination

A method available in literature was adapted and proposed for treating scatter and nonlinearity effects in fracture toughness of polymers in the ductile‐to‐brittle transition regime. The materials used were polypropylene homopolymer (PPH) and a polypropylene‐elastomeric polyolefin blend (PPH/POes 20 wt %), at room temperature and at 20‐mm/min test rate. Under such conditions, the fracture toughness presents a large scatter and a mean value can not be used as a design parameter because it leads to toughness overestimation. Then, there is a need to find a threshold of toughness, as a safe characteristic value for design. The toughness was evaluated by using the J‐integral approach. Large sets of specimens, 53 samples per each material, were tested with the purpose to reveal a reliable tendency in fracture behavior. As the toughness was considered nonuniform throughout the material, a weakest link model was assumed, and then results were analyzed statistically by means of a three‐parameter Weibull model (3P‐W). The PPH responded well to this 3P‐W model, whereas some deviations from the original model were observed in the PPH/POes blend. However, lower‐bound toughness values could be determined for both materials by censoring nonvalid data (Δa > 0.1b₀). From an engineering point of view, the results are very encouraging, since this methodology allows to obtain a threshold of fracture toughness from a given population, that is suitable to characterize the material fracture toughness at a given temperature and strain rate. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3674–3684, 2005     

Effect of blend composition and related morphology on the quasi-static fracture performance of LLDPE/PP blends

In the present work, the effect of composition and related morphology on the fracture behavior of LLDPE/PP blends was thoroughly investigated. Fracture behaviors evaluated under quasi-static loading conditions and different fracture mechanics methodologies were applied to assess fracture toughness depending on the materials behavior. For pure PP and 2575 blend, J at instability was chosen whereas for blends which exhibited completely ductile behavior (such as LLDPE, 7525 and 5050), the EWF methodology was used. Fracture mechanisms were elucidated with the aid of scanning electron microscopy, and results correlated with blends morphology. It was observed that fracture properties are mostly dominated by the majority component properties. In addition, for the 5050 blend, the presence of a co-continuous morphology is responsible for the high scatter of experimental data obtained.     

Deformation mechanisms in nylon 6/ABS blends

The deformation mechanisms during fracture of a Nylon 6/ABS blend compatibilized with an imidized acrylic polymer were compared to those of an uncompatibilized blend. A postmortem examination of deformed zones in samples loaded to failure in a double-notch four-point-bend geometry was made using transmission electron microscopy (TEM). For the compatibilized blend, cavitation of the rubber particles followed by massive shear yielding of the polyamide matrix was concluded to be the sequence of events leading to toughness; while, for the brittle uncompatibilized blend, the evidence indicated that a lack of adhesion at the Nylon-ABS interface prevented the rubber particles from cavitating and the subsequent plastic deformation of the polyamide matrix. © 1994 John Wiley & Sons, Inc.     

Influence of SEBS-g-MA on morphology, mechanical, and thermal properties of PA6/PP/organoclay nanocomposites

Polyamide 6/polypropylene (PA6/PP = 70/30 parts) blends containing 4 phr (parts per hundred resin) of organically modified clay (organoclay) toughened with maleated styrene-ethylene-butylene-styrene (SEBS-g-MA) were prepared by melt compounding using co-rotating twin-screw extruder followed by injection molding. X-ray diffraction (XRD) and transmission electron microscope (TEM) were used to characterize the structure of the nanocomposites. The mechanical properties of the nanocomposites were determined by tensile, flexural, and notched Izod impact tests. The single edge notch three point bending test was used to evaluate the fracture toughness of SEBS-g-MA toughened PA6/PP nanocomposites. Thermal properties were studied by using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). XRD and TEM results indicated the formation of the exfoliated structure for the PA6/PP/organoclay nanocomposites with and without SEBS-g-MA. With the exception of stiffness and strength, the addition of SEBS-g-MA into the PA6/PP/organoclay nanocomposites increased ductility, impact strength and fracture toughness. The elongation at break and fracture toughness of PA6/PP blends and nanocomposites were increased with increasing the testing speed, whereas tensile strength was decreased. The increase in ductility and fracture toughness at high testing speed could be attributed to the thermal blunting mechanism in front of crack tip. DSC results revealed that the presence of SEBS-g-MA had negligible effect on the melting and crystallization behavior of the PA6/PP/organoclay nanocomposites. TGA results showed that the incorporation of SEBS-g-MA increased the thermal stability of the nanocomposite.     

