Design characteristics of the IsoTruss geometry
The patented IsoTruss geometry, named for its repeating pattern of isosceles triangles, reduces the amount of composite material needed to achieve excellent mechanical performance. The lattice pattern of the IsoTruss geometry places the fibers in the composite in the direction of forces to withstand axial, bending, and torsional forces with less material, lowering overall product cost.
What engineers ask about IsoTruss
IsoTruss is a composite structure with a breakthrough geometric design that increases performance while reducing cost and weight. Here are answers to some of the questions engineers ask us about IsoTruss advantages compared to other materials: Strength, bending and axial loading applications, E, deflection, weight savings, damage tolerance, heat resistance and more.
Composites are materials comprised of strong load-carrying materials (known as reinforcement) imbedded in a weaker material (known as matrix). Reinforcement provides strength and rigidity, helping to support structural load. The matrix, or resin, maintains the position and orientation of the reinforcement, balances loads between the reinforcements, protects the reinforcement from environmental degradation, and provides shape and form to the structure. Steel reinforced concrete, fiberglass reinforced plaster casts and wire reinforced tires are all examples of composites. The term composite may also describe newer technology products made from very strong fibrous materials imbedded in thermosetting and thermoplastic resin matrices. The three most common types of reinforcing fiber are fiberglass, carbon and Aramid (Kevlar). Many varieties of each of these classes of fibers are available and each variety may differ from the others in various ways, such as strength, stiffness, fiber diameter and cost. Glass fibers are the heaviest, have the greatest flexibility and the lowest cost. Kevlar is a moderate cost and stiffness fiber and is the lowest density. Carbon is moderate to high in price, slightly heavier than Kevlar but lighter than glass, and features certain varieties that have exceptionally high stiffness. All three types have grades with very high strengths although carbon is the strongest. Hybrids are composites with more than one reinforcing material. Advanced composite is a term used to describe composites reinforced with very high performance fibers such as carbon and Kevlar. Fiberglass is usually included.
Reinforcement – Weight, Stiffness, Cost
Glass- heaviest, most flexible, and cheapest
Aramid (Kevlar)- lightest, moderate stiffness, moderate cost
Standard tow carbon- moderate weight, most rigid, moderate-high cost
Large tow carbon- moderate weight, most rigid, low-moderate cost
A filament is an individual fiber of reinforcing material. A primary bundle of filaments is a strand or tow. A collection of strands spooled such that they appear to be on bundle is called roving. Fibers are used by composite manufacturers in the form of roving, cloth, mat and chopped fiber. The most common types of matrices are thermosetting resins. This is a class of materials that chemically react under certain time and temperature conditions. As soon as this reaction takes place, the material becomes fused in a solid state and will not melt with the application of heat. Epoxy resins, similar to those sold as adhesives, are the most widely used thermosetting resins in the advanced composites field. Others resins include polyester, vinyl ester, phenolic, bismaleimide, epoxy novolac, and polyimide. Each has practical applications and is selected on the basis of required performance and manufacturing technique. Reinforced plastics, glass-reinforced plastics (GRP), and fiber reinforced plastics (FRP) are often used to describe composites in a more descriptive way. Similarly, more specific acronyms such as C/E, a carbon fiber composite with an epoxy resin matrix, are commonly used in the industry.
IsoTruss is a composite structure with the performance and weight advantages of expensive aerospace materials yet creates structural solutions that are cheaper than steel. This break-through geometric design increases the material strength and stiffness, reduces the weight and, for the first time in composite structures, decreases the cost. The cost reduction is achieved through three means: 1) the 50% removal of expensive composite material relative to traditional tubes, 2) an improvement in strength and stiffness via the geometry, and 3) the simplification of a long proven manufacturing process using off the shelf machinery. The product can be manufactured in virtually any diameter and length from bike frame tubes to the largest wind turbine and transmission towers.
Composite products have existed for approximately forty years. Two factors prevented the mass commercialization of composites in structural applications until today: cost of structural geometry and the cost of material. The IsoTruss open lattice geometry eliminates up to 50% of expensive composite material when compared to ordinary solid wall composite structures and enables the use of cost effective fiberglass. Combined with lower cost material due to large volume pricing, IsoTruss products can now be cost competitive with traditional materials. Matter of fact, in certain heavily loaded applications, fiberglass IsoTruss can be cheaper than steel.
