Chung,2001b)in the stress direction during repeated compressive loading (Wen and Chung, 2002a,b).The effect is particularly large when the cement paste contains short steel fibers (8 um diameter,0.18 vol.%and polyvinyl alcohol (0.16 vol.%)In this case,the longitu- dinal piezoelectric coupling coefficient is 2.5 X 10-1mV-!and the piezoelectric voltage coefficient is 1.1 X 10-3m2C-I(10kHz)(Wen and Chung,2002a). 8 Damage sensing Figure 10.6 (Bontea et al.,2000)shows the fractional change in longitudinal resistance, strain and stress during repeated compressive loading at increasing and decreasing stress amplitudes for carbon fiber concrete at 28 days of curing.The highest stress amplitude is 60%of the compressive strength.A group of cycles in which the stress amplitude increases cycle by cycle and then decreases cycle by cycle back to the initial low stress amplitude is hereby referred to as a group.Figure 10.6 shows the results for three groups.The strain returns to zero at the end of each cycle for any of the stress amplitudes,indicating elastic behavior.The resistance decreases upon loading in each cycle,as in Fig.10.2.An extra peak at the maximum stress of a cycle grows as the stress amplitude increases,resulting in two peaks per cycle.The original peak(strain induced)occurs at zero stress,while the extra peak (damage induced)occurs at the maximum stress.Hence,during loading from zero stress within a cycle,the resistance drops and then increases sharply,reaching the maximum resist- ance of the extra peak at the maximum stress of the cycle.Upon subsequent unloading,the resistance decreases and then increases as unloading continues,reaching the maximum resistance of the original peak at zero stress.In the part of this group where the stress 6.0 0.0005 品 4.0 0.0003 2.0 0.0001 0.0 -0.0001 2.0 -0.0003 .0 -6.0 -0.0005 Time(s) Figure 10.6 Fractional change in resistance and strain during repeated compressive loading at increasing and decreasing stress amplitudes,the highest of which was 60%of the compressive strength,for carbon fiber concrete at 28 days of curing(Bontea et al., 2000). ©2003 Taylor&FrancisChung, 2001b) in the stress direction during repeated compressive loading (Wen and Chung, 2002a,b). The effect is particularly large when the cement paste contains short steel fibers (8 m diameter, 0.18 vol. %) and polyvinyl alcohol (0.16 vol. %). In this case, the longitu￾dinal piezoelectric coupling coefficient is 2.5  1011mV1 and the piezoelectric voltage coefficient is 1.1  103m2C1 (10 kHz) (Wen and Chung, 2002a). 8 Damage sensing Figure 10.6 (Bontea et al., 2000) shows the fractional change in longitudinal resistance, strain and stress during repeated compressive loading at increasing and decreasing stress amplitudes for carbon fiber concrete at 28 days of curing. The highest stress amplitude is 60% of the compressive strength. A group of cycles in which the stress amplitude increases cycle by cycle and then decreases cycle by cycle back to the initial low stress amplitude is hereby referred to as a group. Figure 10.6 shows the results for three groups. The strain returns to zero at the end of each cycle for any of the stress amplitudes, indicating elastic behavior. The resistance decreases upon loading in each cycle, as in Fig. 10.2. An extra peak at the maximum stress of a cycle grows as the stress amplitude increases, resulting in two peaks per cycle. The original peak (strain induced) occurs at zero stress, while the extra peak (damage induced) occurs at the maximum stress. Hence, during loading from zero stress within a cycle, the resistance drops and then increases sharply, reaching the maximum resist￾ance of the extra peak at the maximum stress of the cycle. Upon subsequent unloading, the resistance decreases and then increases as unloading continues, reaching the maximum resistance of the original peak at zero stress. In the part of this group where the stress Figure 10.6 Fractional change in resistance and strain during repeated compressive loading at increasing and decreasing stress amplitudes, the highest of which was 60% of the compressive strength, for carbon fiber concrete at 28 days of curing (Bontea et al., 2000). Fractional change in resistance (%) Strain Time (s) 0.0005 0.0003 0.0001 –0.0001 –0.0003 –0.0005 6.0 4.0 2.0 0.0 –2.0 –4.0 –6.0 0 500 1,000 1,500 2,000 500 1,000 1,500 2,000 © 2003 Taylor & Francis