乙烯-丙烯嵌段共聚物的鉴别

应用扫描电子显微镜(SEM)研究乙烯-丙烯嵌段共聚物和聚乙烯/聚丙烯共混物的冲击断裂表面形态。结果表明,这二种材料力学性能的差别,明显地反映在它们的断面形态上。前者显示出一种典型的韧性断裂特征;后者则呈现出一种剥离层状结构。~(13)C-NMR分析结果表明,在上述乙烯-丙烯嵌段共聚物中,确实存在着乙烯-丙烯共聚链段。这是该材料具有良好的抗冲击性能的主要原因。     

Effect of β-phase on the fracture behavior of dynamically vulcanized PP/EPDM blends studied by the essential work of fracture approach

The effect of β phase polypropylene (PP), induced by β-nucleating agent (β-NA), on the fracture behavior in dynamically vulcanized thermoplastic elastomers (TPVs) based on dynamically vulcanized PP/ethylene-propylene-diene rubber (EPDM) blend was studied. Differential Scanning Calorimetry (DSC) and Wide-angle X-ray diffraction (WAXD) were employed to study the melting behavior and crystalline structures, and the results indicated that the β-NA induced β phase of PP effectively in TPVs. With the increasing dosage of the β-NA incorporated in, the content of β phase increased while the total crystallinity of the blend kept constant. The fracture behavior of the TPVs with different β phase content was studied with double edge notched tensile loaded specimens (DENT) using the essential work of fracture (EWF) approach. The specific essential work of fracture (w_e) increased with the increasing of β phase content, indicating that the presence of β phase could effectively enhance the fracture toughness of TPVs.     

The time and temperature dependence of fracture toughness for thermoplastics

A method for obtaining comparative and intrinsic toughness of thermoplastics involves deriving two apparent material properties, yield stress and a critical value of stress field intensity factor (fracture toughness). These are obtained at various times under load and temperatures. A ductility factor can then be derived and is the ratio of the square of fracture toughness to yield stress. Ductility factors can create a league table of toughness for thermoplastics which correlates reasonably with impact toughness energies.Individual values of fracture toughness and yield stress provide an intrinsic measure of toughness. Consequently, the relative toughness of a range of thermoplastics are explained in these terms. Materials discussed include homopolymers and copolymers of propylene, uPVC, PMMA (and rubber toughened versions) and PES.There is an importance in obtaining geometry independent fracture toughness measurements, otherwise likely artefacts are introduced. Such considerations are discussed for uPVC and polypropylene materials.     

Determination and comparison of the essentialwork of fracture (EWF) of two polyester blends

The fracture toughness of blends of polypropylene terephthalate (PPT) with polyethylene terephthalate (PET) and polybutylene terephthalate (PBT) were investigated. Binary blends were prepared comprising 10:90, 30:70, 50:50, 70:30 and 90:10 mass/mass%. The fracture toughness was determined for each blend using the essential work of fracture (EWF) method and thin film double edge notched tension (DENT) specimens. The specific essential work of fracture, w _e, values obtained for blends of PET/PPT ranged from 27.33 to 37.38 kJ m^–2 whilst PBT/PPT blends yielded values ranging from 41.78 to 64.23 kJ m^–2. Differential scanning calorimetry (DSC) was employed to assess whether or not crystallinity levels influence the mechanical properties evaluated. The fracture toughness of PPT deteriorated with PET incorporation. However, high we values exceeding that of pure PPT were obtained for PBT/PPT blends across the composition range studied.     

