The crack growth mechanism is dependent on environment, frequency and temperature as shown by some examples. Two cracking modes are observed in aluminium alloys: the tensile mode and the shear mode. Examples show that inert environments promote the shear mode whereas aggressive environments promote the tensile mode. Information from constant-amplitude tests need not be applicable to service loading conditions as illustrated by an example. Significant environmental effects on crack growths are observed in aircraft flight-simulation tests. Growth delays induced by severe flights occur in all environments. As a consequence the truncation of the load spectrum at the high level end is a difficult question. The problem of producing relevant information in the laboratory is discussed.
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