Traction-separation relationship for polymer-modified bitumen under Mode I loading: Double cantilever beam experiment with stereo digital image correlation

Abstract

The traction-separation law under Mode I loading for a polymer modified bitumen commonly used as an adhesive for 3–tab roofing shingles is quantified. Double cantilever experiments are performed at three different loading rates (0.03 mm/min, 0.3 mm/min and 3 mm/min). Load-displacement measurements are synchronized with speckle image acquisition and the data analyzed to determine (a) J-integral, (b) cohesive zone end opening displacement and (c) the crack opening displacement (COD) as a function of crack extension. Imaging is performed using two stereo vision systems equipped for capturing speckle images of the DCB specimen at two different magnifications with fields of view of (a) 14 mm × 12 mm and (b) 170 mm × 140 mm. The stereo vision system used to acquire data at higher magnification is mounted on a translation stage and focused on a region close to the crack tip as the crack propagated. This stereo vision system is used to measure the strain fields near the crack tip and for measurement of crack opening displacement (COD) at 1 mm behind the current crack tip for a range of crack extensions. The second stereo vision system is used to visualize the process zone size and measure the strain fields over larger fields of view ahead of the crack tip. Assessment of the fracture surface for lower loading rates (0.03 mm/min and 0.3 mm/min) revealed a ductile type fracture (rough surface due to void growth in the process zone). For the highest loading rate (3 mm/min), the fracture surface is typical of the type seen during brittle fracture (shiny surface with striations), with intermittent rapid crack propagation events followed by crack arrest along clearly visible arrest fronts.Analysis of the data obtained for the two slower loading rates that exhibited ductile fracture throughout the entire growth process shows that the energy release rate Gcd = 70 J/m2, and the final separation, δcd = 250 µm for the slowest loading rate of 0.03 mm/min and, Gcd = 120 J/m2, and the final separation, δcd = 250 µm for 0.3 mm/min loading rate. For the highest loading rate, which exhibited nominally brittle fracture, the energy release rate Gcb = 10 J/m2, and the final separation δcd = 1.5 µm. The peak cohesive stresses during ductile and brittle fracture are determined to be σd = 0.7 MPa and σb = 9.6 MPa, respectively, for loading rates of 0.3 mm/min and 3 mm/min, respectively. Finally, COD at the onset of crack growth measured at 1.00 mm behind the moving crack tip for the loading rate of 0.3 mm/min, CODd ≈ 0.30 mm, remaining constant for crack lengths, a, in the range 25 mm ≤ a ≤ 165 mm.