Application of essential work of fracture concept to toughness characterization of high-density polyethylene (original) (raw)
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Toughness of high-density polyethylene in shear fracture
International Journal of Fracture, 2007
This paper evaluates the validity of a new test methodology for measuring shear fracture toughness (mode II) of high density polyethylene (HDPE). The methodology adopts Iosipescu test for the shear loading, and determines the toughness based on the essential work of fracture (EWF) concept. The results show that even under the Iosipescu loading, tensile deformation (mode I) is still involved in the fracture process, possibly due to the significant work hardening that HDPE develops during the plastic deformation. The study found that the mode II fracture toughness can be determined through data analysis using double linear regression, i.e., by extrapolating specific work of fracture to zero ligament length and zero ligament thickness. The paper demonstrates that the new test methodology can be used to evaluate mode II fracture toughness of ductile polymers like HDPE in which significant work-hardening may be involved in the fracture process. The paper also provides quantitative comparison of the fracture toughness for HDPE in mode II with its mode I counterpart.
This paper presents a cohesive methodology for quantifying the fracture behavior of structural polymers. We accomplish this task by reviewing the complexities of polymer fracture mechanics and associated J-integral fracture toughness testing as well as by conducting appropriate nonlinear-elastic fracture mechanics measurements with comprehensive analysis. J-based crack-initiation and crack-growth fracture toughness testing is performed on ten clinically relevant formulations of ultra-high molecular weight polyethylene (UHMWPE). This polymer is chosen for its extensive literature base in terms of its mechanical properties and fracture toughness behavior, as well as its safety-critical importance and broad use in total joint replacements. One of the current limitations in the fracture toughness characterization of polymers is the use of " engineering " constitutive behavior to determine the crack-initiation toughness, as compared with the " true " constitutive properties. UHMWPE offers a plethora of true tensile stress-strain data that serves as a template and predicate base for fracture analysis. This paper aims to demonstrate why using true stress-strain behavior for polymer fracture mechanics is so important and why a justified comprehensive analysis method is needed in order to reliably measure the fracture toughness of polymeric materials.
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.
Polymer Engineering & Science, 2010
Toughness and deformation capacity of six polyethylene copolymers in plane-strain fracture were characterized using the concept of essential work of fracture (EWF). Two types of regression analysis were considered: one based on the traditional, total work of fracture, while the other on the energy partition to extract the portion that is relevant to the plane-strain fracture. In particular, the latter analysis excludes energy that is for the final stretch of the surface flanks and produces toughness values that are smaller than those determined based on the total work of fracture. The study found that two types of analysis rank the copolymers in different orders. Based on the results from the energy-partition approach, the article discusses the influence of material characteristics (molecular weight, branch concentration, density, etc.) on the plane-strain fracture toughness of the polyethylene
Fracture Behavior of Medium Density Polyethylenes by Essential Work of Fracture (EWF)
2007
ABSTRACT The fracture behavior of Medium Density Polyethylenes (MDPE) with different melt flow indexes (1.9 to 5.0 dg/min) employed in rotational molding were studied by Essential Work of Fracture (EWF). The EWF parameters obtained were equivalent for all samples examined regardless of their differences in comonomer content and molecular weights. Annealing of samples resulted in lower ductility values according to EWF parameters, in spite of minimum crystallinity changes.
Scientific reports, 2017
This paper examines the effect of a broad range of crosshead speed (0.05 to 100 mm/min) and a small range of temperature (25 °C and 45 °C) on the failure behaviour of high density polyethylene (HDPE) specimens containing a) standard size blunt notch and b) standard size blunt notch plus small sharp crack - all tested in air. It was observed that the yield stress properties showed linear increase with the natural logarithm of strain rate. The stress intensity factors under blunt notch and sharp crack conditions also increased linearly with natural logarithm of the crosshead speed. The results indicate that in the practical temperature range of 25 °C and 45 °C under normal atmosphere and increasing strain rates, HDPE specimens with both blunt notches and sharp cracks possess superior fracture properties. SEM microstructure studies of fracture surfaces showed craze initiation mechanisms at lower strain rate, whilst at higher strain rates there is evidence of dimple patterns absorbing t...
Polymer, 2005
In this paper it is shown that the resistance to slow crack propagation in polyethylene can be predicted from a simple tensile measurement performed at 80 8C. It is shown that for different types of polyethylene homopolymers and copolymers the slope of a tensile curve above its natural draw ratio (i.e. strain hardening) correlates well with the measured stress crack resistance. The data presented in this paper confirm that the slow crack resistance in polyethylene is determined by the failure of the fibrils within the craze, which is shown to be determined by the strain hardening of a tensile curve. A material with a strong strain hardening will reduce the strain rate and consequently the time to failure will be strongly increased. Considering the fact that the slow crack resistance of polyethylene is usually assessed by tedious and time consuming testing methods performed on the notched samples in contact with specific fluids, the findings reported in this publication offer a possibility to assess the information on slow crack propagation in much simpler and faster way. q Polymer 46 (2005) 6369-6379 www.elsevier.com/locate/polymer 0032-3861/$ -see front matter q
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
The J integral fracture toughness and damage zone morphology in polyethylenes
Polymer, 1991
The J integral analysis of compact tension samples has been used to evaluate plane strain fracture toughness of various polyethylenes at room temperature. Crack propagation commences from a razor notch in high-density polyethylene at Jt¢ = 1.7 kJ m-2. Plastic deformation is confined to a small craze region (about 300/~m long, 20/tm high) through which the crack subsequently propagates. A tough copolymer of PE3408 material resists crack advance until J~c = 8.2 kJ m 2. Here again crack propagation is through a craze, though craze length exceeds l mm. Toughness is also imparted by the formation of shear bands near the notch tip. Low-density polyethylene does not really fracture under the present test conditions; this material responds by general yielding and blunting of the notch tip. 11 and has a characteristic size of about 0.2 mm. This is a factor of 2 5 smaller than the plastic zone sizes