Essential Work Of Fracture Concept Research Articles

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

AbstractThis 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 βwp 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.

Fracture Toughness Characterization of High-Density Polyethylene Using Essential Work of Fracture Concept

In this paper, the concepts of fracture mechanics are used to determine experimentally the work essential to fracture in high-density polyethylene (HDPE) pipes. The tests were carried out on DENT specimens taken from HDPE pipes intended for the distribution of water. The monotonic mechanical characteristics of this material for two strain rates 0.1 and 0.01 s−1 show that the curves obtained are typical of semicrystalline polymers. Their shape shows that the fracture is ductile for this type of polymeric material with plasticization of the ligament. The evaluation of the toughness under plane stress conditions was carried out by this method. This method has been extensively performed in order to assess the performance of two parameters in fracture, i.e., essential work of fracture and nonessential work (βwp) ones.

Post‐yield Fracture Mechanics as a Measure of the Degradation of Artificially Aged Biodegradable Mulch Films

An accelerated approach to assess the long‐term performance of biodegradable agriculture films such as mulch films based on poly(butyrate adipate terephthalate) (PBAT) is presented. PBAT films were aged in water and in air at 80 °C for different times and morphological changes were analysed using Fourier‐transform infrared spectroscopy and light microscopy. For exposure in air a Xenon‐arc test chamber was applied. The essential work of fracture concept was used to determine the resistance against crack initiation and propagation as a measure of the degree of degradation. Only at wet conditions as in real applications the films degrade easily. Thus, the films can break in increasingly small parts to become a part of the ground.

Numerical modeling of essential work of fracture on ductile polymeric films

Essential work of fracture method has been successfully used to characterize the fracture behavior of ductile polymeric films. The observed essential and non-essential fracture process zones are related to the toughness of the films. Three-dimensional finite element method (FEM) modeling has been carried out in this study to elucidate the film fracture process based on the essential work of fracture concept. Experimental observation on a model linear low-density polyethylene (LLDPE) blown film was used to validate the FEM model. The numerical model facilitates an in-depth understanding of the development of stress and strain fields and the fracture process zones at different loading steps. The FEM findings show that film thickness and ligament length have significant influence on the development of stress and strain fields at and around the ligament area. Furthermore, the numerical analysis reveals how various material properties, such as yield stress and strain hardening slope, can influence the development of fracture process zones. Usefulness of FEM modeling for developing tough polymeric films is discussed.

Determination of Traction Separation Curve for Copper Using Essential Work of Fracture (EWF) Approach

The essential work of fracture concept has become very popular to characterize the plane stress toughness of ductile material. The essential work characterizing the resistance of a structure to ductile fracture is important for many applications, from structural design to impact protection. The main objective of the present work is to experimentally derive the traction separation curve also known as cohesive law for copper (UNS C11040) under plane stress condition. To this end a double edge notched tension (DENT) specimens of thickness 1.5 mm were prepared. The ligament of the specimens varied between 6 to 24 mm and each specimen was tested under displacement controlled loading rate of 0.5 mm/min using miniature tensile tenting machine with a load cell capacity of 20 KN. The load displacement data were recorded and the traction separation curve was derived using normalization process suggested by Cottrell et al., 2005. The area under the traction separation curve is believed to the specific essential work of fracture and this fact was further confirmed from the experiments.

Post-Yield Fracture Behavior of Zeolite-Reinforced High Density Polyethylene Annealed Composite

The fracture behavior of annealed composites of zeolite-filled high density polyethylene has been investigated using the essential work of fracture concept. The crack opening displacement (COD) was estimated starting from the essential work of fracture value. The morphology of the fracture surface was examined using the scanning electron microscopy and the fracture mechanisms were discussed. The results showed that the composite with a 5 wt.% zeolite concentration has a unique fracture behavior characterized by the highest resistance to crack initiation in accordance with COD but a low resistance to crack propagation. The non-essential work of fracture as the resistance to the stable crack propagation has shown a maximum of 2.5 wt.% of zeolite followed by a sharp drop at a higher content of zeolite indicating a zeolite induced ductile-to-brittle transition. The fractured surface morphology revealed that the zeolite particle clusters arrested the plastic crack growth, which was more effective at 5 wt.% zeolite concentration. At a higher zeolite content, the amount of particle clusters rose, leading to a decrease in the average distance between them. Consequently, a large-scale plastic deformation of the matrix effectively favored the small strain damage and the fracture eventually occurred in the ductile-to-brittle transition.

