Search

In a previous study a model to predict the fatigue S–N behavior of glass fiber reinforced thermoplastics by using a fracture mechanics approach was presented. Using a single flaw size model, some degree of success was observed, particularly for a reinforced polyamide. The model was not successful in predicting the S–N behavior of a reinforced polyester. The earlier study also employed flexural fatigue rather than tensile fatigue data in the calculations because the calculated flaw sizes were more nearly constant as a function of stress level. Subsequently it was shown that the flexural fatigue stress calculations were in error for these types of short glass fiber reinforced plastics owing to the nonlinearity of their stress–strain behavior. In this report we reexamine the utility of the fracture mechanics approach to predict fatigue S–N behavior for both materials using an improved model. Whereas previously a single initial surface flaw was assumed, here we assume multiple flaws growing simultaneously across the sample thickness. The new model is applied to both flexural and tensile fatigue loading. Results demonstrate that this new approach provides accurate predictions of the S–N behavior for both materials under both loading conditions. This reflects the fact that the calculated initial flaw sizes are relatively independent of stress level. No additional adjustable parameters are required if one uses the initial breaking strength of the material as part of the model calculations.

The present study describes a model to predict fatigue S-N behavior, and thus fatigue life, of glass fiber reinforced thermoplastics by using a fracture mechanics approach. The model assumes the presence of an inherent initial flaw in the molded plastic parts and thus ignores crack initiation contributions. In this paper we describe how fatigue crack propagation rate data were obtained for the same three glass fiber reinforced plastics whose S-N behavior was previously described in detail. Using the measured constants from the crack growth data, and corresponding S-N data for uncracked specimens, the validity of the single initial flaw hypothesis was evaluated. From the analyzed results it is concluded that accurate S-N predictions are possible using this simple fracture mechanics model for some materials. The best results are obtained for glass filled polyamide, PA (nylon 66) and polycarbonate, PC; however, with polybutylene terephthalate, PBT, predictions were poor. It is also shown that S-N data for different glass fiber orientations can be predicted by combining the single flaw model with predicted fatigue crack propagation rate measurements. The latter are calculated from a generalized crack growth rate expression utilizing the strain energy release rate fracture mechanics parameter, which was previously described.

Glass-reinforced nylon 66 polyamide (PA), polybutylene terephthalate (PBT), and polycarbonate (PC) were injection molded in a ribbed plaque. Flat strips, tensile bars, and rib sections were cut from the plaque in the flow and crossflow directions and subsequently fatigue tested under both tensile and flexural loading conditions to obtain conventional fatigue S-N (stress versus number of cycles to fail) or Wohler curves. Both the fiber alignment and the loading mode strongly influenced the resultant S-N curve. As expected, longer fatigue life results when the imposed stress is parallel to the flow direction, which in turn parallels the predominant fiber orientation direction. Also, at the same stress level, it was observed that flexural loading resulted in a much longer fatigue life compared to tensile loading. Similar S-N data were obtained in flexure for different geometries, such as rib or flat cross sections, as long as the local fiber orientation was similar. Attempts to use an empirical relationship to rationalize predicted fatigue data based upon tensile or breaking strength were not successful. Polym. Compos. 25:489–498, 2004. © 2004 Society of Plastics Engineers.

Acetal is known for its fatigue resistance, however, it is also susceptible to degradation at high temperatures. Commercial resins therefore contain complex stabilizer systems that, although present in relatively small amounts, provide thermal stability both for processing and subsequent in-service aging. The present study was undertaken to determine how sensitive the fatigue resistance of an acetal copolymer was to changes in stabilizer concentration. Blends containing a series of increasing stabilizer levels were investigated and compared to the commercial material. Fatigue resistance was assessed using accelerated fatigue crack propagation rate measurements for injection molded plaques. Results demonstrate that the fatigue resistance is dependent upon the amount of stabilizer and in fact suggest that a critical level of stabilizer is needed to provide adequate fatigue resistance in the molded part. It is also shown that room temperature aging further aggravates the poor fatigue resistance of samples with low stabilizer levels. Moreover, the results from this study demonstrate that the varying concentrations of heat stabilizer affect even short term mechanical properties such as tensile modulus, strength, and elongation to fail. In addition, fracture toughness tests clearly distinguish the various materials. The short term mechanical tests provide added support that a critical concentration of stabilizer exists.

