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

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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
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  Effect of annealing process in water on the essential workof fracture response of ultra high molecular weight polyethylene Sinan Yilmaz  • Taner Yilmaz  • A. Armagan Arici Received: 8 July 2010/Accepted: 8 October 2010   Springer Science+Business Media, LLC 2010 Abstract  The toughness of ultra high molecular weightpolyethylene (UHMWPE) before and after annealing pro-cess in water was investigated by the essential work of fracture (EWF) method. Annealing in water at 80   C forvarious aging periods of the 5 mm thickness with variousligament lengths single edge notched tension (SENT)specimens was performed in hot water. Tensile tests wereperformed at 2 mm/min constant deformation rate at roomtemperature in order to determine EWF parameters. Aftertensile tests, the fracture cross-sections of SENT specimenswere investigated by scanning electron microscope (SEM).From the 60th day of annealing process in water, it wasseen that the fracture toughness of the material decreasedwhile water absorption in samples increased. Introduction Thermoplastic polymers are widely used in engineering dueto their beneficial properties but the quality of the thermo-plastic product changes during service. Property degrada-tion owing to annealing in water may be a severe constraintfor the targeted application. That is the reason why greatefforts are devoted to the stabilization of thermoplastics,especially for those which are sensitive to environmentalinfluences. It is known that annealing in water manifestsfirst in a color change without noticeable effect on themechanical properties. Progressing annealing results later insubstantial deterioration of the mechanical characteristicsthat is well reflected by the toughness response. Changes inthe toughness of polymers can well be followed by conceptsof the fracture mechanics. Note that a suitable fracturemechanical method yields a material parameter which doesnot depend on the specimen type and dimension [1, 2]. It is well established that when failure of notchedspecimens is brittle in nature, the area in which energy isdissipated near the crack tip is so small that the entirespecimen can be assumed to exhibit Hookean elasticity.This type of failure can be studied using linear elasticfracture mechanics (LEFM) [3]. The critical stress intensityfactor ( K  Ic ) and critical strain energy release rate ( G  Ic )proved to be material constants under certain testing con-ditions that ensure small scale yielding in the specimens.However, this approach cannot be adopted for toughenedpolymers which exhibit gross yielding during fracture. Forthe assessment of the fracture toughness of such ductilepolymers, the essential work of fracture (EWF) method isgaining acceptance due to its experimental simplicity [4].The EWF assumes that the non-elastic region at the tip of acrack may be divided into two regions: an inner regionwhere the fracture process takes place, and the outer regionwhere the plastic deformation takes place. The total work of fracture can then be partitioned into two components:work that is expended in the inner fracture process zone(IFPZ) to form a neck and subsequent tearing, and the work which is consumed by various deformation mechanisms inthe surrounding outer plastic deformation zone (OPDZ).The former is referred to as ‘‘Essential Work of Fracture’’and the latter as ‘‘Non-Essential Work of Fracture’’ [5, 7]. S. Yilmaz    A. Armagan AriciDepartment of Mechanical Engineering, Kocaeli University,Umuttepe, 41380 Izmit, TurkeyT. Yilmaz ( & )Civil Aviation College, Kocaeli University, Arslanbey Campus,41285 Izmit, Turkeye-mail: taner.yilmaz@kocaeli.edu.trT. YilmazNatural and Applied Sciences, Kocaeli University,Umuttepe Campus, 41380 Izmit, Turkey  1 3 J Mater SciDOI 10.1007/s10853-010-4996-0
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