Influence of winding pattern on the mechanical behavior of filament wound composite cylinders under external pressure

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Abstract

The influence of winding pattern on the mechanical response of filament wound glass/epoxy cylinders exposed to external pressure is studied by testing cylindrical specimens having stacked layers with coincident patterns in a hyperbaric testing chamber. Different analytical models are evaluated to predict buckling pressure and modes of thin wall cylinders (diameter to thickness ratio d/h of 25) and satisfactory predictions are obtained which are in the same order of magnitude that those obtained in experimental results. Test results show no evident pattern influence on either strength (implosion pressure) or buckling behavior (buckling modes) of thin wall or thick wall (d/h of 10) cylinders.

Introduction

Marine and oceanographic research uses unmanned instrumented vessels for deep ocean research; some of them are made using composite materials and fabricated by the filament winding process. These vessels are mainly exposed to external pressure during service.

Design and analysis practices for this kind of structure use the main assumptions of classical laminate theory [1]. In reality the reinforcement structure of filament wound cylinders is more complex than a classical laminate, because fibers form a pattern which is absent in laminates. These patterns have some zones of undulations and others where the material can be considered as a laminate. Filament wound composite cylinders may have different reinforcement patterns but the same global physical characteristics, such as volume fractions, thickness and number of layers. Several works have been made in order to evaluate these properties as a function of process parameters, a remarkable research is the one made by Koussios [2], other researches concern modification of FEM packages by taking in to account process parameters, this is the case in the work of Zhao et al. [3].

The present work investigates the influence of pattern architecture and dimensions on the mechanical behavior, under external pressure, of filament wound cylinders. Such an influence may be revealed by a loss of strength or by a change in buckling or failure modes.

Among the many research papers dealing with buckling of cylindrical shells, those of Donnell [4], Flügge [5], and Cheng and Ho [6] are regularly cited. Several studies have shown that buckling behavior is sensitive to geometrical defects. These defects may be thickness variations due to the fabrication process. Modification of buckling theories to take into account geometrical defects on the cylinder wall was studied for example by Peterson et al. [7], Smithses [8] in the 1970s and 1980s and Fuchs et al. [9]. In the 1990s, there is the work of Hahn et al. [10] concerning compression buckling and Messager [11] concerning thickness defects on external pressure buckling behavior. In those studies, imperfections were taken into account as axial thickness harmonic variations. Imperfection sensitivities of naval structures have been studied by Elghazouli et al. [12] who performed compression. In the same way, Carvelli et al. [13] tested buckling behavior for technological demonstrators at sea. Those studies are based, mainly, on experimental measurement of thickness or surface topography, some of them also represent reinforcement structure through a thickness variation, but in filament wound cylinders material heterogeneity is not necessarily coupled with thickness variation.

Hahn et al. [10] observed a dependency of buckling modes on winding pattern: when pattern size was similar to the expected buckling mode, the critical buckling stress reached a minimum value. Although that work deals with pattern influence on composite cylinders [10] under uniaxial compression loading, one might suppose that a similar pattern sensitivity exists for biaxial compression (external pressure). In order to examine this, in the present study, a series of implosion tests was carried out in a hyperbaric chamber, on cylindrical specimens of two pattern sizes and two wall thicknesses, made of continuous glass roving and epoxy resin. In parallel, several theoretical models to predict buckling pressure and buckling modes have been evaluated.

In the present paper, the winding pattern architecture produced by the filament winding process is presented first. Next, an evaluation of several models is presented using theoretical properties and, finally, results from axial compression and hyperbaric implosion tests are presented.

Section snippets

Winding and pattern architecture

The filament winding process consists of winding a glass roving around a cylindrical mandrel. The roving is impregnated with resin before being wound, and roving tension can be adjusted in order to control composite compaction. The roving dispenser displacement and mandrel rotation are synchronized by numerical control equipment similar to that used in machine tools.

This fabrication process can produce three types of winding, circumferential, helical and polar [14], [15], [16], [17]. Here, only

Specimen characteristics and conditioning

Cylindrical specimens used in this research were 350 mm long, 125 mm internal diameter, thickness 4.4 mm (thin walled) or 12.6 mm (thick walled), 250 mm long in the central parallel section, and with a 90° winding reinforced section at both extremities. Dimensions are presented in Fig. 3. Winding angle in the central section has a value of ±55°, which is a classical winding angle for pressure vessels, where hoop stress is twice the value of axial stress. Two pattern architectures were selected, 1 or

Mechanical properties

Mechanical properties and constitutive relations are initially calculated, in order to have a first approach for the cylinder’s behavior, taking into account as much as possible the winding architecture. For this, a unit cell is the starting point (see Fig. 5). As was stated in the previous section, the filament winding unit cell is actually formed by two layers, each layer has balanced fiber orientations, half of the volume of each layer shows a fiber orientation +α and the other half −α, as

Experimental results

External pressure tests (13 tests in total) were carried out in a hyperbaric testing chamber (see Fig. 11) at the IFREMER facilities in Brest. Four specimens (references 05VE5CNNI-22, 05VE1CNNI-25, 15VE1CNNI-29 and 15VE5CNNI-30) were instrumented with strain gages; four gages placed in the axial direction and four placed around the circumference, at mid-length on the inner wall. An angular separation of 45° was specified between consecutive gages, alternating axial and circumferential.

Conclusions

Results from this study show no strong influence of the two chosen winding patterns on the implosion pressure of filament wound composite cylinders. Buckling behavior does not seem to be sensitive to these two winding patterns. Buckling modes for specimen dimensions and characteristics used in this research are all of the m = 1 and n = 3 type, independent of winding pattern. Surface damage morphology of thick walled cylinders is not influenced by winding pattern. The choice of 1 and 5 unit cells

Acknowledgement

Hilario Hernández Moreno wishes to thank the National Council of Science and Technology of Mexico (CONACYT) and the National Polytechnic Institute of Mexico (IPN) for their scholarship sponsoring. The authors thank Mrs. Ivan Fernandez Hernandez, Jérémie Bauw, Felipe Afonso, and Erik Vargas Rojas for their collaboration during their internship at LGMT/PRO2COM. Also many thanks to Mr. Matthieu Mulle, PhD at LGMT/PRO2COM, for his collaboration during the instrumented implosion test, and IFREMER

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