Repair or Replace?

Jan. 1, 2020
Determining whether to repair or replace a damaged part is a decision made daily in collision repair facilities and estimating bays.

The information you need to take to make the right choice

Determining whether to repair or replace a damaged part is a decision made daily in collision repair facilities and estimating bays.

Because of the number of variables that exist between damaged vehicles, it is difficult to establish firm "black or white" rules that would not be excessively restrictive. That is why it is actually better to become familiar with all of the issues involved with structural parts to make the best decisions on a case-by-case basis.

A growing number of vehicles use a variety of materials in their structures. The criteria for determining whether a part should be repaired or replaced can vary greatly depending on the material in the part and the part structure. The focus of this article will be on steel parts and steel structures.

Material matters

I-CAR has always recommended following OEM recommendations when doing collision repairs, but when it comes to the decision on whether a part can be straightened or whether it should be replaced, many vehicle makers tend to leave that decision up to the collision industry. Making this decision carries a great deal of responsibility due to safety implications and liability exposure. A collision-damaged vehicle must be repaired to the original level of safety the vehicle was designed for.

To better understand how to make the best repair decision, we need to know exactly why new materials and designs are being used. The challenges to the automotive manufacturing industry today are significant. Consumers expect an increasing array of convenience, infotainment and safety systems, along with improved performance and increased fuel economy without giving up vehicle size. They also want safety. Car buyers look for "Five-Star" crash protection.

Incorporating all of these features adds a lot of weight to a vehicle. Overall vehicle weight is trending heavier rather than lighter. To keep vehicles from getting too heavy, automakers offset added weight from vehicle systems by reducing the weight of the body structure. Hence, automakers actually engage in weight mitigation, or weight control, rather than weight reduction.

The new steels used today are not actually lighter. They are much stronger. Some of the ultra high-strength steel (UHSS) being used today can be four times the strength of the mild steels that were used in the past. This allows stamped parts to be made thinner, which reduces the weight of the part.

Note, however, that in some areas of the vehicle, parts are not being made thinner. They actually may be made thicker, even with the use of high-strength steel (HSS) and UHSS. This may be done to improve rigidity and increase side-impact and rollover protection. That's where some of the weight is added back into the structure.

Additional attributes of the new steels are important to remember. Dual-phase (DP), transformation induced plasticity (TRIP) and complex-phase (CP) steels are easier to form than conventional HSS of comparable strength during the stamping process. As advanced high-strength steel (AHSS) is stamped into a die to form a part, the steel work hardens and becomes much stronger than the original strength of the base metal in sheet form. This makes for an extremely strong part.

Other types of AHSS such as Martensitic, DP980 and Boron steels are used in pillars, rockers and roof rails for their strength and resistance to bending rather than energy absorption. This prevents intrusion into the passenger compartment in a collision. These steels tend to be very high in strength, resist bending and transfer collision forces away from the point of impact rather than absorb it by bending and buckling. These UHSS parts are more likely to create indirect damage to adjacent structural parts because of their rigidity. If these parts get damaged in a collision, they often require replacement because they tend to crack during attempts to straighten them.

Because they are critical to occupant safety, it's best to err on the safe side with these parts. Specific steels are selected for specific applications in the vehicle structure. The characteristics of these steels must be preserved during collision repair.

Heat can change the properties of AHSS dramatically. These steels can be extremely heat-sensitive. The extent that some of the new steels can be straightened therefore is somewhat limited. Remember that the goal in a collision repair is not just limited to returning a part to its original physical shape. The part also must maintain its original, pre-crash properties. I-CAR training describes this as returning a part to its original shape and state.

Extent of damage

In 1989, I-CAR introduced a simple guideline for making repair\replace decisions on steel body panels known as the Kink Versus Bend Rule. Even though the guideline was created almost 20 years ago, the fundamentals still apply.

The rule simply states, "If it's bent, repair it; if it's kinked, replace it."

The I-CAR definition of a kink and a bend is as follows: A part is bent when the change in the shape of the part between the damaged and undamaged areas is smooth and continuous and straightening it restores its shape to pre-accident condition without any areas of permanent deformation.

Even with the stronger steels being used today, this part of the rule applies. When a steel part bends slightly without buckling, the steel has not undergone the extensive work hardening that takes place with more extensive damage conditions. Because of this, it is likely that a "bent" part can be reshaped, with the steel properties preserved. This damage type also is more likely to be corrected with cold straightening, which is more likely to preserve the steel characteristics.

A part is kinked when it has a sharp bend with a small radius, usually more than 90 degrees over a short distance, about 3mm, and/or after straightening, there is a visible crack or tear in the metal, or there is a permanent area of deformation which will not straighten to its pre-accident shape without the use of excessive heat.

