Fatigue and Damage Tolerance Questions and Answers

Answer: A

The first two describe the phenomenon. The third is the reverse: widespread damage undermines the fail-safe assumption that alternative load paths remain intact, which is why ageing aircraft programmes address it specifically.

Answer: B

Correct answer: A-2, B-1, C-4, D-3

The pairing turns on which quantities are material properties and which describe a particular situation. The stress intensity factor \( Y\sigma\sqrt{\pi a} \) describes the crack tip field of a specific crack under a specific stress; the fracture toughness is the material’s critical value of it. The stress ratio is the ratio of the extremes, and the fatigue notch factor the effective concentration a notch produces in fatigue.

Answer: C

Between them these cover the two halves of the subject: the first and last belong to stress-life analysis, and the Paris law to crack growth. Goodman bridges them by correcting constant-amplitude data to real mean stresses.

Answer: D

Three of the four work by residual stress or by reducing the range actually experienced, and only the last attacks the geometry itself. Combining them, as a fatigue-critical joint usually does, is more effective than any one alone.

Answer: A

The three fatigue philosophies form a historical sequence, each answering a shortcoming of the last. Modern transport certification rests on damage tolerance, with safe life retained only where redundancy cannot be provided.

Answer: B

Correct answer: A-2, B-1, C-4, D-3

Each phenomenon belongs to a distinct condition. Fretting fatigue arises at clamped interfaces where micro-slip damages the surface, corrosion fatigue in humid environments where pits become initiation sites, widespread fatigue damage at the many similar details of an ageing airframe, and overload retardation in a spectrum containing occasional large cycles, whose plastic zone slows the subsequent growth.

Answer: C

Correct answer: Leaving a sharp corner at a section change

A sharp re-entrant corner is a stress raiser, and fatigue cracks start at stress raisers - so leaving one shortens the life rather than extending it. The other three all help: shot peening and cold working put the surface into residual compression, and a generous radius reduces the concentration factor at source.

Answer: D

Correct answer: It removes the need for scheduled inspection

Damage tolerance depends on inspection rather than replacing it. The philosophy assumes a crack is present, calculates how long it takes to grow to critical, and sets the inspection interval so that at least two opportunities to find it fall within that period. Remove the inspections and the whole argument collapses.

Answer: A

Correct answer: It is a property of the material

The stress intensity factor is a property of the situation, not the material: it combines the applied stress, the crack length and the geometry, and any material would experience the same \( K \) in the same configuration. The fracture toughness \( K_{IC} \) is the material property, being the critical value of \( K \) at which that material fractures.

Answer: B

An overload leaves a compressive residual stress at the crack tip that retards subsequent growth, so it lengthens life rather than shortening it. That retardation is why test spectra must represent such cycles carefully.