Evolution of complex modulus and higher harmonics of stress response of asphalt concrete in strain-controlled four-point beam fatigue tests

The modulus of asphalt concrete that is conventionally obtained from large amplitude oscillations (LAOs) in fatigue tests cannot be considered a material property since the response changes from cycle to cycle. It is necessary to determine how viscoelastic properties vary with the extent of damage, and they cannot be obtained from LAOs without making specific constitutive assumptions. Moreover, the higher harmonics of the stress response can be used to quantify the distortion of the stress waveform and, therefore, the point of failure in fatigue tests. This should also be explored further. With these as the objectives, strain-controlled four-point bending (4PB) tests were conducted in this study with the LAOs regularly interrupted to apply a few cycles of small amplitude oscillations (SAOs). The stress response from both SAOs and LAOs were subjected to Fourier analysis. The fundamental harmonic was used to define the apparent storage and loss moduli, and the moduli from the SAOs and LAOs were compared with each other. The ratio of the amplitude of the third harmonic to the first harmonic was found to sharply increase after a certain point in the test. This was used to define a new failure criterion for 4PB fatigue tests.