I-CAR Structural cheat sheet

Inspection sequence: pre-scan customer interview, visual/moisture check, measuring, then hidden-damage teardown; damage found later can change the repair plan and estimate.

Symmetric measuring compares left and right control points; a difference beyond OEM tolerance indicates sway, mash, or diamond rather than a simple bolt-on issue.

Sway damage: the body is displaced laterally at one end relative to the other; diamond: a parallelogram distortion in plan view; mash: vertical collapse or shortening.

Direct damage is at the impact point; indirect damage appears at hinges, seams, glass openings, and adjacent panels where energy traveled.

Stress risers to inspect: welded seam ends, hole edges, panel bends, and sectioning joints; cracks and elongation concentrate there.

Gap and flush checks around doors, hood, and deck lid reveal body shift; uneven gaps plus a level panel suggest hinge or pillar movement.

Structural vs. cosmetic: parts listed by the OEM as structural (rails, aprons, pillars, rockers, floor reinforcements) require structural repair procedures and welding.

Corrosion and prior-repair screening: look for non-factory welds, filler thickness, panel-bonding adhesive, and rust before approving a repair path.

A repair plan must account for occupant-protection systems: never straighten, heat, or section a component the OEM restricts, and disable SRS before work.

Unitized body: panels and rails are welded into a load-bearing structure; body-over-frame: a separate frame carries loads and the body mounts to it.

Crush zone (front/rear) absorbs energy by controlled deformation; occupant safety cage (pillars, rockers, roof, floor) resists intrusion.

Monocoque/unibody parts: rails, aprons, strut towers, cowl, rockers, floor pan, pillars, roof; bolt-on parts include fenders, doors, hood, bumpers.

Boron and martensitic steels are press-hardened to roughly 1,000–1,500 MPa tensile; mild steel is roughly 270–350 MPa.

Advanced high-strength steels (AHSS) include dual-phase (DP), transformation-induced plasticity (TRIP), complex-phase (CP), and martensitic grades.

Sectioning joint: a cut-and-join location chosen by the OEM; it must avoid crush zones, heat-affected zones of original welds, and high-stress areas.

Heat-affected zone (HAZ): base metal altered by welding heat; its strength drops and it becomes the preferred failure location if not controlled.

SRS components: airbags, pretensioners, sensors, and control module; handling requires disabling power and following OEM wait times.

Repair vs. replace decision: OEM position statements and procedures define when a part may be repaired; non-OEM procedures may not be used as justification.

Datum: OEM reference points in a defined plane; centerline (symmetry) plane and datum height establish the three-dimensional reference.

Control points are specific holes, bolts, and features with OEM X/Y/Z coordinates; measure at control points, not at arbitrary panel surfaces.

Tolerance rule: a reading outside OEM specification by more than the stated tolerance requires further diagnosis, not automatic pulling.

Universal measuring systems (e.g., self-centering gauges, laser/electronic) compare to OEM data; dedicated fixtures bolt to control points for a specific vehicle.

Three-point principle: a component must be located in three planes; missing or loose fixturing lets the structure move during pulling.

Measure before, during, and after pulling; re-measure after each pull increment so you do not overshoot the target dimension.

Diagonal and tram measurements detect diamond and sway; height measurements detect mash and sag; compare side-to-side for symmetry.

Fixture/bench anchoring: clamp the vehicle at OEM control points and verify the anchors are tight before applying load.

Gauge repeatability: confirm the measuring device is zeroed and the same reference points are used each time so readings are comparable.

Pull direction should oppose the direction of impact; pulling at an angle to the damage path adds new distortion instead of reversing it.

Pull in increments with re-measurement; a single large pull concentrates stress and can overstretch metal beyond recovery.

Anchor the vehicle so the reaction force is carried by the bench or floor; unsecured anchoring lets the body slide and defeats the pull.

Heat reduces strength: many AHSS and boron parts prohibit heat; where allowed, stay within OEM temperature limits and never quench.

Stress-relief and hammering: controlled planishing after pulling reduces work-hardening and helps restore panel shape.

Overpull is not a substitute for correct anchoring; correcting diamond and sway may require multiple opposing pulls and push/pull combinations.

Chain, clamp, and puller ratings must exceed expected load; inspect chains, hooks, and clamps for deformation before each use.

After straightening, re-check control-point dimensions, panel gaps, and door/glass fit; structural alignment is confirmed by measurements, not appearance.

Document pull sequence and readings; a repair that cannot be measured back to OEM tolerance is not a completed structural repair.

STRSW passes current through stacked panels held by electrodes; resistance heat at the faying surface forms the nugget, so clean mating surfaces are essential.

Weld schedule parameters: current, squeeze time, weld time, hold time, and electrode force are set by the machine and tip configuration.

Electrode tips must be dressed and matched to the welder; tip face size and condition change current density and nugget size.

Shunting: current takes a path through an adjacent existing weld or conductive adhesive; space welds per OEM and check for shunting.

Weld-through adhesive is permitted only where the OEM allows it; adhesive between panels changes resistance and requires a validated schedule.

Destructive test (chisel/peel) on a test coupon of the same stack-up verifies nugget diameter and failure mode before production welding.

Nugget acceptance is typically a minimum diameter relative to sheet thickness; a button pull or plug failure indicates a sound weld.

