Research Bulletin 1995-7

Important Repair Concerns when Sectioning Structural Components

Introduction

This bulletin identifies repair concerns and recommends precautions for replacing only the damaged portion of a structural component on unitized vehicles. The replacement of only the damaged portion of a component is generally referred to as a "section repair." A section repair involves: straightening and aligning the unibody structure, removing only the damaged portion, and replacing that portion with an undamaged member of the same type (generally from a new service part). Section repairs are performed to minimize technician intrusion or when replacing the entire component at factory seams is impractical or not a cost effective repair alternative.

These concerns and precautions stem from Tech-Cor's collision repair research and case studies on actual vehicle repair situations. Also included in this bulletin are guidelines regarding when to use a section repair, where to find published structural sectioning procedures, and what to look for when determining a proper sectioning location.

Replacing the damaged portion of structural components is very important in maintaining the structural integrity of the vehicle. In most cases, the damaged portion of a structural component includes the "crush zones." These crush zones are designed into the rail structure to initiate and control the absorption of impact energy during a collision. To ensure the vehicle structure has the same energy absorption characteristics after the repair, the damaged crush zones and rail structure frequently require replacement. This is essential to maintaining the structural integrity of the repaired vehicle and proper operation of the vehicle occupant safety systems.

Failure to replace damaged structural components, or the damaged portions, can affect the ability of the vehicle's structure to manage impact energy properly. As a result, the occupant restraint systems may not operate as originally designed and occupant safety may be jeopardized.

For additional information on repairing and replacing structural components, refer to Tech-Cor Information Bulletin 1995-6 titled "Important Repair Concerns When Replacing Structural Components and Subassemblies."

Structural Sectioning Replacement

When practical and cost effective, the first choice is to replace damaged body structural members at factory seams, as recommended by the manufacturers. Replacing structural members at factory seams using new parts creates a straight forward repair procedure. In some cases replacing damaged components at factory seams disrupts many undamaged factory applied welds, disturbs OEM applied corrosion protection beyond the damaged area, and requires replacing a larger assembly than the damage affected. Also, limited access to some factory seams may prevent the ability to remove the entire damaged component economically, or without damaging adjacent components. Sectioning is a repair alternative which involves splicing a portion of a new OEM service part or a quality aftermarket part or salvage component onto the undamaged portion of a structural body component at a location other than at a factory seam.

Structural Sectioning:

Must only be performed by an experienced and properly trained technician.
Requires the highest quality of workmanship.
Should only be performed using tested procedures to maintain the structural integrity of the vehicle.
Limits the disruption of both factory welds and corrosion protection.
Has proven to be a practical and cost effective repair alternative to replacing damaged components only at factory seams.

When damage to one or more structural components is extensive, or located inboard on the component, replacement at factory seams is preferable. However, this method may not always be practical.

Sectioning repair methods minimize vehicle disruption and damage to OEM corrosion protection, by avoiding the removal of undamaged parts to gain access to replace the entire damaged component at factory seams. Sectioning is most effective on vehicles where impact energy is absorbed in a progressive manor, limiting damage to the outboard portions of structural members.

Before performing any section repair, consult the vehicle manufacturer's body repair manual or other collision repair manuals for specific recommended sectioning methods for the particular vehicle and model. Always use tested repair recommendations when available.

It is important for collision repair shops to actively pursue current information on repair methods and keep up to date with new sectioning repair techniques. In some cases, repair techniques can be carried over and used on other similar components.

If specific section procedures are not available, a section repair can be performed by a qualified technician in the right situation. Items to consider when determining whether or not to section a service part are listed below.

Performing a section replacement of only the damaged portion of a structural component:

Requires the highest quality workmanship.
Requires the use of tested procedures whenever available, to ensure proper joint design and location.
Must restore the original crush zones, leaving the remaining undamaged portion of the structural member undisturbed.
Must maintain the original energy management characteristics intact to ensure the proper functioning of passenger safety devices.
Can have a major impact on the strength and serviceability of the vehicle.

When choosing a section location, look for a uniform area (an area having a constant cross sectional shape) inboard of the damage, with enough clearance to perform the necessary cutting and welding operations, without obstructions. Identify and locate any internal reinforcements before choosing a possible sectioning location. Use extreme caution not to cut any internal reinforcements. An additional consideration is the accessibility to properly corrosion protect the welded section joint, both internally and externally.

The vehicle's corrosion protection MUST be properly restored in all repaired areas. Proper corrosion protection is of vital importance in maintaining the structural integrity of the repaired vehicle. The proper function of vehicle safety systems also depend on the structural integrity of the repaired vehicle. Improper corrosion protection can have a detrimental effect the long term performance of the vehicle's structure to manage impact energy properly. As a result, occupant restraint systems may not operate as originally designed and occupant safety may be jeopardized.

For detailed information on restoring corrosion protection, refer to Tech-Cor Research Bulletin 1986-7 "Restoring Corrosion Protection During Vehicle Repair" or to I-CAR Collision Repair Courses on "Replacement of Structural Parts" and "Restoring Corrosion Protection."

