The collision repair industry continues to see drastic changes in vehicle construction which in turn requires equally drastic changes in repair methods and equipment. The corporate average fuel economy standard of 54.5 miles per gallon by the year 2025 play a large role in influencing these changes. Other factors driving these changes include safety and crash test performance, reparability, ride dynamics, ease of manufacture and of course, manufacturing costs.
As manufacturers search for new ways to accomplish these performance improvements, we will continue to see an increase in the use of adhesives for both vehicle assembly and vehicle repair. Industry sources have predicted a structural adhesive market growth of about 33% annually. One example is the Cadillac CTS which uses 120 meters of structural adhesive in areas like the frame rails, rockers, strut towers, wheel houses and floor pans (Fig. 1).
Structural adhesives have been used in some OE structural repairs for the past few years; however, the availability of these products and these types of procedures were limited to a small group of vehicles. The more mainstream type of adhesives available for repairs has been panel bonding adhesives. Panel bonding adhesives are primarily limited to bonding only exterior non-structural panels. This type of adhesive does not meet specifications for bonding structural parts. Typically, structural applications require greater peel strength, impact strength, and lap shear strength than a panel bonding adhesive can offer. Vehicle manufacturers also desire an adhesive that can absorb collision energy as the bonded parts deform during a collision. This requirement for higher performance standards for structural bonding has inspired more adhesive makers to create what they deem Impact Resistant Structural Adhesives (IRSA).
During both vehicle assembly and collision repairs, structural adhesives are typically used along with mechanical fasteners or spot welding which is referred to as cold joining using a hybrid joint. The joint is considered a hybrid because it uses both an adhesive and a fastener together. Hybrid joining combines the strengths of both the adhesive and the fastener together to overcome their individual vulnerabilities.
Adhesives have many useful attributes but they are vulnerable in peel mode (Fig. 2). Typically adhesives do not encounter peel forces in a collision, they encounter shear (Fig. 3) and impact (Fig. 4) forces but the possibility of adhesive peel is still a concern. Therefore, the addition of a rivet or other mechanical fastener in the joint, prevents the adhesive from peel failure.
Mechanical fasteners may be spot welds, flow drill screws, rivets or bolts. This type of fastener primarily has the advantage of “pull-out” or peel strength; however, fasteners are only local, meaning all the stress load is placed on one small footprint. Adding adhesive to the joint with a mechanical fastener helps distribute the stress load over a much larger area increasing joint strength. (FIG. 5)
A hybrid joint (Fig. 6) combines the benefits of both the adhesive and the fastener together to complement their strengths and overcome any weaknesses. The contributions the adhesive makes to the joint include; completely sealing the joint with adhesive to avoid corrosion, reducing noise vibration and harshness (NVH) and forming a continuous bond along the entire flange, which increases the bond area for better stress distribution. The contributions the mechanical fastener provides to the joint are; increased peel strength, immediate fixturing (no cure delay) and the fact that fasteners are known and trusted (Fig. 7).
It’s important to understand that the OEM factory adhesive is a single component chemistry that is cured at high temperatures that are not attainable in the collision repair shop, around 400 F. The high bake temperature makes it possible for these adhesives to achieve extremely high shear, peel and impact strength numbers. Until recently, automotive aftermarket adhesive makers had not unlocked the technology to achieve similar strength properties because they are limited to utilizing a two component, chemical cure adhesive that cures at room temperature. Recently, adhesive makers have overcome many limitations of two component adhesive chemistry with remarkable innovation.
To understand why a special adhesive is required for structural bonding applications we need to know how a structural adhesive is different than a panel bonding adhesive. A structural adhesive uses chemical technology to enhance peel strength, lap shear strength, and impact strength. These manufacturing processes could include additives or temperature treatments to toughen or rubberize the adhesive to allow for some give or flexing before fracture (Fig. 8). This is a very simplified explanation because the chemical engineering process is very complex and it may take years to develop an adhesive with the physical properties of what is commonly referred to as a “crash toughened” or “impact resistant structural adhesive”.
A standard industry test called a “wedge impact test” is performed to measure the stress load the adhesive can withstand as a weighted wedge is driven through bonded strips of steel or aluminum (Fig. 9). “Wedge impact test” samples that are bonded with a panel bonding adhesive, separate with far less resistance than structural adhesive samples which eventually break apart, but with much greater resistance. The amount of energy absorbed as the samples separate can be measured and viewed on a graph and will indicate that a structural adhesive will absorb considerably more energy. Structural adhesives also realize significantly better performance in; extremely high and low temperatures, resistance to corrosion, and stress durability. Simply put, an Impact Resistant Structural Adhesive is a completely different class of adhesive than a panel bonding adhesive.
Now that we’ve established what an “Impact Resistant Structural Adhesive” is, we must understand where it can be used during collision repairs. Be mindful that in most, if not all cases, this type of adhesive will be used with some type of mechanical fastener such as a rivet or spot weld when attaching structural panels. Do not get the idea that because this is a very strong adhesive that you can substitute it for mechanical fasteners and make a “bond-only” repair. There are currently some vehicle makers that use a structural adhesive on non-structural panels such as quarter panels. In this case, if you are certain it is a non-structural panel and no mechanical fasteners are present, it would be acceptable to bond-only unless the OEM states otherwise.
When determining which type of adhesive is recommended for a repair the technician should seek out the OEM repair information. Obviously, if the OEM makes a specific recommendation for an adhesive, there is no question as to what adhesive to use. Technicians should make every effort possible to acquire the OEM repair manual instructions to avoid making a mistake that could make the repair unsafe; after all, these are structural repairs with safety implications.
However, in many cases OEM information is not easy to find or is simply non-existent. One clue that an adhesive may be a structural adhesive is the color. For example, GM uses a purple colored structural adhesive and recommends replacing any purple adhesive found during repairs with an aftermarket Impact Resistance Structural Adhesive. To avoid confusion or errors, GM has recently modified their repair manuals to recommend a structural adhesive for ALL repairs requiring an adhesive, regardless of whether the part is a structural part or not. Not all manufacturers make it that simple to know which adhesive to use. In the absence of OEM repair information there may be cases in which a decision is based on what area of the vehicle is being bonded. The vehicle maker may state to use a structural adhesive if there is any doubt as to whether the part is structural or not, or to err on the side of the stronger adhesive (Fig. 10).
As vehicle construction continues to evolve and become more complex, it becomes more apparent that in order to make proper repair decisions you must have OEM information. The days of attempting to repair a vehicle by simply reverse-engineering the factory assembly methods are over. It is becoming much more common that the repair attachment method will be completely different than the original assembly method, especially when adhesives are in play. With this in mind the trend is moving towards the OEMs providing more information, especially if the repair procedure is unique or specific. It may also be wise to consult the adhesive maker to inquire as to the OEM recommendations, adhesive substitution or equivalent recommendations. Adhesive maker’s websites should contain directions for use, videos, a list of OEM approvals and other information.
The collision repair industry is changing drastically and we are just beginning to see the tip of the iceberg. As structures become lighter and utilize more mixed construction materials, structural adhesives will become as common as sandpaper and masking tape. It will be more important than ever to use these products correctly and acquire the necessary information from both the OEM and adhesive manufacturer.
