Finite element simulation of asphalt fatigue testing

The traditional interpretation of fatigue tests on asphalt mixes has been in terms of a logarithmic linear relationship between the constant stress or strain amplitude and the number of load repetitions to cause failure, often defined as a decrease in modulus to half the initial value. To accomodate non-constant stress or strain, a mode factor may be introduced or the dissipated energy may be used instead of stress or strain.Cracking of asphalt (or other materials) may be described as a process consisting of three phases. In phase one diffuse microcracking is formed in the material. In the second phase some microcracking propagate to form macrocracks and, finally, in phase three the macrocracking propagates until fracture.The last phase may be described using fracture mechanics (e.g. Paris's law), but the emergence of microcracking in the first two phases is better descrie using continuum damage mechanics.The paper describes how continuum damage mechanics may be used with a finite element program to explain the progressive deterioration of asphalt mixes under laboratory fatigue testing. Both constant stress and constant strain testing are simulated, and compared to the actual results from three point and four point fatigue test on different mixes. It is shown that the same damage law, based on energy density, may be used to explain the gradual deterioration under constant stress as well as under constant strain testing.Some of the advantages of using this method for interpreting fatigue tests are that the extrapolation from laboratory tests to insitu conditions is facilitated and that the gradual deterioration of the asphalt may be predicted, which is useful for pavement management purposes.