International Journal of Applied Glass Science--Stickel and Nagarajan Vol.3,No.2,201 required for Type 3 or 4 vessels to place windings paral lel or near-parallel to the primary axis along the length 4500 y=09x38 of the tank. These tanks require complex winding pro- grams with layers at a variety of angles, ranging from pure hoop(88-89, as measured relative to the primary y axis)to low-angle helicals of approximately 10. As the angled windings must anchor around the pressure vessel end bosses to prevent slipping, the composite thickness 51500 is greater here than in the cylindrical section. Ideally fill- ing valves, pressure relief devices, and other accessories are placed before filament winding so that the cured Fig. 10. Impregnated strand tensile to hoop strength translation Why Tensile Strength Matters to impregnated strand or roving form. Figure 9 shows In a cylindrical pressure vessel, hoop or circumfer- a correlation between pristine, single filament and pressure by the cylinder radius and dividing by the wall are shown in the chart, and relative per .ass types ntial stress is calculated by multiplying the internal impregnated strand tensile strength. Four glass types thickness. Hoop stress governs pressure vessel design as clearly illustrated. Efficient strength translation like this it is twice the stress in the axial or longitudinal orienta- suggests that the glass fiber sizing has done its job: the tion. One of the key qualification tests for pressure glass has been well protected and shows good compati- vessels is burst performance, which involves internally bility with the epoxy resin. Figure 10 is an extension of Fig. 9 showing how In a Type 2 cylinder, there is equal load share between impregnated strand tensile strength translates to perfor the steel liner and the composite hoop wrap, however mance in a finished pressure vessel as measured by as the composite reinforcement does not cover the hoop strength. Additional reinforcing fibers are domed end caps, the steel liner is fully responsible for included in this plot, and generally lie along the same all axial stress. In Type 3 and 4 cylinders, the liner trendline, again suggesting that the sizing is functioning ries very little, if any, of the internal pressure, so the as intended to provide good protection, wet out, and tolerable hoop stress or burst pressure is almost exclu- fiber-matrix bonding. The single data point that is off sively a function of the composite' s strength of the curve, K49 aramid, likely has an issue with its To provide hoop strength to a pressure vessel, a sizing that has caused processing difficulty or inade lass fiber must first translate its pristine tensile strength quate resin compatibility. Failure must occur at a burst pressure that is at least the service pressure (3000 or 3600 psi are typical or approximately 200 or 250 bar) multiplied by 02 afety fa afety factors prescribed by Table Ill. Design Stress Ratios( Safety Factors for Composite CNG Cylinders from ANSI NGV2 and ISO 11439 Type 2 Type 3 Type 4 ANSI ISO ANSI ISO ANSI ISO Glass Pristine Filament Tensile Strength (MPa) Glass2.652.753.503.653.503.65 Aramid2.252.353.003.103.003.10 Fig. 9. Pristine flame pregnated strand tensile strength Carbon2.252352.252.352.252.35required for Type 3 or 4 vessels to place windings paral￾lel or near-parallel to the primary axis along the length of the tank. These tanks require complex winding pro￾grams with layers at a variety of angles, ranging from pure hoop (88–89°, as measured relative to the primary axis) to low-angle helicals of approximately 10°. As the angled windings must anchor around the pressure vessel end bosses to prevent slipping, the composite thickness is greater here than in the cylindrical section. Ideally fill￾ing valves, pressure relief devices, and other accessories are placed before filament winding so that the cured composite laminate can anchor them in place. Why Tensile Strength Matters In a cylindrical pressure vessel, hoop or circumfer￾ential stress is calculated by multiplying the internal pressure by the cylinder radius and dividing by the wall thickness. Hoop stress governs pressure vessel design as it is twice the stress in the axial or longitudinal orienta￾tion. One of the key qualification tests for pressure vessels is burst performance, which involves internally pressurizing a cylinder at a constant rate until it fails. In a Type 2 cylinder, there is equal load share between the steel liner and the composite hoop wrap, however as the composite reinforcement does not cover the domed end caps, the steel liner is fully responsible for all axial stress. In Type 3 and 4 cylinders, the liner car￾ries very little, if any, of the internal pressure, so the tolerable hoop stress or burst pressure is almost exclu￾sively a function of the composite’s strength. To provide hoop strength to a pressure vessel, a glass fiber must first translate its pristine tensile strength to impregnated strand or roving form. Figure 9 shows a correlation between pristine, single filament and impregnated strand tensile strength. Four glass types are shown in the chart, and relative performance is clearly illustrated. Efficient strength translation like this suggests that the glass fiber sizing has done its job: the glass has been well protected and shows good compati￾bility with the epoxy resin. Figure 10 is an extension of Fig. 9 showing how impregnated strand tensile strength translates to perfor￾mance in a finished pressure vessel as measured by hoop strength. Additional reinforcing fibers are included in this plot, and generally lie along the same trendline, again suggesting that the sizing is functioning as intended to provide good protection, wet out, and fiber-matrix bonding. The single data point that is off of the curve, K49 aramid, likely has an issue with its sizing that has caused processing difficulty or inade￾quate resin compatibility. Failure must occur at a burst pressure that is at least the service pressure (3000 or 3600 psi are typical, or approximately 200 or 250 bar) multiplied by a design safety factor. Safety factors prescribed by Fig. 9. Pristine filament to impregnated strand tensile strength translation. Fig. 10. Impregnated strand tensile to hoop strength translation. Table III. Design Stress Ratios (Safety Factors) for Composite CNG Cylinders from ANSI NGV2 and ISO 11439 Type 2 Type 3 Type 4 ANSI ISO ANSI ISO ANSI ISO Glass 2.65 2.75 3.50 3.65 3.50 3.65 Aramid 2.25 2.35 3.00 3.10 3.00 3.10 Carbon 2.25 2.35 2.25 2.35 2.25 2.35 130 International Journal of Applied Glass Science—Stickel and Nagarajan Vol. 3, No. 2, 2012