High strength glass reinforcements still being discovered

High strength glass fibre reinforcements have been around for more than 40 years but they are still being discovered by design engineers and fabricators looking for improved performance in composite parts for applications ranging from aerospace to wind energy. The appeal of this unique class of fibreglass stems in part from the fact that high strength glass fibres have an exceptional balance in performance criteria including high strength, stiffness, energy absorption, fatigue and high temperature performance, and they can be a relatively easy drop-in substitute for E-glass with modest cost impact compared to aramid and carbon fibres.

High strength glass fibres may not have the absolute highest rating in a single performance category but they offer a blend of performance characteristics that is unsurpassed by the well-known alternative reinforcements typically considered to be high performance materials. For example, if you only need improved stiffness, carbon fibre will be the clear winner. But if you need stiffness, strength and impact resistance, high strength glass fibres will provide the best combination of those properties.

Strength in the composition

The enhanced performance of high strength glasses results from their chemical composition that is characterised in part by higher silica content. The higher silica content – as much as 64-66% for S-2 Glass® – means the glass batch must be melted and processed at a much higher temperature. S-2 Glass, for example, has a melting temperature that is about 200°C higher than E-glass.

This higher silica content not only makes the glass harder to melt, it makes it harder to fiberise as well. The precious metal alloy content of the bushings is higher and the throughput is slower.

The higher temperature requirements also mean different furnace designs must be employed from the standard, high-throughput refractory-lined glass furnaces that are relatively standard in the E-glass world. Smaller, more flexible melters have been designed for high strength glasses. While this combination makes the fibres more expensive to produce, it also makes the products ideal for speciality applications that require product differentiation and specific properties.

The manufacturing technology also offers the flexibility to follow market demand. Incremental capacity increases do not require large investment and can be implemented in a few months time. Temporary market swings can often be absorbed by restarting or curtailing the appropriate number of bushings as the market requires. This creates a secure supply situation with quickly available upside capacity. This flexibility and the potential for relatively low-volume increases in capacity have made pricing very stable and predictable during the past few decades.

Most high strength glass compositions also include a metallic element such as magnesium. Glasses made with magnesium aluminosilicate formulations include S-2 Glass (AGY), U Glass (NSG) and T Glass (Nittobo). R-Glass fibres (Vetrotex) are made with a calcium aluminosilicate glass containing less silica and added oxides to reduce the melting temperature. The compositions of glass batch formulations classified as producing high strength fibres are included in ASTM D 578.

^a Advantex is a registered trademark of Owens Corning. ^b S-2 Glass is a registered trademark of AGY.

Tensile leads attributes

The biggest single advantage high strength glass fibres have compared to E-glass is high tensile strength. In layman's terms that means the fibres are strong when pulled in the direction of the strands. Under such tension, high strength glass fibres are typically 40% stronger than E-glass fibres. Resin-impregnated strands are also stronger than conventional E-glass, resulting in improved strength for fabrics, prepregs and laminates. This property is put to good used in a variety of applications including filament wound structures such as rocket motor casings, aircraft fuel tanks, pressure vessels and firefighters' air bottles.

Following close behind tensile strength is tensile modulus or stiffness. High strength glass fibres typically provide 20% more stiffness than conventional glass fibre.

 This property has to do with the stress/strain curve of materials and in general terms, the more stiffness a material has the less it will deform, deflect or bend under a given load. When designing a structural beam, for example, using a stiffer material can enable the beam to carry a heavier load with the same amount of deflection.

Increased stiffness, coupled with other properties, makes high strength glass fibres an ideal choice for aircraft flooring applications and the fibre-metal laminates used in the fuselage. High strength glass fibres were originally developed by Owens Corning and the US Air Force for aerospace applications and aerospace continues today to be the number one market for high strength glass fibres, accounting for about 45% of their total use. The combination of higher strength and lighter weight continues to be an appealing mix for the makers of both commercial and military aircraft.

Impact resistance is another important property of high strength glass fibres, which have impact deformation capability that exceeds both K-49 aramid and AS4 carbon. The impact resistance of high strength glass fibres is about double the performance of E-glass. This enhanced performance is exemplified in ballistic armour used to protect crews and passengers on land, sea and in the air.

A primary mode of troop transportation today is the M1114 High Mobility Multi-Purpose Wheeled Vehicle, known globally as the Humvee. To protect the occupants of these vehicles the military relies on a patented armour system based on the use of S-2 Glass roving and phenolic resin. The composite provides lightweight protection against both projectiles and spall. Certain limited-use vehicles also use the AGY system for blast mitigation protection.

The advantages of high strength glass in this case are in weight savings, structural integrity and ease of processing contour shapes and specially designed parts for the existing vehicle platform.

The US Army has commissioned the armouring of many thousands of vehicles over the past 10 years and recently contracted for additional vehicles that will be used to protect servicemen for years to come. Tests conducted on armoured vehicles in constant service for 10 years verified that their high-strength-glass-based armour system is essentially equal to the performance expected when the vehicles were new.

Originally put into service in the early 1990s, the CAV-100 Up-Armored Light Wheeled Vehicle was developed for use in both battle zones and near-battle areas for peacekeeping and relief missions. The concept is to provide a lightweight, composite vehicle used to safely transport personnel into and out of harm's way. Protection from sniper fire, homemade bombs and land mines, as well as a wide variety of known munitions around the world was provided by the design and function of the armour system. Proven performance in the field from the UK government, and from relief agencies such as the UNHCR (United Nations High Commission for Refugees), has provided a solid base of support for these vehicles.

Testing performed on vehicles that have been in constant service for 10 years was carried out by NP Aerospace to assess the life of the vehicle platform and to verify that the protection provided was up to the standards previously established. Not surprisingly, the the armour system's performance was the same as the original performance when the vehicle was brand new.