Many structures exhibit bending loads. For example, a self supported cell tower, transmission tower, or a utility pole experiences primarily bending loads. In these types of applications, a carbon IsoTruss is the lightest of all structural tubes and beams. For example, when compared to traditional metal tubular structures, IsoTruss’s weight vs. strength advantage is striking. When compared against optimally designed tubular structures, a carbon IsoTruss in bending is 8% the weight of steel, 24% of aluminum, and 54% of the weight of traditional carbon composite forms. At these significant weight savings, the product still exhibits the same strength characteristics as comparable metal or traditional composite forms. IsoTruss can compete with virtually any material or geometric combination of structural poles, beams, tubes, and trusses. Even against the I-Beam, one of the most efficient structures, IsoTruss weighs much less. Many products IsoTruss replaces are not designed for optimum tube geometry so your weight savings will probably be better than these numbers indicate. In many tall structures, engineers reduce the amount of material usage by designing lattice triangular structures typically made with steel. These structures in general reduce the weight by 40% to 50%. Even with these increases in material efficiency, the structures can’t compete on a weight savings or cost basis. Carbon IsoTruss is still for example less than 17% the weight. Even though these structure comparisons are not designed around a deflection limited design but are load limited only, the carbon IsoTruss is much stiffer than the aluminum, carbon and glass tube products. For instance, IsoTruss is over 2 times stiffer than the best designed carbon tube product to handle the load.
What is the weight savings of the carbon IsoTruss® compared to other materials in axial loading applications?
Many structures exhibit primarily axial loads, from the low tech tilt-up wall brace, high tech aerospace structures, and guyed communication towers, these structures experience primarily axial loads. For these, buckling is the primary design criteria. In these applications, IsoTruss reduces the weight considerably over traditional tubular structures. When compared against optimally designed tubular structures, a carbon IsoTruss in buckling is 12% the weight of steel, 17% of aluminum, and 30% of the weight of traditional carbon composite tubes.
While the best compromise between weight and cost will ultimately depend on your application, in general a fiberglass composite IsoTruss yields significant weight savings over steel, aluminum, and traditional composite structures. The weight of a IsoTruss will be no more than 21% the weight of steel, 67% the weight of glass tubes, and 58% the weight of Aluminum. As significant, this glass product is only about 30% heavier than a comparable carbon IsoTruss. Since, glass is at least 1/6 the material cost of carbon, the structure is more economical. In axial or buckling applications the relative weight savings are: 37% the weight of steel, an identical weight to expensive carbon tube, 54% of aluminum, and 32% the weight of fiberglass composite tube. These numbers are significant from several aspects: fiberglass IsoTruss can be substantially less than carbon tube at about the same weight. Also of note is that fiberglass tube in buckling applications can actually be heavier than steel. Therefore, fiberglass composite products are usually not found in pure buckling applications as steel is a lighter weight product. However, IsoTruss is the first glass structural buckling product much less than the weight of steel. It is the first major glass composite product to offer a competitive solution to steel.
The strength and weight of a material are directly related. Thus, the IsoTruss structure, at equivalent weight, can be up to 12 times stronger than steel, 4 times stronger than aluminum, and twice as strong as carbon composites tubes.
The strength and weight of a material are directly related. Thus, the IsoTruss structure, at equivalent weight, can be up to 12 times stronger than steel, 4 times stronger than aluminum, and twice as strong as carbon composites tubes.
Many ask: What is E or modulus of your product? This is not quite the accurate question. The question is usually asked to ascertain the relative stiffness of the structure compared to other materials. However, stiffness is a function of E times I. E or modulus is a function of the fiber properties that are governed in most part by the combination of fibers, the resin system, and the orientation of the fibers. For a pultruded rod for example, all the fibers are axial and therefore the modulus (E) is nearly 100% of E as determined by rule of mixtures. In a typical low cost carbon matrix this modulus is near 20 Msi. However, since the rod is solid with a small diameter its moment of inertia is low. For example, think of a fishing pole …very flexible due to small diameter and thick walls. This moment of inertia is directly proportional to the square of the radius. In order to increase this inertia without increasing weight, engineers design structural products to be tubular or something similar such as a box or I beam. This puts the material near the outside where it is needed the most. However, in composites, in order to place the fibers more on the perimeter requires more and more helical or hoop fiber wraps instead of axial fibers to support the thin walls. Unfortunately, this has the effect of reducing modulus (E). Therefore for a typical thin wall composite tube, while the moment of inertia may be fairly large, the modulus will reduce typically over 50%. This places the modulus below 10 Msi in many cases using the least expensive carbon fiber. Since, minimum wall thickness governs in any tube product; the relative inertia will be similar in both composites and metal wall tube like structures so a relative comparison with E usually suffices as a close approximation of stiffness. However, this rationale applies only if both products have relative diameter to wall thickness ratios that are similar.
