Theoretical and Experimental Analysis of Inter-Layer Stresses in Filament-Wound Cylindrical Composite Structures

This paper analyses the issues relative to the modelling of tubular (cylindrical) composite structures. This paper aims to describe the design of a multi-layer structure of filament-wound composite pipes where, after loading, the hoop-stress distribution would be as uniform as possible. That would allow the mass of the composite to decrease while maintaining the proper mechanical strength. This publication presents the development of a calculation model dedicated to mono- and multi-layered tubular composite structures. The equations describing the stress pattern were based on the Lamé Problem, whereas to describe the modelled structures, an anisotropy coefficient was introduced and interlayer pressures values were determined. To verify the calculations, experimental studies were performed. The test specimens were fabricated by winding fibre bundles around a steel core (as rings with an internal diameter of 113 mm and a height of 30 mm). For the test, the method of pressing a conical ring into a split ring, which acts on the internal surface of the tested cylindrical sample, was selected. The operation of the test rig (test stand) was simulated using the Finite Element Method (FEM). Measurements with strain gauges were conducted during the experiments.

CNT/epoxy interleaves for strengthening performance of filament wound composite structures

The failure mechanism of composites dominates the matrix, fiber and interface, and in general, the matrix corresponds to the definitive cause of damage. A filament–wound composite structure involves a notable bridging effect owing to the matrix between the layers, and particle additives are generally adopted to strengthen the matrix. However, particle additives exhibit a low performance when applied to structures, owing to the dispersibility and particle agglomeration. In this study, the strengthening performance of carbon nanotube (CNT)/epoxy interleaves was experimentally verified to facilitate their implementation in the structural design of a filament–wound cylinder structure. The burst pressure, compression, bending and interfacial bonding strength of the cylinder improved by approximately 20%, 161%, 16% and 36%, respectively, and the positioning of CNT/epoxy interleaves was a more notable influencing factor compared to the proportion of CNTs in the entire winding layer. The number of macro voids decreased inside the epoxy modified CNT. The findings demonstrated that the incorporation of CNTs through CNT/epoxy interleaves could facilitate the matrix strengthening and enhance the interfacial bonding.

Experimental damping ratio evaluation using Hilbert transform in filament-wound composite plates

The aim of this work is the evaluation of damping ratio in composite materials with orthogonal fiber orientation based on experimental and numerical techniques. In this study, the logarithmic decrement and the envelope techniques calculated using Hilbert transform are used. Carbon fiber/epoxy composites manufactured by filament winding are dynamically tested in free vibration. Post-processing and data analysis are performed with the developed codes. These comprise the use of a band-pass filter to isolate the first fundamental frequency from the other modes of vibration and noise present in the acquired signal. Then, the Hilbert transform is used to estimate the envelope of the vibration signal and the exponential curve is adjusted to obtain the envelope, in order to evaluate the structural damping ratio. Comparisons with a fitted finite element model are used for validation. The results revealed that damping varied proportionally with the number of layers, the ply orientation and, less evidently, with the length of the samples.