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.     

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.     

Influences of molecular weight and crystalline structure on fracture behavior of controlled-rheology-polypropylene prepared by reactive extrusion

The molecular weight of ethylene-block-co-polypropylene (co-PP) was adjusted by reactive extrusion with the incorporation of dicumyl peroxide (DCP), and the effect of molecular weight on the crystallization behavior, crystal morphology, and fracture behavior was investigated. It was found that, with increasing DCP content, the molecular weight (MW) decreased and the polydispersity (M_w/M_n) slightly decreased. After modification, the number of spherulites with obscure boundaries increased, and the size of the spherulites was more even due to increasing amount of grafting and micro-cross-linking structures, generated in co-PP degradation, which were acting as nucleating agents. Evaluated by essential work of fracture method, the specific essential work of fracture, w_e, was found to be strongly dependent on the molecular weight, especially, on the number average molecular weight (M_n) linearly, while the specific non-essential work of fracture, βw_p, was enhanced with decreasing z-average molecular weight (M_z), probably owing to the reduction of ultra-high molecular weight component in degraded co-PP.     

Essential work of fracture and j-integral measurements for ductile polymers

In the ductile tearing of polymers that neck before failure it is shown that the specific essential fracture work (w_e), consisting of the energies dissipated in forming and tearing the neck, is a material property for a given sheet thickness and is independent of specimen geometry. Work of fracture experiments using both double deep-edge notched (DENT) and deep-center notched tension (DCNT) geometries with different ligament lengths yielded almost identical w_e values for a grade of high-density polyethylene. These measurements for w_e are in fairly good agreement with the theoretical values based on the J integral evaluated along a contour surrounding the neck region near the crack tip. Under J-controlled crack growth conditions, it is shown that J_c obtained by extrapolation of the J_R curve to zero crack growth and the slope dJ/da are identical, respectively, to w_e and 4βw_p obtained from the straight line relationship between the specific total work of fracture (w_f) and ligament length (l).     

Dynamic fracture testing of polymethyl-methacrylate (PMMA) single-edge notched beam

Dynamic fracture in single-edge notched polymethyl-methacrylate (PMMA) beams have been investigated by three-point-bending impact testing with a drop-weight machine. A high-speed camera combined with the digital image correlation (DIC) method is used to capture the impact-induced crack initiation and propagation, as well as the beam deformation fields and the open mode strain at the original notch tip. The crack propagation length is recorded and the instantaneous crack velocity is calculated. Furthermore, the dynamic fracture toughness K_Id is quantified from the loading-displacement relations at different impact velocities. The effects of the impact velocity and impact energy on dynamic fracture toughness, fracture initiation strain, as well as the corresponding influences on the fracture propagation velocity, are discussed.     

Essential work of fracture,crack tip opening displacement,and J-integral relationship for a ductile polymer film

A unique set of double-edge notched tension specimens of a Polyethylene Terephthalate Glycol-modified film was tested in mode I, plane stress. The load was registered on a universal testing machine. The displacements, ligament lengths, and video frames were recorded by a Digital Image Correlation system. With these registered data, the essential work of fracture, J-integral, and crack tip opening displacement (CTOD) fracture concepts have been applied. The onset of crack initiation was through a complete yielded ligament. The analysis showed that the intrinsic specific work of fracture, w_e, is the specific energy just up to crack initiation, which is an initiation value. w_e has both a coincident value and the same conceptual meaning as J_o, the J-integral at the onset of crack initiation. The relationship between J_o and CTOD is also determined. The influence on the notch quality when the specimens were sharpened by two different procedures, femtosecond laser ablation and razor blade sliding, was analysed in detail.     

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     

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.     

Influence of block composition on crack toughness behaviour of styrene-b-(styrene-random-butadiene)-b-styrene triblock copolymers

The deformation and fracture behaviour of symmetric and asymmetric styrene-b-(styrene-random-butadiene)-b-styrene (S-SB-S) triblock copolymers with variations in their molecular architectures in terms of their outer PS block and the random SB middle block composition ratios have been investigated using essential work of fracture approach based on post yield fracture mechanics concept. The present investigations on crack resistance behaviour of these S-(S/B)-S triblock copolymers where effective interaction parameter (χ_eff) is systematically varied through the variation of block compositions and architecture is in continuation to our earlier communicated short article highlighting the phase behaviour-morphology and mechanical property interrelation. The crack initiation and propagation behaviours are correlated to morphology and dynamic mechanical properties as obtained from TEM, SAXS and DMA measurements. The influence of interaction parameter (χ-parameter) space which has been manipulated through the variation of block compositions has clearly manifested in their morphologies and in their mechanical properties. Further the kinetic aspects of fracture mechanical response have also been investigated where all the materials have clearly revealed block composition dependence. SEM analysis was carried out to understand the fracture modes prior to failure.     

