Sheet Metal notch design: preventing stress concentration at sharp geometry transitions
Content Menu
● Why Sharp Notches Kill Sheet Metal Parts
● Material Behavior — What Actually Happens at the Notch Root
● Practical Notch Geometries That Actually Work
● Simulation Tips That Match Reality
● Edge Condition Is Half the Battle
● Testing — Don’t Trust Sim Blindly
Introduction
Sharp corners and abrupt cutouts in sheet metal parts cause more headaches on the shop floor than almost anything else. One day the prototype bends fine, the next batch cracks right at the notch during assembly or after a few thousand cycles in the field. I’ve lost count of how many times we’ve had to scrap dies or rework tooling because somebody forgot that a 0.1 mm laser-cut V-notch acts like a crack starter in DP780 or 6061 aluminum.
The problem gets worse every year because we’re all chasing lighter weight: thinner gauges, higher-strength steels, more complex stampings. A geometry that worked perfectly in 1.5 mm mild steel becomes a fatigue nightmare in 1.0 mm DP1180. The stress doesn’t care about your weight-saving targets — it piles up exactly where the radius is too tight or the transition too sudden.
This article is written for the guys who actually have to make the parts — tool engineers, die designers, CAE analysts, and manufacturing engineers who sign-off on drawings every day. No fluff, no textbook theory without context. Just what works, what doesn’t, and why, pulled from real programs, real failures, and the latest papers that aren’t behind paywalls for no reason.
We’ll go through the mechanics without the academic jargon, show actual Kt drops you can expect from different relief strategies, and give examples from automotive BIW, truck frames, yellow-goods cabs, appliance enclosures, and battery trays — places where notch fatigue kills warranties.
Why Sharp Notches Kill Sheet Metal Parts
When you put a notch in a sheet, you’re forcing all the load paths to detour around it. The shortest path is right at the root, so the stress lines bunch up. In a 1 mm thick sheet under plane stress, that bunching is brutal — theoretical Kt can hit 4-6 on a punched square corner before you even form the part.
Add forming strains and things get ugly fast. Pre-strain from stretching over a die radius hardens the material locally, raises yield strength, and makes the steel much more notch-sensitive. A corner that was marginally OK in the flat blank becomes a crack initiator after 90° bending.
Real-world example from a 2023-2024 truck frame program: longitudinal rail in 8 mm thick S700MC, laser-cut lightening holes with 2 mm corner radii. Parts passed static tests no problem. Put them on the proving ground for 100 000 km durability cycle — 40% cracked exactly at the hole corners. FEA showed peak stress 68% above yield at the sharp transition. Fix was simple: changed to 8 mm full radii + teardrop ends. Zero cracks in the next fleet.
Same story repeats in thinner gauges. Battery tray side members in 1.4 mm DP980 — the supplier left 0.3 mm radii on the flange lightening slots because “the laser can do it.” Result: cracks after 5000 charge-discharge thermal cycles. Changed to 1.0 mm radius minimum, problem gone.
The pattern is always the same: higher strength = lower ductility = higher notch sensitivity.
Material Behavior — What Actually Happens at the Notch Root
Mild steel forgives a lot. You can get away with 0.2-0.3 mm radii in DX56 or DC04 because the material yields locally and blunts the notch itself. The peak stress never reaches theoretical Kt because plastic flow redistributes it.
AHSS doesn’t forgive. Martensite islands can’t deform much. Once local stress hits ~1100-1300 MPa in DP steels, micro-voids open around the ferrite-martensite interface and it’s game over. The notch root sees 3-4× nominal stress while the rest of the section is still elastic.
Recent work on warm-formed steels (WL750, WL980, WL1150) shows something interesting — because they’re formed at 400-600 °C, they end up with much higher fracture toughness (w_e > 250 kJ/m²) than quench-hardened 22MnB5, even at similar strength levels. Result: dramatically lower notch sensitivity. We’ve switched several chassis parts from PHS to warm-formed grades just for this reason — same weight, half the cracking issues on sheared edges.
DC04 deep-drawing steel behaves almost opposite to AHSS. A 2025 study on holed DC04 plates showed that circular holes give Kt ≈ 2.1-2.3 in finite plates, oval holes slightly higher, but rectangular and diamond holes shoot up to 4.5-5+ because of the sharp corners. The fracture always starts exactly at the 90° corner, then propagates at 45° shear bands. Lesson: never use square or rectangular cutouts in cold-formed mild steel if you can avoid it.
Practical Notch Geometries That Actually Work
Forget textbook U-notch vs V-notch debates. Here’s what we use every day:
- Full radiused ends on slots Minimum radius = sheet thickness, preferred 1.5-2× thickness. For 1.2 mm sheet, use 2 mm radius. Kt drops from ~4.5 to ~2.1 instantly.
- Teardrop or keyhole slots instead of elongated holes The straight parallel sides are fine, but blend the ends with a full radius larger than half the slot width. Standard trick on battery trays and seat belt anchors.
- Dog-bone or double-D relief on bend lines When you need a cutout that interrupts a bend, don’t use a simple rectangle. Use two opposing semicircles connected by straight lines. The bend line runs through the straight section, no sharp corner perpendicular to the bend.
- Coining the notch root after forming Common in progressive dies for hinge brackets. After piercing, a coining station squeezes the edge with 0.2-0.3 mm depth. Introduces compressive stress, blunts micro-burrs, drops effective Kf by 20-30%. Cheap and effective.
- Trochoidal relief pockets For tight inside corners where you can’t fit a radius (common in deep-drawn boxes), use a trochoidal tool path in the pierce station. Leaves a wavy edge that has no sharp direction change. Kt typically 1.4-1.6 instead of 3+.
