Fracture toughness of gamma irradiated polycarbonate sheet using the essential work of fracture

Polycarbonate (PC), a ductile polymer, has been found by both linear elastic fracture mechanics and impact tests to present a ductile-brittle transition, which depends on notched specimen thickness, test speed and gamma irradiation. Owing to large amounts of plastic deformation, fracture toughness measurements by these test methods are not precise. In the present communication, a better method, the Essential Work of Fracture (EWF), to assess the fracture characteristics in plane state of stress was for the first time used to evaluate the fracture toughness of PC sheets subjected to gamma irradiation dose. Three-points bend tests of sharp pre-cracked specimens with different ligament lengths were 340 kGy gamma irradiated. EWF results showed that the total fracture work increased linearly with length for both non-irradiated and gamma irradiated conditions. A significant decrease in EWF fracture toughness was associated with brittleness promoted by gamma irradiation. This brittleness was also confirmed by macro and microscopy (SEM) evidence.     

Determination of the tear properties of thermoplastic polyester elastomers (TPEEs) using essential work of fracture (EWF) test method

In this paper, three commercial thin thermoplastic polyester elastomer (TPEE) samples are prepared by the injection molding process. Essential work of fracture (EWF) tests, which have been extended to cover the ductile tearing of polymeric materials that neck before fracture, in both the machine direction (MD) and the transverse direction (TD) are executed based on the ESIS TC-4 protocol as well as conventional trouser tear tests. Through an examination of the EWF test results, the tear properties of TPEE samples can be determined quantitatively. Partitioning of the test results is attempted for identifying the contribution of yielding and necking/tearing on the overall tear properties of TPEE samples. In addition, the fracture surfaces of TPEE samples following EWF tests are investigated through scanning electron microscopy (SEM). The root causes of the differences in tear properties are studied using the morphology of the fracture surfaces of TPEE samples.     

Application of the essential work of fracture concept to nanostructured polymer materials

The first part of the paper deals with a critical discussion of the methodical basis of essential work of fracture (EWF) concept with respect to the specimen geometry (especially the notch depth) and application to polymers. In the second part, an in situ testing device, which combines a tensile testing machine with an optical strain-field measuring system, has successfully demonstrated possibility of characterization of fracture behaviour of polystyrene-polybutadiene block copolymers and block copolymer/homopolymer blends as examples of nanostructured polymer materials. It has been shown that knowledge of the time evolution of the strain field close to the crack tips leads to a simple verification of the basic precondition for the applicability of the EWF concept, the precondition “plastic zone coalescence-before-stable crack propagation”.     

Refined fixture design for effective essential work of fracture toughness characterization of m-LLDPE thin films

Characterization of Mode-I fracture toughness of ductile polymeric thin films is nontrivial. Proper specimen preparation and experimental procedures are required to ensure in-plane tensile loading. In this study, a custom-built double-edge notched tensile test fixture was employed to characterize the Mode-I essential work of fracture (EWF) toughness of metallocene linear low-density polyethylene (m-LLDPE) films. Effects of specimen geometry, strain rate and film orientation on the specific essential work of fracture, w_e, and the specific non-essential work of fracture, w_p, were investigated. Results indicate both EWF parameters are independent of the crosshead speed, gauge length (distance between upper and lower clamps) and specimen width within the ranges tested. w_e is significantly higher for thinner films and for crack propagation perpendicular to the blown film machine direction (MD). The usefulness of EWF for evaluating m-LLDPE fracture toughness is discussed.     

Microstructure and fracture behavior of semicrystalline polymer–clay nanocomposites

The relationships between the microstructure and the fracture behavior of three polymer/clay nanocomposites were studied. Two different polymer matrices were chosen, namely polyamide‐6 and polyethylene (compatibilized with PE‐g‐MA or PE‐g‐PEo), to reach very different clay dispersion states. The microstructure was characterized in terms of polymer crystallinity, orientation of the polymer crystalline lamellae, clay dispersion state, and orientation of the clay tactoids. The mechanical behavior was characterized by tensile tests. The essential work of fracture (EWF) concept was used to determine the fracture behavior of the nanocomposites. Both tensile and EWF tests were performed in two perpendicular directions, namely longitudinal and transversal. It is shown that the fracture behaviors of the matrices mainly depend on the polymer crystalline lamellae orientation. For the nanocomposites, the relationships between the matrix orientation, the clay dispersion states, the values of the EWF parameters (w_e and βw_p), and their anisotropy are discussed. The results show that the lower the average clay tactoid thickness, the lower is the decrease of fracture performance for the nanocomposite and the more consumed energy as longer the path of the crack. Besides, a linear dependence of the anisotropy of the EWF parameters of the nanocomposites on the average clay aspect ratio is found. The more exfoliated the structure is, the less pronounced the anisotropy of the EWF parameters. Interestingly, it is thought that the average clay aspect ratio is the parameter representing the clay dispersion state that governs the fracture anisotropy of the nanocomposites (as the elastic properties determined by tensile tests). © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1820–1836, 2008     