Determining a Proper Structural Section Joint Location

When published section procedures are not available for the particular model vehicle being repaired, and a section replacement of the frame rail is the best repair alternative, refer to the following for guidelines.

Specific guidelines on where to section a particular member on a specific model may vary significantly, depending on the amount and type of damage. To avoid choosing the wrong location, Tech-Cor recommends against any sectioning in or near the locations described below unless specifically directed by a tested section procedure:

When sectioning a structural component, generally avoid:

Designed-in "crush zones" (i.e., holes, buckling initiator beads, accordion sections) at the outboard ends of rails, unless specifically directed by tested sectioning procedures. Crush zones include convolutions, stamped-in shapes and cut-outs in the frame rail, intended to initiate and control the rail's deformation during a collision.
Reason: Changing crush zones can effect the designed-in energy management system of the vehicle structure during a subsequent collision.
Suspension, drivetrain or structural part mounting locations.
Reason: These areas usually consist of multiple layers for extra strength to support the high vehicle loads and vibration fatigue.
Internal and external structural reinforcements, particularly if it is flush against the rail inner or outer panel.
Reason: The inability to properly weld and corrosion protect the section joint. Reinforcements are located at high stress points in rail assemblies, and altering a reinforcement can effect the designed-in energy management system of the vehicle structure. Some section procedures may be located at the end of an internal reinforcement or utilize a portion of the reinforcement within the section joint.
Areas of a structural component that have compound shapes, multiple component overlaps, structural reinforcements and dimensional reference holes.
Reason: It is difficult to fabricate the section joint, to fit-up of new replacement section, and prepare the weld joint.

NOTE:
Structural members with a full length internal reinforcement may preclude sectioning of the component. A full component replacement may be the only option.

When sectioning a structural component, first identify:

The locations of internal and external rail reinforcements, prior to determining a possible section joint cutting location.
Reason: To determine the design and complexity of the rail prior to selecting a section joint location and to verify the feasibility of performing the sectioning procedure.
An area of the structural member with a consistent cross sectional shape for the potential section joint location, inboard of the damage.
Reason: Allows for easier joint fabrication, better replacement part fit-up and results in better quality welds.
Removal of adjacent components necessary to allow adequate working room.
Reason: Allows for easier access to repair area during all operations, and results in a higher quality repair.
Accessibility to apply the appropriate corrosion protection materials to the section joint, after the replacement section is welded in place.
Reason: Corrosion can have a detrimental effect on structural components, especially on thinner gauge alloyed metals. Proper corrosion protection MUST be applied to the section joint to maintain structural integrity over the vehicle's life time.

Structural Section Joint Design Guidelines

Treat all steel like high strength low alloy (HSLA) steel.
When possible, use a minimal overlap joint weld seam, with approximately 1/8 - 1/4 inch (3 - 6 mm) overlap at the section joint to allow for: variances in cut lines, easier joint fit-up, and proper corrosion protection of the section joint.
When using a butt joint (see Photograph 1 below) weld seam for the section joint, cut a small weld backer that is approximately 1/2 - 3/4 inch (13-19 mm) wide from the damaged component. Position the backer strips inside the section joint about 1/4 inch (6 mm) past the joint cut line and tack weld them into place in the corners. This will prevent the molten weld puddle from burning through as the root gap is filled.
Do not make the section repair joint area stronger by adding reinforcements. Maintain the original strength of the rail in the area to be sectioned. Note: Overlaps and weld backers provide assistance for welding proper and do not add significant reinforcement to the rail.
Secure inner panels or reinforcements so they are flush against the outer panel before welding operations. To avoid corrosion, a zinc rich weld-though coating should be applied between the mating surfaces before fit-up.
Use an offset type section joint (see Photograph 2 below) when the rail design allows, as in the C-channel rail design with a separate outer closure panel.

The offset section joint will allow for:
- Better distribution of the heat affected zone, by having offset weld seams.
- The ability to tie the section joint into a rail reinforcement.
- Improved accessibility during joint fabrication and welding operations.
- Replacement of the closure panel at a factory joint.
The repair joint must be cosmetically acceptable when refinishing is completed.

Welding and Related Operations

All welding in this text will refer to fusion welding, using Gas Metal Arc Welding (GMAW) equipment. A more recognized term which is used in collision repair facilities is the Metal Inert Gas (MIG) welder, also referred to as a "wire feed welder." The focus will be on the MIG welder and its usage to weld structural body components during collision repair and structural component replacement. This is appropriate, since the collision repair industry standard as stated by the automobile manufacturers, Tech-Cor and I-CAR all advise MIG welding to be the only recommended method for replacing factory welds on structural unibody components.

Never use any oxyfuel (oxyacetylene) Gas Brazing/Welding or Resistance Spot Welding equipment for making structural welds on unitized automobiles.

