Essential Work Of Fracture Parameters Research Articles

Processing–microstructure–fracture toughness relationships in PP/EPDM/SiO2 blend‐nanocomposites: Effect of mixing sequence

AbstractThis study investigates the mechanical properties and fracture behavior of polypropylene (PP)‐based blend‐nanocomposites comprising 30 wt.% ethylene–propylene–diene monomer (EPDM) and 5 wt.% SiO2 nanoparticles. Different mixing sequences were employed to prepare the nanocomposites, and the resulting morphology development and dispersion states of modifiers were analyzed. Mechanical performance of the nanocomposites was evaluated through quasi‐static and high‐speed dynamic fracture tests. The dispersion and distribution of SiO2 nanoparticles within the nanocomposites were significantly influenced by the mixing protocol. In impact fracture tests, the presence of nanoparticles exhibited a beneficial efffect on fracture energy, demonstrating a synergistic toughening effect of the soft EPDM and rigid SiO2 particles. Conversely, adverse effects were observed in quasi‐static tests. Essential work of fracture (EWF) parameters indicated an increase in the yielding component and a decrease in the necking‐to‐tearing component with SiO2 incorporation into the PP/EPDM blends. During impact loadings, the highest improvement in crack propagation resistance was observed in nanocomposites with nanoparticles localized around the rubbery domains forming a network‐like structure of EPDM/SiO2‐nanoparticles. Morphologies where rubber domains and nanoparticles were separately distributed in the PP matrix resulted in the lowest fracture parameters. Energy dissipation mechanisms were elucidated, revealing multiple void formation followed by matrix shear yielding as the primary source under both quasi‐static and impact fracture conditions. In the latter case, stress‐concentrating percolated structures in the PP matrix facilitated the nucleation of dilatational bands evolving into highly stretched void‐fibrillar structures upon further loading. These findings contribute valuable insights into tailoring nanocomposite morphologies for enhanced mechanical performance in different loading scenarios.Highlights Fracture behavior of PP/EPDM/SiO2 ternary systems was evaluated by EWF methodology and Izod impact test. Rubber particles surrounded by silica nanoparticles led to a percolated morphology and as a result to superior impact resistance. EWF parameters were mostly controlled by the tearing‐related parts regardless of phase morphology. The impact toughness was mainly controlled by the dispersion and distribution characteristics of the SiO2 nanoparticles.

Evaluation of different specimen geometries and testing configurations for the assessment of the degree of gelation of unplasticized poly(vinyl chloride) pipes using the essential work of fracture approach

An unplasticized (U-PVC) pipe sample (ABNT NBR 5647-2, nominal diameter DN 100, outside diameter 110 mm) is studied using essential work of fracture (EWF). This pipe sample is also tested for the quality of processing (degree of gelation) via differential scanning calorimetry (DSC) and tensile strength. The comparative evaluation of different specimen configurations – flat (SENB) and curved (CTPB) specimens in three-point bending, using different support spans, and C-shaped (SNRB) and full rings in tension (SNRT), shows significant differences in their suitability for EWF testing. While SENB- and CTPB-type specimens, independently of the support span ratio S/W adopted, confirm previous findings that both specimen configurations are very suitable for EWF testing of pipe samples, SNRB- and SNRT-type specimens, by their turn, do not show suitability for the purposes of this study, at least using the testing configurations here adopted. When comparing results from this study with previous publications from the authors, it is shown that EWF seems to be an interesting alternative approach for gelation assessment, since it is evident from the comparative results that there is some high degree of correlation between EWF parameters and the DSC degree of gelation.

