Issue 40

Z.S. Metaxa et al, Frattura ed Integrità Strutturale, 40 (2017) 61-73; DOI: 10.3221/IGF-ESIS.40.06 68 Fig. 9 shows the respective test results for the type II specimen with the surface attached fiber. Strain measurements also show a small reduction on the amplitude after some test cycles; ERC measurements seems to be vague and definitely lower sampling rate should have been used. Though the surface-attached fiber gives readings during mechanical testing, optimization will be performed in the near future regarding the preparation of the surface, medium for the proper attachment of the fiber [19], as several vague recordings might be due to local loss of coherency of the interphase of the connecting medium (glue) with the substrate and therefore loading transfer of the specimen to the sensor is not optimal. Figure 9 : Mechanical response of the Type II specimen with surface attached coated PVA-CNT fiber under constant amplitude loading – unloading loops and electrical response of the attached fiber. Incrementally increasing loading - unloading loops In this section the test results of continuous increasing loading – unloading loops will be reported. Tests were performed with continuous increasing maximum load of 40 N after every unloading and hereafter will be called as progressive damage accumulation tests. Typical applied bending force protocol over testing time can be seen in Fig. 10a, while Fig. 10b shows the typical mechanical response (stress-strain) of a specimen for the fourteen cycles till fracture. Residual strain values are noticed every single unloading and the higher the loading value is, the higher the residual axial strain is. Additionally, hysteresis loops are evident for the latest loading cycles. (a) (b) Figure 10 : (a) Applied increasing loading – unloading protocol and (b) mechanical response per different loading loop under four point bending test. Fig. 11 shows the comparison of the mechanical response from readings of the embedded FOBG (red curves) as well as form the attached surface strain gauge (black curves). For the extremely low loading values (Figs. 11a to c), actually no 0 200 400 600 800 1000 1200 0 100 200 300 400 500 600 14th loading 2nd loading 1st loading 3rd loading 4th loading 5th loading 6th loading 7th loading 8th loading 9th loading 10th loading 12th loading 13th loading 11th loading Applied bending force [N] Testing time [sec] 0 50 100 150 200 250 300 350 400 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 fracture 4th loading 5th loading 6th loading 7th loading 8th loading 9th loading 10th loading 11th loading 12th loading 13th loading Axial stress at the bottom surface [MPa] Axial strain at the bottom surface [με]