Hoop-wrapped Cylinders Research Articles

Hydrogen damage in 34CrMo4 pressure vessel steel with high tensile strength

Various efforts have been made to improve the safety of high-pressure gas cylinders for hydrogen or natural gas with high strength steel liners. Metal liners with high tensile strength have a safety concern, particularly with hydrogen gas or hydrogen generating environments. The hydrogen can permeate into the liner material, and make the material brittle, causing hydrogen damage. This study investigated resistance to hydrogen damage for two kinds of 34CrMo4 steel with different strength levels. Hydrogen was charged with the electrochemical method, and the material strength was measured by the small punch testing technique. Hydrogen concentration of the specimen was also measured for every testing condition, with various charging periods. The specimens with high tensile strength absorbed more hydrogen than the regular tensile strength specimens. The absorbed hydrogen caused internal damage of intergranular cracking and blistering. Material ductility at failure decreased, as the hydrogen concentration of the specimen increased. But the hydrogen concentration had virtually no effect on the strength of the materials with hydrogen. These results confirm that the susceptibility to hydrogen damage of the high tensile strength materials is much higher than that of the materials with regular strength. If the metal liner of a hoop-wrapped cylinder vessel of type II has high tensile strength, general corrosion at the liner surface can cause a hydrogen rich environment, and the cylinder can suffer hydrogen damage and embrittlement. Therefore, controlling the strength level under an optimal level is critical for the safety of a cylinder made with 34CrMo4 steel.

P009 減肉を有する周方向被覆鋼管の破裂内圧(フェロー賞表彰対象ポスターセッション)

In this study, the efficiency of hoop-wrapping approach as the countermeasure for pipe strength decrease due to local wall-thinning is studied. For that purpose, the effects of the flaw length, the flaw depth and the hoop-wrapped material thickness on the burst pressure of the steel-lined hoop-wrapped cylinder are investigated by using finite element analysis(FEA).

Stress intensity factor calculation of steel-lined hoop-wrapped cylinders with internal semi-elliptical circumferential crack

The purpose of this paper is to present mode I stress intensity factor for a circumferential semi-elliptical crack on the inner surface of a hoop-wrapped steel-lined CNG cylinder. The stress intensity factors along the crack front are directly computed by 3D finite element method for a wide range of variations of the crack geometry. Also influence of many parameters such as cylinder internal pressure, composite layer thickness, composite material properties and undertaking Auto-Frettage pressure are studied on the stress intensity factor of the crack and some conclusive results are drawn. For the sake of validation of the results and because of lack of the results for a circumferential semi-elliptical crack in the literature, a semi-elliptical axial crack in a composite hoop-wrapped cylinder has been modeled and the results have been compared with those in the literature showing a good agreement.

Fracture analyses on the all-metal cylinders and the metal-lined hoop-wrapped cylinders with axial cracks

The objective of this paper is to report on the research about the effects of the cylinder materials, the hoop-wrap materials and the crack size on all-metal cylinders and metal-lined cylinders with axially oriented internal cracks through comparisons of the fracture behaviors between the all-metal cylinders and the metal-lined hoop-wrapped cylinders. The three-dimensional finite element methods were used to model these different cylinders. The different fracture behaviors of these cylinders are presented by comparing the calculated normalized stress intensity factors and the crack-tip opening displacements of these cylinders. The effects of the crack sizes, the cylinder materials and the hoop-wrap materials on the crack driving forces are studied and discussed.

The fracture behavior of the aluminum lined hoop-wrapped cylinders with internal axial cracks

Aluminum lined hoop-wrapped cylinders with internal axial semi-elliptical cracks in the cylindrical portion center of the aluminum-liner are modeled by three dimensional finite element method. Crack front regions are modeled using singular elements, whereas the rest of the cylinder is modeled using twenty-node hexahedron elements. Not only the cylindrical body but also the neck and the transition areas of the cylinder are considered in the modeling. The stress intensity factor K I and the crack mouth opening displacement (CMOD) are calculated. The influence of the hoop-wrapped materials, the internal pressure and the crack sizes on the fracture behavior of the cylinder are discussed.

