Feasibility study of adhesive bonding reinforcement by electrospun nanofibers

Abstract In previous works, the authors showed that the interleaving of an electrospun nylon nanofibrous mat at the interface between adjacent plies of a composite laminate increases the delamination strength. In particular, the nanomat acts a net-like reinforcing web, enabling a ply-to-ply bridging effect. This reinforcing property of the nanomats can be potentially used in other applications which need to improve the fracture resistance of interfaces, such as adhesive bonding. The present work analyses the feasibility of an electrospun polymeric nanomat as adhesive carrier and reinforcing web in industrial bonding. Thus the adhesive is used to pre-impregnate a nylon nanofibrous mat that is then placed at the interface between two metal pieces and then cured. The aim of the work is first to assess the effectiveness of this procedure, by comparison of the mode-I fracture toughness measured with DCB (Double Cantilever Beam) tests with and without the reinforcement in the adhesive layer. For this purpose, a 2024-T3 aluminum alloy will be bonded using a general purpose, one-part epoxy resin with low viscosity

Interlaminar Fracture Toughness Evaluation in Glass/Epoxy Composites Using Acoustic Emission and Finite Element Methods

© 2014, ASM International. Delamination is one of the most common modes of failure in laminated composites and it leads to the loss of structural strength and stiffness. In this paper, mode I, mode II, and mixed of these pure modes were investigated using mechanical data, Finite Element Method (FEM) and Acoustic Emission (AE) signals. Experimental data were obtained from insitu monitoring of glass/epoxy laminated composites with different lay-ups when subjected to different modes of failure. The main objective was to investigate the behavior of delamination propagation and to evaluate the critical value of the strain energy which is required for onset of the delamination (GC). For the identification of interlaminar fracture toughness of the specimens, four methods were used: (a) ASTM standard methods, (b) FEM analysis, (c) AE method, and (d) sentry function method which is a function of mechanical and AE behaviors of the specimens. The results showed that the GC values obtained by the sentry function method and FEM analysis were in a close agreement with the results of nonlinearity methods which is recommended in the ASTM standards. It was also found that the specimens under different loading conditions and various lay-up have different GC values. These differences are related to different stress components distribution in the specimens which induce various damage mechanisms. Accordingly, stress components distribution obtained from FEM analyses were in agreement with SEM observations of the damaged surfaces of the specimens

Delamination evaluation of composite laminates with different interface fiber orientations using acoustic emission features and micro visualization

This study aims to investigate the sequence of initiation and evolution of different damage mechanisms during the DCB standard test procedure on specimens with different interface fiber orientation using Acoustic Emission (AE) data analysis and microscopic imaging methods. For this purpose, a series of experiments based on standard ASTM D5528 were performed on 24 layer glass epoxy multidirectional specimens with \ub1\u3d1/05 As As layup, in which \u3d1 is 0. 30. 45 and 60. The acoustic data were then acquired with two AE sensors and the whole test procedure was observed by two digital cameras, which were focused on specimen edges. The results show that the initiation and evolution process of matrix cracking as the first activated damage mechanism greatly depends on the interface fiber orientation. Also, load-displacement curves and AE data can be well correlated with microscopic observation in all stages of damage initiation, evolution and propagation. Although the standard nonlinear point as crack initiation onset predict an equal fracture energy for all cases, however AE analyses and microscopic observations show early damage initiation and evolution in 45 and 60-degree interfaces

Experimental investigation on delamination in nanocomposite drilling

This article addresses the influence of cutting parameters on drillinginduced delamination of woven glass fiber-epoxy composites reinforced with functionalized multi-walled carbon nanotubes (MWCNTs). The input parameters include feed rate, cutting speed, drill size, and wt. % carbon nanotubes present in nanocomposite laminates. Experiments were conducted based on Taguchi L16 orthogonal array and analysis of variance was conducted to determine the significance of each parameter. The results indicate that the main effects of nano content, feed rate, and spindle speed are significant, while the effect of drill diameter is negligible. Furthermore, the optimum drilling conditions for minimum delamination were determined according to the Taguchi's S/N ratio analysis

Study on Mode I fatigue behaviour of Nylon 6,6 nanoreinforced CFRP laminates

The benefits of interleaving polymeric electrospun nanofibers in between laminae of composite structure have been widely demonstrated in the past several years. Among the work that still has to be done, this paper aims to study delamination propagation of virgin and nanomodified specimens under Mode I fatigue loading. A 40-micron thick layer of Nylon 6,6 nanofibers have been produced and interleaved in carbon fiber-epoxy resin composite laminates; static and dynamic double cantilever tests have been performed to determine delamination growth onset and crack propagation rate vs. maximum energy release rate respectively. Nanomodified specimens exhibited improved delamination resistance during both the tests: delamination toughness increased 130% and cracks propagated 36\u201327 times slower than virgin interfaces. The benefits of the nanointerleave and its working mechanism have been explained using micrographs and SEM images, which revealed a double-stage reinforce mechanism

Tensile and fatigue characterisation of textile cotton waste/polypropylene laminates

Abstract Short fibre based cotton flocks from end-of-life jeans fabric (denim twill weave) were introduced in an amount of 16 wt.% in a polypropylene (PP) matrix using a specifically designed manufacturing process to preserve as much as possible the properties of the cotton waste during injection moulding. This involved a first phase of binding the cotton flocks on polyvinyl acetate (PVAc) support, then pelletizing them with PP and finally extruding the final composite. The resulting composites were subjected to morphological, tensile and fatigue characterisation with stress levels from 50 to 90% of ultimate tensile strength. Results indicated that injection moulding offered a sufficient uniformity of properties to the composite, albeit with some occurrence of pull-out during loading. In particular, the tensile performance exceeded that of the pure matrix in a measure compatible with the amount of fibres introduced. In addition, tensile fatigue loading up to 5000 cycles evidenced a limited amount of degradation for maximum applied stresses up to 70% of composite tensile strength

Mechanical behavior of jute cloth/wool felts hybrid laminates

This experimental work is aimed at the characterization of new fibre reinforced composites based on epoxy resin with both protein (wool) and lignocellulosic (jute) natural fibres. Wool-based and hybrid (wool/jute) composites with two different stacking sequences (intercalated and sandwich) were developed. Their microstructure has been investigated through optical and scanning electron microscopy, whereas their quasi-static mechanical behaviour has been evaluated in tension and bending. In addition, the impact behaviour under low-velocity impact at three different impact energies, namely 6 J, 8 J and 9 J has been addressed. The tensile and flexural tests have been monitored using acoustic emission (AE) in order to elicit further information about failure mechanisms. AE monitoring showed that development of damage was due to nucleation of matrix microcracks and subsequent debonding and pull-out phenomena in wool fibre composites and that only in hybrid composites a sufficient stress transfer across the jute fibre/matrix interface was achieved. The results confirmed the positive role of hybridization with jute fibres in enhancing both the tensile and flexural behaviour of wool-based composites, though highlighting the need for an improved adhesion between wool fibres and epoxy matrix