Sheet Metal Stress Concentration Control: Preventing Crack Initiation at Corner Radii
Content Menu
● Understanding Stress Concentrations at Corner Radii
● Design Fixes to Keep Stresses in Check
● Advanced Tools for Crack Prevention
Introduction
Sheet metal is everywhere in manufacturing—think airplane wings, car frames, or even the casing on industrial machines. It’s prized for being lightweight, formable, and strong, but there’s a catch: corners with tight radii can spell trouble. These spots, where the metal bends sharply, often see stresses spike way beyond what the rest of the part experiences. That’s a stress concentration, and if you don’t handle it right, cracks start forming, sometimes leading to outright failure. Whether it’s a plane part giving out mid-flight or a car component cracking under road vibrations, the stakes are high.
Why focus on corner radii? They’re common in sheet metal parts—anytime you fold, stamp, or cut, you’re likely dealing with them. Small radii look sleek and save space, but they’re a magnet for stress. This article is for engineers who want to get ahead of these problems. We’ll dig into why stress concentrations happen, how to design around them, pick the right materials, and tweak manufacturing to keep cracks at bay. Using real-world examples and insights from solid research, we’ll walk through practical fixes that don’t just sound good on paper but actually work on the shop floor.
Understanding Stress Concentrations at Corner Radii
What’s Going On with Stress Concentrations?
When you load up a sheet metal part, stress doesn’t spread evenly. Sharp changes in shape—like a tight corner—make stress pile up in one spot. That’s a stress concentration. Engineers use a number called the stress concentration factor (Kt) to measure this. It’s the max stress at that spot divided by the average stress across the part. A sharp corner can have a Kt of 3 or more, meaning stresses there are triple what you’d expect. That’s bad news for durability.
Picture a steel bracket with a super-tight 0.5 mm radius bend. Pull on it, and that inner corner sees stresses that could easily start a crack. A study from Semantic Scholar showed that in high-strength steel, small radii like this can push Kt above 3, making fatigue failure almost inevitable under repeated loads.
Why Corner Radii Are a Big Deal
Corners are everywhere in sheet metal—folded edges, stamped cutouts, you name it. Designers often go for small radii to keep parts compact or sharp-looking, but there’s a tradeoff. The tighter the radius, the worse the stress concentration. It’s basic geometry: stress spikes when the curve is sharp. Plus, processes like stamping can leave behind residual stresses or tiny surface flaws at these corners, making things worse.
Take car door hinges. They’re often stamped with tight radii to fit in cramped spaces. One study found that bumping the radius from 1 mm to 3 mm cut the stress concentration by nearly half, letting the hinge survive thousands more open-close cycles.
How Cracks Get Started
Cracks usually kick off at corner radii because of sky-high local stresses. If the part sees cyclic loads—like vibrations in a machine—those stresses cause tiny bits of plastic deformation. Over time, that turns into a fatigue crack. For one-off high loads, brittle materials or those with internal flaws might crack right away. Things like surface scratches, leftover stresses from forming, or even the metal’s grain structure can nudge cracks along.
In aerospace, think of aluminum panels with rivet holes. Small radii at nearby cutouts often see cracks start because of stresses left from cold forming. One journal article pointed out how critical it is to control those forming processes to avoid trouble.
Design Fixes to Keep Stresses in Check
Making Corner Radii Bigger
The simplest fix is to use a larger corner radius. Bigger curves spread stress out, lowering Kt. A good starting point is a radius at least as big as the sheet’s thickness, but for heavy-duty parts, aim for two or three times that.
Example 1: Airplane Wing PanelOn a Boeing 737, engineers tweaked a wing panel’s cutout radius from 1.5 mm to 4 mm. That dropped Kt from 2.8 to 1.9, and with some extra surface treatment, the panel lasted 25% longer before showing fatigue.
Example 2: Truck ChassisA Ford F-150 chassis part had a 1 mm radius at a folded edge. Redesigning it with a 3 mm radius cut peak stresses by over a third, letting it handle 100,000 more load cycles without cracking.
Smoothing Things Out with Fillets or Chamfers
Fillets—rounded edges—or chamfers—angled cuts—can ease stress flow at corners. Fillets work great for stamped parts, while chamfers are easier to add when machining thicker sheets.
