An Extended FEM Approach for Determining Crack Growth Parameters from the Texas Overlay Tester




Alrashydah, Esra'a

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Transverse cracking is a common form of distress in asphalt concrete pavements. There has been a multitude of efforts in studying this problem using fracture mechanics principles. Crack propagation is typically described through Paris' fracture parameters, namely A and n for elastic materials and A' or C3 and n' or C4 for viscoelastic materials. These parameters define the rate of change in crack length per strain cycle (da/dN) as a function of the change in the stress intensity factor (SIF) denoted by ΔK or alternatively, the pseudo-energy release rate (ERR) at the crack tip denoted by ∆JR. The corresponding Paris power law cracking parameters are A, n and A', n', respectively. These material properties are obtained from laboratory testing, such as the indirect tension creep test (IDT) or the Texas overlay cyclic test (OTR). A variety of empirical and numerical methods (i.e., FEM) has been proposed for estimating the SIF. Modelling crack propagation using the FEM poses several computational challenges (e.g., particular element types needed to capture the discontinuities and the mesh needs to be redrawn as the crack grows). The Dissertation at hand proposes a new approach that allows simulating transverse cracking growth in asphalt concretes using the extended finite element method (XFEM) in ABAQUS. It describes three 3-D XFEM modelling approaches for simulating the OTR; Model 1 involving stationary cracks of various preset lengths, Model 2 involving the virtual crack closure technique (VCCT) and Model 3 involving the low cyclic fatigue (LCF) approach. Six asphalt concrete mixes used in Texas were simulated, a Stone Mastic with a type D aggregate (SMAD), Thin overlay mixture (TOM), two Standard TxDOT mixtures with type C and D aggregate gradations (Type C and Type D), and two Superpave mixes with a type D aggregate gradation (SPD1 and SPD2).Data for these six mixtures were obtained from two sources, namely the Texas Transportation Institute (TTI) and the University of Texas at El Paso (UTEP). The data obtained was analyzed to obtain the material model inputs, including the tensile Young's modulus, and critical fracture energy (GC). These models were validated by comparing the simulation results with the laboratory test results. It was shown that Model 2 is best suited for simulating the OTR for the purpose of obtaining the Paris' power low cracking parameters. Its advantages are that it allows modelling the crack propagation, it outputs the ERR directly at the peak of each cycle and finally, it requires only the tensile modulus and the GC value as input. For the six mixes analyzed, the results of the XFEM-VCCT coupled approach (i.e., Model 2) were further processed to obtain the modified Paris' power law parameters (C3 and C4). This was done by fitting a linear equation to log of the crack propagation rate (da/dN) versus the log of the estimated ERR (i.e., ∆JR). For these six mixes, the K values were estimated analytically from the estimated ERR output. Finally, the log of da/dN was plotted versus the log of K to obtain the A and n values. The results indicated that the XFEM coupled with the VCCT approach is well suited in simulating the OTR. Using the proposed model, it is possible to efficiently estimate the Paris' power law parameters for viscoelastic materials. In addition, it could be possible to differentiate between mixtures with good cracking resistance and poor cracking resistance, as reflected by the field performance of these six mixtures. The methodology used provides a straightforward tool that allows estimating directly the cracking parameters of asphalt concretes from their tensile modulus and critical fracture energy. This model could be extended by considering the asphalt concrete microstructure and the viscoelastic nature of the binder. It can be further extended in modelling the field behavior of overlays by specifying loading rates and temperature conditions that reflect field rather than laboratory conditions.


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Cracking, Energy release rate, Fracture parameters, Overlay tester, Stress intensity factor, XFEM



Civil and Environmental Engineering