Characterization of Kevlar-29 fibers by tensile tests and nanoindentation

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Abstract

Kevlar-29 fibers are being used in different applications due of their exceptional mechanical properties. More mechanical information on these fibers is needed for better understanding of their complex mechanical behavior. This article presents results from tensile tests on single Kevlar-29 filaments, to characterize their intrinsic behavior under quasi-static loading, and nanoindentation tests, to investigate their cross-section mechanical properties. The results reveal that the elastic modulus measured in the fiber cross-section is lower than that obtained in the longitudinal direction due to the high anisotropy of the fibers.

Highlights

► Single Kevlar-29 fibers characterized mechanically. ► A micro universal fiber tester used for the longitudinal tensile testing. ► Ultra-low load indentation used for the cross-section properties evaluation. ► Kevlar fibers have high anisotropy.

Introduction

Aramid fibers, produced under the commercial name of Kevlar by DuPont de Nemours, have a remarkable combination of high strength, high modulus, toughness and thermal stability compared to many other organic fibers [1]. These impressive properties are due to their molecular structure, developed during their production process which is based on liquid crystal technology, as the rigid molecular chains form a mesophase in solution. The spinning process aligns the molecular chains parallel to the fiber axis leading to a highly ordered structure with a high degree of crystallinity [2]. Kevlar fibers were developed for demanding industrial and advanced-technology applications, such as ballistic protection armor, helicopter blades, pneumatic reinforcement, and sporting goods. The mechanical properties of aramid fibers are related to their particular microstructure characterized by several features such as fibrils, radial pleated sheets and skin–core differentiation [3], [4], [5]. A variety of techniques have been used to elucidate the microstructure of the aramid fibers and several models have been proposed with a common feature being the differentiation of a core and skin region within each individual fiber [6]. Although there is some confusion in the literature, it is generally accepted that the core is less well aligned than the skin [7], [8], [9] but that this difference disappears during tensile loading due to alignment of the molecular structure. It is clear that the molecular morphology of the fibers is responsible for the favorable properties of aramid fibers and it would prove extremely valuable to evaluate the mechanical properties of the individual regions.

The longitudinal behavior of single fibers has been studied for a long time, including Kevlar fibers [2], [10], and some test standards have been formulated for this intention [11], [12]. In addition to the longitudinal direction, it is necessary to study the fiber response to mechanical loads in other directions, such as the cross-section. An understanding of a material's properties on a nanometer-scale provides insight and understanding into that material performance on a macroscopic scale. Ultra-low load indentation, also known as nanoindentation, is a widely used tool for measuring the mechanical properties of thin films and small volumes of material [13]. One of the great advantages of the technique is its ability to probe a surface and map its properties on a spatially resolved basis, sometimes with a resolution of better than 1 μm. On the contrary, nanoindentation does not permit the calculation of the ultimate tensile strength. In the present paper the fiber tested was Kevlar-29, and to obtain a better and more comprehensive understanding of its mechanical behavior, nanoindentation and longitudinal tensile testing were jointly investigated.

Section snippets

Materials and methods

The poly(p-phenylene terephthalamide) (PPTA) fibers analyzed in this work were Kevlar-29 from DuPont de Nemours. Single fibers were subjected to tensile tests at room temperature using a Universal Fiber Tester developed originally by Bunsell et al. [14], equipped with a load cell of 250 g calibrated from 0 to 100 g, with a precision of 0.01 g. The fiber specimens were extracted manually from the bundles and glued to card supports so as to give a gauge length (Lo) of 30 mm. The card protected the

Results and discussion

Fig. 1 shows micrographs of the as-received Kevlar-29 fibers. Fibers appear essentially as smooth cylinders (Fig. 1a) although some of them present flaws, roughness, striations, and even swarf on the surface (Fig. 1b). These imperfections seem to come from the fiber manufacturing process. Tensile test results carried out in this work are summarized in Table 1; they are the average of thirty measurements. It can be seen that the values of failure stress (σR), Young's modulus (E), and failure

Conclusions

Tensile test and nanoindentation technique have been used to determine the mechanical properties of single Kevlar-29 fibers. The samples tested exhibited a stress–strain behavior almost perfectly straight. They have high strength and modulus but show considerable scatter in these properties. The fracture morphology under quasi-static loading condition presents severe splitting of the structure. The elastic modulus of the fibers evaluated by nanoindentation was homogeneous in all the fiber

Acknowledgements

This research was supported by CONACYT FOMIX-Chihuahua (147982). JABC was supported as a graduate student by CONACYT (239769). ATO is grateful to CONACYT-Red Temática de Nanociencias y Nanotecnología for his scholarship. The technical assistance of K. Campos-Venegas, W. Antúnez-Flores and O.O. Solís-Canto is greatly appreciated.

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