Fracture characterization of recycled high density polyethylene/nanoclay composites using the essential work of fracture concept

The effect of nanoclay on the plane-strain fracture behavior of pristine High density polyethylene (HDPE) and recycled HDPE blends was studied using the essential work of fracture (EWF) concept. The failure mode of EWF tested specimens was found to be associated with the specific non-EWF (βBwp,B). Adding 6-wt% of nanoclay to pristine HDPE and 2-wt% to recycle-blends greatly decreased the βBwp,B values and led to a transition from ductile to brittle failure mode. A fractographic study revealed that the difference in failure modes was caused by the changes in micro and macro morphologies, which could be related with the specific EWF (we,B). In the ductile failure, we,B is governed by the fibril size; adding nanoclay and recycled HDPE to pristine HDPE decreased the fibril size and subsequently lowered the we,B value. In the brittle failure, the we,B value was enhanced by creating a rough fracture surface. Adding nanoclay to pristine HDPE, a steadily decrease in we,B was measured until 4-wt% after which the change was insignificant. Conversely, nanoclay content more than 2-wt% in recycle-blends greatly decreased the we,B value. A transition map was constructed to illustrate the potential failure mode and the associated fracture morphology based on the tested material compositions. POLYM. ENG. SCI., 56:222–232, 2016. © 2015 Society of Plastics Engineers

Fracture characterization of pristine/post-consumer HDPE blends using the essential work of fracture (EWF) concept and extended finite element method (XFEM)

The effect of recycled HDPE on plane-strain fracture properties of pristine HDPE was studied using the essential work of fracture concept. The plane-strain specific EWF decreased linearly as recycled content increased while the plane-strain specific non-EWF exponentially decreased. The specific non-EWF was investigated using the extended finite element method. The nonlinearity of specific non-EWF was influenced by the shape factor (βD) and the specific non-essential fracture work per unit volume (wp,D), both of which were non-linearly related to recycled content. Blending brittle recycled material to ductile pristine HDPE decreased the fracture toughness and the crack growth resistance.

Fracture behavior of boehmite‐filled polypropylene block copolymer nanocomposites as assessed by the essential work of fracture concept

ABSTRACTThe essential work of fracture (EWF) method was adapted to determine the fracture toughness of poly(propylene‐block‐ethylene) (EPBC) based nanocomposites with different amounts (from 0 up to 5 wt %) of synthetic boehmite alumina (BA). The dispersion of BA in the matrix was studied by transmission and scanning electron microscopies. Agglomerated micronscale along with well dispersed nanoscale BA particles were present in the EPBC matrix. By contrast to the neat EPBC, all nanocomposites failed by unstable necking. Therefore the energy partitioning concept of the EWF was adapted and attention paid to the yielding‐related term. Both specific yielding‐related essential and nonessential work of fracture parameters increased linearly with the product of the yield stress and elongation at yield derived from static tensile tests. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40447.

Characterization of the hot cracking resistance using the Essential Work of Fracture (EWF): application to duplex stainless steels

Duplex stainless steels (DSS) involve two ductile phases, i.e. ferrite and austenite, with a proportion of each phase around 50%. The main advantage in comparison with other austenitic and ferritic stainless steels is the excellent combination of high strength and corrosion resistance together with good formability and weldability. Unfortunately, DSS present in general a poor hot workability. Standard hot ductility tests like hot tensile or hot torsion tests are always helpful to compare the fracture resistance of two very ductile materials. A new method based on the essential work of fracture (EWF) concept has been used in order to determine the hot cracking resistance. The EWF concept was introduced to address ductile fracture based on the entire load-displacement response up to the complete fracture of a specimen and not from the initiation measurements such as in classical fracture mechanics concepts. The aim of the method consists in separating, based on dimensional considerations, the work performed within the plastic zone from the total work of fracture in order to provide an estimate of the work spent per unit area within the fracture process zone to break the material. This method proved to be very well adapted to high temperature cracking. Two different duplex stainless steels have been characterized by the essential work of fracture method. Examination of the fracture micrographs and profiles match the EWF results. This method turns out to be a discriminating tool for quantifying hot cracking and to generate a physically relevant fracture index to guide the optimization of microstructures towards successful forming operations.

