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Mechanical fatigue: what it means for the inspector

Long Seam editorial teamSources checked 3 min read

Mechanical fatigue is cracking caused by repeated stress cycles, each one far below what would break the part in a single pull. A crack starts at a stress concentration such as a notch or weld toe and grows a little every cycle. Vibration and pressure or load cycling are the usual sources.

Graph of stress rising and falling over time in repeated cycles, beside a plate cross-section with a small notch at its edge and curved arcs spreading from the notch showing where the crack front stood at earlier times
Each cycle is small. Over many cycles a crack starts at the notch and grows in steps.

How it works

A part loaded once, below its strength, carries the load. A part loaded and unloaded thousands or millions of times can crack even though no single cycle is large. Three things control fatigue life: the size of the stress swing, the number of cycles, and the stress concentration at the spot where a crack can start.

Fatigue has stages. A crack starts at a stress concentrator such as a sharp notch, a weld toe, a poor-fitting weld, a surface scratch or a small bore connection. It grows a small step each cycle. Finally the remaining wall can no longer carry the load, and the part fails, sometimes suddenly. The fracture surface often shows curved marks (beach marks) where the crack paused or changed pace, with a rough final-fracture region.

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Sources of cycling

  • Vibration. Pumps, compressors and flow-induced vibration shake attached piping. Small-bore connections, branch connections and unsupported pipe are common victims.
  • Pressure and thermal cycling. Repeated start-ups, shutdowns and load swings cycle the stress. Thermal fatigue, from repeated temperature swings, is a related form.
  • External loads. Wind, mechanical movement or equipment loads on supports and attachments.

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Where it shows and how it is found

Cracks form at welds and attachments, so inspection concentrates there. Wet fluorescent magnetic particle testing finds surface cracks in magnetic steel, and penetrant testing handles other metals. UT can look at depth. A good field inspector also looks for the cause: loose supports, missing bracing, vibrating small-bore piping, and anything that suggests unusual movement. Fixing the cause matters more than welding the crack.

Mix-ups to avoid

  • Fatigue versus creep. Fatigue needs cycling. Creep needs steady load and high temperature.
  • Fatigue versus stress corrosion cracking. SCC needs a chemical environment and works under static stress. Fatigue does not need a chemical, though corrosion can speed it up (corrosion fatigue).
  • Fatigue versus brittle fracture. Fatigue is a slow, repeated build-up. Brittle fracture is a single sudden event in cold steel.

On the API exam

Mechanical fatigue (RP 571 paragraph 3.43, which covers vibration-induced fatigue) was on API's list for the September 2024 to May 2025 windows but is not one of the 15 paragraphs on the sheet for September 2026 to May 2027. Questions on RP 571 for API 510 come only from the listed portions, so treat this as background and check the sheet for your window. The idea still helps you reason about cracking at supports and small-bore connections.

Practice it

See what is and is not on the RP 571 list for your window in the API 571 damage mechanisms list for API 510. Compare it with Brittle fracture and Creep, or browse every term in the damage mechanism glossary.

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Facts checked 25 October 2026 against the sources listed above. If something has changed, tell us and we will correct it within 2 business days.

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