Fracture analysis on the metal-lined hoop-wrapped cylinders with internal axial cracks

The metal-lined (steel-lined and aluminum-lined) hoop-wrapped cylinders with internal axial semi-elliptical cracks in the cylindrical portion center of the metal-liner are modeled by a three-dimensional finite element method. Crack front regions are modeled using singular elements, whereas the rest of the cylinder is modeled using twenty-node hexahedron elements. Not only the cylindrical body, but also the neck and transition areas of the cylinder, are considered in the modeling. The stress intensity factor K I and crack mouth opening displacement (C MOD) for the metal-lined hoop-wrapped cylinders are calculated. The influence of the hoop-wrapped materials, the internal pressure and the crack sizes on the fracture behavior of these cylinders are discussed and the different fracture behaviors of the steel-lined hoop-wrapped cylinder and the aluminum-lined hoop-wrapped cylinder are discussed.

Effect of composite wrapping on the fracture behavior of the steel-lined hoop-wrapped cylinders

The steel-lined hoop-wrapped cylinders with internal axial semi-elliptical cracks in the cylindrical portion center of steel-liner have been modeled by a three dimensional finite element (FE) method. Crack front regions were modeled using singular elements, whereas the rest of the cylinder was modeled using twenty-node hexahedron elements. Not only the cylindrical body but also the neck and transition areas of the cylinder were considered in the modeling. The stress intensity factor, K I, and crack mouth opening displacement (CMOD) were calculated. The influence of the hoop-wrapped materials, the internal pressure and the crack sizes on the fracture behavior of the cylinder are discussed.

Sustained-load-cracking characteristics of aluminum-lined NGV cylinders

The aluminum liners of composite-wrapped natural gas vehicle (NGV) cylinders are manufactured from either AA6351-T6 or AA6061-T6 alloy. This study involved determining whether the two aluminum alloys are susceptible to sustained-load-cracking (SLC). A fitness-for-service evaluation of the AA6351A-T6 lined cylinders, based on fracture mechanics methods, is also discussed. SLC tests were performed using pin-loaded pre-cracked compact tension specimens, machined from the three different cylinder heads. Tests were conducted in two stages using the constant crack mouth opening displacement approach. It was determined that both the newer and the older AA6351A-T6 liner materials were susceptible to SLC. SLC had occurred even when the specimen was loaded at as low as 40% of the material fracture toughness value (KQ). No SLC features were found with the AA6061-T6 test samples, even with those that had been loaded to 100% of the material fracture toughness value. Susceptibility to SLC reduces the safe operating life of the 6351 lined hoop-wrapped cylinders from 10700 refuelling cycles to 2100 refuelling cycles.

Effect of composite damage on the fatigue behavior of the metal-lined hoop-wrapped cylinders: Bhuyan, G.S. J. Pressure Vessel Technol., Trans ASME114, 1 ((Feb 1992) pp 120–123

Some popular concepts for reducing three variable stress components σx(t), σy(t), τxy(t) to one equivalent amplitude spectrum, and the use of the linear damage accumulation hypothesis, have been evaluated as not fully correct when these components vary non-proportionally and arbitrarily. A different approach is suggested: computing damage accumulation by means of an integral directly on the non-radial arbitrary path, called the ‘trajectory’, described in the σx−σy plane when τxy(t) = 0, in the σx−τxy plane when σy(t) = 0, or in a special coordinate space where this trajectory is invariant of stress directions x, y. If the trajectory is random, it may be replaced by a statistical two-dimensional density of distribution. The integrand, called the R-function, is derived from various S−N fatigue curves under different determined loadings. Thus the traditional S−N function is replaced by the R-function for direct damage summation with differential analysis, which allows the loading to be arbitrary (non-cyclic, multiaxial and non-proportional). The method works by means of computer programs and is applicable to real structures.

Effect of Composite Damage on the Fatigue Behavior of the Metal-Lined Hoop-Wrapped Cylinders

Effects of an axial defect in the composite of hoop-wrapped steel and aluminum cylinders on the hoop stress as well as on the bulging of the liners were investigated through the numerical and experimental stress analysis. Comparison with the experimental results (reported in reference [5]) suggests that relative fatigue performance of the cylinders, containing damages to the composite wrapping, can be assessed without conducting prototype tests.