Therefore, when we say that a carbon tube has a typical modulus of 9 and steel has a modulus of 29, the relative stiffness difference between the two is about a factor of a little over 3 ie 29/3 for similar diameter tubes. From this you would know that in order to increase the stiffness of the carbon product to the equivalence of steel you would need to beef up the wall to increase inertia since the tube is at the thinnest wall possible or conversely the steel product is 3 times stiffer. Beefing up a wall decreases the efficiency or relative weight per foot directly proportional to the thickness unlike the much better process of increasing diameter which increases the modulus by diameter^4 and increases the weight by only diameter^2. This has the effect of decreasing the wall thickness. Therefore, increasing diameter is a much more efficient method of increasing inertia. However, diameter can only be increased to the point where the wall becomes very thin prior to thin shell buckling occurring. Think of crushing a coke can. This is the limit of tube structure wall thickness and diameter resulting in the most efficient moment of inertia. In contrast to this, the IsoTruss uses uniaxial fibers in the primary load bearing longitudinal members. As mentioned, uni-axial fibers do not experience a knockdown; thus the modulus (E) always remains high regardless of overall diameter. These longitudinal members are supported in space by helical elements that wrap around the structure. From lattice geometry methodology, these helical elements do not increase in weight with increase in diameter. Therefore purely from an efficiency standpoint a IsoTruss should be designed with ever larger diameters subject of course to the constraints of the application. These larger diameters increase the moment of inertia. Unlike tubes, the moment increases with the square of the diameter but the weight does not increase. Also, unlike tubes, the maximum diameter is constrained by buckling of a local longitudinal segment. Therefore this fixes the maximum diameter at the greatest structural efficiency nearly two to three times larger than a tube of the same design strength. Merely from diameter geometry, since efficiency goes up with the square of the diameter, a IsoTruss can be 4 to 6 times stiffer than its tube counterpart. Combining this with a modulus of elasticity close to 20 Msi compared to a thin wall carbon composite modulus of less than 10 Msi reveals the inherently superior stiffness of IsoTruss. So the more accurate question is…“What is the EI of comparable products?”
As mentioned above, the deflection, or the stiffness, of IsoTruss material is a function of the unique geometry. This geometry allows an increase in moment of inertia for a given weight of material that is 2 to 3 times larger than traditional tube structures. This moment combined with the modulus of elasticity of the structure provides a high stiffness. When these fibers are incorporated into the IsoTruss geometry while using a larger diameter, the resultant IsoTruss product can be stiffer than its metal or composite tube equivalent. Traditionally, fiberglass products are cheaper than solid wall carbon tube counter parts but much weaker with a corresponding very high degree of deflection. Until IsoTruss. For the first time in history, fiberglass products can be manufactured to be stiffer than their solid wall carbon tube equivalents at a lighter weight and less cost. For example, lets assume that your application requires a lightweight product but requires the stiffness of steel. When composite structures are designed with stiffness greater than the stiffness obtained just from load capabilities, the structure traditionally must be designed either with very thick tube walls at less than the optimal structural efficiency, i.e. the weight increases or very expensive high modulus carbon fibers are incorporated. Both solutions are not optimal in cost competitive civil engineering or commercial applications. However, using the superior stiffness geometry of IsoTruss, the stiffness can be obtained with relatively little increase in weight while using low cost carbon or fiberglass. For example, the weight comparisons for a steel equivalent stiffness product in carbon IsoTruss would be: 9% the weight of steel, 22% the weight of carbon tube, and 13% the weight of aluminum. Fiberglass IsoTruss would be: 26% the weight of steel, 65% the weight of carbon tube, and 38% the weight of aluminum. Most revealing from this comparison is the weight of glass IsoTruss compared to carbon tube. The weight is less than what has traditionally been the lightest known structural material – carbon. All in glass at less than 1/6 the material cost offered as a direct steel replacement.
Carbon fiber IsoTruss can be priced to be lower in cost than traditional carbon fiber composites and aluminum, while still delivering respectable company gross margins. And depending on the application, fiberglass IsoTruss is less expensive than steel.
The open geometry of the IsoTruss structure is frequently confused with other lattice structures. The IsoGrid is a shell-like grid structure that is particularly suited to skin stiffener applications, although there are considerable manufacturing issues. Other lattice structures are metal, similar to truss structures used in towers. The IsoTruss is different from both of these structures. Unlike the IsoGrid it is truly 3-dimensional, requires no skin, and can be wound from a single continuous fiber. Unlike traditional lattice towers, it is fabricated from advanced composite materials and is quite different in geometry — the simplest IsoTruss is a bit like an overlapping double triangular lattice structure. While there are similarities among all truss — or lattice — structures, the IsoTruss has very unique characteristics.
