The fully-wrapped carbon fiber reinforced cylinders with an aluminum liner are currently the most common high-pressure hydrogen storage tank for vehicles. However, it is difficult to detect it by traditional detection methods effectively. X-ray digital radiographic inspection technology is currently a feasible way to detect the typical defects of the winding layer and aluminum liner of the fully-wrapped carbon fiber reinforced cylinders with an aluminum liner for vehicles. In order to verify the detection ability of digital radiographic inspection for the main defects of carbon fiber wound gas cylinders with aluminum liner and to provide quantitative data of the detection ability, this paper aims at the fiber layer fracture, scratches, and pores that may occur during the use and manufacture of gas cylinders. In order to simulate the above-mentioned natural defects, the images of artificial defects were taken through the optimized transillumination parameters. The test results show that the digital radiographic inspection method can detect the axial notches, circumferential notches, oblique notches with a depth of 0.2 mm on the fiber layer, and axial and circumferential notches with a depth of 0.2 mm on the liner. Furthermore, it can also detect artificial defects such as 0.5mm fiber layer flat-bottom hole and 0.2mm depth /Φ1mm aluminum liner flat-bottom hole. This proves that digital radiographic inspection can be used for the early stage of a significant failure of the fully-wrapped carbon fiber reinforced cylinders with an aluminum liner and provides strong support for its safe use.

Tube trailers with large capacity glass fiber wrapped composite cylinders with steel liners have entered the Chinese market to deliver compressed gases since 2011. The tube trailers with the composite cylinders can carry more gases than those with full steel cylinders, and are welcomed by the market. However, the outer surfaces of the composite layers of the cylinders are susceptible to abrasion damages as resin-based glass fiber composites have low surface hardness. Effect of outer surface defects on the large capacity composite cylinders for tube trailers was investigated in this paper using finite element analysis. First, a mesh model of the large capacity composite cylinder without any defects was built according to its actual geometrical dimensions, in which each layer of the composite overwrap was modeled using a layer of solid elements. Second, the meshes corresponding to surface defects with different surface sizes and different depths were killed using element birth and death method to generate surface defects. Then, the models under internal pressure were solved to obtain stress distribution and predict burst pressure. Numerical results show that the surface defects decrease the burst pressure of the cylinders, and the defect depth has a more important effect on the burst pressure than the surface sizes of the defects. Based on the numerical results, three suggestions to deal with the surface defects were proposed: 1) the cylinders having surface defects with depths less than 1 mm can be acceptable, 2) those having surface defects with depths greater than 3 mm should be condemned, and 3) those having surface defects with depths between 1 mm and 3 mm may be repaired but the inspection interval of the cylinders must be shortened.

Tube trailers with large capacity gas cylinders have been widely used to deliver compressed natural gas (CNG) in China in recent years. Most of the large capacity cylinders are full steel cylinders (Type CNG-1), and the tube trailers with large capacity glass fiber hoop-wrapped composite cylinders with steel liners (Type CNG-2) have entered into Chinese market since 2011. The tube trailers with CNG-2 cylinders can carry more gases than those with CNG-1 cylinders, but need more loading and cooling time to control their wall-temperatures according to the experiences of on-site operators in gas stations. In this paper, the wall-temperatures of those two types of large capacity cylinders on two tube trailers have been tested using a thermal infrared imaging camera during CNG loading processes in two natural gas stations in China. And numerical simulation on the CNG loading processes has been carried out to investigate the temperature changes of the gases and the shells of the cylinders. The results show that CNG-2 cylinders can have less maximum temperatures of the gases and the shells and less wall-temperature increments at the cost of lower loading rate and longer loading time.