## 1 Introduction
With regards to the material aspects, thin-walled composite cylinders have been studied for mechanical applications such as pressure vessels [23; 24; 25; 26] and components that support energy absorption [27; 28; 29; 30]. Composite materials have been exploited in pressure vessels in order to store chemicals [24; 25]. These components were obtained by filament winding by coupling an inner liner with an outer composite laminate. They are designed to contain the fluid working at high pressure and occasionally, to carry partially the load. They also avoid the composite material corrosion. The technological constraints for the filament winding process on the fiber orientation are discussed in [31; 32]. The possibility of using composite materials for hydraulic actuators seem to be meager; in fact, a few contributions are noticeable, see [7; 8; 9; 10]. In [7], the authors present an experimental test of hydraulic actuators on static and fatigue cyclic pressurization and, also aspects regarding wear. In [8; 10], a methodology is presented for designing the cylinder tube and the rod in composites of a hydraulic actuator, and the analytical results and the experimental data are compared. In [9], the feasibility of a hydraulic cylinder in composites is investigated by numerical FE analyses. Static analyses are exploited to investigate the stress and the strain field caused by oil pressure. In addition, the buckling occurrence of the rod is analyzed. Optimization techniques have been applied to cylinder composite stacking sequence and shape in order to increase the critical buckling load. In [10], a hydraulic actuator made up of composites has been applied to a telescopic hydraulic cylinder where the weight reduction is \(\sim\)96% compared to the cylinder in steel, and it costs half of the steel counterpart. For a double-acting cylinder, cylinders manufactured by composites have been investigated that do not exceed the bore diameter and the axial length of 200 mm and 1946 mm, working at maximum operating condition of 380 bar, see [7]. The present work defines a methodology to evaluate a weight efficient alternative design for an actuator with a bore diameter of 300 mm and an axial length of 2300 mm working at high pressure (350 bar). To the best of knowledge, no such methodology for an actuator of these dimensions has been presented before, in the technical literature. The paper is organized as follows. Section 2 analyses the reference steel actuator. A Lame's solution [33] is obtained for orthotropic materials in Section 3, and the formulas developed for composite thick-walled cylinder stresses and strains are presented. In Section 4, three types of fiber were investigated and the composite lamina structural properties are illustrated. In Section 5, the thickness of the composite cylinder tube is calculated by evaluating the required circumferential and axial load-bearing requirements. The selection of the most appropriate carbon reinforcement is analyzed, and the influence of the stiffness-to-weight and the strength-to-weight ratio of the reinforcement on the design is discussed. In Section 6 advanced topics such as buckling and dynamic behavior are discussed comparing the original solution and the composite replacement. The optimal solution is a multi-material design component that adopts an inner and thin-walled steel liner and a composite barrel 13.3 mm thick. This solution is particularly promising in terms of wear resistance and reducing fatigue as a result of the inner metallic liner.