Essential Work Of Fracture Method Research Articles

Influence of low-density polyethylene blown film thickness on the mechanical properties and fracture toughness

Recent research has shown that the resistance against crack initiation and propagation of polyethylene blown films depends on density, chain branching and crystal orientation next to the processing parameters. To assess low-density polyethylene blown film fracture toughness with different thicknesses, the essential work of fracture method has been conducted in this study. The thickness of the investigated low-density polyethylene films was regulated by the draw down ratio, which causes low-density polyethylene crystal orientation with the chain axis parallel to the machine direction at higher draw down ratios, influencing mechanical parameters and the fracture toughness. The crystal orientation was measured by X-ray diffraction. Blown films with thicknesses equal to or greater than 200 µm exhibit no preferred orientation, which is consistent with various calculated mechanical and fracture mechanics parameters. Films of less than 200 µm thickness show preferred machine direction crystal orientation and a distinct thickness influence on mechanical and fracture mechanics parameters.

An assessment of essential work of fracture testing method applied to medium density polyethylene (MDPE)

The increasing usage of polyethylene in high-demanding structural applications, e.g., in the production of pipes for water and natural gas distribution networks, leads to the growing interest in the characterization of the material’s fracture toughness. Polyethylene, however, is ductile at room temperature and, hence, elastoplastic fracture toughness concepts are most convenient for its characterization. The present work describes results obtained in a systematic investigation of the applicability of the essential work of fracture (EWF) test protocol to a medium density polyethylene (MDPE) resin designed for the production of extruded water pipes. Typical issues of this method, as for example, influence of sample preparation, sample geometry, and different pre-cracking methods are investigated and their influence on the obtained results are described. The results obtained show that EWF method is well suited for the characterization of this MDPE resin and the test parameters allow, for example, the evaluation of process induced anisotropy effects in the sample. Results show that pre-cracking method, sample thickness and sample processing method affect the obtained results within 5% statistical significance level and that ‘rougher’ pre-cracking methods affects the results by increasing data dispersion.

On the valid ligament range of specimens for the essential work of fracture method: The inconsequence of stress criteria

The Essential Work of Fracture (EWF) method is widely used for the toughness determination of thin, ductile sheets under quasi plane-stress conditions. To define the valid ligament range of tests empirical role of thumbs and theoretical approaches are generally used. The lower ligament limit is typically ascribed to the “plane-strain/plane-stress transition”, which is accompanied by an increase in maximum net-section stress. Nevertheless, during the measurements neither the assumed plastic-rigid behavior nor true geometric similarity exists, which unfits the theoretical stress criteria based on these approximations. In the present study several materials were considered to explain the experimental results and to clarify the possible ambiguities related the observed maximum stresses.

Morphological properties of impact fracture surfaces and essential work of fracture analysis of cellulose nanofibril‐filled polypropylene composites

AbstractScanning electron microscopy (SEM) was employed to investigate crack initiation and propagation process in notched and unnotched Izod impact fracture surfaces of the cellulose nanofiber (CNF) and microfibrillated cellulose (MFC)‐filled polypropylene (PP) composites compared with microcrystalline cellulose (MCC)‐filled composites. CNF is in the form of short fibers 50–300 nm in diameter and 6–8 in aspect ratio, MFC is in the form of long fibrils 50–500 nm in diameter and 8,000–80,000 in aspect ratio, and MCC is in the form of particles 50 μm in average diameter and 1–2 in aspect ratio. The reinforcement material size of CNF and MFC are smaller than that of MCC which means that the larger interfacial area between filler and matrix leading to larger energy dissipation at the interface during the impact fracture. The reinforcement‐matrix debondings nearby MCC particles caused easy crack propagation which contributes smaller energy dissipation at the interface. A slip‐stick behavior and stress whitened area during the fracture were observed. Morphological investigation helps to explain impact fracture behavior. According to essential work of fracture (EWF) analysis of Izod impact results, EWF method is applicable to analyze impact fracture behavior and the energy consumed in crack initiation and propagation during the fracture process can be calculated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

