Issue 40

Z.S. Metaxa et al, Frattura ed Integrità Strutturale, 40 (2017) 61-73; DOI: 10.3221/IGF-ESIS.40.06 67 interphase between the embedded fiber and surrounding matrix [19] that gives a sudden drop in ERC values as the fiber is partially unloaded. This pattern of sudden drops in ERC values can be noticed for even higher stress level and can be additionally due to micro-cracking [20] that approximately starts at the 30 % of the maximum bending stress (around 1 MPa) and fracture for higher applied stresses. (a) (b) Figure 7 : (a) Mechanical response of the prismatic specimens and electrical resistance response of the different embedded PVA-CNT fiber type (a) coated and (b) coated and annealed. Constant amplitude loading - unloading loops The test results for the constant amplitude tests will be described in this section. Ten (10) loading – unloading loops were performed with maximum bending load being 80 N that corresponds to approximately 0.5 MPa that is definitely within the elastic loading regime without the presence of micro-cracking. Fig. 8 shows the mechanical response of the type I specimen (stress in red color while strain in black color) as well as the ERC values of the embedded fiber (in blue color) over testing time. Despite the low maximum loading stress, residual strain values can be noticed after several cycles that can be attributed to developed damage inside the specimen. The strain loops seems to take low amplitude values; after the tenth loading loop strain amplitude goes from 1 to 2.5 % axial strain that is almost half when compared with the first loading loop. ERC values show increased scatter values that might be a problem of the high sampling frequency. The peak and valleys of the ERC values can be hardly distinguished in the diagram; however, the trend shows that ERC is continuously increasing with the constant amplitude loadings. Such a trend was also noticed for the case of monitoring glass fiber reinforced plastic with embedded PVA-CNT fiber and ERC was directly compared with the residual strain of the composites that is clearly evidence of induced damage in the matrix of the material [20]. This is obviously the case with the existing test results. Figure 8 : Mechanical response of the type I specimen with embedded coated PVA-CNT fiber under constant amplitude loading – unloading loops and electrical response of the embedded fiber. 0 50 100 150 200 0 1 2 3 4 Monotonic 4pt bending coated PVA-CNT fiber stress resistance change Axial strain at the bottom surface [με] Maximum bending stress [MPa] 0 1 2 3 4 5 6 Electrical resistance change Δ R / R 0 [%] 0 50 100 150 200 0 1 2 3 4 Monotonic 4pt bending coated and annealed PVA-CNT fiber stress resistance change Axial strain at the bottom surface [με] Maximum bending stress [MPa] 0,0 0,2 0,4 0,6 0,8 Electrical resistance change Δ R / R 0 [%]