Characterization of Kevlar-29 fibers by tensile tests and nanoindentation
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.
References (21)
- et al.
Polymer
(2000) Mater. Sci. Eng. A
(1998)- et al.
Composites A
(2004) Technical Guide
(2011)- et al.
J. Mater. Sci.
(1982) - et al.
J. Mater. Sci.
(1992) - et al.
- et al.
Macromolecules
(1999)
Cited by (42)
Ballistic performance and protection mechanism of aramid fabric modified with polyethylene and graphene
2023, International Journal of Mechanical SciencesNanoindentation of freestanding single Kevlar® fibers with an adjusted indentation area function
2022, Journal of Materials Research and TechnologyCitation Excerpt :One important aspect is the anisotropic nature of Kevlar fibers especially as related to the crystalline orientation of the polymer phase. The mechanical characterization and response of Kevlar fibers can be studied using experimental and computational techniques in which axial (tension [7] and compression [8,9]), indentation, and nanoindentation [10–13] tests are employed in conjunction with modeling tools, especially in the case of dynamic loading [14,15]. Load-controlled indentation experiments evaluate the indentation moduli of a single fiber at specified indentation depths, while compression testing relies on globally deforming fibers with two flat plates to calculate the elastic modulus [8,16].
An investigation of deformation and failure mechanisms of fiber-reinforced composites in layered composite armor
2022, Composite StructuresCitation Excerpt :Due to elongations and defibrations, the transverse sections of some fibers are reduced. These fracture morphologies are typically found in the aramid fibers upon tensile failure [38,39]. On the front layer, it is noticed that the fractured surface of the fiber bundle is relatively flat (Fig. 11b).
The experimental and numerical analysis of the ballistic performance of elastomer matrix Kevlar composites
2021, Polymer TestingCitation Excerpt :This constraint allowed us to restrict the movement of areas of a model to the motion of a reference point. Table 6 shows the properties of the Kevlar and elastomer, individually [51,64]. This table is also used for numerical modeling.
State-of-the-art review on recent advances and perspectives of ballistic composite materials
2021, Composite Solutions for Ballistics