Composites have different toughness, fatigue properties, fracture propagation modes and failure mechanisms compared to metals. Composites can enable lighter weight design, but they have limitations and differences compared to metals as well. Design engineers need to understand the differences between composites and metals and develop automotive design methods. Methodologies can be borrowed from the aerospace industry. For instance, ESDU’s Automotive Design Methods package provides several sections on composite laminate design, analysis and failure criteria.
Carbon fiber reinforced plastic (CFRP) and glass fiber reinforced plastic (GFRP) composites have exceptional properties, which has led to their use in aircraft components, boat hulls, racing cars, professional bicycles, rehabilitation equipment, luxury goods, watches, wallets, computer cases, motorcycle bodies, bicycle frames, skis, tennis rackets, fishing rods, yacht masts, wind turbine parts and jet engine fan blades. High-performance applications such as wind turbine or jet engine blades take advantage of composites' high strength-to-weight ratio or specific strength and specific modulus. Carbon fiber composites are known to have excellent fatigue properties.
Understanding how a composite material will deform and break is important in damage tolerant automobile design to provide the highest possible crashworthiness and passenger safety. A major concern in designing automotive composite parts is assessing how an impact or out plane forces on a panel will alter the composite properties. Compression after impact (CAI) tests such as ASTM D7137 can be used to assess damage tolerance. ASTM D7137, the “Standard Test Method for Compressive Residual Strength Properties of Damaged Polymer Matrix Composite Plate” testing standard, measures the compressive residual strength and modulus of a multidirectional FRP composite laminated plates after simulated impact damage from an indentation or drop-weight impact per ASTM D6264 or ASTM D7136. ASTM D6484, “Open-Hole Compressive Strength of Polymer Matrix Composite Laminates,” is another test for composite damage tolerance. An open hole in a composite laminate simulates a structural defect. Design engineers can use the damage resistance and damage tolerance properties from the ASTM D6484 and ASTM D7137 tests to guide development, design safety factors and material selection.
Metals are mostly isotropic in properties, while composites are anisotropic because properties vary due to fiber orientation. Composites with orientated fibers can have very high tensile strength when stress parallel to the fibers, but poor strength response with loads perpendicular to the fibers. For instance, a glass or carbon fiber pultrusion can have four times the strength in the pultruded length or longitudinal dimension compared to the transverse dimension. The strength of wood along the grain versus perpendicular to the grain would be analogous. Metals have more isotropic, predictable properties. Design engineers must account for the anisotropy in their structural analysis of composite parts and systems.
The percent elongation of carbon and glass fiber composites is no more than 2 percent, which means composites are low in ductility and do not have the intrinsic toughness of an aluminum or steel material. Completely linear stress-strain curves indicated elastic, brittle nature. Stress-stress curve that becomes non-linear (line bends over) indicates plastic deformation, which provides more energy absorption and a tougher material. A material can fail by deformation. For instance, a severely deformed metal part may no longer function. Metal and composite parts are typically designed so they will only be exposed to stress levels in the elastic region. If CFRP or GFRP body panel cracks during a car accident impact, then the whole part may need to be replaced. A high-end luxury and sport car owners can afford to just replace a damaged composite panel with a new OEM part. Repair procedures for automotive composites need to be developed for mass market vehicles. A dent in metal body panel can sometimes be pulled, filled and repainted.
Thermal and Electrical Properties
Some of the properties of composites can limit use in certain applications. Composites are typically not electrically conductive compared to steel or aluminum, so the automotive body and frame components cannot carry an electrical ground or provide RFI/EMI shielding. The low electrical conductivity of composites can be an advantage for wireless communications noise reduction and the isolation dissimilar metals (e.g., aluminum and steel). A conductive coating can be applied or conductive fillers can be added to the resin matrix to provide some electrical conductivity and RFI/EMI shielding.
While the glass and carbon fiber themselves can be used at high temperatures, the plastics or matrix resins in composites limits end-use temperatures. Low alloy steels, stainless steels, and cast irons maintain their strength at higher temperatures. Certain epoxy resin composites can be used up to 230 degrees C (446 degrees F). Phenolic resin composites maintain their properties during short temperature durations up to 500 degrees C (932 degrees F), but phenolics are generally weaker and more brittle compared to epoxies. Manifolds, catalytic converters and exhaust pipes on some vehicles can reach 650 degrees C (1200 degrees F). Turbocharger system components using exhaust gas to compress intake air are often as hot or hotter than other exhaust system components.
Automobiles must be designed with sufficient gaps between the composite members and hot surfaces. If the design requires the composite component to be in contact or close proximity to hot surfaces, then heat shielding or insulation of the composites might be required. Matti Holtzberg, an entrepreneur famous for his “Pollimotor” plastic motor and composite process inventions, has more worked with several carbon fiber and high-temperature resin manufacturers to make a carbon fiber engine. Holtzberg’s plastic engines have cylinder liners and aluminum caps on the plastic pistons to overcome the low heat resistance of plastics. Carbon-carbon or composites would retain mechanical properties at elevated temperatures, but CFRCs are more expensive than CFRPs. High-temperature oxidation of carbon is another at issue, which might require protective coatings or cylinder liners.