Diglycidyl ether of bisphenol-A epoxy resin–polyether sulfone/polyether sulfone ether ketone blends: phase morphology, fracture toughness and thermo-mechanical properties

The properties of diglycidyl ether of bisphenol-A epoxy resin toughened with poly(ether sulfone ether ketone) (PESEK) and poly(ether sulfone) (PES) polymers were investigated. PESEK was synthesised by the nucleophilic substitution reaction of 4,4’-difluorobenzophenone with dihydroxydiphenylsulfone using sulfolane as solvent and potassium carbonate as catalyst at 230 °C. The T _g–composition behaviour of the homogeneous epoxy resin/PESEK blend was modelled using Fox, Gordon–Taylor and Kelley–Bueche equations. A single relaxation near the glass transition of epoxy resin was observed in all the blend systems. From dynamic mechanical analysis, the crosslink density of the blends was found to decrease with increase in the thermoplastic concentration. The storage modulus of the epoxy/PESEK blends was lower than that of neat resin, whilst it is higher for epoxy/PES blends up to glass transition temperature, thereafter it decreases. Scanning electron microscopic studies of the blends revealed a homogeneous morphology. The homogeneity of the blends was attributed to the similarity in chemical structure of the modifier and the cured epoxy network and due to the H-bonding interactions between the blend components. The fracture toughness of epoxy resin increased on blending with PESEK and PES. The increase in fracture toughness was due to the increase in ductility of the matrix. The thermal stability of the blends was comparable to that of neat epoxy resin.     

Polyimide incorporated cyanate ester/epoxy copolymers for high-temperature molding compounds

The rapid development of high-power devices has driven the requirement for high-temperature stable epoxy molding compounds. In this work, a designed polymer blend system consisting of cyanate ester/epoxy copolymers modified by polyimide (CE/EP-PI) has been studied. Polyimide used in this study has shown excellent dispersity in the cyanate ester and epoxy copolymer network (CE/EP), exhibiting homogeneous phase with a denser polymer network structure. With this polymer blend structure, CE/EP-PI system was proved to have a glass transition temperature as high as ~270 °C, increased modulus, and largely enhanced fracture toughness up to 2.06 MPa m^1/2. CE/EP-PI resins showed outstanding long-term stability at high temperature with low mass loss and increased fracture toughness after aging at 200 °C. This work provides a novel insight into the development of molding compounds based on polymer blends system with excellent high-temperature properties. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2412–2421     

Role of matrix-particle interface adhesion on fracture toughness of dual phase epoxy-polyethersulfone blend

Compatibility or miscibility of a polyethersulfone (ICI: Victrex 100P) and a tetrafunctional epoxy (Ciba-Geigy: MY-720), cured with an aromatic anhydride, has been studied using scanning electron microscopy, x-ray microanalysis, and dynamic mechanical spectroscopy. Fracture toughness of epoxy and a blend of epoxy and polyethersulfone has been measured using three point bend tests (ASTM: E-399-81). Fracture surfaces were examined by x-ray microanalysis for detecting concentration of sulfur, present in polyethersulfone, in the matrix and precipitated phase. The influence of the morphology of the epoxy/polyethersulfone blend on its fracture toughness and toughening mechanism has been studied. Fracture toughness values of unmodified and modified cured epoxy resin were evaluated as a function of test temperature.     

Extruder‐made TPO nanocomposites. I. Effect of maleated polypropylene and organoclay ratio on the morphology and mechanical properties

PP/PP‐g‐MA/MMT/EOR blend nanocomposites were prepared in a twin‐screw extruder at fixed 30 wt % elastomer and 0 to 7 wt % MMT content. Elastomer particle size and shape in the presence of MMT were evaluated at various PP‐g‐MA/organoclay masterbatch ratios of 0, 0.5, 1.0, and 1.5. The organoclay dispersion facilitated by maleated polypropylene serves to reduce the size of the elastomer dispersed phase particles and facilitates toughening of these blend nanocomposites. The rheological data analysis using modified Carreau‐Yasuda model showed maximum yield stress in extruder‐made nanocomposites compared with nanocomposites of reactor‐made TPO. Increasing either MMT content or the PP‐g‐MA/organoclay ratio can drive the elastomer particle size below the critical particle size below which toughness is dramatically increased. The ductile‐brittle transition shift toward lower MMT content as the PP‐g‐MA/organoclay ratio is increased. The D‐B transition temperature also decreased with increased MMT content and masterbatch ratio. Elastomer particle sizes below ～1.0 μm did not lead to further decrease in the D‐B transition temperature. The tensile modulus, yield strength, and elongation at yield improved with increasing MMT content and masterbatch ratio while elongation at break was reduced. The modified Mori‐Tanaka model showed better fit to experimental modulus when the effect of MMT and elastomer are considered individually. Overall, extruder‐made nanocomposites showed balanced properties of PP/PP‐g‐MA/MMT/EOR blend nanocomposites compared with nanocomposites of reactor‐made TPO. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

