Morphological and local mechanical surface characterization of ballistic fibers via AFM

Abstract

As-received morphologies, defect structures, and contact moduli of Kevlar KM2 Plus and three other ballistic fibers varying in chemistry and processing, were observed and compared using atomic force microscopy (AFM) and instrumented nanoindentation (NI) techniques. Surface features and defects were defined and measured for each fiber chemistry: p-phenylene terephthalamides (PPTA including KM2 Plus and Twaron), co-polymer aramid (AuTx), and ultra high molecular weight polyethylene (UHMWPE including Dyneema). Although a multitude of surface defects were observed in each fiber, the types of defects were similar from one fiber type to another. It was found that surface defects generally map to a more compliant local modulus value. Contact modulus values were compared with NI elastic modulus values to demonstrate validity for the AFM technique. Challenges and limitations of the AFM technique for cataloging defects are discussed. This study is the first which attempts to outline the various morphologies found on several fiber surfaces. These local property studies will enable future comparisons with single filament and bulk fiber properties.

Structural hierarchy and surface morphology of highly drawn ultra high molecular weight polyethylene fibers studied by atomic force microscopy and wide angle X-ray diffraction

“…The nanoscale fibrillar crystals which compose the fiber are often considered the fundamental building block of these fibers, but knowledge of their characteristic size and interconnectivity in high tenacity fibers is lacking. In addition, the appearance of epitaxial structures on the surface of highly drawn fibers has been noted [23,24] and a deeper understanding of their origin may provide valuable insight into the evolution of the interior structure of a fiber under common processing conditions. Through the comparison of high resolution microscopy techniques and wide angle X-ray diffraction, the research herein aims to quantify dimensions of fibrillar crystals and epitaxial features as well as provide some insight into the nano and mesoscale interactions between fibrils.…”

“…The type of high draw ratios and elevated temperature at which this occurs is key to the development of the extended chain morphologies from which the fiber derives high strength and stiffness. Other research has revealed the presence of these epitaxial structures on the surface of highly drawn fibers [23,24], raising the question of their origin and significance in the discussion of structure/property relationships. As these fibers are incorporated into composite materials, a key metric in determining their effectiveness in load bearing and impact resistance is their ability to absorb and dissipate energy.…”

Improved quasi-static twin-fiber transverse compression of several high-performance fibers

“…9 The internal structure of AuTx consists mostly of large corrugated fibril bundles rather than single fibrils, with a typically cylindrical fiber shape. 28 At lower strains, the experimental results were consistent; at larger strains past approximately 40%, stress deviations become significantly larger (Figure 13). This may be attributed to the sudden failure and fibrillation of the fibers past the 40% strain mark, as described by Singletary et al 6 for PPTA fibers.…”

Recent advances in modeling and experiments of Kevlar ballistic fibrils, fibers, yarns and flexible woven textile fabrics – a review

“…Nano-fibrils consist of stacks of crystallites separated by thin non-crystalline plates, portions of which are spanned by inter-crystalline bridges giving a shish-kebab structure 130 . Polyethylene ballistic fibers exhibit a shish-kebab crystalline structure reported through AFM characterization by Strawhecker and Cole 131 . It was mentioned earlier that post failure investigation of impacted fabrics showed deformation and failure modes including permanent transverse compressive deformation, fibrillation and transverse shear (cutting) failure.…”

“…130 PE ballistic fibers exhibit a shish-kebab crystalline structure reported through AFM characterization by Strawhecker and Cole. 131…”