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have variable amplitudes, such as the loads on a beam in a bridge carrying traffic composed of cars and trucks of various weights. For variable amplitude loads, no lower bound on the fatigue strength is believed to exist, but some design codes use one half of the constant amplitude fatigue limit as the fatigue limit for variable amplitude loading.

Fatigue strengths of aluminum alloys are 30-40% those of steel under similar circumstances of loading and severity of the detail.

Fatigue is also affected by environmental conditions. The fatigue strength of aluminum in corrosive environments such as salt spray can be considerably less than the fatigue strength in laboratory air. This may be because corrosion sites such as pits act as points of initiation for cracks, much like flaws such as dents or scratches. The more corrosion-resistant alloys of the 5xxx and 6xxx series suffer less reduction in fatigue strength in corrosive environments than the less corrosion-resistant alloys such as those of the 2xxx and 7xxx series. 0n the other hand, fatigue strengths are higher at cryogenic temperature than at room temperature. There isn't enough data on these effects to establish design rules, so designers must test specific applications to determine the magnitude of environmental factors on fatigue strength.

The fatigue strength of castings is less than that of wrought products, and no fatigue design strengths are available for castings.

9.2.3 Property Ratings

Ratings for properties such as corrosion resistance, weldability, and machinability are given in tables for wrought (Table 9.20) and cast alloys (Table 9.21). See Chapter 12 for more information about corrosion.

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