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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.

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 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.

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.