Metal Fatigue Failure: Reading the Fracture

Metal fatigue failure is a part cracking under repeated loads well below its static strength. A crack starts at a stress raiser on the surface, grows a little with each load cycle, leaving curved "beach marks" behind the crack front, and then the shrunken remaining section snaps. Fatigue causes the majority of in-service mechanical failures.

Fatigue is the quiet killer of rotating and cyclically loaded equipment: shafts, springs, gears, welds, fasteners, and anything that vibrates. It is dangerous precisely because it happens at stresses a static strength calculation says are safe, and it gives almost no warning, the part looks fine until the day it is in two pieces. But a fatigue fracture is also one of the most readable failures there is. The broken surface records how the crack started, how long it grew, and how hard the part was loaded. This guide teaches you to read that surface, use the S-N curve behind it, and design fatigue out.

What is metal fatigue failure?

Fatigue is progressive, localized cracking caused by repeated or fluctuating stress. The key word is repeated: a stress that a part survives easily once will crack it after enough cycles. Failure proceeds in three stages. First, crack initiation a microscopic crack nucleates, almost always at the surface, at a point of stress concentration. Second, crack propagation the crack advances a tiny amount on each cycle, and this stage consumes most of the part's life. Third, final fracture once the crack has eaten enough of the cross-section, the remaining metal can no longer carry the load and fails in a single overload event.

How do you read a fatigue fracture?

Hold the broken surface to the light and look for three things. The origin is where the crack started, trace the beach marks back to their common center, and you will usually find a stress raiser there: a keyway, a fillet, a tool mark, a weld toe, or a corrosion pit. Beach marks (also called clamshell marks) are the curved bands that record the crack front pausing and advancing as the load changed, they mean slow, progressive growth. The final fracture zone is the rough, dull region where the part finally overloaded; a larger rough zone means the part was carrying a higher load when it broke.

That reading is what separates fatigue from a one-time overload, and the distinction changes your whole investigation. A fatigue break is flat and smooth with little visible deformation; an overload break shows gross plastic deformation, bending, necking, a fibrous or slanted surface, from a single excessive load. Getting this right early points your root cause analysis at the right question: fatigue asks "what cyclic load and stress raiser?", overload asks "what single event?"

What is the S-N (Wöhler) curve?

The S-N curve, introduced by August Wöhler in the 19th century, so also called the Wöhler curve, plots the stress amplitude S a material can take against the number of cycles N it survives before fatigue failure. Higher stress, fewer cycles; lower stress, more cycles. It is the fundamental design tool for fatigue: given the cyclic stress a part sees, the curve tells you roughly how many cycles of life to expect.

The curve also splits fatigue into two regimes. Low-cycle fatigue (roughly under 1,000–10,000 cycles) happens at high stress with plastic deformation each cycle, think of a part loaded near its yield strength a few thousand times. High-cycle fatigue is the long, low-stress right-hand side of the curve, where deformation stays elastic and parts survive millions of cycles. Most rotating equipment lives in high-cycle territory, which is why the shape of the curve out at high N matters so much.

What is the endurance limit, and do all metals have one?

For steel and most ferrous alloys, the S-N curve flattens into a horizontal plateau, the endurance limit (or fatigue limit). Below that stress, the material can theoretically survive an unlimited number of cycles without failing. Design a steel shaft so its cyclic stress stays under the endurance limit and, in principle, fatigue is off the table. As a rough rule, the endurance limit of steel is around 35–50% of its ultimate tensile strength, often approximated at half.

Crucially, not all metals have one. Aluminum, magnesium, and most non-ferrous alloys show no plateau, their S-N curve keeps sloping down, so there is no stress low enough to guarantee infinite life. For those materials you must design to a finite fatigue strength at a defined number of cycles and plan to retire or inspect the part before it gets there. This is a core reason aircraft aluminum structures have hard life limits and mandatory inspections, and why predictive monitoring and crack inspection matter more as parts approach their design life.

Where do fatigue cracks start?

Almost always at a surface stress concentration, because that is where cyclic stress is locally amplified. The usual suspects: sharp fillets and steps where a shaft changes diameter, keyways and splines, threads, weld toes, machining and grinding marks, stamped part numbers, and, a big one in process plants, corrosion pits. Corrosion and fatigue together (corrosion fatigue) are far worse than either alone, because each pit is a fresh crack starter. Surface finish and residual stress matter too: a smooth, shot-peened surface (which puts the skin in compression) dramatically raises fatigue life, while a rough or tensile-stressed surface slashes it. The practical lesson is that fatigue is usually designed and machined in at features you can see, and often designed out the same way.

How do you prevent fatigue failures?

Fatigue is one of the most preventable failure modes once you know where it starts. Work this list on critical cyclically loaded parts:

1. Kill the stress raisers. Generous radii instead of sharp fillets, chamfered keyway ends, ground-smooth weld toes, no sharp tool marks. Most fatigue lives or dies at these features.
2. Improve the surface. Better finish on highly stressed zones, and put the surface in compression with shot peening or rolling where it counts. A compressed skin resists crack initiation.
3. Design below the endurance limit, or to a life. For steel, keep cyclic stress under the endurance limit. For aluminum and other non-ferrous parts with no limit, design to a finite life and set a retirement or inspection interval before it.
4. Control corrosion. Every pit is a crack starter, so coatings, materials, and environment control that stop corrosion directly buy fatigue life.
5. Manage the loads. Cut the cyclic amplitude at the source, fix misalignment, imbalance, resonance, and vibration. A shaft running through a resonance sees far more damaging cycles than the design assumed.
6. Inspect for cracks before final fracture. Because propagation is slow and most of life is spent growing a crack, non-destructive testing (dye penetrant, magnetic particle, ultrasonic) and vibration monitoring can catch a fatigue crack while it is still a crack, not a failure.
7. Feed findings back. Log every fatigue break with its origin and mode so the pattern shows up in your MTBF data and drives the design or PM change that ends it.

What the numbers say

• Fatigue is not a niche failure mode, it is the dominant one. It is widely estimated to cause the large majority of in-service mechanical failures (commonly cited as over 90%, per ASM International), because so much equipment is cyclically loaded and fatigue strikes below static strength. The underlying mechanics, the S-N curve, the endurance limit near half of tensile strength, and crack growth consuming most of the life, are laid out in MIT's fatigue module (MIT, Fatigue of Materials, D. Roylance).
• Because fatigue failures are typically sudden and unplanned, catching them early is where the money is. The U.S. Department of Energy's FEMP O&M guidance, maintained by PNNL, finds condition-driven maintenance saves 8–12% over preventive-only and the opportunity versus reactive, run-to-failure operation can exceed 30–40% (PNNL, O&M Best Practices: Maintenance Approaches). Finding a fatigue crack on an inspection instead of a catastrophic break is that saving in action.

A fatigue failure looks like bad luck and reads like a confession. The beach marks point to the origin, the origin points to the stress raiser, and the stress raiser points to the fix, a bigger radius, a smoother surface, a corrosion coating, or a load taken out of resonance. Do that on your critical rotating parts and fatigue stops being the failure code that ends shafts and gears without warning, which is the heart of building real equipment reliability. Pair it with a CMMS that keeps the failure history, so the next crack of the same kind gets designed out. For how one manufacturer built floor data trustworthy enough to catch failures early, see the CLS case study.