Fatigue is an effect consistent with the formation of cracks in material caused by a series of repetitive tensile stress cycles that do not exceed the tensile strength of the material. (For more information on fatigue, refer to the many publications with descriptions of this phenomena [e. g., SHRP Reports A-312 or A-404].)
12.3.1 Test Methods
There are many methods for testing fatigue; for example, the European standard on testing fatigue EN 12697-24 has quoted the following ones:
• Two-point bending test on trapezoidal-shaped specimens (2PB-TZ)
• Two-point bending test on prismatic-shaped specimens (2PB-PR)
• Three-point bending test on prismatic-shaped specimens (3PB-PR)
• Four-point bending test on prismatic-shaped specimens (4PB-PR)
• Indirect tensile test on cylindrical-shaped specimens (IT-CY)
Note: the standard EN 12697-24 has clearly stipulated that results obtained with various methods are not comparable; also, the standard EN 13108-20 has limited fatigue tests exclusive to AC mixtures (as a part of initial type testing).
Extended comparisons of fatigue test methods can be found in the literature (di Benedetto et al., 1997; Said and Wahlstrom, 2000).
Fatigue tests are carried out under one of the following modes of loading:
• Stress controlled
• Stress is induced in a specimen and is held throughout the test; strain steadily increases with the loading cycles until failure of the specimen occurs, which signals the end of testing.
• The fatigue limit is proportional to the mixture’s stiffness.
• Strain controlled
• Strain is induced in a specimen and is held throughout the test; stress steadily decreases with the application of loading cycles until the specimen’s stiffness reaches 50% of its initial level.
• The fatigue limit is inversely proportional to the mixture’s stiffness.