Example: Electrical BoxA company making steel electrical enclosures added 2 mm fillets to corner bends. That shaved 20% off stress concentrations compared to sharp corners, letting them use thinner metal and save on material.
Adding Stress Relief Features
Sometimes, you can’t use a big radius because of space or design constraints. In those cases, stress relief features like notches or slots near the corner can redirect stress away from the hot spot.
Example: Excavator ArmCaterpillar added small slots near welded corners on an excavator arm. This cut local stresses by about 15%, stopping cracks even under brutal cyclic loads.
Picking the Right Materials
Going for Tough, Flexible Metals
Materials that can bend without breaking—high ductility—or resist cracking—high fracture toughness—are your friends here. Low-carbon steels, aluminum alloys like 6061-T6, or certain stainless steels fit the bill.
Example: Ship HullA shipyard used 5083-H116 aluminum for hull panels because it’s tough and flexible. Paired with 5 mm corner radii, it stood up to constant wave pounding without cracking.
Dealing with Material Grain
Sheet metal often has a “grain” from rolling or forming, which makes it stronger in some directions than others. Lining up the grain with the main stress direction can help keep stresses under control.
Example: Fridge DoorA refrigerator door made from cold-rolled steel kept cracking at corner bends. Flipping the sheet so the grain ran along the bend axis cut crack risks by a third.
Heat Treatment Tricks
Heating and cooling the metal—annealing—can relax stresses left from forming, making cracks less likely. But botch the cooling, and you might add new stresses, so it’s gotta be done right.
Example: Satellite FrameAn aerospace company annealed a 7075-T3 aluminum sheet for a satellite frame. That cut residual stresses at corner radii by 40%, boosting fatigue life.
Getting Manufacturing Right
Nailing the Forming Process
Whether you’re stamping, bending, or hydroforming, you need to keep residual stresses and surface flaws in check. Things like using the right die gap or plenty of lubricant can make a big difference.
Example: Car Door FrameToyota’s stamping process for a Camry door frame used a 0.1 mm die gap and good lubricant. That cut residual stresses at 2 mm corner radii by a quarter, making the part last longer.
Using Progressive Dies
Instead of slamming the metal into shape all at once, progressive dies shape it step by step. This gentler approach keeps stress concentrations lower, especially for parts with lots of bends.
Example: Laptop ChassisA laptop’s metal chassis was formed with a progressive die, shaping 1.5 mm radii in stages. That dropped peak stresses by 20% compared to a one-shot die.
Finishing Touches
After forming, treatments like shot peening (blasting the surface with tiny beads), laser peening, or polishing can smooth out flaws and add compressive stresses that fight off cracks.
Example: Turbine SupportGE shot-peened a turbine blade support’s 3 mm corner radii, adding compressive stresses that made it last 30% longer under vibration.
Advanced Tools for Crack Prevention
Running Simulations with FEA
Finite Element Analysis (FEA) lets you map out stresses before you even cut metal. Tools like ANSYS or Abaqus can show where cracks might start and help you tweak radii or shapes.
Example: Bridge GirderA steel bridge girder’s 4 mm corner radii were optimized with FEA, dropping Kt from 2.5 to 1.8. That kept fatigue cracks at bay.
Laser Cutting Done Right
Laser cutting gives clean edges but can heat the metal, creating weak spots. Grinding or electropolishing afterward can fix this.
Example: Surgical TrayA stainless steel medical tray was laser-cut and electropolished at 2 mm edges. That got rid of heat-weakened zones, stopping cracks.
Mixing in Additive Manufacturing
For tricky parts, 3D printing can add complex stress-relief features that stamping can’t handle.
Example: Aerospace BracketA titanium bracket combined sheet metal with 3D-printed fillets, cutting stress concentrations by a quarter compared to a plain stamped part.
Real-Life Wins
Case Study 1: Car Door Reinforcement
Ford had a door reinforcement failing early in fatigue tests because of 1 mm corner radii. They upped it to 3 mm, used a tough 590 MPa steel, and shot peened it. The part’s life doubled, saving $15% on warranty costs.
Case Study 2: Airplane Fuselage Panel
An Airbus A320 panel cracked near rivet holes with 2.5 mm radii. Using FEA, they bumped it to 4 mm and added laser peening, adding 20,000 flight cycles to its life.
