Deciding whether to repair or replace isn't as simple as kinked vs. bent

Al Thomas ABRN collision repair auto body repair repair vs. replace structural parts repair integrity welding repair time Like many good guidelines, the statement "If it's kinked, replace it, and if it's bent, repair it," is a sound basic principle. But, as with other generalities, simply following this guideline will not always produce successful results automatically.

To make the correct repair-plan decision, there are many other areas a repair facility must analyze. The foreman or technician must ask an array of questions:

Is the bent part structural?
What type of material is the part made from?
Should the repair be done using heat or not?
What are the manufacturer's recommendations?
Will the repair cause excess repair stresses?
Is it likely the area being repaired will have unseen defects after the repair?
What's the liability of the repair technique?
What impact will repairing vs. replacing have on the time needed to complete the repair?

Repair vs. replace

One might ask, which damage is considered a bend and which is considered a kink? As a general rule, a damaged area where the bend is greater than 90 degrees in the space of one-eighth of an inch is considered a kink and should be replaced. (See Fig. 1) However, there may be other factors that should be considered:

If the area is a structural part, can the repair be done without further damaging the structural part?
Is the damaged area in a crash zone?
Can the repair be made without heating it?
How will the repair affect the crashworthiness?

All these factors must be taken into consideration when making a replace vs. repair decision.

Structural parts

Determining which parts are considered structural and which aren't isn't always easy. 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're the front and rear unirails, rockers panel assemblies, suspension crossmembers, strut-tower assemblies and radiator support. Body pillars, while not primary load-bearing structures, also require special treatment and are considered structural components.

Structural parts differ, depending on the construction of the vehicle. Some manufacturers consider some parts as structural that others don't. The best way to identify a part is by consulting the body repair manual. The vehicle's manufacturer may recommend which parts should be repaired and which should be replaced when damaged. Though it's ultimately up to the repair facility to make a decision about the severity of the damage, sometimes seemingly minor damage to an area that doesn't meet the kinked criteria may be on a structural part and will need to be replaced (see Fig. 2). Though this damage isn't greater than 90 degrees and is in an area greater than one-eighth of an inch, the part needs to be replaced because of its location and the possibility of repair damage as it's straightened.

Repair integrity

When damaged or repaired, steel work hardens. As the part is repaired, that work hardening continues. Because of this condition, the surfaces become brittle, and small, sometimes invisible, cracks develop. Aluminum also can develop this defect when welded, and though the repair appears to be intact, its crashworthiness – the ability to properly withstand a second crash – has been compromised. Even if the visible cracks have been repaired, others may still remain. If the part is replaced and welded as recommended, the repair's crashworthiness will remain intact.

When repairing structural parts cold (pulling without heat) or with the use of heat, the part can develop microscopic fractures and suffers from repair fatigue.

Welding

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.

Heat vs. cold

As vehicle components have continued to evolve, new, lighter, stronger materials have been developed. One example is stronger steels with higher yield points. This higher strength steel is much more brittle and subject to cracking. Higher strength steel also is more subject to heat affect. If the recommended heat is exceeded, the steel becomes weaker than designed and more subject to deformation if a second collision happens. Material that doesn't exceed its yield point will return to its original shape without deformation. Mild steel has a 30,000-pound yield point. On higher strength steel, it could be 50,000 to 100,000 pounds.

On some vehicle models, the manufacturers have been making nonstructural parts, such as fenders, from high-strength steel, which limits the fenders' reparability. They suffer from repair fatigue, heat affect and cracking the same way structural parts do; therefore, such a part with even minor damage may not be a candidate for repair.

Type of structure

Even though some structural damage may occur outside 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 (see Figs. 3 and 4). The manufacturer's recommended repair procedures will indicate the correct replacement procedure to avoid these internal structures.

Removing welds, such as factory spot welds, can be accomplished with various spot weld cutters (see Fig. 5). On the surface of the part to be removed, the spot weld is drilled out partially (see Fig. 6), then the weld cut is completed by separating the weld with a splitting tool. This keeps the integrity of the structural part that's 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's 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.

By following the vehicle manufacturer recommendations, the repair facility and the technicians can limit their repair liability. Some of these recommendations are 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 all parts will be replaced under these regulations, this isn't the case. Good repair skills, such as metal finishing and applying the correct amount of filler, are still a significant part of collision repair.

Repair time

Another aspect of deciding whether a part is 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.

Rental car costs and the inconvenience caused to the customer should be taken into consideration. The faster a vehicle can be repaired, the more satisfied both customers (vehicle owner, primary customer, and the insurance company, the secondary customer) are.

To help with cycle time, some shops have labeled which vehicle areas are to be repaired and which replaced before sending it to the technician.

The decision to replace a part instead of repairing one is complex. It's a much bigger question than simply bent vs. kinked.

The type of material the damaged part it is made from, its location, the repair integrity, the effect of heat during repair, the manufacturer's recommendations, the skill of the technician and cycle time become a part of the process.