Research On Fatigue Failure Mechanism Of Asphalt Mixtures Using Digital Speckle Correlation Method

Fatigue failure due to repeated loading is one of the major distress in asphalt pavements.That is also the major issue in basic theory of strcture and design in asphalt pavement. Studieshave shown that traffic load randomness, environment complexity and the mesostructurevariability of asphalt mixture lead to the randomness of asphalt pavement fatigue response. Sothere are big differences between indoor fatigue test and fatigue damage accumulation processof the actual pavements. Therefore, it is necessary to acquire more insight into the fatiguebehavior of asphalt mixture from the mesoscopic level,to obtain a better understanding of thefatigue failure mechanism of asphalt mixtures.The displacement fields and strain fields of asphalt mixtures in four-point bending fatigueloading process were obtained in this paper using digital speckle correlation method andMatlab platform. Furthermore, the local characteristics of the strain fields were analyzed; localfailure strength of asphalt concrete was proposed; the asphalt mixture fatigue failure criterionwas discussed. And finally this paper proposed the ratio of dissipated energy change toindicate the damage accumulation in the mixture as asphalt mixtures fatigue failure criterion,giving a theoretical proof. Considering the existing test conditions and equipment, the maininnovative work completed in this paper is included as follows:1) In order to obtain the deformation distribution and evolution characteristics of asphaltconcrete under cyclic loading, the basic principle of the digital speckle correlation method(DSCM) and numerical methods were introduced in detail.2) The subpixel displacement and strain fields were calculated based on Matlab platformcombining numerical calculation method. And then Numerical simulations were carried out onthe calculation results; the accuracy of displacement and strain measurement in actual testenvironment was discussed.3) Four-point bending fatigue test was carried out on asphalt concrete beam specimens oftwo particle size matrix. By the DSCM measuring system, the displacement field and strainfield of asphalt concrete were monitored in real time under4PB test. And then the deformationdistribution characteristics of asphalt concrete were obtained under4PB test.4) The size of asphalt concrete in the fracture process zone was defined, according to thedistribution characteristics of the strain field before the unstable fracture and digital images.The relative position between fatigue crack growth path and aggregates was analized. In4PBstrength test, the crack initiation time of asphalt concrete was captured by DSCM measuringsystem, awaring of the asphalt concrete starting to crack before peak load and then failure strain was calculated.5) Based on the fracture statistical theory, damage threshold must be pre given beforeanalyzing material fracture behavior. Therefore, based on the basic idea of the asphalt mixturefatigue damage threshold, combining4PB fatigue test and flexural strength test, microdamagehealing effect of the asphalt mixture was verified and fatigue damage threshold was calculated.6) During4PB fatigue test, there are different degrees of damage accumulated at differentlocations in the middle of specimen which is equal stress zone. Therefore,the presence ofpartial failure strength in asphalt concrete was inferred, giving the approach of calculatingpartial failure strength.This approach appeared to be rational and to provide the followingadvantages: strain rate was used in the determination of the dissipated creep strain energy limit,which accounts for the effects of viscoelasticity during repeated loading test;the actual stressdistribution near the crack tip was considered;dissipated creep strain energy per cycle wasdetermined by integrating the stress and strain functions associated with the actual loadingcondition(i.e.,the harvesine load).7)The failure probability of asphalt concrete fatigue failure was analized using statisticalfracture mechanics model named weakest link model. And then the fracture probabilityfunction was derived under some stress level.8) Presently, it is not reached consensus for asphalt mixtures fatigue failure criterion.Themost universally accepted fatigue criterion is a decrease in initial stiffness by50percent inflexural fatigue test. However, the50percent reduction in initial stiffness is an arbitrarynumber with no relation to damage accumulation in the material. This paper presents ratio ofdissipated energy change to indicate the damage accumulation in the mixture as asphaltmixtures fatigue failure criterion, giving a theoretical proof. This procedure shows similaritybetween constant stress and constant strain modes of testing. Finally, this paper verifies thefatigue performance prediction model, based on the ratio of dissipated energy change, isindependent of the load control mode.