Physical assessment of essential work of fracture parameters based on m-LLDPE blown films

A new experimental approach based on essential work of fracture (EWF) method and an in-situ digital image correlation strain mapping analysis has been developed to quantify different aspects of fracture energy on model metallocene linear low-density polyethylene (m-LLDPE) blown films. Three distinctive fracture process zones have been identified from the photoelastic observation of the model m-LLDPE blown films. These three zones include the essential work (We) zone due to necking and crack propagation, the non-essential plastic deformation zone (Wp), and a newly proposed recoverable viscoelastic deformation zone (Wv), which is found to be nontrivial. Furthermore, in contrast to the assumption involved in the EWF theory, We is found to be proportional to the volume of the materials involved in the inner fracture process zone. The current approach allows for exact physical interpretation of the EWF parameters and their correlation to material characteristics. Usefulness of the present work for development of stronger polymeric blown films is discussed.

Analysis of the effect of the test rate on polymer essential work of fracture parameters for the small punch test

ABSTRACTIn recent decades, one of the non‐standard tests that has been consolidated as a viable alternative in those cases where there is not sufficient material to carry out standard tests is the small punch test. This test basically consists of deforming a miniature specimen using a high strength punch. It is possible for this miniature specimen to have an initial pre‐notch with the aim of improving the fracture behavior estimation of the material analyzed. Recently, to characterize the fracture properties of polymer sheets under plane stress conditions, there has been an attempt to establish the feasibility of applying the essential work of fracture (EWF) method in polymer pre‐notched miniature specimens. This article intends to go one step beyond and focuses on the test rate, which is an important aspect in the EWF application. Its effect on the EWF parameters in polymer pre‐notched miniature specimens has been analyzed and its correlation has been established with the results obtained from standard specimens. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43314.

Determination of the fracture energy in polymeric films by in situ photoelasticimetry on double edge notch specimen

ABSTRACTThe fracture toughness is key parameters to select polymeric films. The essential work of fracture (EWF) is a phenomenological but efficient way to characterize this resistance to fracture. One can gain valuable information on the resistance to perforation and propagation of flaws. A new technique was developed to better understanding the EWF experiments. A tensile test combined to photoelasticimetry allows following in situ the geometry and amount of plastic deformation on double edge notched specimen. The EWF parameters are determined when the plastic deformation appears constant, so when the fracture energy Wf only contributes to rupture filament. This new methodology requires just a single sample, whereas at least five specimens are required for general method. It will help characterize expensive polymeric films or reveal the heterogeneous behavior, for instance after polymer ageing. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 132, 42854.

New fracture characterization in polymer miniature specimens based on the essential work of fracture

ABSTRACTThe fracture characterization under plane‐stress conditions in polymer sheets in recent decades has usually been done through the application of the essential work of fracture (EWF) method. However, when deeply double‐edged, notched tensile standard specimens cannot be obtained, the use of alternative small pieces, such as prenotched miniature specimens, could be a viable solution. This is why we examined the new fracture characterization in polymer‐prenotched small punch test (SPT) specimens in this study. With the results that we obtained, the feasibility of using prenotched SPT specimens for evaluating the EWF parameters and their correspondence with the results from standard specimens were established. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 132, 42837.

Polymer pre-notched small punch specimens to evaluate the essential work of fracture (EWF) parameters

Abstract Standardized deeply double edge notched tensile specimens (DDEN-T) have been usually used to assess fracture properties in thin polymer plates under plane stress conditions. The methodology is based on a multi specimen technique and is well known as the essential work of fracture (EWF) method. In some special circumstances these standard specimens cannot be machined as could be the case if the material is limited. A possible solution proposed in this paper could be the use of pre-notched small punch test specimens, in which the aim is to establish the feasibility of using pre-notched SPT specimens for evaluating the essential work of fracture parameters. The results obtained with these miniature specimens have been compared with those obtained from standard DDEN-T specimens.