The role of ultimate elongation in the determination of valid ligament range of essential work of fracture tests

In this study, the feasibility of a previously proposed ultimate elongation-based criterion of essential work of fracture (EWF) method was reconsidered for several materials. Suitable ductility level (DL) ranges for the EWF testing of polymers were discussed, too. It was shown that there is a relationship between the DL terms and the experimentally estimated crack opening displacement values. Based on Coterell’s estimate of the size of the fracture process zone a lower ligament limit of EWF concept was also presented. This lower bound should be considered for thin samples where the original thickness-dependent criteria seemed to be not severe enough.

Evaluation of the tear properties of polyethylene blown films using the essential work of fracture concept

In the case of very thin materials such as blown films, the applied stress state in front of the crack tip is normally a plane stress condition, and the deformation around the crack tip due to the remote stress is very large. However, current standard test methods for quantifying the fracture toughness of thin films, such as the Elmendorf tear test, cannot explain or represent the tear characteristics accurately. The common way of interpreting the test results from the Elmendorf tear test is to develop an empirical correlation and then compare the average values. In this paper, essential work of fracture (EWF) tests for five commercial polyethylene (PE) blown films have been conducted, and the fundamentals of their tear properties based on fracture mechanics have been studied. The results from the EWF test are interpreted based on two important parameters, i.e., the essential work of fracture (We) and the non-essential work of fracture (Wp). Further, the relationship between these parameters and the current standard Elmendorf tear test is shown.

Deformation mechanism under essential work of fracture process in polycyclo-olefin materials

The fracture toughness of a glassy polycyclo-olefin (PCO) was investigated by the essential work of fracture (EWF) method using a double-edge notched specimens. It was shown that the PCO follows the EWF concept in the temperature range between room temperature and glass transition temperature Tg where the ligament yielding appear at a maximum point on the stress-displacement curves and subsequently the necking and tearing processes take place in the post yielding region. The essential work of fracture required for the ligament yielding drops as the temperature approaches Tg. The non-essential work of fracture attributed to tearing process after yielding is consumed to expand the plastic region and causes molecular chains to orient to the stretching direction.

Mechanical and Fracture Toughness Behavior of TPNR Nanocomposites

Thermoplastic natural rubber (TPNR) nanocomposites containing organophilic layered silicates were prepared by melt blending method at 180°C using internal mixer (Haake 600P). The aim of this study is to determine the influence of the organoclay filler on the mechanical and fracture properties. In this study, two mixing methods were employed to incorporate filler into matrix, namely the direct (DM) and indirect (IDM) method. The mechanical properties of TPNR nanocomposites were studied using tensile, flexural, and impact tests. The tensile and flexural tests revealed that the optimum loading of organoclay was at 4 wt% using the indirect method of mixing. Plane stress fracture toughness of thermoplastic natural rubber (TPNR) nanocomposite was determined by the essential work of fracture (EWF) concept using tensile-loaded deeply double-edge notched (DDEN-T) specimens. The EWF measurements indicated that the specific essential work of fracture (we) decreased in the presence of nanoclay. Nevertheless, these TPNR nanocomposites met the basic requirement of the EWF concept of full ligament yielding, which was marked by a load drop in the force—displacement curves of the DDEN-T specimens.

Application of essential work of fracture concept to toughness characterization of high‐density polyethylene

AbstractDeformation and fracture toughness of high‐density polyethylene (HDPE) in plane‐stress tension was studied using the concept of essential work of fracture (EWF). Strain range for necking was determined from uniaxial tensile test, and was used to explain the deformation transition for 2‐staged crack growth in double‐edge‐notched tensile test. Through work‐partitioning, EWF values for HDPE were determined for each stage of the crack growth. Appropriateness of these EWF values to represent the material toughness is discussed. The study concludes that the EWF values for ductile polymers like HDPE may not be constant, but vary with the deformation behaviour involved in the crack growth process. POLYM. ENG. SCI., 47:1327–1337, 2007. © 2007 Society of Plastics Engineers

