Questions: Fatigue and Cyclic Stress Failure

Question 1

A steel shaft is designed so that the operating stress is 60% of yield strength — well within the elastic range. An engineer says the shaft is safe indefinitely. What is wrong with this reasoning?

• A. Nothing — elastic deformation is fully reversible, so no damage accumulates
• B. The shaft may be safe statically but can fail under repeated cyclic loading even though each cycle stays elastic
• C. 60% of yield strength exceeds the fatigue limit for all steels
• D. The engineer should have used ultimate tensile strength as the reference, not yield strength

Answer: (B)

This is the fundamental fatigue insight: staying below yield strength does not guarantee safety under cyclic loading. Each load cycle opens and extends microscopic cracks at stress concentrations (notches, surface defects, inclusions) — even when the bulk material remains elastic. After enough cycles, the crack reaches critical size and fast fracture occurs. The material hasn't yielded in any individual cycle, yet it has failed. 'It's elastic, it's safe' is the most dangerous misconception in fatigue design, and it's responsible for some of history's worst structural failures.

Question 2

Why does shot peening improve the fatigue life of a metal component?

• A. It smooths surface defects, removing potential crack initiation sites by abrasion
• B. It induces compressive residual stresses in the near-surface layer, which must be overcome before a crack can propagate
• C. It increases the hardness of the bulk material, raising the yield strength throughout
• D. It coats the surface with a protective layer that prevents corrosion-induced fatigue

Answer: (B)

Fatigue cracks initiate and propagate by opening under tensile stress. Shot peening bombards the surface with small steel balls, plastically deforming the near-surface layer and leaving it in compression. A crack cannot open under compressive stress — the compressive residual must first be overcome by the applied tensile stress before crack-tip loading can occur. This effectively raises the stress threshold for crack propagation, extending fatigue life by factors of 2–5× in typical engineering metals. Option A is a common misconception: shot peening roughens the surface, it does not smooth it. The benefit is mechanical (residual stress), not geometric.

Question 3

Aluminum alloys, unlike steels, do not have a true fatigue limit and will eventually fail under any cyclic stress if given enough cycles.

• T. True
• F. False

Answer: True

The fatigue limit is a feature of materials whose S-N curves flatten at long lives — primarily steels and some titanium alloys. For these materials, there exists a stress amplitude below which the material appears to tolerate infinite cycles without failure. Aluminum alloys (and most non-ferrous metals) have S-N curves that continue declining without flattening. Engineers therefore use a fatigue strength at a specified life (commonly 10⁷ or 10⁸ cycles) as the design allowable. This distinction is critical for aircraft and aerospace design, where aluminum structures are dominant and infinite-life design is not achievable.

Question 4

A steel component that rarely exceeds its yield strength during service cannot fail by fatigue.

• T. True
• F. False

Answer: False

This is the central misconception the topic is designed to correct. Fatigue failure occurs under repeated cyclic loading at stresses well below yield strength — the defining feature is the accumulation of microscale crack damage over many cycles, not any single exceedance of yield. A steel shaft cycled at 60% of yield strength can fracture after millions of load reversals. The elastic regime is not a 'safe zone' for cyclic loading; it only guarantees no plastic deformation in any single cycle. Proper fatigue design requires the S-N curve, not just the yield strength.

Question 5

Why do fatigue cracks almost always initiate at the surface, and how does this explain why surface condition is the most controllable fatigue variable in design?

Model answer: Fatigue crack initiation requires a stress concentration — a local region where stress exceeds the nominal applied stress. Surfaces are where geometric discontinuities (notches, holes, machining marks, weld toes) and microstructural discontinuities (grain boundaries at the free surface, inclusions exposed by machining) are most abundant and most exposed to the cyclic loading. The surface also lacks the constraint of surrounding material, making crack opening easier. Because initiation consumes most of total fatigue life, controlling the density and severity of surface initiation sites directly controls fatigue resistance. This is why polished surfaces outperform machined, which outperform corroded, and why treatments like shot peening (compressive residual stress) and nitriding (hardened surface layer) produce dramatic improvements without changing the bulk material.

Surface fatigue dominance has direct engineering implications: specifications for fatigue-critical parts focus heavily on surface roughness Ra values, prohibit certain machining operations that introduce tensile residual stresses, and require post-processing steps like shot peening or roller burnishing. Interior defects (inclusions, porosity) matter mainly in very-high-cycle fatigue or in materials where surface treatments have been used to harden the surface — in those cases, the initiation site shifts inward to the next weakest location.