Not using excessive heat is an important point because several vehicle manufacturers recommend cold straightening to ensure that the properties of the metals are not compromised. Cold straightening limits the extent of damage that can be straightened because the stronger, harder steels used today cannot easily be straightened without using heat. Note too that "excessive heat" may mean any heat on some newer vehicles.

A safe way to proceed is avoiding using heat altogether when straightening, especially on newer vehicles, unless the vehicle maker allows heat and specifies the allowable temperatures and heating times. If heat is allowed, you must monitor the steel temperature with a non-contact thermometer, heat crayons or thermal paint so the steel characteristics are not compromised. If damage is too severe to be straightened cold, part replacement may be necessary, especially when heating is not recommended.

It may be arguable that the 90-degree angle specification (from 1989) that differentiates a kink from a bend may be too sharp when attempting to straighten an AHSS panel, especially with some of the steels on the higher end of the strength range and some of the thicker, heavier gauge panels. Although it may be possible to straighten a 90-degree deflection in a small area of a mild steel trunk floor, harder steels that are bent at a right angle probably will not straighten very well, even in flat areas of a part.

In formed areas of the part, any buckling, regardless of the angle, will have to be monitored very carefully and evaluated on a case-by-case basis to determine whether the part can be returned to the original shape and state.

The 90-degree specification is intended to be used only as a guide. Focus on the wording "after straightening, there is a visible crack or tear in the metal, or there is a permanent area of deformation which will not straighten to its pre-accident shape without the use of excessive heat." That guideline still applies today. Regardless of the bend angle, if the part won't return to the original shape without excessive heat, replace it.

Even in the case where a buckled AHSS part can be returned to the correct shape, there still may be micro cracking or flaws in the granular structure of the steel that could change the performance of the part in another collision or could cause the part to fail over time if it is subjected to repeated stress during regular vehicle operation.

Welding cracks

While it may seem that small cracks resulting from a collision or by straightening could be repaired by GMA (MIG) welding, this usually does not produce a satisfactory repair. The problem is that the area on a part next to the crack or tear is work hardened, and damage to the steel microstructure is present. Because of this, welding the crack does not fix the problem. The area next to the weld is still flawed and is prone to cracking from fatigue later. Welding a crack often results in another crack forming right next to the weld.

The other problem with GMA (MIG) welding is the high heat input. Some of the parts used in structures today are UHSS. The strength of UHSS is destroyed by heat. Replacing cracked parts by partial or full panel replacement is usually the best solution.

Using heat to straighten parts

If the vehicle maker allows heat during straightening, be mindful of the hazards that exist with the heating process, and exercise care. Consider using an induction heater because of the flameless heat it generates, rather than using an open flame to heat the part. You will also need to replace the anti-corrosion coatings that the heating destroys if you do choose to use heat during straightening.

Years ago when HSS began to be used in limited applications for vehicle structures, I-CAR adopted the phrase "Treat all steel like high-strength steel." The idea was that if the location of heat-sensitive steel was unknown to the technician, it would be more efficient and safer to proceed as if the entire vehicle were made from HSS, which basically meant that heating parts should be done in a controlled manner.

Unfortunately, it isn't that simple. Treating all steel like UHSS would be extremely restrictive and not the best approach for a multitude of reasons.

The best approach is to know, at the very least, exactly where the UHSS is. This can be done by either checking the vehicle maker collision repair information or by using steel identifiers such as the one available from JNE Boron. That way, you can be mindful of the parts that are likely going to require replacement versus the parts that have more latitude for straightening.

Part design and availability

You may need to determine if an individual replacement part is available, or if the part is only serviced as an assembly. In some cases, vehicle makers recommend replacing an assembly rather than a single part so technicians will not weld directly on a UHSS part. For example, the B-pillar reinforcement on a Buick Enclave is serviced with the B-pillar assembly. The reinforcement is made from DP 980, which is a UHSS, but it is spot-welded to an HSLA reinforcement. Replacing the HSLA part does not involve welding directly onto the UHSS, thereby maintaining the integrity of the UHSS reinforcement. Replacing an assembly rather than an individual part needs to be considered.

Complete or partial replacement

Another question that needs to be answered is whether there are options to partially replace a part at a factory seam or by sectioning. Referencing the OEM collision repair information will allow you to determine that. A quick reference tool to find out if an OEM partial replacement procedure exists for a vehicle is the online I-CAR Partial Replacement Matrix on I-CAR's Web site at www.i-car.com/partialreplacement. This site does not contain the procedures, but it will let you know if an OEM procedure exists. To access the procedure you need to go to the OEM Web site and download it.

Final word: Training

Proper training on new steels is integral to making the right repair\replace decision. All your techs and estimators should attend classes. To learn more about new vehicle designs and the new steels that are being used, attend the I-CAR Steel Unitized Structures Technologies and Repair (SPS07) training program that is available nationwide and in Canada. Also available from I-CAR is the Advanced High-Strength Steel Overview (AHS01) online training program.n

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