MIG welders can be used with different shielding gases and wire types, or flux core wire with no shielding gas. The recommended shielding gas is a mixture of 75% Argon and 25% Carbon Dioxide gas (the gas mixture may vary slightly, depending upon the supplier), since it provides the best shielding and weld penetration for collision repair welding conditions. There are many types and sizes of welding wire that can be used in various MIG welders. It is very important that welding wire meets the American Welding Society (A.W.S.) standards for welding wire type and quality. The recommended MIG welding wire for unibody repair is 0.023 inch (0.584 mm) or 0.025 inch (0.635 mm) diameter, that meets A.W.S. wire specification "AWS-ER70S-6." This welding wire can produce quality welds on high strength and mild steel body components.

There are many other factors that will affect weld quality, such as: the technician's experience & technique, metal thickness & joint configuration, joint fit, weld sight preparation, weld site location & orientation, quality & condition of the welder, available line voltage to welder, and the proper welder adjustment settings.

When using MIG plug or continuous welding techniques, the technician MUST alternate welding locations in order to limit heat build-up in the base metal. Both temperature and length of time heat is applied are critical factors which affect the amount of heat build-up in the base metal during the welding operation. Heat build-up in high strength alloy steels (HSLA & HSS) or special heat treated steels can significantly reduce the strength of the base metal adjacent to the weld joint. This loss in strength is due to a change in the metal's molecular structure, caused by being heated above its critical temperature for too long.

The typical plug weld hole diameter should be 5/16 inch (8 mm) minimum, with a minimum of 1 inch (25 mm) spacing between plug welds. Continuous butt or edge welding should be performed in 1/2 - 3/4 inch (13-19 mm) alternating increments. Aside from the general guidelines above, follow the same weld spacing and location as the factory installed welds in areas other than section joints.

A multi-layered flange on the service part can be "pre-welded" together at the same locations where the part will be welded to the vehicle, so the weld will effectively penetrate all metal layers. Complete weld penetration is accomplished since the additional weld will be made on top of the previous weld location, to form a single weld nugget which attaches all metal layers together.

Another option is using a "step" plug weld, by step drilling the multi-layered flange on the service part, prior to fit up. Step drilling is a procedure for using successively larger holes, for each metal layer away from the base metal when plug welding. The top piece will have the largest hole. The plug weld hole diameter next to the base metal should be 5/16 inch (8 mm) for the first piece being attached, then use progressively larger holes in each additional layer. For the second piece being attached, use a 5/16 - 3/8 inch (8-10 mm) diameter hole, dependent on the specific metal thicknesses being welded. On thin panels 5/16 inch (8 mm) will be adequate, when it is a non-structural part. For thicker panels, roughly the same thickness as the base metal, when it is a structural part use a 3/8 inch (10 mm) diameter hole. This procedure allows for adequate weld penetration into the base metal and first metal layer being attached, without just filling the hole at the start of the plug weld operation. As the weld continues, the weld pool is tied into the second (or top) metal layer being attached. The end result of either method is a weld nugget connecting all metal layers at each weld location, similar to factory applied resistance spot welds during vehicle production.

Proper joint fit is essential to obtaining good weld quality. Always test fit the replacement part onto its intended location to test for correct weld joint fit prior to tack welding the part in place. Metal layers to be welded should be clamped tight against each other.

Prior to any welding operations, it is imperative that the technician properly adjust the welder by making practice welds on like material and perform peel tests. These practice welds should use the same metal type, metal thickness and joint configuration as in the repair application being welded. When peeling apart the sample weld pieces, the metal pieces should first deform, then tear apart leaving the complete weld nugget on one piece of base metal. Perform additional weld test samples to establish proper welding technique and welder adjustment settings to obtain adequate weld penetration and quality.

Refer to the I-CAR Collision Repair Course on "Welding in Collision Repair" for additional information.

Note:
The material contained in this Information Bulletin does not supersede previous repair information published by Tech-Cor Please refer to specific Tech-Cor bulletins when performing sectioning on vehicles for which Tech-Car has provided recommendations.

The following lists previous bulletins on sectioning.

Unibody Full Sectioning Procedure (1986-1)
Rear Sectioning Procedure For The GM 200 Van (1990-2)
Front Sectioning Procedure For The GM 200 Van (1990-3)
Front Sectioning Procedure For Ford Taurus/Sable (1990-6)
Front Sectioning Procedure For The GM '10' Series (1991-1)
Saturn Rear Unirail Sectioning Procedure (1993-6)
Saturn Front Unirail Section Procedure (1993-7)
1992-94 Ford Ranger/Explorer Front Frame Horn Replacement (1993-8)
1990 and Later General Motors C/H Body Front Lower Rail Replacement (1994-2)
GM N-Body Left Front Lower Rail Section (1995-1)
1992-1995 Ford Econoline Front Frame Horn Replacement (1995-2)
Chrysler L/H Series Front Unirail Section Procedure (1995-5)

The information provided in this bulletin is for educational purposes only. Although every effort has been made to ensure the accuracy of the information contained in this bulletin, Tech-Cor assumes no responsibility or liability for any repairs performed using information from any publication issued by Tech-Cor.

Any person performing repairs must determine whether any suggested or recommended procedures or repairs are suitable or appropriate for the particular vehicle being repaired. The repairer remains solely responsible for such determination, as well as for the proper completion of the repairs.

Reproduction of this bulletin is not permitted without the written approval of Tech-Cor.