The influence of various standard (ASTM, ISO) and experimental moisture conditioning methods on mechanical performance of injection molded nylon 6 is discussed as a result of an in-depth, comprehensive investigation.These methods covered a wide range for two basic process parameters for conditioning: temperature (from 23 to 100 C) and relative humidity (from 50% RH to water immersion). The variation of these parameters may result in significantly different moisture absorption rates, equilibrium levels and mechanical properties. The kinetics of mechanical performance and microstructure were evaluated prior to tests and during conditioning in this comprehensive analysis.The results from this investigation may provide comprehensive, up-to-date information and recommendations concerning accelerated nylon conditioning methods for test specimens and various molded parts, preselection of nylon based plastic for design, and prediction and optimization of mechanical performance.

To support the fatigue design of the cyclically stressed plastics parts, such as automotive under-the-hood and exterior components, we analyzed the short-term and long-term mechanical performance (tensile strength, fatigue strength, and fatigue life) of short glass fiber reinforced polyamides PA 6 and PA 66. Comprehensive tension-tension fatigue tests were conducted with reference to the latest ASTM, ISO, and Japanese industrial standards for plastics, at temperatures from 4 0°C to 121°C. on materials aged at 121°C for 0, 100, 500, and 1000 h. Tests were conducted at a loading frequencyf = 5 Hz, stress ratio R = 0.1, and in a wide range of cycles to failure from 2 x lo3 to 2 X lo6. Without aging and for both PA 6 and PA 66, the highest fatigue strength or fatigue life was found at -40°C; it decreased significantly at 23"C, and decreased further at 121°C. The fatigue strength of PA 6 was found to be higher than that of PA 66 at -40°C. but the reverse was seen at 23°C.At 12 1°C. the fatigue strengths of PA 6 and PA 66 were virtually the same. Aging at 12 1°C improved the tensile strength of PA 6 and PA 66 as aging time increased from 100 to 1000 h, and this process seemed to be more influential for PA 6.

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Constant‐load creep‐rupture tests were performed on single Si‐C‐O fibers (Nicalon). Test environments included pure carbon monoxide (CO), pure argon gas, and a mixture of CO and argon gas with a CO partial pressure of 40 kPa. Fibers were tested at temperatures of 1200°‐1400°C and at nominal applied stresses of 0.15–0.7 GPa. The as‐received and crept specimens were characterized by means of scanning electron microscopy, transmission electron microscopy, X‐ray photoemission spectroscopy, electron‐probe microanalysis, Auger electron spectroscopy, and thermo‐gravimetric analysis. In pure argon, the microstructure of the Nicalon fiber was unstable, which was attributed to the decomposition of the silicon oxycarbide phase, which resulted in CO and silicon monoxide gas evolution and silicon carbide grain growth. Fiber shrinkage was observed at temperatures <1300°C at low applied stresses. At high stresses, fibers exhibited only primary creep. In the CO/ argon‐gas environment, very limited grain growth and a smooth carbon coating were observed at the fiber surface at temperatures <1350°C. At all applied stresses, fibers exhibited steady‐state creep whose rates, strains, and times to failure were higher than those observed in argon. The apparent activation energy for creep of Nicalon fibers in the CO/argon‐gas environment was 435 kj/mol. At temperatures >1350°C in the CO/argon‐gas environment, however, the fiber behaved as in pure argon. Tests in pure CO only resulted in lower strains to failure and thicker carbon layers on the fiber surface. A rheological model based on the viscous flow of a concentrated suspension was proposed to describe the fiber deformation. The continuously decreasing creep rate in argon was suggested to be related to the continuous increase of the total solid volume fraction, which affects the fiber viscosity. On the other hand, the steady‐state creep of Nicalon with a stable microstructure in the CO/ argon‐gas environment was characterized by a Newtonian‐type viscous flow, which supports the predictions of the model.

Tensile creep tests of single Si-C-0 fibers (Nicalon, Nippon Carbon Co., Tokyo, Japan) were conducted in argon at 1300°C and 300 to 700 MPa. Fibers exhibited only primary creep, where the creep strain E and creep time t could be empirically fitted by E = (Up) In (1 + p&,t). The fiber deformation was described by a rheological model for the viscous flow of a concentrated suspension. Under the test conditions, the microstructure of Nicalon was unstable, resulting in weight loss and S i c grain growth. This instability was attributed to the decomposition of the amorphous SiC,O, phase in the fiber, forming S i c and CO as products. As a result, the viscosity of the fiber increased because of an increase in the S i c volume fraction. The continuous increase in viscosity caused a continuously decreasing creep rate, which made steady-state creep impossible under these conditions. Because of the instability in the microstructure, the chemical environment was found to have a profound influence on the mechanical properties of Nicalon at eievated temperatures.