Welding vs. Bolts vs. Rivets: Which one fails first?
Welding, bolts, or rivets: Which joint resists best when everything shakes? An analysis from fatigue and fracture perspective.
In the previous two posts, we discussed practical advantages: when to use welded studs and when rivets are irreplaceable. But today we’re getting to the heart of the matter: What happens when these joints age, vibrate, or receive an unexpected impact?
Because in engineering, it’s not enough for a joint to hold on the first day. What matters is how long it lasts without failing… and how it fails when it does.
| Technology | Typical failure modes | Does it warn before breaking? |
|---|---|---|
| Welding | Fatigue in the heat-affected zone, cold cracking, lack of penetration | Almost never (brittle and sudden failure) |
| Bolts | Fatigue failure in the thread, loss of preload, shear failure of the shank | Sometimes (if they loosen beforehand) |
| Rivets | Shear failure of the shank, plate bearing failure, base material fatigue | They usually warn (progressive deformation) |
“Tell me how you fail and I’ll tell you who you are.”
The Wöhler curve (S-N): The electrocardiogram of joints
Just as a cardiologist measures heart activity, engineers use the Wöhler curve (also called the S-N curve or fatigue curve) to understand how long a joint can withstand before failing due to fatigue.
- S (from Stress) is the applied stress on the joint in each cycle, like the force a rivet withstands every time a beam vibrates.
- N (from Number of cycles) is the number of cycles that joint can withstand before failing from fatigue.
The basic equation governing this behavior is Basquin’s law: σ = C · N^(-1/k)
Welding
Welding has the worst fatigue curve because heat and internal defects create microcracks from the first cycle, reaching only 20-30% of the base material’s strength.
Weak point: The heat-affected zone (HAZ) and stress concentration at the weld root.
Behavior: Welds have a very low fatigue limit. Internal microcracks can grow undetected.
Typical failure: Catastrophic and without warning (like Aloha Airlines flight 243, where a fatigue crack tore off part of the fuselage).
Bolts
Bolts live or die by preload. If they maintain it, they last a long time; if they lose it, the thread acts like a knife and service life plummets. The first thread root bears 40% of the load, which is why quality bolts have rolled threads.
Weak point: The thread acts as a stress concentrator (Kt factor > 3).
Behavior: If designed to work below the fatigue limit and preload is maintained, they can last indefinitely.
Typical failure: Fracture at the first thread root (highest concentration area). Sometimes they warn if preload is lost and they start to wiggle.
Rivets
Rivets are the kings of fatigue. When installed, they compress the hole and block crack propagation. Their curve is the closest to the base material, as demonstrated by World War II bombers flying thousands of hours without failure.
Weak point: The hole in the plate weakens the net section.
Behavior: When installed by deformation, they generate compressive residual stresses around the hole, which delays the appearance of fatigue cracks.
Typical failure: Rivet shear or plate bearing failure. There is usually prior deformation (the rivet “bites” before breaking).
When joints failed: Three historical lessons
Aloha Airlines (1988) – The plane that lost its roof
A Boeing 737 was flying along peacefully until part of its fuselage peeled away. The rivets? They held. What failed was the surrounding sheet metal, which after thousands of flights said “I’m done here.” Lesson: The rivets didn’t fail, the metal around them did.
Silver Bridge (1967) – The bridge that collapsed at Christmas
46 cars were crossing when the bridge came down. The culprit: invisible corrosion inside a component that no one could see. Lesson: Bolts and mechanical joints can be rotten inside while looking perfectly fine on the outside.
Titanic (1912) – Cheap rivets
The iceberg didn’t open a huge gash. What happened was that the bow rivets, full of impurities, popped like corks when hit. Lesson: A poorly manufactured rivet isn’t just another link—it’s the weak link.
Con más de 40 años de experiencia en el sector de la soldadura y las técnicas de unión Bearcat extiende sus conocimientos a todos los sectores industriales: automoción, ingeniería de ferrocarriles, ingeniería industrial, industria naval, obra civil.
