68 A. Tessema et al. 9.2 Specimen Preparation Laminates with three different stacking arrangements are used in this study. These laminates are made from carbon fiber prepregs provided by Toray Composite Inc. Each ply in the laminate is stacked using fiber placement technique. Once the prepreg is stacked in the required orientation and arrangement, it is wrapped with vacuum plastic bags and placed in the oven to cure. Eight plies Quasi-isotropic laminates with three different laminate stacking sequences are manufactured. The three ply arrangements of laminate are (0/ 45/90/C45)s, (0/ 45/C45/90)s and (0/90/C45/ 45)s. Rectangular coupon test samples are extracted from the cured laminates using a CNC water-jet cutting machine. The final coupons have a size of 150 22mm2 and thickness of 1.6 mm. The side edges of the coupons are polished with different grade of sandpapers to remove any kind of flaws (i.e. notches and indentations) that are formed during water-jet cutting. Following, fiberglass tabs are glued on the width face of the coupon at the two gripping ends, as shown in Fig. 9.1a, to protect the fiber damage from the gripping pressure of the testing machine. In order to facilitate the digital image correlation, black speckle patterns are made on the white background of the specimen surface where full-field strain is acquired. The speckles are made on two distinct regions of the specimen. The first one is on the thickness edge and these are micro scale with an approximate size of 6 16 m. The second one is on the width side of the samples and had an approximate size of 250 300 m. The speckles on edge side are used to capture the local residual strain concentration and damage formation across the plies. On the other hand, the speckles on the width side is used to obtain the laminate global average strain. 9.3 Test Setup Uni-axial tension test is conducted using Material Testing System (MTS 810) machine based ASTM D3479 standard. The MTS 810 machine is adjusted to perform displacement controlled loadings at a rate of 0.01 mm/min. The monotonic load is applied until the final failure of the specimen. As shown on Figs. 9.1b, 2D-DIC setup is applied for both speckled regions. These setup is used capture consecutive images of speckled regions during loading. For the micro speckled region that has an approximated area of interest 4 mm 1mm. A3 magnification lens is coupled with a 9 MPixels CCD (Grasshopper) camera to capture the images at a framing rate of 1fps. For the macro speckled region, the images are acquired simultaneously with the micro speckled region using 5MPixels CCD camera incorporated with 60 mm Nikon lenses. The images of the micro and macro speckle are post processed using VIC-2D software separately. Thereby, from the micro speckle the local full-field axial, transverse and shear strain data are obtained levels. Fig. 9.1 Schematic diagram of (a) Test specimen and speckles and (b) experimental setup
RkJQdWJsaXNoZXIy MTMzNzEzMQ==