Buckling analysis of thin-walled metal liner of cylindrical composite overwrapped pressure vessels with depressions after autofrettage processing

The cylindrical filament wound composite overwrapped pressure vessels (COPV) with metal liner has been widely used in spaceflight due to their high strength and low weight. After the autofrettage process, the plastic deformation of the metal liner is constrained by composite winding layers, which introduce depressions to the metal liner that causes local buckling. To predict the local buckling of the inner liner with depressions of the pressure vessel after the autofrettage process, a local buckling analysis method for the metal liner of COPV was developed in this article. The finite element method is used to calculate the overall stress distribution in the pressure vessel before and after the autofrettage process, and the influence of local depressions on the buckling is evaluated. The axial buckling of the pressure vessel under external pressure is analyzed. The control equation of the metal liner with depressions is developed, considering the changes in the pressure and the bending moment of the liner depressions and its vicinity during the loading and unloading process. Taking the cylindrical COPV (38 L) with aluminum alloy liner as an example, the effects of liner thickness, liner radius, the thickness-to-diameter ratio, autofrettage pressure, and the length of straight section on the autofrettage process are discussed. The results show that the thickness of the inner liner has the most significant influence on the buckling of the liner, followed by the length of the straight section and the radius of the inner liner, while the autofrettage pressure has the least influence.

Structural Design and Stress Analysis of a Fully-Wrapped Composite CNG Gas Cylinder With Nominal Working Pressure of 30 MPa

China is the world’s largest user of compressed natural gas vehicles, with a total of nearly 6 million compressed natural gas (CNG) vehicles. The nominal working pressure of the cylinders used in the CNG vehicles in China is 20 MPa, as a result, CNG vehicles have a short range. In order to improve the range of CNG vehicles, the development of CNG vehicles with higher pressure is promoted by the CNG vehicle industry in China nowadays. In this paper, structural design of a fully-wrapped composite CNG gas cylinder with nominal working pressure of 30 MPa are carried out. The steel liner is made of 4130X seamless steel with design wall thickness of 5.9 mm, and the outer surface of steel liner is wrapped with resin based glass fiber composite material. The fully-wrapped composite adopts mixed fiber winding mode: low-angle helical winding, high-angle helical winding and hoop winding. Stress analysis and autofrettage pressure optimization of the designed composite gas cylinder are carried out with finite element method. The results show that the designed composite gas cylinder meets the requirements of ISO 11439-2013, and the best autofrettage pressure of the gas cylinder is 52 MPa after optimizing the autofrettage pressure.

A Finite Element Method Study of Combined Hydraulic and Thermal Autofrettage Process

Autofrettage is a metal working process of inducing compressive residual stresses in the vicinity of the inner surface of a thick-walled cylindrical or spherical pressure vessel for increasing its pressure capacity, fatigue life, and stress-corrosion resistance. The hydraulic autofrettage is a class of autofrettage processes, in which the vessel is pressurized using high hydraulic pressure to cause the partial plastic deformation followed by unloading. Despite its popularity, the requirement of high pressure makes this process costly. On the other hand, the thermal autofrettage is a simple method, in which the residual stresses are set up by first maintaining a temperature difference across the thickness of the vessel and then cooling it to uniform temperature. However, the increase in the pressure carrying capacity in thermal autofrettage process is lesser than that in the hydraulic autofrettage. In the present work, a combined hydraulic and thermal autofrettage process of a thick-walled cylinder is studied using finite element method package ABAQUS® for aluminum and SS304 steel. The strain-hardening and Bauschinger effects are considered and found to play significant roles. The results show that the combined autofrettage can achieve desired increase in the pressure capacity of thick-walled cylinders with relatively small autofrettage pressure. For example, in a SS304 cylinder of wall-thickness ratio of 3, an autofrettage pressure of 150 MPa enhances the pressure capacity by 41%, but the same pressure with a 36 °C higher inner surface temperature than outer surface temperature can enhance the pressure capacity by 60%.