Galvanized and coated steels need appropriate schedules and tip maintenance; coating can increase resistance and contaminate tips.

Do not substitute MIG plug welds for STRSW unless the OEM procedure permits it; substitution changes crash performance.

MIG uses a consumable wire electrode with shielding gas; MAG uses reactive gas mixes; both are gas-shielded arc processes (GMAW).

Sectioning joint location is OEM-specified; typical rules avoid the crush zone, original weld HAZ, and high-stress bends.

Butt joint with backing or insert: OEM inserts/backing plates are used where specified; do not fabricate substitutes unless the procedure allows.

Stitch vs. continuous welds: follow OEM weld length, spacing, and location exactly; extra weld does not equal extra strength and can alter crush.

Heat input control: use lowest practical amperage, shortest arc, and skip-weld sequences to limit HAZ softening in AHSS.

Plug weld hole size and count follow OEM procedure; the hole must be in the outer panel and the weld must fuse both layers.

Weld-through primer is used only in the joint where the OEM permits it; it must be removed from the weld zone if the procedure says so.

Galvanized steel requires zinc removal at the weld zone or a procedure that accounts for porosity and fume; ventilate and use proper PPE.

Post-weld: inspect for porosity, undercut, and incomplete fusion; dress welds only as the OEM allows, then apply corrosion protection.

Replacement sequence: remove trim and adjacent parts, locate and drill original welds, cut sectioning joints, then trial-fit the new panel before welding.

Spot weld removal: use a spot-weld cutter sized to the weld; avoid cutting through the lower panel, which must remain intact.

Trial fit checks gaps, flushness, and control-point alignment before any welding; correcting fit after welding is far more difficult.

Clamping and fixturing hold the new panel in position; verify door/glass fit with the panel clamped, not just tacked.

Weld count and placement must match the OEM procedure; replacing welds with fewer or differently placed welds reduces structural integrity.

Bolt-on panels (fenders, doors) still require gap/flush adjustment and corrosion protection at mating surfaces.

Sectioning a new panel requires transferring OEM measurements and joint locations; mark the cut line from the original part, not by eye.

After welding, apply seam sealer and cavity wax per OEM; new panels often lack factory coatings on mating flanges.

Record replaced panels and weld locations for the customer and for any future repair or inspection.

Structural adhesive bonding distributes load over the entire joint, unlike spot welds that concentrate load at points; it complements welding.

Surface prep is decisive: remove coatings to bare metal where specified, abrade, clean with the approved cleaner, and bond within the open time.

Adhesive bead geometry (height, width, continuity) follows OEM procedure; gaps or voids create stress concentrations and water paths.

Clamping time and full-cure time are separate; do not pull or weld on the joint before the specified handling strength is reached.

Rivet bonding combines adhesive with rivets: the adhesive carries shear and the rivets clamp the joint during cure and add peel resistance.

Self-piercing rivets (SPR) require access to both sides and a die/anvil; blind rivets are used where only one side is accessible.

Rivet selection (diameter, length, material) must match the stack-up thickness and OEM specification; wrong grip length causes loose or crushed joints.

Dissimilar-material joints need isolation to prevent galvanic corrosion; adhesive and sealant act as the barrier between steel and aluminum.

Cure temperature matters: some adhesives require elevated temperature; cold shop conditions extend cure and can cause early failure.

Galvanic corrosion occurs when dissimilar metals contact in the presence of an electrolyte; steel and aluminum joints need isolation.

Restoration layers: conversion coating/primer, then OEM-specified topcoat and clear; bare steel must never be left exposed.

Cavity wax is applied inside rails, pillars, and rockers after welding; access holes and drain paths must remain open.

Seam sealer replaces factory sealant at welded seams; it must be applied over primed surfaces and tooled to match OEM appearance.

Weld-through primer protects the faying surface but must be used only where the OEM permits; it can affect weld quality.

Zinc-rich primers protect bare steel at welds and abrasions; they are not a substitute for the full coating system.

E-coat (electrocoat) is the factory dip primer; repairs must replicate its protection at welded and cut areas with approved primers.

Drainage: never block drains or weeps with sealer or wax; trapped water is the primary driver of repeat corrosion.

Post-repair inspection should confirm coating coverage at flanges, sectioning joints, and inner cavities that cannot be seen later.

AHSS grades: DP (dual-phase), TRIP, CP (complex-phase), MS (martensitic), and press-hardened boron; each has different formability and repair limits.

Strength vs. ductility trade-off: higher tensile strength generally means lower elongation, so AHSS cracks rather than stretches when pulled.

Many boron and martensitic parts are not repairable; OEM position statements often require replacement rather than straightening or heating.

Heat limits: AHSS may lose strength above OEM-specified temperatures; never use torch heat to shrink or straighten without OEM approval.

Mixed-material vehicles use aluminum closures, magnesium castings, and steel structures; each needs dedicated tools to avoid cross-contamination.

Aluminum and steel must be isolated with adhesive, sealant, or coated fasteners; bare contact plus moisture causes galvanic corrosion.

Dedicated tooling: aluminum tools, bits, and abrasives must be kept separate from steel tooling to prevent embedded particles and corrosion.

Sectioning AHSS requires OEM-approved joint locations and weld procedures; HAZ softening means weld placement and heat input are critical.

Fastener and rivet selection must match base material; steel rivets in aluminum panels need isolation or coated rivets per OEM.