A shipboard example of ballistic protection with high strength glass is the US Navy's LHD WASP Class ships, which rely on high strength glass armour for deckhouse ballistic protection. The combination of ballistic protection and structural load-bearing capability inherent in high strength glass armour systems make them ideal for blast mitigation and other structurally demanding ballistic applications.

Another performance benefit resulting from the higher silica content and higher melting temperature of high strength glass fibres is their enhanced temperature resistance. High strength glass fibres retain more of their tensile strength at elevated temperatures than conventional glass fibres, and they perform at up to 760°C (1400°F). This attribute has been put to use in a variety of automotive applications where the reinforcements must perform at the elevated temperatures created by today's smaller engines and hotter exhaust gasses. Examples include Ford choosing high strength glass for the gaskets of their catalytic converters, and Owens Corning using high strength glass in the Silentex® muffler systems supplied to Toyota.

Another important performance attri-bute of high strength glass fibres is enhanced fatigue resistance, a measurement based on tolerance to damage accumulation. Composite parts made with high strength glass fibres can withstand high levels of tension and flexural fatigue, and major ballistic impact, without catastrophic failure. And composite systems regularly demonstrate a longer hour life, typically by a factor of two or three, over metals.

This attribute makes high strength glass fibres the material of choice for helicopter blades and rotor assemblies. Major helicopter manufacturers such as Bell and Sikorsky continue to favour high strength glass fibre over metals in their rotor systems. The use of high strength glass fibres in rubber timing belts also demonstrates significant advantages in fatigue performance over metal chains, especially in the hot, wet and corrosive environment found inside automobile engines.

Less well known but equally important in some applications is the enhanced radar transparency of high strength glass fibres. Compared to conventional E-glass, for example, S-2 Glass delivers a 20% lower dielectric constant. This improved transparency – coupled with its inherent stiffness, strength, impact resistance, temperature resistance and fatigue resistance – makes S-2 Glass fibre a frequent choice for radome applications. Superior mechanical perfor-mance allows thinner structures, which further enhances transparency.

Drop-in substitution

An appealing attribute of high strength glass fibres is their ease of substitution in processes that are already using another glass fibre such as E-glass. By choosing a high strength glass with a similar yield and finish, composite fabricators can often simply drop the new material into the process with very little change or adjustment.

High strength glass fibres will process in the same machinery in the same way and have similar visual properties and aesthetics, although the fibres will be a little whiter than conventional E-glass.

Switching to aramid or carbon fibre on the other hand, may require significant process change and a lot of learning on the shop floor. Carbon fibre can be very hard on equipment and requires special handling considerations. One fabricator we know had to install an air lock between the office and the place where carbon fibres are processed to protect electrical equipment from damage and premature failure.

There is a cost associated with such process changes, required learning and protective equipment that must be considered when moving up to a higher performance level. If you can get the performance you need from a high strength glass fibre, it is definitely the easier and less expensive route to travel.

New applications evolving

Despite their long history, high strength glass fibres continue to show up in many of the newest composite applications as designers and engineers simultaneously push the performance and cost envelopes.

The very first market for high strength glass fibres, aerospace, continues to grow as aircraft manufacturers embrace more com-posite applications. High performance glass fibres have an advantage in their strength, of course, but that strength advantage can also yield weight savings as lower density (about 3% less than E-glass) and higher strength fibres enable parts to be made with less glass reinforcement. Durability and fatigue resistance are also important in these applications. Truck transportation is also making greater use of high strength glass fibres in the fight to save weight, add payload capacity and improve fuel economy.

Several new ballistic applications are being made with high strength glass including Humvee Helmet Hardtop® for retro-fitting troop-carrying vehicles previously made with open tops, a new light armoured vehicle (LAV) that is at the heart of the US military's rapid-deployment transformation plans, and small arms protective inserts (SAPI plates) for body armour. In wind energy, a focus on larger and more efficient blade designs is prompting a search for new reinforcement materials. High strength glass fibres may never replace E-glass fibres in whole blades but certainly those parts of the blade that experience the most stress and strain are candidates for a higher performing fibres.

High strength glass fibres are also making inroads in the marine market in boats that typically take a severe pounding, such as offshore racing boats. Boat manufacturers who are using high strength glass fibres report improvement in impact and fatigue properties, which is extending the life of their hulls.

Historically, demand for high strength glass fibre reinforcements has come almost exclusively from thermoset applications but we see that beginning to change and emerging technology could very well accelerate the shift to high performance reinforced thermoplastics. Examples of that trend are DRIFT® (Direct Reinforcement Fabrication Technology; DRIFT is a registered trademark of PolyComp Inc) and other long-fibre thermoplastic (LFTP) processes. Long-time reinforced thermoset fabricator Gordon Composites (Montrose, Colorado, USA) has been working in reinforced thermoplastic processing and sees so much potential in the material that they started a whole new business – named Polystrand Inc – to focus on reinforced thermoplastics using a proprietary process they perfected. Polystrand is producing a ballistic material using S-2 Glass and thermo-plastic resin that has a much faster processing rate than traditional ballistic material using epoxy and other thermoset resins. The application offers a tremendous amount of cost-effective protection and we expect it to open a lot of eyes to the potential for high performance thermoplastic applications.

Growth to continue

So what does the future hold for high strength glass fibres? We think this 40-something reinforcement is still very young at heart. As designers, engineers and composite fabricators continue to push for higher performance and competitive cost, high strength glass fibres will continue to be an attractive alternative to traditional E-glass and higher-cost aramid and carbon fibres.

The two factors that make high strength glass fibres so attractive are easy substitution in most common glass fibre processes, and their blend of performance attributes that competitive materials find hard to match. High strength glass fibres are a material that both engineers and accountants can love.