Sufficient testing and simulations need to be performed to provide an understanding of how composite components will perform under crash conditions or on-board fires. The U.S. Automotive Materials Partnership (USAMP) and the U.S. Department of Energy (DOE) have a four-year “Validation of Material Models for Crash of Carbon Fiber Composites” project to evaluate the crash performance of composites. Fires in vehicles will continue to occur from leaking gas lines or tanks ignited by a mechanical spark, auto-ignition from a hot surface part, or electrical shorts. Electrical vehicles fires started after lithium batteries are punctured or crushed during an accident. Composite materials might have insufficient fire resistance or compromised strength when a fire occurs within the vehicle or in contact with a hot exhaust system or engine part. Fire retardant additives, intumescent coatings, heat shielding and fireproofing insulation could be utilized to protect composites during an onboard fire, but not without added weight and costs. Engineers designing with existing composites or developing composites with improved fire resistance often use cone calorimetry testing per ASTM E1354 to measures the response of materials to controlled levels of radiant heating with or without an ignition source. The cone calorimeter test determines critical factors for predicting the spread of fire such as intensity of and speed to reach the peak rate of heat release (PRHR).
Corrosion and Environmental Performance
Composites are immune to rust or corrosion. Steel components without protective surface coatings rapidly degrade due to rusting. In hybrid metal-composite structure, galvanic corrosion could occur on metals parts in electrical contact with CFRP composites because carbon is more noble or high on the galvanic scale compared to metals. GFRP plastics should not promote galvanic corrosion because GFRP composites have much higher electrical resistivity compared to GFRP or polymer fiber reinforced plastic. A high resistivity barrier (plastic, GFRP, etc.) between the carbon FRP and metal parts should electrically isolate and prevent galvanic corrosion.
The resins within the composite may also degrade or become brittle over time due to weathering, oxidation and UV light exposure. The colorants or dyes with the polymer composite can fade or change color upon UV exposure. UV inhibitor additives in the plastic matrix and external protective coatings could eliminate this challenge. Colorants could be added to the composite resin to eliminate the coating steps altogether. If the composite is exposed to higher temperatures within or near the combustion engine or exhaust system then creep properties can be a factor to consider. Solar simulators can be used to test for the impact of UV or sunlight exposure on a plastic matrix composite.
Mismatches and Joint Performance
Increased use of composites will require alternatives to weld joints such as adhesives and mechanical fastening, co-curing, stitching or a combination. How will composite-to-composite or composite-to-metal joints perform over time? The dynamic or fatigue strength of materials is always less the static strength (UTS). Mechanical cyclic loads are induced from rotary mechanical components in the vehicle and by roadway induced vibrations. Thermal mismatches or differences in coefficient of thermal expansion can induce residual stress, cyclic strains and loads during thermal cycling. Carbon fibers can have negative CTEs. The CFRP CTE will vary depending on the specific carbon fiber type and fiber orientation. In general, CFRP composites have much lower CTEs compared to steel and aluminum. GFRP composites have CTEs similar to steel and aluminum. High thermal strain and cyclic stresses are more likely to occur in CFRP composite to metal joints compared to GFRP to metal joints. Mechanical fastening or adhesive bonding systems need to be selected, which can accommodate the thermal strains. Cyclic loads can loosen mechanical joints over time, which increases fretting or wear and generates rattling or noise (reducing NVH quality).
The modulus of a composite material can be much different from the metal The metal sections in a metal-to-composite joint may require tapering or perforations to improve the stiffness match between the higher modulus metal and lower modulus composite. The researchers describe using a transitional zone of stiffness in perforated steel in “Biomimetic-inspired CFRP to perforated steel joints”.
A loosened mechanical joint or delaminated adhesive bond can lead to mechanical system failures. The composites and composite joints in the vehicle must have sufficient fatigue strength or endurance limits to maintain performance to the end-of-life under static and cyclic loading. Some adhesive bonds can compensate for thermal mismatch, damage and flaws in composite bonds are difficult to detect especially across multi-layer laminates and sandwich constructions. New non-destructive testing techniques need to be developed to consistently find and quantify difficult to detect defects such as zero-volume (kissing bond), incomplete curing, cracking, void or porosity, and debonding flaws.
Automotive composite materials can be stronger and stiffer compared to steel parts with similar thicknesses with weight savings of 40 to 70 percent. Continuous fiber composites have excellent fatigue resistance. Fiber composites have superior corrosion resistance compared to the steel and aluminum. However, composites and carbon fiber composites can be relatively brittle compared to metals with limited ductility, no yielding and low impact resistance.
For more information, read "How Composite Materials Challenge the Automotive Manufacturing Industry - Part 1 Cost Barriers."