Fracture studies of polypropylene/nanoclay composite. Part I: Effect of loading rates on essential work of fracture

Polypropylene (PP)/Montmorillonite (MMT) nanoclay based composite was prepared by melt compounding with maleic anhydride grafted polypropylene (MA-g-PP) as a compatibilizer in a twin-screw extruder, and the test specimens were injection molded. Mechanical properties such as tensile modulus, flexural modulus, yield strength and maximum percent strains were measured for pure PP and PP based nanocomposite to establish the effect of clay platelet reinforcement. The fracture properties were measured by using the essential work of fracture (EWF) method. PP/clay nanocomposite shows 25% improvement in specific EWF compared to pure PP. The variation of EWF parameters with loading rate is discussed, whilst the mechanisms of fracture are considered in a subsequent paper.     

Impact fracture toughness of polyamide‐6/montmorillonite nanocomposites toughened with a maleated styrene/ethylene butylene/styrene elastomer

Polyamide‐6 (PA6)/montmorillonite (MMT) nanocomposites toughened with maleated styrene/ethylene butylene/styrene (SEBS‐g‐MA) were prepared via melt compounding. Before melt intercalation, MMT was treated with an organic surfactant agent. Tensile and impact tests revealed that the PA6/4% MMT nanocomposite fractured in a brittle mode. The effects of SEBS‐g‐MA addition on the static tensile and impact properties of PA6/4% MMT were investigated. The results showed that the SEBS‐g‐MA addition improved the tensile ductility and impact strength of the PA6/4% MMT nanocomposite at the expenses of its tensile strength and stiffness. Accordingly, elastomer toughening represents an attractive route to novel characteristics for brittle clay‐reinforced polymer nanocomposites. The essential work of fracture (EWF) approach under impact drop‐weight conditions was used to evaluate the impact fracture toughness of nanocomposites toughened with an elastomer. Impact EWF measurements indicated that the SEBS‐g‐MA addition increased the fracture toughness of the PA6/4% MMT nanocomposite. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 585–595, 2005     

聚丙烯/POE共混组成对材料断裂行为的影响

采用基本断裂功(EWF)方法对聚丙烯(PP)/聚烯烃弹性体(POE)共混物的注射双边缺口拉伸试样的断裂行为进行了研究,比较了不同POE含量对共混物各断裂参数的影响.结果表明,PP和用量为5phr POE的共混物都可完全满足EWF方法的要求,共混物的断裂韧性-比基本断裂功we,较PP有显著提高;POE用量为10phr以上的共混物则出现明显的成颈现象而限制了EWF方法的应用;PP和各种POE用量的共混物都得到了其屈服所需要的比基本断裂功we,y和比塑性功β′wp,y.     

Determination and comparison of the plane stress essentialwork of fracture of three polyesters PET, PPT and PBT

The essential work of fracture (EWF) method has been used to study the relationship between molecular structure and thin film fracture toughness for three ductile polyesters at ambient temperature. The fracture toughness of PPT is of particular interest. Successful fracture characterisation of thin film polyesters has been achieved by the EWF method using double edge notched tension (DENT) specimens. The specific essential work of fracture, w _e, for polyethylene terephthalate (PET), polypropylene terephthalate (PPT) and polybutylene terephthalate (PBT) films is found to be 35.54±2.56, 41.03±3.23 and 31.34±8.60 kJ m^–2, respectively. Differential scanning calorimetry (DSC) has been employed to investigate the crystallinity of the polymers concerned and the effect of this on their EWF values.     

Morphology and fracture behaviour of polyethylene/Mg-Al layered double hydroxide (LDH) nanocomposites

Fracture behaviour of polyethylene (PE)/Mg-Al layered double hydroxide (LDH) based nanocomposites has been studied by essential work of fracture (EWF) approach. Transmission electron microscopy (TEM and X-ray diffraction (XRD) analysis have been used to investigate the morphological features of these nanocomposites. A maximum in the non-essential work of fracture was observed at 5 wt.% LDH demonstrating enhanced resistance to crack propagation compared to pure PE. Morphological analyses of the nanocomposites show that the dispersed LDH platelets are partially exfoliated and also forms clusters with polymer chains remaining entrapped within. Rheological analyses show that the typical low-frequency Newtonian flow behaviour, as observed in unfilled polymer, shifts to shear-thinning behaviour with increasing LDH concentration. At 5 wt.% LDH a ductile-to-brittle transition has been observed. Fracture surface investigation by SEM reveals the arresting of the plastic crack growth by the LDH particle clusters, which is more significant at 5 wt.% LDH content. At higher LDH concentrations, the number of such particle clusters increases causing decrease in the average distance between them. As a result large-scale plastic deformation of the matrix at higher LDH concentration is effectively arrested favouring small strain failure and this in turn reaffirms the possible existence of a ductile-to-brittle transition. The study in general reveals that the resistance against crack initiation (essential work of fracture: EWF) and crack propagation (non-essential work of fracture: βw_p) in these nanocomposites are structurally correlated with the matrix behaviour and the morphology (state of LDH particle dispersion) respectively.     