Fracture behavior of core-shell rubber-modified crosslinkable epoxy thermoplastics

The fracture behavior of a core-shell rubber (CSR) modified cross-linkable epoxy thermoplastic (CET) system, which exhibits high rigidity, highT _g, and low crosslink density characteristics, is examined. The toughening mechanisms in this modified CET system are found to be cavitation of the CSR particles, followed by formation of extended shear banding around the advancing crack. With an addition of only 5 wt.% CSR, the modified CET possesses a greater than five-fold increase in fracture toughness (G _IC) as well as greatly improved fatigue crack propagation resistance properties, with respect to those of the neat resin equivalents. The fracture mechanisms observed under static loading and under fatigue cyclic loading are compared and discussed.     

Interlaminar fracture toughness of unidirectional DCB specimens: A novel theoretical approach

A novel theoretical approach is presented to calculate the mode I interlaminar fracture toughness (G_Ic) of double cantilever beam (DCB) specimens with low ratio of initial crack length-to-thickness (a₀/2h). This method is based on a sixth-order beam theory, namely Reddy-Bickford beam (RB), on Winkler elastic foundation (WEF) to account for both transverse shear deformation of the beam and local effects at the delamination front (root rotation). RB with only two generalized displacements w and ?; and three boundary conditions at ends and loading points of a shear deformable beam gives more accurate results than the fourth-order Timoshenko beam theory. The accuracy of the proposed method in prediction of initiation G_Ic values is evaluated together with other available models considering the experimental fracture toughness for moderately thick unidirectional E-glass/epoxy DCB specimens with small initial delamination lengths.     

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.     

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.     

添加磷酸氢钙对硅酸三钙骨水泥性能的影响

本文将磷酸氢钙(CaHPO₄·2H₂O,DCPD)添加到硅酸三钙(Ca₃SiO₅,C₃S)骨水泥中,采用X射线衍射(XRD),扫描电镜(SEM),万能力学测试机等手段对不同添加量的骨水泥进行表征,考察添加DCPD对硅酸三钙骨水泥性能的影响。实验表明,C₃S材料中添加10% DCPD有着优于单纯C₃S骨水泥的水化性能,骨水泥的初凝时间从92 min缩短到80 min;添加20%~30% DCPD能提高材料的短期力学强度,可以实现其短期抗压强度的优化;添加30%~40% DCPD的材料有着优良的生物活性与适中的可降解性能。结果表明,通过添加DCPD优化C₃S水泥的性能,对各种不同性能具有DCPD添加量的依赖性。通过进一步优化DCPD添加量,将可能获得优良的生物活性骨缺陷填充材料。     

Improving the interlaminar fracture toughness of carbon fiber/epoxy composites using clustered microcapsules

The composite laminates are susceptible to delamination between reinforcing plies during their long-term service. In this paper, we propose a modified carbon fiber/epoxy composite laminate with embedded clustered dual-component microcapsules in order to increase the interlaminar fracture toughness of the lamina. The details of microcapsules were illustrated using scanning electron microscope (SEM). The modified CF/EP composite laminates were fabricated using hot-compaction technique. Mode I interlaminar fracture tests were conducted using double cantilever beam specimens, then the values of opening fracture toughness G_IC were calculated to evaluate the toughening effect of modified laminates. The toughening mechanism was revealed and discussed through micrographs of the fracture surfaces obtained by ultra-depth microscope and SEM. The results show that clustered microcapsules after polymerization are equal to special Z-pinning, significantly enhancing the ability of crack arrest, and largely and roundly improved the G_IC values of resultant composite laminates. Meanwhile, the clustered microcapsules and matrix resin formed a second-phase material layer, which also absorbed the fracture energy and suppressed the expansion of cracks.     

Epoxy/CNT@X nanocomposite: Improved quasi-static,dynamic fracture toughness,and conductive functionalities by non-ionic surfactant treatment

The present work investigated the effects of non-ionic surfactant treatment on the dispersibility, surface chemistry and structure of carbon nanotube (CNT) particles. Subsequently, the fracture experiments of as-prepared epoxy/CNT@X nanocomposites were carried out under quasi-static and dynamic loading conditions. By simply introducing the steric repulsive force between CNT@X filler and epoxy matrix, improved mode-I critical-stress-intensity factor (K_Ic) and dynamic crack initiation toughness (K_Ii^d) of the epoxy/CNT@X nanocomposite were simultaneously obtained without compromising other desired physical properties, such as electrical properties and electro-thermal behavior. In the case of SHPB impact loading, high-speed imaging along with digital-image-correlation (DIC) technology was utilized to determine dynamic fracture parameters. The results showed a notable reinforcement for the epoxy/CNT@X nanocomposite category, producing maximum increase of ~79% and ~153% in K_Ic and K_Ii^d values relative to epoxy/CNT nanocomposite at such maximum content of 1.0 wt%, respectively. The most delayed crack initiation time (59.9–68.4 μs) and slowest crack-tip velocity (229 ± 28 m/s) were also observed in the epoxy/CNT@X_1.0 case. These results may be explained by improved dispersibility and interfacial adhesion after surfactant treatment.     

