Essential work of fracture: application for polymers showing ductile-to-brittle transition during fracture (original) (raw)
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Polymer Engineering and Science, 1990
We have extended the essential work of fracture technique to allow for the determination of the plane-strain essential work of fracture. The new technique is to measure the specific work of fracture as a function of ligament length in deeply double edge notched samples. This type of data is then experimentally corrected to remove the plastic work of fracture and leave only the essential work of fracture as a function of ligament length. By extrapolating the essential work of fracture to zero-ligament length, we claim to be measuring the plane-strain essential work of fracture. This new technique was applied to two rubber toughened nylons and to a series of polyethylenes. The plane-strain essential work of fracture was found to be independent of thickness. Where comparison can be made to J-integral testing, the plane strain essential work of fracture was similar to the critical J-integral, J Ic. * Analysis done using full set of experimental data spanning both the mixed-mode and the plane-stress regions ** Analysis done by the double extrapolation procedure
Polymer Engineering & Science, 2000
The plane strain fracture toughness of two ductile polymers, ultra high molecular weight polyethylene (UHMWPE) and acrylonitrile-butadiene-styrene (ABS), was measured by using the essential work of fracture approach. Truly plane strain fracture toughness (Wre) was measured for ABS at quasi-static and impact rates of loading. For UHMWPE, the measured values were only "near" plane strain values (Wre*). It was confirmed both w,~* and wIe were independent of specimen type but dependent on strain rate. For UHMWPE, there was a negative strain rate effect, i.e., wIe* decreased with increasing loading rate. At low quasi-static loading rate (v 5 10 mm/min), wr,* was constant at 55 kJ/m2. It then decreased to 15 W/m2 when the loading rate was increased to 100 mm/min, and remained at that value even up to impact rate of loading (u = 3.7 m/s). For ABS, a mild positive strain rate effect was observed. w,, increased from 13 kJ/m2 at u = 10 mm/min to 17 kJ/m2 at u = 3.7 m/s.
The fracture behaviour of materials in the ductile-to-brittle region is neither completely brittle nor entirely ductile. Besides, scatter in toughness results has been reported in polypropylene and nylon. At the moment there is no general agreement on the methodology to determine the fracture toughness in the transition region. In this work an assessment of different proposed methods based on LEFM, EPFM and statistical approach was carried out over two materials: polypropylene homopolymer (PPH) and a blend of PPH containing 20 wt.% of elastomeric polyolefin (PPH/POes). The methods analysed were Fernando-Williams method, plastic zone corrected LEFM proposed by Gerin et al., G ST /G INST method by V-Khanh and De Charentay, JR curve method by Santarelli et al., and a statistical approach proposed by the authors in a previous work. The results of this analysis indicate that the Fernando-Williams and Plastic zone corrected LEFM methods, based on LEFM, tended to underestimate the fracture toughness, being very conservative. On the other side, JR method may overestimate the toughness, as in PPH/POes blend case. The G ST /G INST and Statistical methods appear to be the most adequate to characterise the fracture toughness of PPH and PPH/POes blend. The values of the characteristic fracture toughness found by both methods were slightly smaller than the minimum determined experimentally and proved very close between themselves.
Evaluation of fracture toughness of ABS polymers via the essential work of fracture (EWF) method
Journal of Materials Science, 2012
The essential work of fracture (EWF) method was employed to determine the fracture toughness of SAN/ PB-g-SAN blends with the compositions of 65/35-0/100. It was found that the plane stress EWF approach is applicable for different SAN/PB-g-SAN blends. During EWF tests, three different types of load-displacement curves were recorded, depending on the blend composition. For the samples containing rubbery phase of 35-65 wt% crack growth occurred immediately after full ligament yielding. Further increase in rubber content and for the samples with 75 and 85 wt% of rubbery phase, the onset of crack growth was preceded by the formation of necking zone in the form of a sharp load drop after full ligament yielding. For the sample with the composition of 0/100, strain hardening behavior was observed without any sign of neck formation. The specific essential work of fracture (EWF) w e increased at first with the increase of PB-g-SAN wt% and then decreased with further increasing amount of rubbery phase. The maximum value of w e was observed for the blend with 75 wt% of PB-g-SAN. The specific non-essential work of fracture bW p increased with increasing of rubbery phase concentration. The analyzing of yielding and necking/ tearing components of essential and non-essential parameters showed that for the samples containing 55 wt% and higher of rubbery phase, w e;nt 1 w e;y andb nt w p;nt 1 b y w p;y , indicating that a majority of fracture energy was dissipated in the necking and tearing stages of fracture process. Finally, it is shown that w e values can be predicted via COD values.
ESSENTIAL WORK OF FRACTURE CONCEPT IN POLYMERS
Plane-stress fracture toughness of amorphous copolyester (COP) sheets of different composition and molecular mass characteristics was determined by the essential work of fracture (EWF) concept using tensile-loaded deeply double-edge notched (DDEN-T) specimens. It was determined that these COPs meet the basic requirement of the EWF concept since their yielding along the full ligament preceded crack growth. A drop in load in the corresponding load-displacement (F − x) curves indicated yielding and allowed us to split both the specific essential and non-essential work of fracture (w e and w p , respectively) into their contributing terms based on yielding (w y ) and necking including fracture (w n ). Development and size of the plastic zone were studied by light microscopy (LM) and infrared thermography (IT).
Engineering Failure Analysis, 2009
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.
Polymer Engineering & Science, 2007
Deformation 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 doubleedge-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.
Influence of the Geometry on the Essential Work of Fracture of Polypropylene Materials
2013
For reasonable application of the essential-work-of-fracture (EWF) concept to polymers some geometrical requirements has to be fulfilled, whereas these conditions, especially that of plane state of stress and self-similarity of the load–displacement diagrams are often handled very noncritical in the literature. A brief discussion of minimum and maximum valid ligament length and the influence of specimen thickness on toughness has been given, therefore, by comparing data empirically determined with the predictions. Furthermore, the applications of small-sized specimens has been shown on example of ethylen-propylene copolymer. Introduction Based on fundamental studies of Broberg [1] as well Cotterell and Reddel [2], the method of essential work of fracture (EWF) as one of the main concepts of the ‘Post-Yield’ Fracture Mechanics has been first applied to polymers by Mai and Cotterell [3]. At present, the EWF methodology is widely used for highly ductile materials, mostly polymers but a...
Polymer Testing, 2019
The effects of initial crack length on the mode I fracture properties of crosslinked elastomers were investigated. A J-integral approach was implemented using single-specimen test procedures. Quasi-static fracture tests were carried out on filled and unfilled acrylonitrilebutadiene rubbers with various pre-cut (initial) crack lengths, , at a fixed rate of extension, using single edge notch rectangular bar specimens. Data from these experiments were analyzed and crack resistance curves (R curves) were obtained. A detailed analysis of the single-specimen method was carried out to establish correlation with the fracture properties derived from the more involved multi-specimen method. The weak dependence of geometry factor on rubber composition and displacement (in the neighborhood of crack initiation displacement) was verified. The crack initiation resistance and tearing modulus of the materials were calculated based on the R curve for each initial crack length. Fracture properties of lightly cracked specimens (/ < 0.4, where is the specimen width), including crack initiation resistance, strain at crack initiation, tearing modulus and maximum load and extension before fracture, were found to show significant variations with. These variations are characterized and discussed for the filled and unfilled rubber materials.