Mechanical characterization of a new Kevlar/Flax/epoxy hybrid composite in a sandwich structure

Natural fibers are inexpensive, biodegradable, and have similar specific properties to some synthetic fibers. Hardly any previous investigations exist of a composite made of multiple layers of pure Kevlar fiber fabric and pure Flax fiber fabric in a “sandwich structure”, but it only measured impact properties. The composite was made of 12 Flax/epoxy layers at the core in 3 possible configurations (i.e. [0]_12F, [0/90]_6F, or [±45]_6F) that were sandwiched by 2 Kevlar/epoxy layers (i.e. plain weave) on each side. This study showed maximum change in the mechanical properties with respect to Flax/Epoxy for tension (+137.85% in E_T, and +171.22% in σ_UT), compression (+171.22% in E_c, and −10.6% in σ_UC), 3-point bending (−11.54% in E_B, and +2.19 in σ_UB), torsion (−5.31% in G, and 395.82% in τ), and water absorption (60.04%). This novel hybrid composite may be useful for research and industry applications.     

Multiaxial mechanical behavior of aramid fibers and identification of skin/core structure from single fiber transverse compression testing

The transverse and longitudinal mechanical properties of aramid fibers like Kevlar? 29 (K29) fibers are strongly linked to their highly oriented structure. Mechanical characterization at the single fiber scale is challenging especially when the diameter is as small as 15 µm. Longitudinal tensile tests on single K29 fibers and single fiber transverse compression test (SFTCT) have been developed. Our approach consists of coupling morphological observations and mechanical experiments with SFTCT analysis by comparing analytical solutions and finite element modeling. New insights on the analysis of the transverse direction response are highlighted. Systematic loading/unloading compression tests enable to experimentally determine a transverse elastic limit. Taking account of the strong anisotropy of the fiber, the transverse mechanical response sheds light on a skin/core architecture. More importantly, results suggest that the skin of the fiber, typically representing a shell of one micrometer in thickness, has a transverse apparent modulus of 0.2 GPa. That is around more than fifteen times lower than the transverse modulus of 3.0 GPa in the core. By comparison, the measured longitudinal modulus is about 84 GPa. The stress distribution in the fiber is explored and the critical areas for damage initiation are discussed. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 374–384     

Ethylene propylene diene monomer rubber‐based heat shielding materials for solid rocket motor: Impact of Kevlar fiber reinforcement on the thermal and mechanical properties

This work scrutinizes the utilization of ethylene propylene diene monomer rubber matrix (EPDM) with an embodiment of aramid fiber for the heat shielding applications in solid rocket motor (SRM). Aramid fibers are aromatic poly‐paraphenylene terephthalamide, here deployed are Kevlar fibers (KF). However, the literature that encompasses the thermal and mechanical behavior with the fiber loading is reported nowhere else. The effect of fiber addition on the surface morphology and density was thoroughly studied, and it revealed that the EHSMs were of lower density to act as an efficient payload for the SRM. In this regard, the thermal conductivity, heat capacity, thermal diffusivity, fire behavior, and mechanical properties of the EPDM/KF‐based EHSMs were explored. The results revealed that the EHSMs are thermally insulating and thermally stable material with balanced mechanical properties that can engender the thermal and mechanical strains of the rocket motor. Furthermore, other analytical techniques such as scanning electron microscopy and energy dispersive X‐ray spectroscopy have been exploited to monitor the performance of the char residues of the EHSM to delineate its performance in the fire atmosphere.     

Influence of hybridization on in-plane shear properties of 2D & 3D thermoplastic composites reinforced with Kevlar/basalt fabrics

This paper investigates the characterization of in-plane shear properties of thermoplastic composites reinforced with Kevlar/basalt fabrics. Different fabrics had architectures of two dimensional plain woven (2D-P) and three dimensional angle-interlock (3D-A). Intralayer hybridization was performed during the weaving of the fabrics with the combination of Kevlar and basalt yarns. Five 2D-P and three 3D-A composite laminates were manufactured with polypropylene (PP) as a matrix, using compression molding. Iosipescu shear tests were carried out to evaluate the in-plane shear properties. The experimental results revealed that the shear properties including shear modulus, shear strength and shear failure strain of homogeneous composites were improved by 6.5–14.9%, 4.3–19.7%, and 3.2–46.7%, respectively. Similarly, change in the fabric architecture from 2D-P to 3D-A also enhanced the shear strength and shear failure strain by 32.0–41.6% and 7.2–22.5%, respectively. Intralayer hybrid composites had better in-plane shear properties than the interlayer hybrid composites. The fracture morphologies of the specimens were examined by scanning electron microscopy (SEM).     