Synergistic Effects of the ZnCl2-SiCl4 Modified TiCl4 /MgCl2 /THF Catalytic System on Ethylene/1-Hexene and Ethylene/1-Octene Copolymerizations简

The copolymerizations of ethylene with 1-hexene or 1-octene by using TiCl4 /MgCl2 /THF catalysts modified with different metal halide additives(ZnCl2, SiCl4, and the combined ZnCl2-SiCl4) were investigated based on catalytic activity and copolymer properties. It was found that the catalyst modified with mixed ZnCl2-SiCl4 revealed the highest activities for both ethylene/1-hexene and ethylene/1-octene copolymerization. The increase in activities was due to the formation of acidic sites by modifying the catalysts with Lewis acids. Based on the FTIR measurements, the characteristic C―O―C peaks of the catalysts modified with metal halide additives were slightly shifted to lower wavenumber when compared to the unmodified catalyst. This showed that the modified catalysts could generate more acid sites in the TiCl4 /MgCl2 /THF catalytic system leading to an increase in activities as well as comonomer insertion(as proven by13C-NMR). However, Lewis acidmodifications did not affect the microstructure of the copolymers obtained. By comparison on the properties of copolymers prepared with the unmodified catalyst, it was found that polymers with ZnCl2 and/or SiCl4 modification exhibited a slight decrease in melting temperature, crystallinity and density. It is suggested that these results were obtained based on the different amount of α-olefins insertion, regardless of the types of Lewis acids and comonomer.     

Mechanical and fracture behavior of polypropylene/poly(ethylene-co-vinyl alcohol) blends compatibilized with ionomer Na

In this work the deformation and fracture behavior of PP/EVOH blends compatibilized with ionomer Na⁺ at room and low temperature was studied. Uniaxial tensile tests on dumb-bell samples and fracture tests on single-edge notched bending (SENB) specimens were performed for 10 wt.% and 20 wt.% EVOH blends with different ionomer content at 23 °C and −20 °C. The incorporation of EVOH to PP led to less ductile materials in tension as judged by the lower values of the ultimate tensile strain displayed by all PP/EVOH blends in comparison to neat PP. In contrast, the ionomer Na⁺ addition partially counteracted this effect. The compatibilizing effect of ionomer Na⁺ was also evident in fracture results since higher values of the fracture parameter were obtained for the ternary blends. SEM observations also confirmed this effect. On the other hand, PP/EVOH blends exhibited different fracture behavior with test temperature. All blends showed “pseudo stable” behavior at room temperature characterized by apparently stable crack growth that could not be externally controlled. On the contrary, blends behaved as semi-brittle at −20 °C with some amount of stable crack growth preceding unstable brittle fracture. Finally, irrespectively of the temperature or the ionomer content all PP/EVOH blends exhibited more ductile fracture behavior with a higher tendency to stable crack propagation than neat polypropylene.     

Plane-strain fracture energy instability and mixed mode crack propagation in polycarbonate at room temperature

A study of crack propagation in double cantilever beam specimens of polycarbonate has revealed a large velocity-dependent instability in the plane-strain fracture energy G_Ic. At a crack velocity of 10^?2 in./min, G_Ic accords with published values obtained from tensile studies of precracked specimens. Crack propagation in doubly grooved double cantilever beam specimens is unstable at higher velocities. The G_Ic's during crack jumping and at crack arrest are estimated to be 0.2 and 2%, respectively, of the low crack speed value, based on the amounts of crazing produced at the various crack speeds. Evidence of plane-strain shear deformation at the low speed crack tip is presented. The G_Ic instability is suggested to arise from differences in the kinetics of shear failure and craze breakdown.     

Fracture Behaviour and Deformation Mechanisms of Polyimide/Silica Hybrids

Films of particulate polyimide-silica hybrids were produced by a sol-gel process and were examined in terms of morphological structure and fracture properties at different temperatures, varying from 20 to 250 °C. The fracture toughness characteristics were studied by the “Essential Work of Fracture” method using double edge-notched specimens of different ligament lengths. The results showed that the fracture toughness, expressed through the essential (w_e) and non-essential work of fracture (βw_p) parameters increased when submicron silica particles were dispersed in the polyimide matrix. In particular, it was found that both w_e and βw_p increased with temperature to a larger extent than the parent polyimide. This enhancement in the fracture toughness was attributed to extensive cavitations and shear yielding originating at the particle interface and within the matrix.