聚苯乙烯及其橡胶增韧聚合物的断裂韧性

用单边缺口法测定了聚苯乙烯(PS)和两种橡胶增韧PS(HIPS-A,10％橡胶,机械共混;HIPS-B,6.5％橡胶,接技共聚)裂纹在亚临界和失稳扩展时的断裂韧性K_c(s)、K_c(f)。实验结果表明,在限制裂纹尺寸范围之后,可用一般单边缺口拉伸方法测定橡胶增韧塑料HIPS的断裂韧性。     

Studies on fracture behaviors of immiscible polypropylene/ethylene‐co‐vinyl acetate blends with multiwalled carbon nanotubes

Immiscible polypropylene/ethylene‐co‐vinyl acetate (PP/EVA) blends with two different compositions, one (PP/EVA = 80/20) exhibits the typical sea‐island morphology and the other (PP/EVA = 60/40) exhibits the cocontinuous morphology, were prepared with different contents of f‐MWCNTs. The fracture behaviors, including notched Izod impact fracture and single‐edge notched tensile (SENT) fracture, were comparatively studied to establish the role of f‐MWCNTs in influencing the fracture toughness of PP/EVA blends. Our results showed that, for PP/EVA (80/20) system, f‐MWCNTs do not induce the fracture behavior change apparently. However, for PP/EVA (60/40) system, the fracture toughness of the blend increases dramatically with the increasing of f‐MWCNTs content. More severe plastic deformation accompanied by the fibrillar structure formation was observed during the SENT test. Furthermore, SENT test shows that the significant improvement in fracture toughness of PP/EVA (60/40) with f‐MWCNTs is contributed to the simultaneous enhancement of crack initiation energy and crack propagation energy, but largely dominated by crack propagation stage. Further results based on crystalline structures and morphologies of the blends showed that a so‐called dual‐network structure of EVA and f‐MWCNTs forms in cocontinuous PP/EVA blends, which is thought to be the main reason for the largely improved fracture toughness of the sample. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1331–1344, 2009     

Compatibilized poly(ϵ‐caprolactone)/poly(L‐lactide) blends for biomedical uses

A binary poly(L ‐lactide)/poly(ε‐caprolactone) (PLLA/PCL) (70/30 w/w) blend and a ternary PLLA/PCL/PLLA‐PCL‐PLLA blend of the same composition which contains 4 wt.‐% of a triblock PLLA‐PCL‐PLLA copolyester as compatibilizing agent were prepared by melt mixing at 200°C. Investigation of the thermal and mechanical properties of the blends and scanning electron microscopy of their fracture surfaces showed in the case of the ternary blend a better state of dispersion of PCL in the PLLA matrix and an improved toughness.     

Fracture energy of automotive clearcoats—I. Experimental methods and mechanics

The fracture energy of five automotive clearcoats was measured using fracture mechanics methods originally developed for measuring the fracture energy of brittle inorganic thin films. The fracture energy of the clearcoats ranged from 12 J m⁻² to 140 J m⁻². A discrepancy between the actual fracture energy and the ‘apparent’ fracture energy was observed and attributed to differences in the tensile stress-strain behavior of the materials, in particular the yield strength. Correcting for differences in their tensile stress-strain behavior, the apparent fracture energy of the films ranged from 13 J m⁻² to 330 J m⁻². Contrary to expectations, the fracture energy of the clearcoats did not decrease with increasing cross-link density. This was ascribed to differences in chemical architecture and additives in these commercial materials. Measuring the toughness in this manner shows promise for evaluating the long-term cracking resistance of automotive clearcoats.     

