Investigation of the tensile-shear failure of asphalt concrete base (ACB) construction materials using a non-linear cohesive crack model and critical crack threshold analysis

The American Society for Testing and Materials (ASTM) has recommended ASTM D8044, an international standard test using Semi-Circular Disc Bend (SCB) specimens, to test only the tensile fracture toughness (mod I) of Asphalt Composite (AC) materials. However, AC materials are exposed to not only tensile stresses but also both compressive and shear stresses under wheel load. Thus, one of the main objectives of this study is to develop a new approach for the mixed mode I-II (tension and shear) testing of AC materials by using the SCB geometry. This study deals with a comparative assessment of the fracture toughness of AC materials by using the results of experimental, analytical, and Linear Elastic Fracture Mechanics (LEFM) and Non-LEFM (NEFM) numerical analyses done on the SCB specimens. The numerical analyses of the stress distribution and the cohesive crack growth were performed using the fracture mechanics program FRANC2D. Experimental results showed that the notched cracks opened at a 30 degrees inclination angle (beta) and kept opening up to beta of 45 degrees. The mode I fracture toughness (KIC) value of Asphalt Concrete Base (ACB) specimens was found at 0.37 MPa root m. On the other hand, the KC value was found to be 0.44 MPa root m for the 30 degrees inclined crack and 0.40 MPa root m for the beta = 45 degrees inclined notch crack. The critical crack length for the mode-I loading was found to be 2-2.5 mm using the experimental and Critical Crack Threshold (CCT) analytical analysis, and it was found to be 2.5-3 mm using the NEFM-based cohesive crack analysis. Moreover, the NEFM-based cohesive crack analyses showed the critical crack length was between 3 mm and 7 mm for the mixed mode (I-II) condition, in which the notch cracks are inclined at 30 degrees and 45 degrees. The feasibility of the SCB test proposed by ASTM and AASHTO standards in determining the mixed mode I-II (tensileshear) fracture toughness value of ACB specimens, which is an important parameter in determining the strength of materials, was verified by comparing the experimental results with LEFM and NEFM numerical analyses. As a result, experimental and numerical analysis results showed that the SCB test could be an appropriate geometry to determine the mixed mode I-II fracture toughness of AC materials when the crack is inclined at 30 degrees. It is believed that this study would be evaluated as initial research on the use of SCB geometry for AC materials not only for mode I test but also for mixed mode loading tests and for the establishment of an international standard test.