The influence of sample thickness on fatigue crack propagation rates in injection molded nylon 66 was determined by preparing 12.7 mm thick plaques along with more conventional 3.0 mm thick samples. Initial results suggested a large effect of thickness as the crack propagation rates were accelerated in the thicker samples and the stress dependence was also increased. Since the calculated thickness for a plane stress to plane strain transition in nylon 66 is 9.0 mm, it was thought that these results were related to the stress state at the crack tip. However, a more thorough study of the thicker plaques has now demonstrated that neither the magnitude nor the stress dependence of the fatigue crack growth rates is necessarily changed under plane strain conditions as similar results can be obtained for thick and thin plaques. It is suggested that the earlier results were confounded by a previously unrecognized processing history effect which does accelerate fatigue fracture. The latter effect is shown by thermal analysis and optical microscopy to be related to a rearrangement of the polymer network during melt processing.

Fatigue crack propagation rates in injection molded nylon 66 were previously shown to be strongly affected by prior processing history. To provide a physical basis for the observed acceleration in crack growth rates, microtomed sections were cut through the tips of stable fatigue cracks and examined by optical microscopy. A reduction in spherulite size occurs with reprocessing along with an accompanying decrease in the amount of deformation at the crack tip. For the initially processed nylon 66 this deformation consists of a vast array of independently initiated craze-like zones. Patchy type regions observed on the fatigue fracture surface are similar in size to the initially formed crazed zones. Crack advance occurs by the breakdown and coalescence of the crazed regions via matrix shearing. The extensive damage zone is believed to result in a reduction in stress intensity at the crack tip thereby reducing the crack propagation rates. For the reprocessed nylon 66, one observes fewer crazes and a sharper fatigue crack tip with a consequent acceleration in crack propagation rates and a smoother fracture surface.

The influence of glass fibres on the fatigue crack propagation rates of injection-moulded nylons has been determined. In contrast to previous results for unreinforced nylons, the cracking kinetics are independent of the oscillating load frequency. The fact that the crack growth rate per cycle is constant, when expressed in terms of the time under load, demonstrates that the contribution of creep crack extension is minimized by the glass fibres. Thus a true fatigue process is suggested for the fatigue fracture of the reinforced system, even when the glass fibres are preferentially aligned parallel to the crack growth direction. A complicating factor in characterizing the fatigue resistance of the glass-reinforced nylons is the tremendous influence of fibre orientation on crack growth rate. It is shown that the anisotropy problem can be handled by simply expressing the crack growth rate data in terms of the strain energy release rate rather than the usual stress intensity factor representation. Results for four different glass-filled nylons show that the diverse crack growth rates for cracking parallel versus perpendicular to the glass-fibre axes collapse on to individual strain energy release rate curves. Each single relationship therefore characterizes the fatigue fracture of the filled material and furthermore permits a prediction of the cracking rates for any glass-fibre orientation based upon the expected change in modulus. Finally it is demonstrated that the increased stress dependence of fatigue crack propagation (slope of the Paris plot) in filled nylons can be duplicated in unfilled samples under certain conditions. It is concluded that the fatigue fracture mechanism is matrix dominated in these chopped glass-fibre reinforced materials.

The susceptibility of nylon polymers to environmental stress cracking in aqueous salt solution was investigated. The time to fail (complete rupture) was recorded for razor-cut precracked samples which were totally immersed in water, sodium chloride, or calcium chloride at various initial stress intensity levels and temperatures. Ductile failures were observed in both water and aqueous sodium chloride at all temperatures. However, in saturated aqueous calcium chloride the nylon failure mode changed from ductile tearing, at room temperature, to crazing and cracking at elevated temperatures (50 to 100 ° C). Lithium and magnesium chlorides were also observed to be potent cracking agents at the elevated temperatures. The temperature interval over which cracking occurred corresponded to the onset of the glass transition temperature, Tg, of the nylon. Thus, nylon 6, having a lower Tg, was more susceptible to cracking than nylon 6,6 at any given temperature. On the other hand, nylon 11, having a lower ratio of amide to methylene groups, was not susceptible to salt-induced cracking. For the nylon 6 and nylon 6,6, the failure kinetics in salt solutions were similar, regardless of whether dry samples or previously moisturized samples were examined. Thus a simple temperature-moisture content equivalence does not apply for salt-induced stress cracking in aqueous solutions.

The susceptibility of polycarbonate, polymethyl methacrylate, and cellulose acetate butyrate to gasoline-induced stress cracking was evaluated by measuring the critical strains of specimens exposed to a variety of commercial gasolines and gasoline components. Cellulose acetate butyrate exhibited the greatest resistance to gasoline followed by polymethyl methacrylate and polycarbonate. Stress cracking was a complex function of gasoline composition. Measured critical strains generally decreased and the severity of cracking of molded parts generally increased as the total aromatic content of the gasolines increased. However, a significant difference in critical strains was observed between the individual aliphatic components of gasoline. No correlation was observed between critical strain and gasoline type, i.e., premium, regular, or unleaded. The effect of time of exposure was examined, and it was observed that for some binary mixtures splashing caused much more severe cracking than continuous exposure.