- Chamfer instead of radius when radius not possible 45° chamfer 0.5-0.8 mm wide works surprisingly well — better than a 0.2 mm radius in many cases because it removes the burr direction entirely.
Case Studies From the Floor
Case 1 — EV battery tray side impact beam, 1.6 mm MS1500 (martensitic) Original design: laser-cut slots for wiring harness with 0.5 mm corner radii. Parts cracked during side pole crash test at the slot corners. FEA showed 1850 MPa peak — way over material capability. Changed to full teardrop with 4 mm end radius + 1 mm 45° chamfer. Peak stress dropped to 1120 MPa, parts passed with margin.
Case 2 — Agricultural cab roof bracket, 3 mm S355 Stamped with rectangular lightening holes. Field failures after 4000 hours — cracks from hole corners under roof load vibration. Changed holes to oval with 12 mm end radii. Failures disappeared. Bonus: part weight dropped another 8% because we could make holes larger without risk.
Case 3 — Appliance washing machine tub bracket, 1.0 mm 304 stainless Deep drawn with pierced holes for mounting bolts. Original square holes with 0.3 mm radius — cracked after 2000 cycles in durability test. Switched to circular holes + 2 mm radius blend where hole meets flange radius. Zero cracks in 10 000 cycle test, and drawing depth could be increased 15 mm.
Case 4 — Truck cross member, 6 mm S700MC Flange had rectangular cutouts for brake line routing. 60% of parts cracked at corners during frame twist durability. Redesign used large radius “racetrack” shape — parallel sides with full semicircle ends. Problem solved, and saved 1.2 kg per frame.
Simulation Tips That Match Reality
Stop using theoretical Kt from Peterson’s for sheet metal. They overpredict badly in thin gauges.
Use this instead:
- Shell elements with 6-8 layers through thickness if you care about bending gradient
- For punched/laser edges, add 0.05-0.1 mm initial root radius to simulate real edge condition
- Include forming history — import thickness and plastic strain from forming sim into fatigue analysis. The pre-strain effect is huge in AHSS
- Use Neuber or Glinka for local plasticity correction at notch root
- For fatigue life, use Smith-Watson-Topper or Fatemi-Socie with Morrow mean stress correction — works much better than plain Goodman in notched AHSS
Quick check we use: if your calculated elastic peak stress > 1.8 × Rp0.2, you’re going to have problems in high-cycle fatigue, regardless of what the global FOS says.
Edge Condition Is Half the Battle
Even with perfect radius, a bad edge kills you.
Punched edges in AHSS above 800 MPa typically have rollover + burnish + fracture zone with micro-cracks 50-100 μm deep. That’s already a notch.
Fixes that work:
- Increase punch-die clearance to 12-15% for grades >1000 MPa (counter-intuitive but reduces fracture zone depth)
- Shave the edge after piercing (adds operation but drops effective Kf dramatically)
- Use fine-blanked edges for critical features (expensive but unbeatable)
- Laser cut + light coin or brush deburr for prototypes
Warm-formed parts win again here — the high-temperature trimming produces beautifully smooth edges with almost no fracture zone.
Testing — Don’t Trust Sim Blindly
Always validate with physical parts.
Our standard stack for notch-critical parts:
- Static tensile with DIC on notched specimens (different radii)
- Strain-gauged component test under service loads
- Accelerated durability on shaker or 4-poster with block cycle from customer data
- If still unsure, cut parts from production tools and run to 10× design life
The 2023 Metals paper showed perfect correlation — materials with essential work of fracture w_e > 250 kJ/m² never failed from sheared edges in their tests, even with 15% clearance punching. Below 200 kJ/m², almost all failed. Simple go/no-go criterion.
Rules of Thumb We Live By
- Minimum corner radius = sheet thickness for t ≤ 3 mm
- For AHSS > 800 MPa, minimum radius = 2 × thickness
- Never use rectangular cutouts that cross a bend line
- Oval/teardrop always better than slot with square ends
- Warm-formed steels beat PHS for notch fatigue every time
- If you must use sharp corner, coin it or shot-peen the hell out of it
- DC0X mild steels can take surprisingly sharp corners if you keep loads low
Conclusion
After twenty-plus years fixing cracked stampings, the conclusion is boringly simple: spend the extra two minutes in CAD putting proper radii on every single notch and cutout. It costs nothing upfront and saves tens or hundreds of thousands in warranty, scrap, and die rework later.
The data backs it up — whether it’s 2025 DC04 hole studies showing circular beats everything, or 2023 fracture toughness work proving warm-formed grades are far more forgiving, or daily experience on the line watching MS1500 parts crack at 0.5 mm radii while the same geometry in WL980 sails through.
Do the radius right the first time. Your future self (and the warranty department) will thank you.
Frequently Asked Questions
Q1: Can I get away with 0.5 mm radius on 1.2 mm DP980 flanges?
No. You’ll get cracks. Use minimum 2.0-2.5 mm or teardrop shape.
Q2: Best way to handle lightening holes in 8 mm S700MC?
Oval or racetrack with end radius ≥ 20 mm. Add 8-10 mm 45° chamfers if needed for packaging.
Q3: Does shot peening help notched AHSS parts?
Yes, but only if coverage is 200% and Almen intensity ≥ 0.35A. Otherwise marginal gain.
Q4: Why do warm-formed parts survive punched edges better than cold-stamped AHSS?
Higher fracture toughness (w_e often 300-400 kJ/m²) and cleaner sheared edges from high-temp trimming.
Q5: Quick FEA check for notch acceptability?
If elastic von Mises at notch root > 1.7 × Rp0.2, change the damn radius.