Fatigue Cracking in Bituminous Mixture Using Four Point Bending Test

Abstract

This paper describes investigation into cracking in bituminous mixture using the four point bending notched fracture (FPBNF) test, which has been developed at the University of Lyon/ Ecole Nationale de Travaux Publics de l’Etat (ENTPE). A special loading path is applied on the notched beam specimen at a constant temperature of - 4.5°C. A monotonic loading was first applied until the peak load and after unloading, many loading/unloading cycles at small amplitude were carried out until the final failure of specimen. Deflection of the beam and crack mouth opening displacement (CMOD) are measured. Crack length is determined experimentally using crack propagation gauges. It is also obtained with an improved method, called Displacement Ratio Crack length (DRCL) method, developed at ENTPE laboratory, which allows back calculating the crack length. This method is based on the relation between two experimental displacement measurements: the crack mouth opening displacement (CMOD) and the deflection of the beam. The results obtained from this method are discussed and compared with the crack length measured with crack propagation gauges. During the test, the fracture behaviour is investigated. The crack propagation is studied as a function of loading/unloading cycle number. The stress intensity factor is evaluated. Two different domains of crack evolution are distinguished: the first domain where pre-existing crack progressively re-opens, the second domain where crack propagates. The Paris fatigue law could be applied in the domain where crack propagates.