Currently, a 3D braiding machine is being developed to produce IsoTruss continuously at 3ft. per minute. At this speed, we can produce IsoTruss that is less expensive than steel. A large scale prototype has been built to prove the feasibility of the braiding machine and IsoTruss Industries is currently raising money to scale up the machine to produce large IsoTruss structures.
The IsoTruss products are currently manufactured using the filament winding process. Filament winding combines an economy of material and flexibility of material placement which make it the clear choice for this type of lattice structure. Filament winding is accomplished on a machine which winds fibers onto a mandrel in a prescribed pattern to form the desired finished shape. A programmable logic controller (PLC) is used in a closed loop control circuit to control machine movements. This system allows for placement of the right amount of material at the right place and orientation. For filament winding, material is purchased in a yarn-like form called roving. When using prepreg roving, the resin is already mixed into the fiber and is applied directly onto the mandrel. Under the wet wind method, dry roving is routed through a bath of liquid resin before it reaches the mandrel. After the fibers and resin are in place, heat and pressure are applied to consolidate the fibers and initiate hardening of the resin. After hardening, the product is removed from the mandrel.
How susceptible is composite material to moisture degradation? If a section is damaged, how much will it degrade and lose strength from moisture?
Sustained moisture can degrade fiberglass fibers very slowly. Moisture degrades the matrix (resin) and/or the fiber/matrix interface. These are both a function of the particular resin system and “sizing” applied to the fibers. These effects can be minimized by proper selection of resin & sizing systems. The resins are the primary source of moisture absorption, governed by Fick’s Law (DC,zz=C,t, where D is the coefficient of moisture diffusion, C is the moisture content as a percentage of the dry mass of the composite, z is the coordinate through the thickness & t is time), although cutting a hole in the material certainly opens up a potential express avenue for “wicking” along the fiber/matrix interface that will depend on the interfacial properties. The time constant is fairly large (compared, say, to thermal effects), although it is accelerated by wicking. IsoTruss grid structures are designed to carry the load primarily through axial tension and/or compression in the individual members. The fibers in the individual members are aligned with the primary load direction (axial). Thus, the behavior is fiber-dominated, rather than matrix- or resin-dominated. Therefore, any reduction in resin properties due to the presence of moisture will have minimal effect on the performance of the IsoTruss grid structures. About the only adverse impact from moisture absorption would be a minimal increase in weight.
Carbon fibers are completely inert and are unaffected by moisture. As stated above, any moisture absorption will be a function of the particular resin system selected.
IsoTruss products are highly damage tolerant due to their highly redundant design. Depending on the type of loading and the particular member(s) removed (i.e., helical vs. longitudinal, etc.), if one or more members is removed, the resulting performance will be reduced substantially less than for a traditional truss design (which could collapse if a member were removed) or a cylinder (which could experience up to 80% reduction in strength due to local damage). For example, removing part or all of a pyramid (two helicals) without damaging a longitudinal should not affect the bending or compressive strength, and should theoretically reduce the torsion and/or shear properties by something less than 25%. Likewise, removing a single longitudinal should reduce the compressive strength by less than 12%. The corresponding reduction in bending performance for this latter case could be anywhere from nothing to 50% (depending on which member was removed, and the direction of the bending load).
Fortunately, carbon composites are completely opaque, and not susceptible to ultra violet (UV) radiation degradation. However, it has been proven that UV can have harmful effects to the aesthetic and functional capabilities of fiberglass-reinforced composites (FRC) after prolonged exposure. As such, IsoTruss Industries, Inc. recognizes the need to provide UV protection of its products for applications that will be exposed to the prolonged effects of UV radiation. UV protection has been studied by engineers, chemists, and FRC manufacturers for 40+ years. Numerous chemical companies have developed UV protectants that have decades of evolutionary field-use and correlation of field measurements to laboratory measurements. These protectants are ranked by a standard termed QUV-rating. QUV is a quantitative measure of a protectants (the higher the QUV-rating the better its protection against UV degradation) capability or performance against the effects of FRC to prolonged UV exposure. Additionally, the National Institute of Standards and Technology has developed a procedure that standardizes testing used to obtain time-accelerated UV degradation data on the performance of protectants. This procedure is ASTM Procedure G 53-96, titled “Standard Practice for Operating Light- and Water-Exposure Apparatus (Florescent UV-Condensation Type) for Exposure of Non-Metallic Materials”. Therefore, it is possible to simulate years of exposure to natural UV degradation in a laboratory in a relatively short period of time thereby allowing FRC product designers/manufacturers to select protectants best suited to the life cycle or life expectancy of their product. As is shown by these few examples, the breadth of FRC application is extensive as is the length time FRC products have been successfully used in various applications exposed to UV radiation. Through information obtained in these and other products/industries, UV protection is a deeply understood phenomenon by FRC manufacturers and their material suppliers. UV protectants available to product designers are numerous and vary widely in regards to their capabilities (life expectancy). In addition, the method of applying UV protectants vary widely allowing manufacturing/process designers significant flexibility in their manufacturing operations. IsoTruss Industries, Inc. has investigated various methods of UV protection. The candidate solutions best suited for IsoTruss products and manufacturing processes are:
UV inhibitors added to the FRC resin.