Fracture toughness assessment of polypropylene random copolymer with multiple stress modes using the essential work of fracture theory

In recent years, the essential work of fracture (EWF) method has been extensively employed for assessing a material's toughness by specific essential fracture work, especially for polymers showing ductile failure. However, most research has studied either the in-plane stress mode or the out-of-plane stress mode. To obtain a more in-depth understanding of the EWF theory, the specific essential and non-essential fracture work of polypropylene random copolymer (PP-R) was investigated in both in-plane stress mode and out-of-plane stress mode. The effects of ligament length, amount of pre-cracking and pre-cracking method on the specific essential and non-essential fracture work were explored. The specific essential fracture work obtained in both stress modes is compared and discussed.     

Essential Work of Fracture of photo-oxidized LDPE/EVA Films

The Essential Work of Fracture approach (EWF) was used to determine how UV-C irradiation alters the fracture behaviour of LDPE/EVA films. Complementary characterization was performed by FTIR, DSC, TOM, and uniaxial tensile testing. The crosslinking reactions that govern photo-oxidation process at initial stage of exposure stiffened the amorphous phase of the polymer, leading to films with enhanced elastic modulus, yield stress and ultimate strength, but impaired strain at break. In the fracture experiments carried out on films irradiated within 0 and 5 days, EWF methodology requirements were met and the corresponding fracture toughness parameters (w_e and βw_p ) turned out to be sensitive to UV-C irradiation. Longer irradiation time triggered the development of microcracks, which not allowed further stable crack growth and invalidated the application of EWF approach.     

Crazing in polyethylene

The tensile stress—strain curves of various types of polyethylene were compared from 77 to 298 K in nitrogen, isopentane, and the inert environment of helium at various strain rates. It was found that in general polyethylene crazes in a gas such as nitrogen at a temperature below 1.6 times its boiling point and in isopentane. Although the behavior of polyethylenes is similar to that of other polymers with regard to crazing in gases at low temperatures, they are in general less sensitive to the gas. The decrease in tensile strength of polyethylene in an environmental gas increases with crystallinity. The differences in the intrinsic low-temperature brittle fracture stress are attributed to differences in the density of tie molecules. The intrinsic yield point at room temperature showed the usual increase with increasing crystallinity, but all the polyethylenes have the same yield point below the γ transition temperature.     

Work of Fracture and Strain-induced Cold Crystallization Behavior of Amorphous Copolyester Sheets

The toughness of amorphous copolyester sheets was assessed by the essential work of fracture (EWF) concept. While the yielding-related work of fracture terms did not change significantly, the necking-related parameters strongly decreased with decreasing entanglement density of the copolyesters having different amounts of cyclohexylenedimethylene (CHDM) units in their backbones. Furthermore, copolyesters with high CHDM content and thus less entanglement density showed full recovery of the necked region beyond the glass transition temperature, i.e. the ‘plastic’ zone in the related specimens formed by cold drawing and not by true plastic deformation. By contrast, the copolyester with negligible amount of CHDM did not show this shape recovery. Modulated differential scanning calorimetry (MDSC) revealed that the necking in the latter system was accompanied by strain-induced crystallization. The superior work hardening in the necking stage of the respective poly(ethylene terephthalate) (PET) specimens can thus be ascribed to stretching of the entanglement network with superimposed crystallization. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of thermal ageing on mechanical and thermal behaviors of linear low density polyethylene pipe

The mechanical and thermal behaviors of linear low density polyethylene (LLDPE) pipe with variation in thermal exposure time were studied. The prolongation of thermal exposure time leads to a progressive increase, until 6000 h, in tensile strength and a slight increase in hardness, while a proportional decrease in elongation at break. These results can be explained by the increase of crystallinity, followed by the increase of crosslinking density and the decrease in chain mobility due to thermal oxidation as the exposure time increases. The additional ageing to the antioxidant-depleted LLDPE pipe induces the formation of T2 endotherm, which leads to a negative effect in mechanical properties. Long-term hydrostatic pressure test result implies the existence of transition point from ductile to brittle fracture in terms of the thermal exposure time. Chemiluminescence (CL) and oxidation induction time (OIT) tests are employed to monitor the thermo-oxidative degradation of LLDPE pipe. The CL emission intensity increases with increasing with thermal exposure time. Furthermore, the OIT result suggests that after 6000 h of the thermal ageing, the depletion of antioxidant originally added in LLDPE pipe occurs. Fourier transform-infrared spectroscopy results show the increase of carbonyl (-CO) and hydroxyl (O-H) function groups on the surface of thermally exposed LLDPE pipe. This result suggests that the hydrocarbon groups locally undergo the oxidation on the LLDPE surface due to thermal degradation.     

