binder phase resulted in a different response. It is to be noted that the effective change in the macroscale piezoresistivity of CNT-polymer nanocomposites is believed to be attributed in large part to the rearrangement, i.e. either disruption or formation, of conductive networks of CNTs within the matrix material and the associated electron hopping in neighboring CNTs, i.e. tunneling resistance change rather than the intrinsic individual CNT piezoresistivity, i.e. CNT resistance change under deformation. 0.5 wt% MWNTs-ammonium perchlorate-epoxy hybrid composites exhibited an increase in the relative resistance change for very low values of applied strains (less than 0.2 %), which can be attributed to local deformations of the inherently piezoresistive MWNT-epoxy binder in absence of interfacial damage. However, the relative resistance change also exhibited a non-monotonic behavior. This observation might be explained by several factors. It is to be noted that the electrical conductivity measurements shows that 0.5 wt% MWNTs-ammonium perchlorate-epoxy hybrid composites are well above percolation threshold. There may not be a disruption in conductive pathways within the local polymer binder as the interfaces separation between surrogate crystals and binder due to saturated MWNT nanotube networks. Complex rearrangements of the current conductive paths makes the hybrid composite exhibit either no change in relative resistance and/or slight reduction in relative resistance change under applied strains due to formation of new conductive paths. Furthermore, for such composites with granular composite microstructure, it is difficult to constrain the developing microcracks to initiate in the gauge section. Microcracks can develop outside of the gauge section, which relax the local stresses in the gauge section. The effective resistance can reduce because of local relaxation of the specimen within the gauge section. 23.4 Conclusion This preliminary study performs the experimental investigation of MWNT-ammonium perchlorate-epoxy hybrid composites to assess their microstructure morphology, effective conductivity and deformation based effective piezoresistive response. The current exploration of nanocomposite bonded polymer surrogate explosives includes several key observations. Neat epoxy-ammonium perchlorate samples are non-conductive and addition of MWNTs increases the effective conductivity of the hybrid composite. The stress-strain response of hybrid composites undergoes a brittle failure going through linear elastic behavior, formation of microcracks leading slight reductions in load carrying capacity and finally macrocracks result in eventual failure. Incorporating MWNTs into local polymer binder improves the effective stiffness about 42 % compared to neat ammonium perchlorate-epoxy samples. Moreover, it is observed that the relative resistance change of MWNT-ammonium perchlorate-epoxy samples captures very low values of applied strains (less than 0.2 %). Complex rearrangements of the current conductive paths makes the hybrid composites exhibit either no change in 0 0.05 0.1 0.15 0.2 0.25 0 2 4 6 8 10 12 0 0.002 0.004 0.006 0.008 0.01 0.012 DR/R0 Stress, MPa Strain, mm/mm Neat 0.5 wt% MWNT S1 0.5 wt% MWNT S2 0.5 wt% MWNT S3 Fig. 23.8 Stress-strain and relative change in resistance-strain response of as-produced the neat ammonium perchlorate-epoxy and MWNTsammonium perchlorate-epoxy hybrid composites (S1, S2, S3 are sample #) 200 E.C. Sengezer and G.D. Seidel
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