128 International Journal of Applied Glass Science--Stickel and Nagarajan Vol.3,No.2,201 Figure 7 illustrates the effect of the sizing selection pre Short-beam shear strength testing was performed per th d air spraying. For ASTM D2344: Standard Test Method for Short-Beam fiberglass flament process is used and Strength of polymer Matrix Composite Materials ar rovings are continuous thre Their Laminate using curved specimens made from chopped. FFU requirements for filament winding rov Owens Corning XStrand S rovings Short-beam shear ngs include good payout, low fuzz, and fast wet out testing does have limitations: it measures only apparent and opening of strands upon introduction of resin interlaminar shear strength, it obtains neither a pure Two strength-dominated composite applications nor uniform shear stress state, and it does not measure will be discussed in the next section. The product shear modulus. For these reasons, other test methods forms, sizings, and processes used to maximize and are preferred when shear properties design data are maintain tensile strength in each will be described. needed. Advantages of ASTM D2344 are that specimen preparation and testing are simple, rapid, and inexpen- Composite Processing and Selected Applications sive. Interlaminar shear strength values obtained are a good indicator of the fiber-matrix interfacial bond, and thus of the quality of the composit Pressure Vessels The two rovings shown in Fig. 7 are identical in Pressure vessels are used to contain pressurized Alu glass formulation and tex, and curved beam test speci- or gases. Most fluid-containing vessels are main- mens,14 per sample, were produced at th e same fib tained at fairly low internal pressure, which means that content(75% by weight) in a vinyl ester resin. Samples tank walls can be relatively thin. Steel is the material of differed only in sizing chemistry. MCX21 is a multi- choice for most low-pressure applications unless the compatible sizing, meaning that it has functionality in vessel will be in contact with corrosive a variety of resin types, including vinyl ester, whereas contained within it or in its external service environ- EPX15 is considered epoxy-compatible only. Data has ment. For such corrosion applications, composite mate- been normalized to the dry or as received shear strength rials are preferred of the MCX2I product. For high-pressure storage of compressed gase The multi-compatible MCX2I roving clearly per- composite materials offer the most competitive forms superior to the epoxy-compatible EPX15 in a strength-to-weight ratios. Applications include com vinyl ester resin system. Furthermore, the MCX21 pressed natural gas(CNG), hydrogen fuel, paintball more than 80% of its original shear bond and breathing air for personal use and emergency res- h after 96 h of hot-wet aging in a boil tank. cue. Within CNG, which this discussion will focus on on the other hand, suffers poor initial bonding is a wide range of sub-segments and pressure vessel strength and rapid deterioration upon aging. As shown, sizes. At the low end, passenger vehicles will have tanks these specimens retained only about 50% of their shear in the 50 L range, whereas bulk hauling modules will ength through the 96 h boil have pressure vessels the size of shipping containers. alos n addition to compatibility with resin, sizing is Although metal is dominant with more than 90% selected for compatibility with the composite man- share, with the current number of CNG pressure ves- facturing process. The same family of resins, and sels on the market close to 30 million and an annual sometimes the same specific resin system, can be used growth rate of greater than 25% in select regions, the in multiple composite processes, such as compression 10% share belonging to composites represents an molding and pultrusion. Although some sizings can also be used in multiple processes, most are tailored to meet the specific fitness-for-use(FFU) requirements of one process. Vessel Types For example, polyester resins are used frequently There are four general pressure vessel types, for boat hulls and for pipes. a glass fiber sizing with described in Table II, that are recognized by domestic polyester resin compatibility would be required for both and international design and qualification standards applications, but the overall sizing package would be Type I vessels, which are all steel (typical)or quite different. Boat manufacture utilizes the spray up Im, are generally the least expensive to manufFigure 7 illustrates the effect of the sizing selection. Short-beam shear strength testing was performed per ASTM D2344: Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminate using curved specimens made from Owens Corning XStrand® S rovings. Short-beam shear testing does have limitations: it measures only apparent interlaminar shear strength, it obtains neither a pure nor uniform shear stress state, and it does not measure shear modulus. For these reasons, other test methods are preferred when shear properties design data are needed. Advantages of ASTM D2344 are that specimen preparation and testing are simple, rapid, and inexpen￾sive. Interlaminar shear strength values obtained are a good indicator of the fiber-matrix interfacial bond, and thus of the quality of the composite.8 The two rovings shown in Fig. 7 are identical in glass formulation and tex, and curved beam test speci￾mens, 14 per sample, were produced at the same fiber content (75% by weight) in a vinyl ester resin. Samples differed only in sizing chemistry. MCX21 is a multi￾compatible sizing, meaning that it has functionality in a variety of resin types, including vinyl ester, whereas EPX15 is considered epoxy-compatible only. Data has been normalized to the dry or as received shear strength of the MCX21 product. The multi-compatible MCX21 roving clearly per￾forms superior to the epoxy-compatible EPX15 in a vinyl ester resin system. Furthermore, the MCX21 retains more than 80% of its original shear bond strength after 96 h of hot-wet aging in a boil tank. EPX15, on the other hand, suffers poor initial bonding strength and rapid deterioration upon aging. As shown, these specimens retained only about 50% of their shear strength through the 96 h boil. In addition to compatibility with resin, sizing is also selected for compatibility with the composite man￾ufacturing process. The same family of resins, and sometimes the same specific resin system, can be used in multiple composite processes, such as compression molding and pultrusion. Although some sizings can also be used in multiple processes, most are tailored to meet the specific fitness-for-use (FFU) requirements of one process. For example, polyester resins are used frequently for boat hulls and for pipes. A glass fiber sizing with polyester resin compatibility would be required for both applications, but the overall sizing package would be quite different. Boat manufacture utilizes the spray up process, so rovings must maintain adequate integrity through chopping and compressed air spraying. For fiberglass pipe, the filament winding process is used and rovings are continuous throughout the part, not chopped. FFU requirements for filament winding rov￾ings include good payout, low fuzz, and fast wet out and opening of strands upon introduction of resin. Two strength-dominated composite applications will be discussed in the next section. The product forms, sizings, and processes used to maximize and maintain tensile strength in each will be described. Composite Processing and Selected Applications Pressure Vessels Pressure vessels are used to contain pressurized flu￾ids or gases. Most fluid-containing vessels are main￾tained at fairly low internal pressure, which means that tank walls can be relatively thin. Steel is the material of choice for most low-pressure applications unless the vessel will be in contact with corrosive media, either contained within it or in its external service environ￾ment. For such corrosion applications, composite mate￾rials are preferred. For high-pressure storage of compressed gases, composite materials offer the most competitive strength-to-weight ratios. Applications include com￾pressed natural gas (CNG), hydrogen fuel, paintball, and breathing air for personal use and emergency res￾cue. Within CNG, which this discussion will focus on, is a wide range of sub-segments and pressure vessel sizes. At the low end, passenger vehicles will have tanks in the 50 L range, whereas bulk hauling modules will have pressure vessels the size of shipping containers. Although metal is dominant with more than 90% share, with the current number of CNG pressure ves￾sels on the market close to 30 million and an annual growth rate of greater than 25% in select regions, the 10% share belonging to composites represents an attractive opportunity. Vessel Types There are four general pressure vessel types, described in Table II, that are recognized by domestic and international design and qualification standards. Type 1 vessels, which are all steel (typical) or aluminum, are generally the least expensive to manufac- 128 International Journal of Applied Glass Science—Stickel and Nagarajan Vol. 3, No. 2, 2012