Issue 35

E. Dall’Asta et alii, Frattura ed Integrità Strutturale, 35 (2016) 161-171; DOI: 10.3221/IGF-ESIS.35.19 165 If the maximum deformation to be measured is less than 30% the use of simple similarity cost function (like SAD) can be used without any trouble with SGM algorithms. However, with these approaches and as far as the displacements are limited, pixel-locking [25] effects, which sometimes occur during the parabolic sub-pixel fitting of the cost function minimization, can produce unwanted biased results, as is evident in some of the results obtained below). Since the 2D displacement search domain should be very limited to improve the computational time required for the analysis, the differences between consecutive frames must be as small as possible. Namely, the frame rate during the test should be small enough to ensure that the maximum displacement between consecutive frame in image space is limited to some (1÷5) pixels. If higher deformation rates are expected, SGM algorithms should be discarded: the use of more complex similarity functions that consider also a patch deformation model, even if in principle can be implemented, will make the entire process computational unfeasible. At the same time, the simplistic regularization function in (8) can be unsuitable to constrain the displacement field of the solution which, with high deformation, can show localized effects more likely. Providing appropriate regularization functions and penalization terms can be tricky and introduce biases in the final results. In this context we found that the use of other cost function (i.e. Census transform cost function [24]) improves to some extent the results. Anyway, in all these cases the use of LSM-like algorithms should be preferred: even with simple affine patch deformation model the accuracy obtainable is very good. The use of not too big templates (13 to 21 pixels) can provide affordable results, capturing localized effects with good computational times. A final remark concerns the possibility to help the matching algorithm through the change of the master (reference) image during the DIC processing: in this way, comparing more similar images should improve the matching algorithm performance (no matter if LSM or SGM is used). Even if, in principle, that is true, the user have to consider that, in this way, accumulating the displacements computed from every reference and slave image, also the errors accumulate; in the end, it is likely that the entire process becomes less accurate than the case where the master image is fixed. M ATERIALS AND SPECIMENS Asphalt Mastic sphalt mixture is a particulate composite material consisting of interspersed aggregates, asphalt binder and air voids. Its cracking behaviour is affected by its heterogeneity, specifically by the interaction between the aggregates and the mastic. The stiffness of the mastics affects the ability of the mixture to resist permanent deformation and their fracture and fatigue strength. Accurate description of strain evolution and distribution in mastics is essential for revealing significant information on the influence of microstructure properties on asphalt mixture macroscopic behaviour. Eight asphalt mastics with very different properties were used in this study. The cracking behaviour of the mastics is investigated using a Modified Direct Tension Test (MDTT) performed in a servo-hydraulic load frame to allow the material to stretch up to rupture. The specimen geometry, previously optimized to obtain uniform stress distribution within the specimen web and adequate gauge length for accurate strain measurements, is 106 mm long, 20 mm wide and 10 mm thick, with an effective gauge length of 46 mm. Stroke applied is 1.68 mm/s until rupture occurs. A digital Basler piA1600-35gm camera (resolution 1608x1308, focal length 8mm, pixel size 7.4 micrometers, 35 fps@max resolution), directly connected to the testing control system, is located on a support inside the climatic chamber where tests are performed. The chamber is provided with a proper LED lighting system which assures good illumination without heating up the specimen. Since the crack phenomenon is very fast and short-lasting (1-2 seconds), the camera is properly set up to acquire the images in a smaller area of the sensor (1600x500 pixel) reducing the bandwidth required for transmitting each frame and, consequently, allowing a higher frame rate (about 80 fps). Thanks to the elongated shape of the specimen, once provided an optimal imaging geometry, the reduced size of the images still allows the complete acquisition of the whole specimen surface. The images are automatically processed by the software, providing accurate displacement/strain fields. To achieve high accuracies in the strain field measurements, the specimen surface must present a well-contrasted grey scale speckle pattern, easily obtainable by a water paint-based treatment. Strains are obtained from DIC system, interpolating all the strain values of the grid points located at the 46x20 mm specimen central area. An example of the test configuration at failure is shown in Fig. 1(a). Plastic materials Polylactic acid or polylactide (PLA) is a biodegradable thermoplastic aliphatic polyester derived from renewable resources. PLA is commonly processed by 3D printing. In this work, PLA specimens are produced by a FFF (Fused Filament A