Fatigue and Damage Tolerance Questions and Answers

Answer: C

The plastic zone left by the overload closes on unloading and puts the crack tip in compression, so the following smaller cycles must first overcome it. Deliberately including such overloads in a test spectrum can therefore give an unconservatively long life.

Answer: D

The stress intensity factor combines the applied stress, the crack length and a geometry factor into a single parameter that governs the crack tip field. Two different cracked bodies with the same value behave identically at the tip.

Answer: A

The square root dependence means a crack must grow fourfold to double the stress intensity, which is why crack growth accelerates so sharply towards the end of life. A geometry factor accounts for the specimen and crack configuration.

Answer: B

It is a material property with units of stress times the square root of length, and it sets the critical crack size for a given applied stress. High-strength alloys typically have lower toughness, which is a central trade in damage-tolerant design.

Answer: C

Plotted logarithmically it is a straight line over the mid-range of growth rates, with a slope typically between three and four for metals. Threshold behaviour at low stress intensity and unstable fracture at high stress intensity bound it at either end.

Answer: D

The threshold defines a stress intensity range below which growth is undetectable in practice. Designing so that service cycles fall below it is one route to a safe structure, though it is difficult to guarantee across a whole spectrum.

Answer: A

The philosophy accepts manufacturing and service damage as inevitable and requires that any crack be found by inspection before it reaches critical length. Inspection intervals follow directly from the crack growth calculation.

Answer: B

Two inspections must fall within the growth period so that a crack missed once is caught the next time. The detectable size depends on the inspection method, which is why improving the method can lengthen the interval.

Answer: C

As an airframe ages, cracks appear at many nominally identical details at once, and the remaining ligaments between them can fail progressively. It undermines the fail-safe assumption that one crack is present at a time, and it is why ageing aircraft receive special attention.

Answer: D

Drawing an oversized mandrel through the hole yields the surrounding material, which then settles into compression as the bulk elastically recovers. Applied cycles must overcome that compression before the hole edge sees tension, and life improvements of several times are routine.