A deformation-mechanism-based energy partitioning approach for measurement of plane-stress fracture toughness of low-density polyethylene

A previously proposed energy partitioning approach, based on deformation mechanisms involved in the fracture process, is applied to double-edge-notched tensile (DENT) test to determine plane-stress fracture toughness for a commercial low-density polyethylene (LDPE). Test results show that mechanical behaviour of LDPE is affected by unloading–reloading cycles introduced in the DENT test. Therefore, modification is made for determining the residual elastic energy. The modified energy partitioning approach yields much lower specific energy consumption for fracture than that from the essential work of fracture (EWF) method. Support to this approach comes from the close agreement between the specific energy consumption for necking in the DENT test and the specific deformation energy in the simple tensile test at the same strain level. The paper also analyzes the energy consumption for each mechanism involved in the fracture process and its dependence on the specimen ligament length. The analysis suggests that the non-essential part of energy consumption for crack growth is not purely a function of the square of ligament length. Therefore, the EWF method should not be used to determine fracture toughness for polymers when necking is involved during the crack growth.

Essential work of fracture study of polymers: a novel criterion for the validation of tested ligament range

The essential work of fracture method is widely used for the determination of fracture toughness of ductile metals and polymers under plane stress conditions. Nevertheless, this method has numerous prerequisites, which are easy to fulfill in metals, but are less certain in polymers. The aim of this study was, therefore, to present a simple, empiric “displacement-criterion” that helps the determination of valid ligament lengths for polymers—one of the biggest sources of error of this theory. This criterion not only facilitates the definition of a lower bound, but also helps the description of an upper ligament limit. Its other advantage is, that combined with a stress criterion, it helps the data validation not only until crack initiation, but also over the entire deformation process.

Amorphous polyamide/maleated styrene–ethylene–co-butylene–styrene nanocomposites: effects of clay loading and compatibilizer content on morphology and mechanical properties

The effect of the organoclay content on the toughness of a rubbery modified amorphous polyamide (aPA)/organoclay-based nanocomposite was studied by changing the modifier (maleic anhydride) content. The dispersed rubber particle size decreased markedly with the addition of the modifier, indicating compatibilization of the nanocomposite. However, the particle size of the dispersed phase increased slightly with the organoclay content due to the interactions between the dissolved surfactant and the compatibilizer. Furthermore, we observed that the organoclay resided in the aPA matrix, and that its dispersion remained constant upon rubber addition. This resulted in materials with high stiffness and extremely large toughness values, as measured by both the standard impact strength and the essential work of the fracture method. Because the rubber content was kept constant, the inorganic part of the clay was proposed to be the main parameter that controls toughness. The impact strength of an amorphous polyamide/organoclay nanocomposite has been increased by means of a rubber modified with maleic anhydride (MAH). The organoclay resided in the polyamide matrix and maintained dispersed rubber particle size upon rubber addition. The changes of the dispersed rubber particle size with modifier and nanoclay addition indicated compatibilization of the nanocomposites and interactions between the dissolved surfactant and the rubber modifier. The obtained nanocomposites had a high stiffness and extremely large toughness values, as determined both by impact strength measurements and by the essential work of fracture method.

Characterization of reactively compatibilized poly( d, l-lactide)/poly(ε-caprolactone) biodegradable blends by essential work of fracture method

In this work biodegradable blends of poly( d, l-lactide) and poly(ε-caprolactone) were studied. The weight fraction of poly(ε-caprolactone) was varied between 0 and 100%. The originally immiscible blends were compatibilized with l-lysine-diisocyanate and l-lysine-triisocyanate, respectively, to increase the fracture toughness of materials and maintain their biocompatibility. The blend morphology was characterized by scanning electron microscopy and differential scanning calorimetry. The fracture properties of blends were analyzed by the essential work of fracture method.

