Polyethylene fatigue

Many initiators attack steels of the AISI 4300 series and the barrels of the intensifiers, which are usually of compound constmction to resist fatigue, have an inner liner of AISI 410 or austenitic stainless steel. The associated small bore pipework and fittings used to transfer the initiator to the sparger are usually made of cold worked austenitic stainless steel. The required pumping capacity varies considerably from one process to another, but an initiator flow rate 0.5 L / min is more than sufficient to supply a single injection point in a reactor nominally rated for 40 t/d of polyethylene. 

The material in use as of the mid-1990s in these components is HDPE, a linear polymer which is tough, resiUent, ductile, wear resistant, and has low friction (see Olefin polymers, polyethylene). Polymers are prone to both creep and fatigue (stress) cracking. Moreover, HDPE has a modulus of elasticity that is only one-tenth that of the bone, thus it increases the level of stress transmitted to the cement, thereby increasing the potential for cement mantle failure. When the acetabular HDPE cup is backed by metal, it stiffens the HDPE cup. This results in function similar to that of natural subchondral bone. Metal backing has become standard on acetabular cups. 

The data presented in Figure 19.7 were obtained on a Sonntag-Universal machine which flexes a beam in tension and compression. Whereas the acetal resin was subjected to stresses at 1800 cycles per minute at 75°F and at 100% RH, the nylons were cycled at only 1200 cycles per minute and had a moisture content of 2.5%. The polyethylene sample was also flexed at 1200 cycles per minute. Whilst the moisture content has not been found to be a significant factor it has been observed that the geometry of the test piece and, in particular, the presence of notches has a profound effect on the fatigue endurance limit. 

Figure 19.7. Fatigue resistance of acetal resin compared with nylon 66 and with polyethylene. Measured as the cycles to failure at a given applied stress. (Du Pont trade literature)...

The mechanisms by which polymers undergo degradation in the human body are not yet completely understood. Examples of breakdown of these materials are illustrated by the embrittlement and excessive wear of polyester sockets exposed to the mechanical, biochemical and thermal stresses of the physiological milieu, as well as by the fatigue fractures, excessive wear and additional cross-linking (embrittlement) that have been observed in polyethylene sockets. 

The performance properties of PEN present opportunities for replacement of rayon or polyamide in carcass construction. The use of PEN cord in these applications is currently being evaluated in both Asia and Europe. PEN has demonstrated acceptable flexural fatigue equivalent to polyethylene terephthalate (PET) and rayon. It has equivalent toughness to rayon, which is important for sidewall impact resistance. PEN s superior mechanical properties also afford opportunities to use less fiber in carcass construction enabling production of lighter-weight, more fuel-efficient tires. 

An increase of molecular weight also has a beneficial effect on the resistance to fatigue fracture in crystalline polymers. For example, as M was increased from low to high values in polyethylene, an appreciable increase was noted in the average fatigue life of unnotched specimens and, in precracked specimens, a marked reduction in the FCP rate was noted as molecular weight was increased Also, in specimens... 

Figure 14.39 Applied stress versus failure time (static fatigue) for a sample of high density polyethylene at various temperatures. The inflection shows the point of change from brittle failure to ductile failure. (From Ref. 49.)...

Keywords polyethylene, microdeformation, slow crack growth, fatigue, TEM, cavitation... 

Regardless of the water content or AK level, no DGB s have yet been observed in any nylon specimens (either Series A or Series B). By contrast, the discontinuous growth of fatigue cracks has been described in studies of both crystalline and amorphous polymers [for example, in polyacetal (29,30), polyethylene ( ), and a variety of poorly crystalline or amorphous polymers ( ) ]. While the process of DGB formation in poorly crystalline or amorphous polymers is fairly well understood ( 5, 12,29,32) the role of well developed crystallinity is not clear. Hence the reason why DGB bands are not observed in nylon 66 is as yet unknown. 

When polymers slide on machined metal surfaces, it is quite possible that steady-state wear Involves a combination of abrasive, fatigue, and adhesive wear mechanisms. To study fatigue wear, it would be desirable to minimize the contributions of the abrasive and adhesive wear modes. In this paper, the following polymers polycarbonate, polyvinyl chloride, ultra-high molecular weight polyethylene, siloxane modified epoxies, and polylmldes are tested in experiments in which the fatigue wear mode is predominant. 

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