numero25

B. Moreno et alii, Frattura ed Integrità Strutturale, 25 (2013) 145-152 ; DOI: 10.3221/IGF-ESIS.25.21 150 Figure 6 : Strain strain field after applying (LHS) 27500 and (RHS) 30000 cycles. Fig. 7 shows the vertical displacement map as well as a vertical displacement profile along crack opening direction. Fig. 7 was obtained by rotating the camera some 60º clockwise so that the crack appears horizontal in the image. A drastic change due to the existence of a discontinuity is observed in the displacement profile (RHS in Fig. 7). Therefore, after applying 33500 cycles (Fig. 7) it is possible to distinguish the crack. At this stage, the crack can be distinguished as a discontinuity in the vertical displacement field (RHS in Fig. 7). The crack is also visible by looking at the painted surface because of the paint being cracked around the fatigue crack mouth. Rotation of the camera also allows easier identification of the crack as a mode I (opening mode) crack. Due to the discontinuity introduced by the crack, the algorithm is not able to provide displacement information just above and below the crack plane, since displacements in these positions are very different. This behaviour is most probably due to the fact that the crack in the metal is longer than that in the paint. When the crack is very short (up to approximately 30000 cycles in the current specimen), the paint is elastically deformed without being cracked. For longer crack lengths (from 33500 cycles onwards) the crack opening displacement in the metal is so large that elastic deformation of the paint is not enough to accommodate such large displacement. This causes the paint to break around the region close to the metal crack mouth. The fact that the crack length shown by the paint is smaller than the real crack length in the metal is studied in Fig. 8. Fig. 8 shows a superposition of the metal surface after being cleaned (i.e. with no paint) and vertical strain fields calculated by DIC. Comparison between Figs. 7 and 8 makes clear that actual crack length in the metal (approximately 3.2 mm) is much longer than that in the paint (approximately 0.86 mm). Therefore, with the current arrangement, it is not possible to directly measure the correct crack length in the speckled surface. Figure 7 : (LHS) Vertical displacement field and (RHS) displacement profile along a vertical line (shown in white) after applying 33500 cycles on the specimen