Mechanical Testing of Polymers in Pressurized Hydrogen: Tension, Creep and Ductile Fracture

The paper aims at characterizing some key features of the mechanical behavior of two semi-crystalline polymers in weakly pressurized hydrogen. The opportunity to use hydrogen as an alternative energy strengthens the need for reliable data on possible coupling effects between gas diffusion and mechanical properties, especially for safe design purpose. However, such effects have not been really quantified in polymers. In the present study, a hydraulic testing machine has been fitted with a pressure hydrogen chamber, and three major aspects of the mechanical behavior have been investigated in polyethylene and polyamide 11: monotonic tension, long-term creep (based on a time-temperature superposition principle) and ductile fracture (evaluated from an essential work of fracture method). Suitable protocols have been defined to take into account specificity like temperature and pressure history dependence and gas saturation kinetics of the sample.

Fracture behavior of quenched poly(lactic acid)

The effect of a quenching treatment applied on heated cast sheet extruded films of two poly(lactic acid) (PLA) commercial grades, with different optical purities, was studied. The thermal and mechanical properties of the films, as well as their fracture behavior, were assessed by differential scanning calorimetry (DSC), tensile tests, and the essential work of fracture (EWF) approach. The heating-quenching treatment causes a de-aging effect with an increase in the free volume of polymer chains evidenced by a decrease in the glass transition temperature (Tg) and a decrease in the tensile stiffness and yield stress. As a result, there is an abrupt increase in ductility, finding a dramatic change in the fracture behavior, from brit- tle to ductile. The use of digital image correlation (DIC) of the strain field analysis during fracture testing has allowed relat- ing the decrease on the yield stress promoted by quenching with the crack propagation kinetics. The use of the EWF method to characterize the fracture toughness of PLA has allowed to measure this enhancement on toughness, finding that the spe- cific essential work of fracture (we) and the plastic term (!wp) parameters increased 120% and 1200%, respectively, after the quenching process.

Characterization of the fracture properties of aragonite‐ and calcite‐filled poly(ε‐caprolactone) by the essential work of fracture method

AbstractBiodegradable polymers, blends, and composites are often investigated during tissue engineering studies, but their fracture properties, which are important mechanical engineering characteristics, are often disregarded or wrongly treated. In this study, essential work of fracture tests were performed on calcium carbonate filled poly(ε‐caprolactone), a very ductile polymer, to determine the effects of different filler shapes (calcite spheres and aragonite whiskers), sizes, and contents on the fracture parameters. Increasing the filler content caused stability problems during crack propagation, and this influenced the self‐similarity of the load–displacement response and resulted in the yielding point being missed. Moreover, the yielding‐related essential work of fracture and the energy dissipating during yielding were found to be almost independent of the filler content and thus could be indicators of matrix–filler adhesion. A shape effect of aragonite whiskers appeared during stable crack propagation; the motion of the particles and the friction on their surface slightly increased the dissipated energy quantum and resulted in a more oriented molecular structure. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Effect of composition on fracture behavior of polypropylene–wollastonite–polyolefin elastomer system

The fracture behavior of polypropylene (PP)–wollastonite–polyolefin elastomer (POE) in the mixed mode region was studied using the essential work of fracture (EWF) method. The relationship between the microstructure and the fracture parameters was analyzed. The effect of wollastonite content on the essential work of fracture and the work of plastic deformation was discussed. The energy dissipation during a double-edge-notched tension (DENT) test was calculated with the EWF method. It was found in the mixed mode region that σn increases with shortening of the ligament length region as plastic constraint effect rises and variation of the specific total work of fracture with ligament length was still reasonably linear within the mixed mode region. With increasing wollastonite content, w e (specific essential work of fracture) increases, while the βw p (specific non-essential work of fracture) decreases. The measurements of energy dissipation show that improvement in the fracture toughness of PP–wollastonite–POE is mainly due to the increase in crack propagation resistance during the necking and tearing processes after yielding, while the plastic deformation capability of the material depends mainly on the properties of fracture behavior before yielding. It is also found that the impact strength of the material decreases with increasing wollastonite content. However, the composition with high impact strength has lower specific essential energy of fracture and lower long-term fracture resistance, indicating that EWF is a better indicator of long-term fracture properties than the impact strength. DSC results show that the presence of wollastonite hinders crystallization of the PP.