Fracture strength and adhesive strength of hydroxyapatite-filled polycaprolactone

Fracture toughness and tear strength of hydroxyapatite (HAP)-filled poly(epsilon-caprolactone) (PCL) with increasing HAP concentration were studied. The toughness was assessed in terms of essential work of fracture (EWF). Adhesive strength between HAP and PCL interfaces was evaluated using T-peel testing. The adhesion between the two components was found to be relatively strong. Double edge notched tension (DENT) and trousers test specimens were used for the EWF tests. The effect of HAP phase in PCL on the fracture and tearing toughness was investigated. The results obtained from the EWF tests for the HAP-filled PCL complied with the validity criteria of the EWF concept, namely, (1) geometric similarity for all ligament lengths; (2) fully yielded ligament and (3) plane-stress fracture condition. Values for specific essential work of fracture (w ( e )) and specific plastic work of fracture (betaw ( p )) were found to decrease with increase in HAP concentration. The testing procedure showed promise in quantifying the tearing resistance and rising R-curve behavior common in natural materials and it can be extended to other biomaterials that exhibit post-yield deformation. A quantitative assessment based on fracture mechanics of the adhesive strength between the bioactive interfaces plays an important role for continued development of tissue replacement and tissue regeneration materials.

Essential work of fracture analysis for starch filled poly(propylene carbonate) composites

Starch filled poly(propylene carbonate) composites are environmental friendly materials. In this study, the fracture toughness of composites under mode I loading was determined by the essential work of fracture concept. The specific essential fracture work of the poly(propylene carbonate)/starch composites decreases with increasing the starch content, while the non-essential work term, βw p increases with increasing the starch content. In addition, the morphologies, thermal properties, thermo-mechanical properties were studied by scanning electron microscope, thermogravimetric analysis, dynamic mechanical analysis, and differential scanning calorimetry, respectively. The thermal and thermo-mechanical measurements revealed that increasing starch content led to an increase in glass transition temperature and thermal stability. Morphology observation indicates that poly(propylene carbonate) and starch have weak interfacial adhesion.

Essential work of fracture of glass bead filled low density polyethylene

Recently, there has been an increasing interest in introducing high modulus inorganic rigid particles into polymer matrices because of the cost-effective character and the ability to reinforce the matrix, different from reduction in tensile strength and modulus caused by the compliant rubber particles. There are also examples of the toughening effect of these particles such as glass bead, CaCO3 etc. filled into polymers such as high density polyethylene (HDPE) [1, 2], polypropylene (PP) [3] and so on in the literature. In these cases, the toughness of the composites was generally evaluated by means of impact tests. When studying the failure of ductile materials, where a large plastic zone at the crack tip develops, and the energy dissipation is no longer confined to a small local zone near the crack tip, the J-integral approach has been used traditionally. Recently, the essential work of fracture (EWF) method, first developed by Broberg [4], has gained popularity due to the experimental simplicity and the intensive works of Karger-Kocsis [5–10], Mai [11–14] and other researchers [15, 16], and has been used extensively to study the fracture behavior of a wide range of polymeric materials. The EWF concept of Broberg proposed that the nonelastic region at the tip of the crack may be divided into an inner fracture process zone (IFPZ) and an outer plastic deformation zone (OPDZ), as shown in Fig. 1. The total work of fracture, Wf , is then partitioned into two terms: the essential work of fracture We expended in the IFPZ to form a neck and subsequent tearing, and the nonessential work of fracture Wp dissipated in the OPDZ where various types of deformation such as shear yielding and microvoiding may be functioning. So,

Essential work of fracture analysis of poly(propylene carbonate) with varying molecular weight

Fracture toughness of poly(propylene carbonate) (PPC) with various molecular weights was determined by the essential work of fracture concept (EWF) using double-edge notched (DENT) specimens. The thermal properties and thermomechanical properties were studied by thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA) and modulated differential scanning calorimetry (MDSC), respectively. The essential fracture work (w e) of PPC tends to increase with increasing molecular weight. The TGA, DMA and MDSC results showed that the decomposition temperature and glass transition temperature of PPC increase with increasing molecular weight. It was indicated that the fracture toughness, thermal stability and glass transition temperature can be improved by increasing the molecular weight of PPC.

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”.