Combination of (1) and (2).
UV inhibitors or resin additives are blended into the FRC resin (matrix) during composite manufacturing. These inhibitors generally take two forms: (1) a stabilizer that acts chemically with the resin rendering the cured resin less susceptible to the effects of UV degradation (cracking, peeling, blooming, et al.). (2) a pigmentation such as TiO2 (titanium dioxide) that acts as a barrier between the resin and harmful effects of UV radiation. Coatings are available in various forms allowing flexibility in their method of application. Some of these application methods include: molding, spraying, and painting. Coatings are available in various base chemistries allowing product designers to specify a UV coating that is chemically similar to the FRC resin thereby ensuring best adhesion characteristics over the life cycle of the product. IsoTruss products use the optimum mix of resin and coatings to provide superior protection against UV.
The temperature capability of the IsoTruss composite will depend on the type of resin used for the matrix and the selected fiber. For a phenolic resin, the temperature capability is greater than 1,000°F. For example, carbon fiber/ phenolic resin composites are used in rocket nozzles where the temperature can exceed 2,000°F. For a thermoset matrix, it will depend upon the cure temperature and the resin. For the thermoplastic, it will depend upon the melt temperature. For some of the typical higher temperature thermosets, the temperature capability is about 325-350°F and for the higher temperature thermoplastics, it will be in the 400-425°F range. In either case, the cost for a high temperature system will be significantly increased in terms of $/Kg than for typical 250°F cure systems.
The effect of high temperature on the composite will depend upon the time of exposure, the additives in the matrix, and the type of fiber being used. For short time duration exposure on the order of 1-5 minutes, the typical resins will usually not heat up fast enough and the structure may survive. As exposure continues, the matrix material modulus drops rapidly, and in the presence of significant compression stresses, the structure may collapse. For higher cost higher temperature capability resins, the structure will last longer but under continued exposure, collapse under compression stresses would still be the concern. If additives are not used in the matrix, many resin systems may begin to burn. Additives can suppress ignition and reduce the risk of fire. Again, the cost of the resin increases. These additives are typically used in aircraft structures but are not generally considered for commercial structures. Carbon fiber would resist heat exposure since the material is processed above 1000°C but the matrix would dominate the performance. Fiberglass fiber would be more susceptible to high temperatures. As stated previously, phenolic resins are extremely resistant to intense heat and can resist this high level of heat for a long length of time. This can be a significant factor in buildings where an intense fire could bring down an ordinary steel structure.
The lifetime of IsoTruss material used in most applications should be on the order of 50 years or more. For example, recent infrastructure composites for bridge decks are quoting 75-year lifetimes using glass, carbon and polyester resins. The matrix and fiber system need to be resistant to the elements (sun, rain/snow, chemicals, etc.). Again, there are additives usually included with the matrix resin to ensure resistance to UV exposure, and other chemical environments associated with road maintenance.
Currently, the usual recycling method for fully cured thermoset composites is to simply break them down into small pieces and reusing them in carbon/epoxy blocks for high performance parts. Thermoplastic composites may be recycled to some extent provided the selected matrix can be re-melted in a chopped pellet configuration and used in an extruder. If re-cycling cannot be done for the thermoplastic, than it would be crushed and dumped in an approved landfill.
Each end of the IsoTruss transitions to a solid wall tube. Using the analogy of PVC pipe, connectors, couplings, and attachment points are being specifically developed for different applications. No loss of strength will occur at these force transition points.
What are the relative freight and installation cost savings for IsoTruss products versus metal, concrete, wood or other materials?
Use of lower weight IsoTruss may directly reduce your transportation and handling costs. Lower product weight means more products per truck or air cargo shipment.
From a structural load perspective, external sheathing on IsoTruss structures are strictly optional. For cosmetic or functional purposes however, numerous materials can be used to cover the IsoTruss depending on the application.