Essential work of fracture analysis of the tearing of a ductile polymer film

Abstract The fracture behaviour of a 0.5 mm thick ethylene–propylene block copolymer, previously evaluated using the essential work of fracture method, has been analyzed again in more detail, using different plots, allowing the determination of the crack initiation displacement and stress. In such plots is evidenced that the specific essential work of fracture, w e , corresponds to the energy just up to crack initiation value that can be related with J 0 . Also, it has been found a novel relationship between the plastic term, βw p and the crack initiation stress, σ i .

A study of the fracture toughness of acrylic composites using the essential work of fracture method

The Essential Work Fracture approach was used in order to calculate the fracture toughness of acrylic composites. The composite materials were formulated with polymethylmethacrylate (PMMA) as a matrix and either core–shell or multi-layer particles with 10 or 20% volume content. Double-Edge Notched Tension (DDENT) specimens were tested in a universal testing machine at 2 mm/min. It was found that the values of the specific essential work of fracture, w e , increased with increase of the particle content. The composites prepared with 10 and 20% core–shell particles reached values of 4.63 and 9.72 kJ/m 2, respectively, whereas the composites formulated with 10 and 20% multi-layer particles reached values of 8.36 and 8.42 kJ/m 2, respectively. The nonspecific value of fracture for the multi-layer particle composites was found to yield a negative slope, however, from Scanning Electron Microscopy (SEM) micrographs, ductile-to-brittle behavior was observed, i.e., unstable crack growth.

The effect of organo‐modifier on the structure and properties of poly[ethylene–(vinyl alcohol)]/organo‐modified montmorillonite composites

AbstractEthylene–(vinyl alcohol) (EVOH)/organo‐modified montmorillonite clay (OMMT) composites were prepared using commercial OMMTs with two different organo‐modifiers. The first one has polar groups, which interact with EVOH chains, whereas the second one is non‐polar. Composites with a high weight percentage of OMMT were prepared by melt‐extrusion in a twin‐screw extruder. Films were prepared in a cast‐film line in order to evaluate the tensile and fracture parameters. The morphology of the EVOH/OMMT composites was characterized using wide‐angle X‐ray scattering and transmission electron microscopy, complemented by image analysis. The mechanical properties were evaluated using uniaxial tensile tests; the fracture behaviour was analysed using the essential work of fracture (EWF) method. Results show differences in composite morphologies and crystalline fraction, depending on the content and the nature of the organo‐modifier. The mechanical and fracture behaviours are strongly influenced by those parameters, which act in opposite senses. The EWF method is shown to be a useful tool for the analysis of the structure–properties relationships of these composites. Copyright © 2010 Society of Chemical Industry

Analysis of the essential work of fracture method as applied to UHMWPE

The validity of the basic assumptions behind the method of essential work of fracture (EWF), as applied to ultra-high molecular weight polyethylene (UHMWPE), is evaluated using finite element modelling. To define a suitable model of constitutive behaviour, the mechanical properties of UHMWPE have been measured in both uniaxial tension and compression over a range of strain rates. The observed strain rate dependence of stress, including the observed differences in strain rate sensitivity between tension and compression, is interpreted in terms of a single Eyring process. The constitutive theory is constructed comprising an Eyring process and hyperelastic networks, the latter having responses symmetric with respect to tension and compression. This theory is implemented within a finite element scheme, and used to model fracture measurements made on the same material using double-edge notch tensile specimens. Calculations of the non-essential work and of the extent of the plastic zones are thus made possible. It is concluded that the specific non-essential work is essentially constant, but that the shape factor β, assumed constant in the conventional analysis, varies significantly with ligament length. The implication of this finding on the derived EWF value is evaluated and found to be slight.

