Can You Glue A Car Together?
I'm not talking about a plastic Revell model of a '57 Chevy, but a real vehicle, one that rolls off an assembly line in 1999 with another 99,999 just like it right behind. Is it possible, or is this just a fantasy of the marketing department at Elmer's?
Okay, I'll cut to the chase and answer the question. Yes, you probably could assemble a car entirely with adhesives, however, no one is currently building a Gluemobile. Adhesive technology has made significant advances in the past decade, but there are still some hurdles to clear when it comes to body engineering. The most basic dilemma is that structural adhesives lack the decades of performance data under their belt that other joining technologies—namely welding—possess. But as engineers continue to cut vehicle weight by using lightweight materials that don't lend themselves to thermal welding (like aluminum and composites), structural adhesives are looking more promising and generating more research. While the most adhesive-intensive vehicles may still be low-volume, technology-heavy cars that use a lot of alternative materials (like the Corvette on our cover: its exterior—closure panels, doors, hood, lift gate—is fabricated with adhesively bonded SMC), structural adhesives have made inroads into more mainstream production vehicles.
Understanding Structural Adhesives
Fundamentally, structural adhesives are epoxies. Without delving too deeply into the chemistry, suffice it to say that epoxies are organic bonding compounds in which a resin develops its bonding properties when combined with a catalyst. Will Repensek, director of Business Development and Strategic Planning at Henkel Surface Technologies(Madison Heights, MI), explains that there are two main types of automotive structural adhesives: glassy matrix epoxies and polyurethane epoxies. Glassy matrix epoxies (like those made with acrylics) are the most rigid and need to be applied in very thin cross sections for best performance. Although these high-strength adhesives have been widely used and can sustain shear forces of 1,500-2,000 psi, they have some glaring drawbacks that prevent them from being the perfect bonding solution. Since they're stronger than the substrates that they bond, they don't "manage energy" very well, acting similarly to conventional thermal welds. When a thermally welded joint is repeatedly loaded, it finally fails not at the weld (which is stronger than the metal), but in the metal right next to the weld. This happens because the load energy does not transfer across the weld, but is instead absorbed by the metal at the edge of the weld. This same principle gives glassy matrix epoxies a tendency to "peel," i.e., once you start to crack them along a seam, they don't stop separating.
When designing the Chrysler Sebring convertible, engineers had to do something to stiffen the body. Traditionally, this would mean adding a lot of steel. By strategically using structural adhesives, however, engineers were able to double the stiffness, while saving approximately 75 lb. of weight.
Polyurethane epoxies, on the other hand, are more flexible and therefore allow energy to be transmitted through the bond. In a vehicle, this allows forces (torque from the engine, suspension feedback, etc.) to be dispersed throughout the bonded structure, which helps resist peel. While this "toughness" is advantageous, polyurethane epoxies are weaker (shear failure occurs at 500-1500 psi). They do, however, hold perhaps the best compromise in performance for current automotive applications, due to their ability to "manage energy." Another advantage is that they can be applied more thickly. For these reasons, these are the types of adhesives that are currently being used to bond windshields.
Structural adhesives are a lot like duct tape. This is not said to denigrate structural adhesives—quite to the contrary, as duct tape is a modern miracle material. Duct tape can be used to stick just about anything to just about anything else; it can be used to bolster the strength of weak materials; and it can also be used to fabricate things nearly all on its own. Structural adhesives are used in vehicles in these same three ways. Here are some examples:
- Sticking things together. Manufacturers first started using sealants between spot welded sheet metal in unibody vehicles to keep water and other contaminants out of the seams and to reduce NVH. Now, low- or medium-strength adhesives are being substituted for these sealants (or adhesive qualities are being added to the sealants). This bonds the entire surface of sheet metal, rather than just fastening it at the point of the spot welds. Processes like this are applied on the assembly line, reducing the number of spot welds. This also increases the strength of the seam, due to the energy management qualities of the adhesive bond.