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.     

Impact‐specific essential work of fracture of maleic anhydride‐compatibilized polypropylene/elastomer blends and their composites

Charpy drop‐weight‐impact and essential work of fracture (EWF) characteristics of maleic anhydride (MA)‐compatibilized styrene–ethylene butylene–styrene (SEBS)/polypropylene (PP) blends and their composites reinforced with short glass fibers (SGFs) were investigated. MA was grafted to either SEBS copolymer (SEBS‐g‐MA) or PP (PP‐g‐MA). The mPP blend was prepared by the compounding of 95% PP and 5% PP‐g‐MA. Drop‐weight‐impact results revealed that the mPP specimen had an extremely low impact strength. The incorporation of SEBS or SEBS‐g‐MA elastomers into mPP improved its impact strength dramatically. Similarly, the addition of SEBS was beneficial for enhancing the impact strength of the SGF/SEBS/mPP and SGF/SEBS‐g‐MA/mPP hybrids. A scanning electron microscopy examination of the fractured surfaces of impact specimens revealed that the glass‐fiber surfaces of the SGF/SEBS/mPP and SGF/SEBS‐g‐MA/mPP hybrids were sheathed completely with deformed matrix material. This was due to strong interfacial bonding between the phase components of the hybrids associated with the MA addition. Impact EWF tests were carried out on single‐edge‐notched‐bending specimens at 3 m s^?1. The results showed that pure PP, mPP, and the composites only exhibited specific essential work. The nonessential work was absent in these specimens under a high‐impact‐rate loading condition. The addition of SEBS or SEBS‐g‐MA elastomer to mPP increased both the specific essential and nonessential work of fracture. This implied that elastomer particles contributed to the dissipation of energy at the fracture surface and in the outer plastic zone at a high impact speed of 3 m s^?1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1881–1892, 2002     

Fracture analysis of annealed PVDF films

The effect of annealing 50 μm thick, extruded poly(vinylidene fluoride) films in form (-PVDF) was investigated by differential scanning calorimetry and mechanical testing on unnotched and double-edge notched tensile specimens. As to the fracture behavior, micromechanisms of cavitation, spherulite breakdown, fiber bundle structure formation and - to β-phase transformation were detected. The progressive structural evolution taking place during annealing affected the deformation and fracture behavior significantly. While the annealing-induced subtle distinctions in microstructure are slightly reflected by the determined mechanical and essential work of fracture (EWF) properties, a distinct differentiation was possible by analysis of the process zone. A correlation between characteristic values of the process zone at necking and the endothermic transitions due to secondary crystallization determined by differential scanning calorimetry (DSC) was established. Annealing of PVDF films facilitates the micromechanism of cavitation, which is presumably related to perfection of morphological superstructures (spherulites) and, thus, interspherulitic stress concentration and failure.     

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.     

Relationships between molar mass and fracture properties of segmented urethane and amide copolymers modified by chemical degradation

This publication highlights the structure–property relationships in several thermoplastic elastomers (TPEs): one poly(ether-block-amide) and two thermoplastic polyurethane elastomers with ester and ether soft blocks. Structural changes are induced by chemical degradation from virgin samples through hydrolysis and oxidation. Molar mass measurements show an exclusive chain scission mechanism for all TPEs, regardless of the chemical modification condition. Mechanical behavior was nevertheless obtained from uniaxial tensile testing and fracture testing while considering the essential work of fracture (EWF) concept. During the macromolecular scission process, elongation at break shows a plateau followed by a drop, while stress at break decreases steadily. Once again, the trend is identical for all TPEs in all conditions considered. The βw_p parameter determined using the EWF concept exhibits an interesting sensitivity to scissions (i.e., molar mas decrease). Plotting elongation at break as a function of molar mass reveals a strong correlation between these two parameters. This master curve is particularly remarkable considering the range of TPEs and chemical breakdown pathways considered (hydrolysis and oxidation at several temperatures). Relevant structure–property relationships are proposed, highlighting that molar mass is a predominant parameter for determining the mechanical properties of thermoplastic elastomers.