Effect of processing parameters on essential work of fracture toughness of LLDPE blown films

Blown film process is one of the most attractive processes for producing low-cost, high-performance polyolefin films. In this study, mode-I essential work of fracture (EWF) analysis and Elmendorf tear test were performed on m-LLDPE films to investigate how different processing conditions influence blown film EWF and tear properties. After the EWF test, the film was carefully characterized, especially within the necked zone. Effects of frost-line height, draw down ratio, blow-up ratio, and haze-zone region on the EWF parameters of the films were determined. Correlation between the EWF parameters and the films' Elmendorf tear properties was also made. The usefulness of the EWF test for investigating ductile polymeric blown films fracture performance is discussed. POLYM. COMPOS., 55:2403–2413, 2015. © 2015 Society of Plastics Engineers

Fractographic analysis of the crack resistance of styrene–acrylonitrile/polybutadiene‐g‐styrene–acrylonitrile blends as evaluated by the essential work of fracture method

ABSTRACTThe fracture surfaces and deformation micromechanisms of styrene‐co‐acrylonitrile (SAN)/polybutadiene‐g‐styrene‐co‐acrylonitrile (PB‐g‐SAN) blends with the compositions ranging from 65/35 to 0/100 were studied with a scanning electron microscopy technique. The results were compared to the essential work of fracture parameters obtained in a previous study conducted on double‐edge notched tension specimens. Different plastic damage mechanisms were observed, and they depended on the blend composition. For blends of 65/35 and 45/55, a high degree of rubber particle cavitation and multiple cracking followed by the massive shear yielding of the matrix were found to be the main source of energy dissipation during crack growth. Within this compositional range, more intense plastic damage in a larger volume of material, especially at the notched region, was observed as the concentration of the rubbery phase increased. For the 25/75 blend, the prevailing mechanism was pure shear yielding without any sign of cavitation inside the particles, and the fracture surface became relatively flat and was covered with aligned small microcracks. This sample showed the highest specific essential work (we) value among the blends examined in the previous study. For the samples containing concentrations of dispersed phase higher than 75%, the shear yielding process gradually became less important with the progressive importance of multiple crazing so that high‐magnification micrographs revealed extensive microcracking/crazing both inside and between the rubber particles, as the only active deformation micromechanism for neat PB‐g‐SAN. The variation we and specific plastic work of fracture with the PB‐g‐SAN phase content were successfully explained in terms of prevalent deformation mechanisms. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40072.

On the toughness of thermoplastic polymer nanocomposites as assessed by the essential work of fracture (EWF) approach

The essential work of fracture (EWF) approach is widely used to determine the plane stress fracture toughness of highly ductile polymers and related systems. To shed light on how the toughness is affected by nanofillers EWF-suited model polymers, viz. amorphous copolyester and polypropylene block copolymer were modified by multiwall carbon nanotube (MWCNT), graphene (GR), boehmite alumina (BA), and organoclay (MMT) in 1 wt% each. EWF tests were performed on deeply double-edge notched tensile-loaded specimens under quasistatic loading conditions. Data reduction occurred by energy partitioning between yielding and necking/tearing. The EWF prerequisites were not met with the nanocomposites containing MWCNT and GR by contrast to those with MMT and BA. Accordingly, the toughness of nanocomposites with homogeneously dispersed and low aspect ratio fillers may be properly determined using the EWF. Results indicated that incorporation of nanofillers may result in an adverse effect between the specific essential and non-essential EWF parameters.

Essential work of fracture analysis of short glass fiber and/or calcite reinforced ABS/PA6 composites

The effect of reinforcing agent type and composition on the fracture behavior of short glass fiber (SGF), CaCO3 particle, and glass fiber/CaCO3 hybrid reinforced ABS/PA6 blend based composites have been studied by using the essential work of fracture (EWF) method. Two millimeter thick rectangular shaped samples were first processed in twin-screw extruder and they were subsequently injection molded. Double edge notched tensile (DENT) specimens with various ligament lengths were subjected to tensile tests at 2 mm/min constant deformation rate at room temperature in order to determine EWF parameters. For the neat matrix and 10 wt% calcite reinforced materials fractured in ductile manner, that is, the ligament fully yielded and the crack stably propagated unlike the other compositions. For the neat matrix, both the specific EWF, we, and the nonessential work of fracture, βwp, values dramatically decreased with increasing reinforcement weight ratio regardless of the agent type. The analyzing of yielding and necking/tearing components of essential and nonessential parameters showed that for the samples reinforced with SGF we,nt > we,y and βntwp,nt > βywp,y, indicating that a majority of fracture energy was dissipated in the necking and tearing stages of fracture process. POLYM. ENG. SCI., 54:540–550, 2014. © 2013 Society of Plastics Engineers