Essential Work of Fracture Evaluation of Compression-Molded Polypropylene/Ethylene and 1-Octene Copolymer Blends

The influence of ethylene and 1-octene copolymer content on the fracture behavior of compression-molded polypropylene/copolymer of ethylene and 1-octene blends was evaluated by means of the essential work of fracture method. The results indicated that the approach worked well for the polypropylene/copolymer of ethylene and 1-octene blends when copolymer of ethylene and 1-octene content was 0∼7.5 wt%. The specific essential work of fracture (we) of the blends increased with increasing copolymer of ethylene and 1-octene content and the dominant factor which affected we was the necking and subsequent fracture term (we,n). The specific non-essential work of fracture (βwp) slightly increased with increasing copolymer of ethylene and 1-octene content, while the term wp showed a decreasing trend with increasing shape factor (β). The trend of w e values obtained from crack-opening displacement was similar to that of essential work of fracture tests.

The Essential Work of Fracture (EWF) method – Analyzing the Post-Yielding Fracture Mechanics of polymers

The Post-Yielding Fracture Mechanics describe the fracture behaviour of pre-cracked films and thin sheets that show yielding phenomenon at the crack tip during fracture. The Essential Work of Fracture method (EWF) has been used for this type of fracture characterization, determining two parameters: the specific work of fracture, w e related with the real fracture process area, and the specific non-essential work of fracture, w p that corresponds with the work done in the outer region of the crack tip. The EWF technique has been successfully employed especially with polymers, allowing the study of the influence of many variables in fracture properties, unavailable using other techniques such us K IC or J IC determination. In this work, the fundamentals of the technique and examples of application are reviewed, presenting a brief summary of the most relevant contributions of our group to the EWF method.

Nonlinear Field Theory of Fracture Mechanics for Paramagnetic and Ferromagnetic Materials

A nonlinear field theory of fracture mechanics is developed for crack propagation in paramagnetic and ferromagnetic materials from the global energy balance equation and the non-negative global dissipation requirement. The crack-front generalized J̃-integral is equivalent to the generalized energy release rate serving as the thermodynamic driving force for crack propagation and also related to the generalized energy-momentum tensor in a way similar to the material force method. On the basis of the developed theory, the generalized energy release rate method, the generalized J̃-integral method, and the extended essential work of fracture method are proposed for quasistatic and dynamic fracture characterization of magnetosensitive materials in the presence of magnetothermomechanical coupling and dissipative effects. The present work overcomes the drawbacks and limitations of conventional fracture mechanics and resolves the controversial issues on magnetoelastic fracture criterion. Especially, the crack-front generalized J̃-integral has an odd dependence on the magnetic induction intensity factor for a Griffith-type crack in a magnetoelastic solid.

Fracture behaviour of poly[ethylene–(vinyl alcohol)]/organo‐clay composites

AbstractBACKGROUND: Ethylene–(vinyl alcohol) (EVOH) copolymer/organo‐modified montmorillonite (OMMT) composites were investigated. Composites with two different percentages by weight of OMMT were prepared using a melt‐extrusion procedure in a twin‐screw extruder, using EVOH as matrix. Films made of EVOH and EVOH/OMMT composites were prepared in a cast‐film extrusion line.RESULTS: The mechanical properties were evaluated by tensile tests and the fracture behaviour was analysed using the essential‐work‐of‐fracture (EWF) method. Fracture characterization was carried out for the two main processing directions: melt flow direction and transverse direction. Fractographic observations were made using scanning electron microscopy.CONCLUSION: The tensile test results indicated good compatibility between EVOH and OMMT. In addition, the fracture tests showed the influence of the clay particle arrangement on the fracture behaviour, showing an increase in the specific essential work of fracture, we, which was attributed to the EVOH–OMMT interaction. The plastic term, βwp, showed different trends depending on the test direction, explained by the size of the plastic zone and the restrictions to the EVOH plastic flow promoted by the clay particles. In this sense, the EWF method is shown to be a very useful tool for the analysis of structure–property relationships in polymer–organo‐clay composites. Copyright © 2009 Society of Chemical Industry