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

Damage analysis for low velocity impacted composite laminates using acoustic emission technique

Characterization of dynamic induced damage is one of the most controversial issues in the application of composite structures. To this aim, Acoustic Emission (AE) technique has been qualified as a robust tool for damage sequence analysis due to its high sensitivity to damage mechanisms. First, AE reference map was created using a couple analysis of Fuzzy C-Means (FCM) clustering and Principal Component Analysis (PCA). The low velocity impact tests were then conducted on composite plates. Finally, Impact AE signals were discriminated and the sequence of damage mechanisms during impact process was discovered. As a result, it is concluded that AE method accompanied with the presented method is a new appropriate approach to discriminate damage mechanism sequences in impacted composite specimens

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

Mechanical and impact characterisation of hybrid composite laminates based on flax, hemp, basalt and glass fibers produced by vacuum infusion

This work concerns the production by vacuum infusion and the comparison of the properties of different hybrid composite laminates, based on glass, flax and hemp fibers in different combinations, keeping constant basalt laminates as the inner core of the layered material and a 21±1% fibers volume throughout. The laminates have been subjected to tensile, flexural and falling weight impact tests. Mechanical tests show quite limited differences between the three hybrid configurations, a fact which is also suggested by the not large variation in material density obtained. The main differences have been observed dealing with falling weight properties, carried out at energies between 6 and 24 Joules using a half-inch impactor with a mass of 1.25 kg. Here, the hybrid configuration containing together flax and hemp fibers shows some energy dissipation properties with the onset of complex damage modes in the laminate