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, we, and the specific non-essential work of fracture, wp, 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. we 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.

Effect of annealing process in water on the essential work of fracture response of ultra high molecular weight polyethylene

The toughness of ultra high molecular weight polyethylene (UHMWPE) before and after annealing process in water was investigated by the essential work of fracture (EWF) method. Annealing in water at 80 °C for various aging periods of the 5 mm thickness with various ligament lengths single edge notched tension (SENT) specimens was performed in hot water. Tensile tests were performed at 2 mm/min constant deformation rate at room temperature in order to determine EWF parameters. After tensile tests, the fracture cross-sections of SENT specimens were investigated by scanning electron microscope (SEM). From the 60th day of annealing process in water, it was seen that the fracture toughness of the material decreased while water absorption in samples increased.

Grafted Polyolefin-Coated Synthetic Mica-Filled Polypropylene-co-ethylene Composites: A Study on the Interfacial Morphology and Properties

Purified maleic anhydride grafted isotactic polypropylene (iPP-MA) and maleic anhydride grafted polyolefin elastomer (POE-MA) were used to coat synthetic mica, which was added to block polypropylene-co-ethylene (cPP) to prepare composites with 10 wt% mica content. The effect of the kinds of grafted polyolefin, their contents and composition ratios on the interfacial morphology, mechanical properties, crystallization behavior, and essential work of fracture parameters for the composites were studied. The results showed that the coating of synthetic mica with grafted polyolefin could effectively lower the surface polarity of the mica and improve the interfacial adhesion between mica and cPP. The sedimentation volume of mica reached a minimum, and the interface adhesion of the composites was comparatively good when the grafted polyolefin was 3 wt% of the weight of the mica. When the content of grafted polyolefin was fixed at 3–5 wt% of the weight of the mica, the sedimentation volume of mica decreased and the interface adhesion of the composites improved with increasing POE-MA content. The coating with grafted polyolefin and the composition change of the grafted polyolefin had little effect on the mechanical properties and crystallization behavior of the mica-filled composites. However, the coating with grafted polyolefin increased the special essential work of fracture (we ) of the composites significantly, whereas the special non-essential work of fracture (βwp ) showed a slight decrease. When the total grafted polyolefin was fixed, the we decreased and the βwp increased a little as the POE-MA content was increased.

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 (we and βwp), 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.

Understanding the Underlying Physics of the Essential Work of Fracture on the Molecular Level

The essential work of fracture (EWF) method is widely used in the experimental characterization of fracture toughness of thin polymer films. However, the underlying physics of this method and the EWF parameters are not completely understood. In the present work, we investigated the correlations between the EWF parameters and the molecular structures of amorphous and ductile polymers. Based on our experimental observations, a physical model describing the EWF method on the molecular level was built. With this model, the variations of the EWF parameters with extrinsic and intrinsic factors under a plane stress condition were explained successfully; the theoretical values estimated with some assumptions and approximations of the EWF parameters agree satisfactorily with the experimental ones. This is the first theoretical model in the development history of the EWF method, which gives reliable physical meanings of the EWF parameters. The precise description of the molecular structure and fracture toughness relationships given by this model can also provide guidelines to molecular design of new polymers with high toughness.

