Influence Of Absorbed Moisture On Fatigue Crack PATCHED Propagation Behavior In Polyamides
DOWNLOAD >>> https://urlin.us/2tvLqo
How Water Affects the Fatigue Resistance of Polyamides
Polyamides are a class of synthetic polymers that have high strength, toughness and resistance to abrasion and chemicals. They are widely used in engineering applications such as gears, bearings, pipes and textiles. However, polyamides also have a tendency to absorb water from the environment, which can affect their mechanical properties and performance.
One of the important aspects of polyamide behavior under cyclic loading is the fatigue crack propagation (FCP) rate, which measures how fast a crack grows under repeated stress. FCP rate is influenced by many factors, such as temperature, stress intensity factor, loading frequency and environmental conditions. In particular, the presence of water can have a significant effect on FCP rate in polyamides, depending on the type and amount of water absorbed.
In this article, we will review some of the research findings on the influence of absorbed moisture on FCP behavior in three common types of polyamides: nylon 66 (N66), nylon 6 (N6) and nylon 612 (N612). These polyamides differ in their molecular structure and water absorption characteristics. We will also discuss some of the possible mechanisms behind the observed effects of water on FCP rate in polyamides.
Bretz et al. [^1^] [^2^] conducted an extensive series of fatigue tests on specimens of N66, N6 and N612 that had been equilibrated at various levels of absorbed water. The specimens were subjected to cyclic loading with a constant stress ratio of 0.1 and a frequency of 10 Hz. The FCP rate was measured as a function of stress intensity factor range (ÎK), which is a parameter that characterizes the severity of the cyclic stress at the crack tip.
The results showed that FCP rate in polyamides was strongly affected by the water content. In N66 and N6, FCP rate decreased as the water content increased from dry to about 2.5 wt%, which corresponds to a relative humidity (RH) of about 50%. This was attributed to the plasticizing effect of water, which increased the ductility and crack-tip blunting of the polymer. However, as the water content increased further to saturation (8.5 wt% for N66 and N6), FCP rate increased sharply and exceeded that of the dry polymer. This was attributed to the reduction in material stiffness and strength due to water absorption.
In contrast, FCP rate in N612 decreased monotonically as the water content increased from dry to saturation (3.2 wt% for N612). This was attributed to the lower degree of crystallinity and higher molecular weight of N612 compared to N66 and N6, which made it less sensitive to water-induced softening.
The fracture surface morphology of the fatigued specimens was also examined by scanning electron microscopy (SEM) to reveal the mechanisms of FCP in polyamides [^3^]. The fracture surfaces showed different features depending on the type and amount of water absorbed by the polymer.
In dry N66 and N6, the fracture surfaces exhibited brittle features such as river markings and radial cracks. As the water content increased to about 2.5 wt%, the fracture surfaces became more ductile and showed classical fatigue striations, which are parallel ridges that indicate the crack growth increments per cycle. These striations were more pronounced in N6 than in N66. As the water content increased further to saturation, the fracture surfaces became rougher and showed signs of extensive plastic deformation such as dimples and microvoids.
In dry N612, the fracture surface showed a mixed mode of brittle and ductile features. As the water content increased to saturation, the fracture surface became dominated by a microvoid coalescence mechanism, which involves the formation and coalescence of microscopic cavities along the crack path. This mechanism was independent of ÎK level and resulted in lower FCP rates than those observed in N66 and N6 at high water contents.
The influence of absorbed moisture on FCP behavior in polyamides is complex and depends on several factors such as polymer type aa16f39245