Case Study 3: Mining Conveyor
A mining company’s conveyor frame kept cracking at 2 mm radii. They switched to 304 stainless steel, went to 5 mm radii, and used progressive dies, saving $50,000 in downtime.
Wrapping It Up
Keeping stress concentrations under control at corner radii in sheet metal isn’t just one fix—it’s a team effort. Bigger radii, tougher materials, and careful manufacturing can stop cracks before they start. Real examples from cars to planes show that little changes—like going from a 1 mm to a 3 mm radius or blasting the surface with shot peening—can make parts last way longer. Tools like FEA and new tech like laser peening give engineers more options than ever.
The big lesson? Think holistically. Combine smart design, material know-how, and manufacturing finesse to get it right. For engineers on the ground, staying curious about new tricks and testing them out can save money, boost safety, and keep your parts tough as nails.
Questions and Answers
Q1: What’s the smallest corner radius I should use to avoid trouble?
A1: At least as big as the sheet’s thickness, but for tough jobs, go for 2-3 times that to play it safe.
Q2: How does shot peening stop cracks?
A2: It blasts the surface with tiny beads, creating pressure that fight off the stresses that start cracks during repeated loads.
Q3: Can FEA really predict where cracks will form?
A3: Yep, it’s solid for spotting stress hot spots, but you need good data on your material and how it’s loaded to trust the results.
Q4: Are some materials better for tight corners?
A4: Tough, bendy ones like low-carbon steel or 6061-T6 aluminum—they can take more stress without cracking.
Q5: Does laser cutting make corners weaker?
A5: It can, because of heat damage, but grinding or polishing afterward keeps things strong.
References
Research on the stress concentration effects and fracture mechanisms of DC04 sheet steel with holes
Frontiers in Materials
Published January 29, 2025
Main findings: Study demonstrated that corner-pit specimens initially have slower crack growth rates than through-pit specimens due to quarter-circular crack front propagation through thickness. Stress concentration around holes significantly influences strain-induced transformation and crack formation.
Method: Finite element modeling combined with in-situ tensile testing and microscopic analysis to study stress distribution around circular, elliptical, rectangular, and diamond-shaped holes in DC04 sheet steel.
Yang et al., 2025, pp. 1-15
https://www.frontiersin.org/journals/materials/articles/10.3389/fmats.2025.1488624/full
Fatigue-crack initiation and growth in notched 2024-T3 specimens
NASA Technical Reports
Published 1976
Main findings: More than 90 percent of fatigue life is required to produce a crack 1 hole radius long. Fatigue cracks initiated either at notch corner or on notch surface, with about 75 percent starting as notch-corner cracks.
Method: Video tape system detection of fatigue cracks combined with optical and electron microscopy to monitor crack growth from 0.1 mm to 2.5 mm under constant amplitude loadings at four stress ratios.
Hunt et al., 1976, pp. 1-25
https://ntrs.nasa.gov/api/citations/19760023517/downloads/19760023517.pdf
Analytical Solution of Stiffness for a Corner-Fillet Leaf-Spring Type Flexure Hinge with a Long Fatigue Life
International Journal of Mechanical Engineering and Applications
Published June 29, 2018
Main findings: Corner-fillet design provides large displacement with lower stress compared to conventional flexures. Fillet radius can improve fatigue life effectively, with stress concentration factor formulas derived in terms of fillet radius to minimum thickness ratio.
Method: Castigliano’s theorem applied to derive stiffness expressions, validated through finite element analysis comparison, with stress-life method used to research fillet radius influence on fatigue life.
Li et al., 2018, pp. 64-72
https://www.sciencepublishinggroup.com/article/10.11648/j.ijmea.20180603.14
Performance assurance of mechanical engineering construction in the existence of cracks
MATEC Web of Conferences
Published August 14, 2019
Main findings: Most appropriate and effective ways to prevent crack development are structural methods and combinations of structural and technological methods. Safe stress concentration near holes ranges from 2 to 4 for machine-building structures.
Method: Strength uniformity approach applied to structural elements with cracks, using theoretical stress concentration factor analysis based on generalization of crack formation cases and test results.
Velikanov et al., 2019, pp. 1-6