Visualization of strain concentrations in composites using advanced image processing techniques

Owing to high tensile strength and toughness even at high temperatures, titanium (Ti) alloys hold great promise for applications in automobiles, aircraft, spacecraft, etc. To investigate their instability behaviours and failure mechanisms, it is indispensable to detect the strain concentration locations before microscale crack occurrence non-destructively [1]. In this study, the microscale deformation distributions of two Ti-6Al-4V specimens were measured by advanced digital image correlation (DIC) and sampling Moiré [2, 3] techniques complementarily to visualize the strain concentrations in tensile tests. The geometric profiles of the two Ti-6Al-4V specimens are the same as presented in Fig. 1(a). The thickness and the minimum width were 1 mm and 1.8 mm, respectively. The difference was that there was a prefabricated notch on specimen #1 to narrow the area of the stress concentration (Fig. 1(b)), while no prefabricated defect existed on specimen #2 (Fig. 1(c)). The deformation distributions of specimen #1 in a small region near the notch were measured by the DIC method, and those of specimen #2 in a large area were measured by a developed sampling moiré method [3]. On specimen #1, the notch with width of 5 μm and length of 100 μm was produced by focused ion beam milling. A 500-nm-pitch grid pattern was fabricated by electron beam lithography in a small area of 500×500 μm² around one end of the notch (Fig. 1(b)). The tensile test was carried out in a Quanta scanning electron microscope, and a series of grid images were recorded when the magnification was 10000× during the test. The deformation distributions in 23×18 μm² near the notch were analysed by the DIC method [1]. Figs. 2(a)-2(d) show an example of the grid image and the distributions of the x-direction strain, the y-direction strain and the shear strain on specimen #1 under 511 MPa. Strain concentrations are observable near the notch from the strain distributions. The x-direction strain is maximum and the y-direction strain is minimum along the oblique line from the middle of the end of the notch. For specimen #2 without any prefabricated defect, because it is not known where the strain concentration happens easier before measurement, a 3-μm-pitch grid was fabricated in a large area of 1.8×15 mm² by UV nanoimprint lithography. The tensile test was performed under a Lasertec laser scanning microscope, and grid images were collected when the magnification of the objective lens was 10×. Full-field deformation distributions in a large area were measured by the developed sampling moiré method. Strain concentrations were found out in a square region near the specimen edge labelled in Fig. 1(c). The grid image in the square region of 219×204 μm² and the x-direction, y-direction and shear strain distributions were illustrated in Figs. 2(e)-2(f) taking the case when the tensile stress was 604 MPa as an example. The absolute values of the x-direction strain and the shear strain are maximum at a prior β grain boundary perpendicular to the tensile direction. The y-direction strain is minimum at lower parts of the grain boundary, but the absolute value is smaller. Our experiments have verified that a crack occurs along the oblique line of strain concentration on specimen #1, and an incipient crack emerges along the strain concentration line at the grain boundary on specimen #2 under a greater tensile load. It indicates that strain concentrations visualized using image processing enables accurate prediction of microscale crack occurrence. Summarily, microscale strain concentrations of Ti alloys were visualized in a small region around a notch root and in a large area from the strain mapping using DIC and Moiré methods complementarily. The crack occurrence locations were successfully predicted and slip lines in oblique angles were found to arise in tensile tests.