Mechanical behavior of nylon 66 fibers under monotonic and cyclic loading

Abstract

The deformation and fracture behavior of partially oriented nylon 66 fibers with a diameter approximately 33 μm exposed to uniaxial monotonic and cyclic loading was studied using a unique test technique. Prior cyclic deformation resulted in a ratchet strain as well as a decrease in residual strength and ductility of the fibers. A description of the fracture process is hypothesized and supported by critical flaw size and energy calculations. The critical flaw size at fracture did not depend on the prior cyclic loading history.

Introduction

Nylon 66 fibers are used extensively in rubber reinforcement due to their light weight, high strength, and toughness. Mallory [1], Schroeder and Prettyman [2], and Draves et al. [3] have all classified nylon 66 as a viable component for tire reinforcement. With the technological trend shifting to lightweight and stronger materials for engineering applications, there is a continuing need to increase the strength of these fibers to give a higher performance, or the same performance with fewer fibers. Under in-use loading conditions, the composite, and subsequently the nylon 66 cord, becomes fatigued and deformed which changes its mechanical behavior. Patterson and Anderson [4] concluded that the in-service flexure loads of the composite cause the individual cords to become substantially weaker and results in an overall reduction in residual strength. Thus, it can be concluded that when the actual reinforcement components become damaged, either extensively plastically deformed or fractured, the strength of the composite is reduced significantly. The degradation of the reinforcing fibers themselves under cyclic loading is also fairly well established. Lyons [5] examined nylon 66 fibers that were exposed to fatigue and determined from X-ray diffraction measurements that an axial lengthening of the crystallites in the material was produced from the fatiguing process. Thus it was concluded that the crystallites in nylon 66 samples elongate in the axial direction as a consequence of the fatiguing process, indicating internal structural changes in the fiber. Prevorsek and Lyons [6] further performed experiments on the effect of the stroke limitations on the fatigue performance of nylon 66 fibers, and in general concluded that increases in the stroke decreased the time of survival, and alternatively the number of cycles to failure, for individual samples. In this study, they also examined the creep extensions that were engendered during the fatiguing process in nylon 66 fibers and found an increasing relationship for the creep extension vs. the number of fatigue cycles. From these experiments they also concluded that creep is only an auxiliary portion of the fatigue process and not the immediate cause of final fracture. They hypothesized that the final cause of rupture in individual fiber samples was believed to be the result of crack propagation. Hearle and Plonsker [7] used an experimental method to measure the elastic recovery as a function of strain and the measurement of strain engendered during cumulative extension cycling. These experiments were performed on monofilament nylon fibers as well as other polymer fiber bundles. The results showed that as the frequency (or strain rate) increases, the elastic recovery increases due to the fact that there is a smaller amount of time in which secondary creep can increase permanent set in the material. The results also indicated that for small imposed strains, the effects of viscoelasticity were important while for large imposed strains, the effects of elastic recovery were dominant.

More recently, several researchers have attempted to elucidate the plasticity phenomenon in nylon fibers. Northolt et al. [8] examined polymeric fibers (aliphatic polyamides, PET, PpPTA) below the glass transition temperature and established a plasticity criterion. They discovered that the resolved shear stress dominates the tensile and compression deformation phases of the fiber. They also determined that the initial orientation distribution of the polymer chains is the most important attribute in determining the tensile behavior (extension) of the fiber below the glass transition temperature, regardless of the morphology of the fiber (i.e., crystalline or non-crystalline). Ahzi et al. [9] made an analysis of the large-scale deformation in a multitude of crystalline bulk polymers and determined that the crystal lattice of nylon 6 is monoclinic in nature and deforms by crystallographic slip. These studies were primarily concerned with the inelastic deformation of nylon fibers and did not address the issues of fracture.

A previous study by Hearle et al. [10] on the deformation and fracture of nylon 66 fibers provides some understanding of the behavior of these small diameter fibers. Specifically, Hearle obtained many fractographs for nylon 66 fibers, as well as other polymeric fibers, under monotonic and cyclic loading, in tension, bending, and rotation. For the uniaxial tensile experiments, the fracture surfaces indicated that nylon fibers have some very distinct characteristics as compared to brittle fibers. The fracture of nylon fibers is primarily a ductile process, and produces two distinct fracture regions: (1) a ductile v-shaped opening phase followed by (2) further crack growth perpendicular to the fiber axis. Hearle determined that the appearance of the fracture surface is altered by changes in elongation rate and changes in the flaw initiation site. Furthermore, the fracture behavior of larger nylon 66 fibers, approximately 900 μm in width and 250 μm in depth (around cross-section), was measured by Michielsen [11]. The critical strain energy release rate (G_Ic) was measured by introducing a flaw in the fiber by means of a sharp razor and then loading in tensile to fracture. The energy release rate for the highly oriented filaments was 17.8 kJ/m². For comparison, the critical energy release rate for DAM (dry as molded), low-orientation bulk samples was 3.9 kJ/m² [12]. Michielsen [13] also confirmed the effects of relative humidity on the mechanical behavior of nylon 66 fibers. The studies confirmed that increases in RH engender a decrease in the initial modulus (E_i) and critical strain energy release rate (G_Ic) of the sample. G_Ic varied from 31.3 kJ/m² at 0% RH to 15.6 kJ/m² at 100% RH.

Most of the prior work on fracture of individual nylon fibers has been qualitative in nature. Thus, a quantitative understanding of the fiber failure from the viewpoint of residual strength and fracture mechanics could be particularly useful for designers in composite reinforcement and manufacturers. In this paper, the mechanical behavior of 33 μm diameter partially oriented nylon 66 fibers was investigated. The goal of the study was not to reinvestigate the previous studies that have already been mentioned. Rather, the primary study focuses on the effect of uniaxial cyclic loading on the mechanical behavior of the fibers. A unique experimental methodology was developed to investigate these small diameter nylon 66 fibers. In addition, ratchet strain and energy analyses have been performed to characterize these samples. Ratchet strain, in the context of this paper, can be defined as the amount of accumulated strain that developed in the uniaxial direction of the fiber during the fatigue process. Also, a stress intensity solution for semi-elliptical surface flaws has been implemented to estimate the size of the critical flaw size that leads to failure.