- Adding strength. High-strength structural adhesives are being used where there is need to reinforce certain areas of the vehicle, primarily for crash safety considerations. Often, metal parts are coated with adhesive by a supplier and "dropped-in" during assembly. In the case of a seat belt anchoring rail, for instance, the adhesive increases the surface area of the part that is actively attached (when compared to welding or mechanically fastening). This makes the anchor less likely to be ripped out of the floor of the vehicle in a crash.
- Fabricating. This is the area where structural adhesives aren't being exploited nearly as well as they could be, mainly because vehicles are designed to be welded together. But because aluminum is difficult to weld and composites don't lend themselves to welding, structural adhesives may find more applications. Still, adhesives won't really become a predominant material unless vehicle structures are designed to be assembled with glue.
All About Fasteners
Anyone who has anything to do with metric fasteners would undoubtedly benefit from having the newly published Metric Fastener Standards, Third Edition from the Industrial Fastener Institute (Cleveland, OH). This is a massive reference tome—on the order of 1,000 pages—that covers: bolts; screws; nuts; pins; other threaded fasteners; rivets; washers; locking fasteners; materials; quality assurance; screw threads; structural bolting; and technical data. ASME, ASTM, and SAE standards are included. The comprehensive scope of this book undoubtedly dwarfs the $140 price tag.—GSV
Saad Abouzahr, manager of Advanced Programs and Processes for Special Vehicle Engineering at DaimlerChrysler, identifies two challenges to using structural adhesives. First, they add complexity to the manufacturing process. Second, they raise environmental concerns over potentially harmful emissions.
Where did all this adhesive technology come from? Much of it is adapted from the aerospace industry. Engineers are using high-strength structural adhesives to bond the car's aluminum body. However, they also used mechanical fasteners to offset the structural adhesive's tendency to peel. Although the adhesive has a shear strength that can withstand up to 2,000 psi before failing, it will peel like a banana at only 30 psi.
While any new materials or methods are bound to create obstacles to manufacturing just due to their learning curve, structural adhesives have a formidable challenge when it comes to quality control. Welding is well understood and documented. But structural adhesives are somewhat "unproven" in the eyes of both the design and process engineers. Regardless of statistical information concerning an adhesive's performance in an ideal (laboratory) situation, process engineers are very concerned with the curing behavior of adhesives, especially considering all the variables in a typical manufacturing facility. Furthermore, finite element analysis (FEA) methods for structural adhesives have been lacking. So from a manufacturing standpoint, the engineers are asking, "How do I know that this adhesive is going to perform like the chemist says it will?"
Of course, the easiest way to test an adhesive is through load testing—a destructive process. Bond the materials, load the joint, and see if (or when) it fails. This is obviously not the most ideal or efficient approach to implement in a high volume manufacturing scenario. To solve this problem, Jessica Schroeder, a staff research scientist at General Motors and chairperson of the USCAR group that is exploring bonding technology, is developing nondestructive testing methods for adhesives, as well as developing better FEA modeling for adhesives. She is focusing primarily on composite-to-composite bonding. However, her research has broader implications for developing full-field, non-contact testing methods that could be implemented in a production setting regardless of the materials.
Schroeder's research has focused on three different testing methods. (1) Thermography uses infrared technology to rapidly scan large surfaces. (2) Optical shearography uses lasers to measure surface strain variations. (3) Free vibration testing analyzes sound waves when a material is "pinged." All of these methods hold possibilities for being used on an assembly line to measure the adhesive application and effectiveness of curing.
While this research may provide solutions to quality issues, environmental concerns are much more complicated. The chemicals in structural adhesives—whether epoxy, urethane, acrylic, or something else—all have properties that are potentially harmful to workers. Overexposure is not limited to the raw adhesives, as they also give off emissions during the curing process. This is an inherent problem with adhesives that can only be managed with proper caution and safe handling.
On a somewhat brighter environmental note, structural adhesives are friendly to recycling. Since they have a lower melting point than most metals, simply heating the scrap causes the adhesive to disintegrate. (Fortunately, this melting point is in the neighborhood of 600° F, so there's no chance that your adhesively bonded vehicle will fall apart in your driveway on a hot summer day. Similarly, adhesives won't crack due to cold until you get to about -40°.)