Effect of PEO‐PPO‐PEO copolymer on the mechanical and thermal properties and morphological behavior of biodegradable poly (L‐lactic acid) (PLLA) and poly (butylene succinate‐co‐L‐lactate) (PBSL) blends

A blend of two biodegradable and semi‐crystalline polymers, poly (L‐lactic acid) (PLLA; 70 wt%) and poly (butylene succinate‐co‐L‐lactate) (PBSL; 30 wt%), was prepared in the presence of various polyethylene oxide‐polypropylene oxide‐polyethylene oxide (PEO‐PPO‐PEO) triblock copolymer contents (0.5, 1, 2 wt%). Mechanical, thermal properties, and Fourier transform infrared (FTIR) analysis of the blends were investigated. It was found that the addition of copolymer to PLLA/PBSL improved the fracture toughness of the blends as shown by mode I fracture energies. It was supported by morphological analysis where the brittle deformation behavior of PLLA changed to ductile deformation with the presence of elongated fibril structure in the blend with copolymer system. The glass transition temperature (T_g), melting temperature (T_m) of PLLA, and PBSL shift‐closed together indicated that some compatibility exists in the blends. In short, PEO‐PPO‐PEO could be used as compatibilizer to improve the toughness and compatibility of the PLLA/PBSL blends. Copyright © 2010 John Wiley & Sons, Ltd.     

Environmentally benign green composites based on epoxy resin/bacterial cellulose reinforced glass fiber: Fabrication and mechanical characteristics

Bio-based bacterial cellulose (BC) epoxy composites were manufactured and their mechanical properties were examined. The BC was initially fabricated from Vietnamese nata de coco by means of alkaline pretreatment followed by solvent exchange. The obtained fibers were dispersed in epoxy resin (EP) by both mechanical stirring and ultrasonic techniques. The resulting blend was used as the matrix for glass-fiber (GF) composite fabrication using a prepreg method followed by multiple hot-press-curing steps. The morphology, mechanical characteristics and mode-I interlaminar fracture toughness of the fabricated composites were investigated. With a 0.3-wt% BC content, the mode-I interlaminar fracture toughness for both crack initiation and crack propagation were improved by 128.8% and 1110%, respectively. The fatigue life was dramatically extended by a factor of 12, relative to the unmodified composite. Scanning electron microscopy images revealed that the BC plays a vital role in increasing the interlaminar fracture toughness of a GF/EP composite via the mechanisms of crack reflection, debonding and fiber-bridging.     

酞侧基聚芳醚酮屈服应力与杨氏模量及断裂韧性关系的研究

研究了酞侧基聚芳醚酮（ＰＥＫ－Ｃ）的屈服应力、杨氏模量和断裂韧性等对温度的依赖性，给出了本文所用ＰＥＫ－Ｃ的屈服应力与杨氏模量及屈服应力与断裂韧性间的定量关系．研究表明，在１２℃～１９０℃的温度范围内，随温度的升高，材料的屈服应力、杨氏模量降低．断裂韧性Ｋ１ｃ和Ｇ１ｃ的对数与屈服应力间存在很好的线性关系．     

Effect of interfacial adhesion on the mechanical properties of poly(vinyl chloride) modified with cross-linked poly(methyl methacrylate) particles prepared by seeded emulsion polymerization

The effect of interfacial adhesion on the mechanical properties of an incompatible polymer blend was investigated. For this purpose, the preparation of non-cross-linked and cross-linked poly(methyl methacrylate) particles having mean sizes of about 0.8 μm was completed by seeded emulsion polymerization, and the number and the distribution of cross-linked points in the particles were varied. The emulsion particles obtained were powdered by a freeze–dry method and dispersed into a poly(vinyl chloride) matrix by melt blending. The non-cross-linked particles were completely dissolved in the matrix because poly(methyl methacrylate) has good compatibility with poly(vinyl chloride). On the other hand, in the case of the cross-linked particles, the mutual diffusion of the polymer molecules was restricted within the particle/matrix interfacial regions owing to the cross-linked points. Additionally, interfacial structures with different concentration slope dependent upon the number and the distribution of inner cross-linked points were developed with the same domain size. Mechanical and fracture properties were measured. As a result, both yield stress and fracture toughness decreased with a decrease in the interfacial adhesion, and the decrease was found to occur as a result of interfacial debonding. When the interfacial adhesion was sufficient it was never observed that the level was lower than that of the components. Received: 6 April 2000 Accepted: 29 September 2000