Essential work of fracture analysis of glass microsphere-filled polypropylene and polypropylene/poly (ethylene terephthalate-co-isophthalate) blend-matrix composites

The essential work of fracture (EWF) concept was successfully applied to the fracture toughness characterization of ductile glass microsphere-filled polypropylene (PP)-matrix composites. Injection-moulded PP and PP/poly(ethylene terephthalate-co-isophthalate) (PET) blend-matrix composites filled with both untreated and silane-treated glass beads (50 wt%) were studied to ascertain the influence of the material composition on the EWF parameters. In addition, the effect of the specimen thickness was also studied on deeply double-edge notched (DDENT) specimens of nominal thickness 1, 2 and 3 mm. The results indicated that the EWF methodology could be applied to this kind of particulate-filled composites whenever high plasticity is developed in the crack tip front before the fracture propagation. PET addition markedly reduced both fracture toughness and plastic work of fracture in untreated and mercaptosilane-treated glass beads, attributed to glass embedding by PET. Concerning specimen dimensions, a noticeable drop in fracture toughness was found in thicker samples, whereas plastic work of fracture showed the opposite trend.

Fracture toughness evaluation of K resin® grafted with maleic anhydride‐ compatibilized polyamide 6/K resin® blends

AbstractThe mechanical behavior and fracture toughness of polyamide 6 (PA6)/K resin® (K) blends, with and without maleic anhydride‐grafted K resin® (K‐g‐MAH) incorporated, have been investigated. The results showed that the tensile strength, elongation at break and impact strength of PA6/K blends were improved considerably on incorporating K‐g‐MAH. This results from the improvement of compatibility between the PA6 and K phases. The essential work of fracture (EWF) method was employed to determine the fracture toughness of PA6/K blends with and without K‐g‐MAH incorporated. The effect of composition on the EWF parameters of the blends was particularly investigated. The results showed that a significant improvement in the specific EWF (we) of PA6/K blends occurred when K‐g‐MAH was incorporated. The effect of K‐g‐MAH content on the fracture toughness of the PA6/K/K‐g‐MAH blends was mainly achieved through its influence on the specific essential and nonessential work of fracture in the yielding process. Copyright © 2007 Society of Chemical Industry

Tenacidade à fratura de blendas PA 6/ABS avaliada através do método EWF (Trabalho Essencial de Fratura) - parte a: avaliação do efeito do compatibilizante

A poliamida 6 (PA6) é um polímero termoplástico semicristalino que apresenta alta susceptibilidade ao entalhe quando solicitado mecanicamente. Apesar de apresentar um comportamento dúctil sob deformação e sob fratura quando solicitado sem entalhes, torna-se essencialmente frágil e quebradiço quando entalhado. Assim, o comportamento pseudo-dúctil dessas poliamidas requer cuidados especiais na confecção do entalhe e da pré-trinca para análise de seu comportamento mecânico. Devem ser observadas ainda as dimensões ideais dos corpos de prova para garantir a condição de estado de tensão plana no ensaio de tração uniaxial para aplicação do método EWF (Trabalho Essencial de Fratura). Neste trabalho, o método EWF foi utilizado para a avaliação da tenacidade à fratura de blendas poliméricas com uma matriz PA6 modificada com ABS e também a eficiência de um compatibilizante acrílico reativo (MMA-MA) sintetizado em laboratório, na tenacificação da blenda PA6/ABS. Através da aplicação do método observou-se que a simples incorporação do ABS à poliamida aumenta o termo relativo ao trabalho específico de fratura e reduz a sensibilidade ao entalhe da blenda embora com perda na capacidade de absorção de energia plástica. A incorporação do copolímero MMA-MA à blenda refletiu-se nos acréscimos do trabalho específico e recuperação da capacidade de absorção de energia plástica em relação à blenda não-compatibilizada. Os resultados ilustram a aplicabilidade do método EWF para determinação da tenacidade à fratura de blendas poliméricas em relação aos polímeros puros, bem como a avaliação do efeito do compatibilizante reativo na tenacidade da blenda PA6/ABS.