Characterization of 3D angle-interlock thermoplastic composites under high strain rate compression loadings

In the present work, dynamic compression response of polypropylene (PP) based composites reinforced with Kevlar/Basalt fabrics was investigated. Two homogeneous fabrics with Kevlar (K3D) and Basalt (B3D) yarns and one hybrid (H3D) fabric with a combination of Kevlar/Basalt yarns were produced. The architecture of the fabrics was three-dimensional angle-interlock (3D-A). Three different composite laminates were manufactured using vacuum-assisted compression molding technique. The high strain rate compression loading was applied using a Split-Hopkinson Pressure Bar (SHPB) set-up at a strain rate regime of 3633–5235/s. The results indicated that the dynamic compression properties of thermoplastic 3D-A composites are strain rate sensitive. In all the composites, the peak stress, toughness and modulus were increased with strain rate. However, the strain at peak stress of Basalt reinforced composites (B3D, H3D) decreased approximately by 25%, while for K3D specimens it increased approximately by 15%. The K3D composites had a higher strain rate as compared to the B3D and H3D composites. In the case of K3D composite, except strain at peak stress, remaining dynamic properties were lower than the B3D composite, however, hybridization increased these properties. The failure mechanisms of 3D-A composites were characterized through macroscopic and scanning electron microscopy (SEM).     

Laser cutting of Kevlar laminates and thermal stress formed at cutting sections

Laser cutting of Kevlar laminates is carried out and thermal stress field developed in the cutting region is predicted using the finite element code. Temperature predictions are validated through the thermocouple data. The morphological changes in the cutting section are examined by incorporating optical and scanning electron microscopes. It is found that temperature predictions agree well with the thermocouple data. High values of von Mises stress are observed at the cutting edges and at the mid-thickness of the Kevlar laminate due to thermal compression formed in this region. The laser cut edges are free from whiskers; however, striation formation and some small sideways burning is observed at the kerf edges.     

Influence of humidity cycling parameters on the moisture‐accelerated creep of polymeric fibers

Accelerated creep is a curious and poorly understood transient moisture effect. The creep rates of most hydrophilic materials increase greatly with moisture content. However, when these same materials are subjected to creep loads in cyclic humidity environments, they often exhibit much higher creep rates than in a constantly humid state. This is called accelerated creep. Previous experimenters reported that accelerated creep was less likely to occur in polymeric fibers. We demonstrate experimentally that this happened only because of their choice of humidity cycling parameters. New results are given for Kevlar, lyocell, nylon‐6,6, and ramie fibers. Other paper scientists have argued that the absence of accelerated creep in single fibers supports a explanation based on fiber network effects for accelerated creep in paper. We argue here that accelerated creep is a more general phenomenon consistent with sorption‐induced stress‐gradient explanations. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2048–2062, 2001     

Surface derivatization of poly(p-phenylene terephthalamide) fiber designed for novel separation and extraction media

The surface derivatization of poly(p-phenylene terephthalamide) fiber was studied. The obtained surface-derivatized filaments were packed into a fused-silica capillary to evaluate its surface characteristics by using GC. As the stationary phase for GC the surface-derivatized fibers showed higher retention for alkanes and alkylbenzenes than that with the untreated Kevlar fiber. The improvements on the retention power and the peak shape were observed on the benzyl-modified fibrous stationary phase. The derivatized fibrous materials were also evaluated as the extraction medium in fiber-in-tube SPE, and the effect of the surface modification on the extraction power was compared to the parent fiber. The results indicated that the modified fiber possessed a higher extraction power than the untreated fiber. Based on the facts, the successful modification of the fiber surface was estimated.     

光谱法研究MC尼龙6/芳纶复合材料的结构

将经甲苯二异氰酸酯(TDI)和己内酰胺改性的Kevlar纤维作为MC尼龙6的增强体,X射线光电子能谱观察到改性后纤维表面C,N和O元素的含量发生了改变,谱峰也相应发生变化。从红外光谱分析发现,Kevlar纤维和MC尼龙6相混所得谱图只是二者红外谱图的简单叠加,而改性Kevlar纤维可作为己内酰胺阴离子开环聚合的活性中心,且接枝链上的酰胺基可以与基体尼龙形成较强的氢键,有利于提高界面结合。XRD测试表明Kevlar纤维的引入并没有明显改变MC尼龙6的晶型,但其晶粒将具有更严格的三维周期性结构。在相同纤维用量时,改性Kevlar纤维增强的MC尼龙6的晶粒较未改性纤维的完善。当纤维含量小于2%时,纤维的加入有利于生成完善的α球晶,纤维含量大于2%时,α球晶结构含量随着纤维用量的增多反而下降。     

干摩擦和水润滑条件下芳纶浆粕/环氧树脂复合材料摩擦磨损性能研究

研究了芳纶浆粕纤维增强环氧复合材料在干摩擦和水润滑条件下的摩擦磨损性能,探讨了纤维含量对复合材料摩擦磨损性能的影响,并分析了复合材料的磨损机理.结果表明:芳纶浆粕纤维能够大幅度提高环氧树脂的摩擦磨损性能;当纤维体积分数为40%时,复合材料的比磨损率最小;在水润滑条件下,复合材料的摩擦系数和磨损率均比干摩擦下的明显降低,这是由于水起到了润滑和冷却作用;干摩擦时的磨损机理为粘着磨损和塑性变形,水润滑时主要为犁削和轻微的磨粒磨损.