A previous investigation (J. Mater. Sci.22 (1987) 1707) has shown that nylon polymers are susceptible to stress cracking by aqueous calcium chloride at slightly elevated temperatures (50° C). In the present study, the nylon-salt interactions were investigated using thin films of both nylon 6 and nylon 6,6. Chemical, thermal, infrared and dynamic mechanical analyses were performed along with tensile property determinations. Weight gain measurements indicated that above 50° C large increases in sorption occurred for films immersed in either aqueous calcium, lithium, or magnesium chlorides. Both salt ions and water were absorbed at the elevated temperature. In contrast to these results, the data for sodium chloride indicated that only water was absorbed at all temperatures. The sorption process was shown to be physically reversible since the absorbed salt could be removed by immersion in pure water. Moreover the sorption of the salt solution at 100° C did not produce chemical degradation or significant crystallization of the nylons. At higher temperatures the absorbed salt did modify the recrystallization behaviour and increase theTg of the nylons. Significant reductions in tensile modulus were observed for nylon films equilibrated in the salt solutions and tensile elongations at rupture were increased. The effect of increasing temperature and salt type on the sorption process parallels the previously observed stress-cracking activity of the aqueous salt solutions. It is therefore suggested that a plasticization-like mechanism is responsible for the salt-induced crazing of the crystalline nylons.

Elastomers susceptible to ozone cracking can be protected by the addition of antiozonants. For polychloroprene (CR) rubber, a mixture of diaryl-p-phenyl-enediamines, Wingstay 100 (WS 100), is one of the most effective antiozonants. Preliminary studies in our laboratory have shown that the effectiveness of WS 100 can be improved even further by the use of an organobentonite clay. To provide more insight into the way in which this antiozonant provides resistance to ozone cracking, a technique for its analysis was required. High performance liquid chromatography (HPLC) is an attractive technique for analyzing WS 100, as well as for a wide variety of additives in polymers. This study describes a newly developed method for the determination of WS 100 in CR. The method is based on acetonitrile extraction of the antiozonant from the rubber, followed by quantitative analysis using HPLC. With this method, subtle differences in concentrations of WS 100 in CR, caused by changes in compounding, mixing sequence, curing conditions, and grease extraction, have been measured.

Tensile properties of injection and compression molded ABS, terpolymer of acrylonitrile, butadiene, and styrene, were measured as a function of aging time at 40 to 90°C. Both air and nitrogen atmospheres were employed to separate the oxidative and thermal effects. Results demonstrate that tensile elongation is drastically reduced by oven aging at 50 to 90°C, even when a nitrogen atmosphere is employed. Aging in air causes changes in the infrared spectra of ABS and these changes can he monitored in thick samples using the frustrated multiple internal reflection (FMIR) technique. The FMIR technique also provides information on the changes in polybutadiene distribution near the surface of molded parts. The loss of tensile elongation after oven aging is attributed to annealing of the glassy styrene-acrylonitrile matrix. This is supported by differential scanning calorimetry. The tensile elongation is found to he recoverable by reheating to processing temperatures.

The effect of liquid type, moisture content and temperature on the stress cracking behavior of semicrystalline nylon 6,6 has been investigated. Dry nylon is highly susceptible to cracking with critical strains as low as one percent in many liquids. Nylon which has been preconditioned in 50% relative humidity or immersed in water shows almost no propensity to stress crack at room temperature. However, moisturized nylon is still susceptible to stress cracking at low temperature (−40°C). The equilibrium weight gains of the liquids in nylon can be represented on a two-dimensional solubility parameter map and also are proportional to the hydrogen bonding solubility parameter alone. However, unlike previous results for glassy noncrystalline polymers the critical strains for cracking of nylon 6,6 cannot be simply related to solubility. This is due to the fact that rapidly sorbed liquids cause homogeneous stress relaxation to occur in competition with crack initiation. Thus, in general, both the solubility and sorption kinetics must be known in order to predict critical strains of nylon 6,6 in organic liquids.

Certain fuel line hoses having neoprene covers and nitrile inner tubes and test slabs of these materials were subjected to accelerated heat, fuel, and ozone exposure. Results indicate some neoprene hose covers can lose ozone resistance after severe test exposure to heat and/or fuel circulation. Chemical analysis indicates that one type of antiozonant, nickeldibutyldithiocarbamate, is extracted out of the neoprene cover by fuel circulation through the inner nitrile tube. Similarly, heat aging causes a migration of this antiozonant from the neoprene cover into the nitrile tube. These two factors are largely responsible for the loss of ozone resistance. Extraction of plasticizing oil also occurs during fuel circulation with subsequent hardening of hoses. However this contributes only in a secondary manner to the observed loss of ozone resistance. Test conditions for candidate fuel line hoses are recommended to provide improved ozone resistance.