In the past, many structural technicians have followed the general guideline of "If it’s bent, repair it; if it’s kinked, replace it." As with other general guidelines, however, simply adhering to it may often not automatically produce successful results.
To make the right repair plan decision, a repair facility must also look at many other areas. The foreman or technician must ask an array of questions: Is the bent part structural? What are the manufacturer’s recommendations? Should the repair be done using heat or not? Will the repair cause excess repair stresses? What impact will repairing vs. replacing have on the time needed to complete the repair? What is the liability of the repair technique? Let's take a look at all these questions.
Structural vs. nonstructural
Determining which parts are considered structural and which are not isn’t always an easy task, especially when structural parts can differ depending on the construction of the vehicle. Generally, structural components are those parts that support the weight of the vehicle and absorb the energy of the impact, as well as road shock. They are the front and rear unirails, rocker panel assemblies, suspension, crossmembers, strut-tower assemblies and radiator core supports. Body pillars, while not primary load-bearing structures, also require special treatment and should be considered structural components.
Some manufacturers consider some parts as structural that others do not. Of course, the best way to identify a part is by consulting the body repair manual, in which the manufacturer may recommend which parts should be repaired and which should be replaced when damaged. With digital repair manuals available to shops on their computers, and often even accessible to technicians on their smart phones, identifying structural parts has become easier. Also, many estimating programs now indicate structural parts with an “S” and even allow for the billing for repairs of structural parts at a different rate than for other, non-structural repairs.
Bent vs. kinked
Though it is ultimately up to the repair facility to make a decision as to the severity of the damage in an individual case, sometimes seemingly minor damage to an area that does not meet the “kinked” criteria may be on a structural part. If this is so, the part will need to be replaced. Though this damage may match the criteria of being not greater than 90 degrees and in an area greater than 1/8 of an inch (Fig 1), because of its location and the possibility of repair damage being caused as it is straightened, the part will need to be replaced.
“Bent”, on the other hand, is damage that is less severe, and is such that the damaged area can be returned to its pre-accident condition, restoring both its shape and state (Fig 2). As shown in Figure 3, though the bend is somewhat gradual, it is in an area that would compromise the part’s strength if repaired (note the hole). If the damage is in a crush zone — which employs a variety of shapes and designs such as dimples stamped into the rails, piercing holes and marking slots in critical areas (Fig 4) — the repair cannot be completed without compromising the vehicle’s ability to withstand a subsequent crash, and therefore such a part should also be replaced.
Shape vs. state
When repairing a bend, it is important to remember that the damage not only changed the shape of the part, but also its state. That is, when the bend occurred, the molecular structure was changed from a relatively inline structure to a deformed one, a structure in which some of the molecules were stretched and others compressed (Fig 5). The molecular structure must be stress relieved either during the repair or after to return the structure to its original state. Stress relieving is most often done by vibration (often during the repairs through hammering) or by heating the repair area. Manufacturers often have heat limitations due to the type of materials used in the part, and the technician should know, understand and follow the manufacturer’s recommendations.
Determining manufacturer recommendations
Even though some structural damage may occur outside of a crash zone, a part’s internal structure may not make it a good candidate for repair parts with stiffening internal parts. Replacing parts at their factory seams will avoid cutting open a structural part and finding internal strengthening structures (Fig 6). The manufacturer’s recommended repair procedures will indicate the correct replacement procedure to avoid these internal structures.
Even locating and removing welds, such as factory spot welds, and the method for removal is sometimes outlined in the procedure recommendation. Weld removal can be accomplished with a variety of spot weld cutters (Fig 7). On the surface of the part to be removed, the spot weld is partially drilled out (Fig 8); then the weld cut is completed by separating the weld with a splitting tool (Fig 9). This keeps the integrity of the structural part that is not removed. The area is cleaned up and prepared for the new replacement part. The mating surfaces should be prepared for repair as recommended by the manufacturer. For example, follow recommended guidelines for corrosion protection to the bare steel, fitting of the surfaces, and confirming the location of the replacement part.
It is also a good practice to perform a few test welds prior to completing the welds on the new part. The steel used for the test welds should be the same thickness and type of material as the replacement part.
Welding in a replacement part, or sectioning a portion of the part, using recommended welding practices, away from the crash zones, can be acceptable. If the technician uses the recommended replacement procedure, the vehicle should maintain its crashworthiness.
By following the vehicle manufacturer recommendations, the repair facility and the technicians can limit their repair liability. Some of these recommendations are very complex, incorporating new manufacturing techniques such as adhesive bonding and in-shop resistance spot welding. All are designed to maintain the repaired vehicle’s crashworthiness. Although it may seem that all parts will be replaced under these regulations, of course this is not the case. Good repair skills such as metal finishing and application of the correct amount of filler are still a significant part of collision repair.
Repairs with heat or no heat
As vehicle components have continued to evolve, new lighter, strong materials have been developed. One example is stronger steels with higher yield points, or strength. Yield strength is the amount of stress that a material will withstand before deforming. Material that does not exceed its yield point will return to its original shape without deformation. Mild steel has a 30,000 lb. yield point. On higher strength steel, it could be from 50,000 to 100,000 lbs.
High strength steel (HSS) is much more brittle and subject to cracking. HHS is also more subject to “heat effect.” If the recommended heat is exceeded, the steel becomes weaker than designed, and thus more subject to deformation if a second collision is experienced.
Some general heat recommendations by vehicle make include:
- GM — Avoid heat when possible; do not exceed 900 to 1000°F.
- Chrysler — Treat all steel as HSS. Do not heat above 1200°F. (Note: On vehicles 1983 and older, do not exceed 700°F.)
- Ford — Avoid the use of heat when possible, and do not exceed 1200°F. Do not heat for longer than three minutes when heat is used.
On some vehicle models, the manufacturers have currently been making non-structural parts such as fenders from HSS. Because of this, the fenders’ reparability has been limited. They suffer from repair fatigue, heat effect and cracking the same way as structural parts do; therefore, such a part, with even minor damage, may not be a candidate for repair.
Another aspect of deciding whether a part is to be replaced or repaired is cycle time. If repair will cause the vehicle to be in the repair facility longer than it would be if replaced, this factor will influence the repair plan. The cost of rental cars and the inconvenience caused to the customer should be taken into consideration. The faster a vehicle can be repaired, the more satisfied both customers — the vehicle owner as the primary customer and the insurance company as the secondary customer — will be.
Shops have adopted many types of internal communications such as repair orders, but one of the most helpful innovations is on-vehicle communications (Fig. 10). Simply marking right on the vehicle the location where it is to be repaired or replaced will speed up both accuracy and cycle time in the shop.
Repair technique liability
No discussion about replacement or repair of structural parts can be complete without mentioning the liability of the repair. Many factors concerning the technician and the repair should be considered. Assuring that technicians are up to date on the latest recommendation and procedures is of great importance. It is not enough just to say that because you have been in the collision repair business for years that your repair technicians are correct and effectively educated for the vehicle you are working on. Staying current on repair techniques and procedures is very important. Knowing and using the manufacturer’s recommendations during the repair, following in-shop standard repair procedures and keeping accurate repair records are also vital.
As you can see, the decision to replace a part instead of repairing one is complex. It is a much bigger question than simply “bent vs. kinked.” The type of material that the damaged part is made from, its location, the repair’s integrity, the effect of heat during repair, the manufacturer’s recommendations, the skill of the technician and even cycle time all become a part of the process.