7.5 Experimental results of instrumented composite specimen with nylon/silver sensor

Three tensile tests were performed with the composite specimen incorporated with sensor wire. The mechanical response including Young's modulus and yield strength of the tested samples showed that it was not affected by insertion of sensor in the sample and the sensor did not act as intrusive element. The mechanical response of composite specimen and electrical response of the sensor were correlated simultaneously. The resistance was changed at the same time as the failure started to initiate, and, as the test progressed, at the point of failure, the resistance of the sensor started to increase and eventually went to infinity, Figure 12. The sensor was reporting what was going on as the fracture formed. The results were

Figure 11. 3D discrete model (a) before failure and (b) after failure.

where σ is the stress due to damage response, D is the damage variable, and σ ́is

The nylon monofilament coated with silver thin film was subjected to tensile elongation until failure. Results showed that it was viable to use one filament to validate the piezo-resistive behavior of untwisted coated yarn. The true stress-strain behavior showed a good correlation with the experimental results in the elasticplastic region, Figure 9. It can be seen in the results that it was fine to use coated monofilament model to verify the result because the plane of stress is same. However, there is a slight difference in the failure initiation and breakage, which is understandable because: in experimental results, the failure shows gradual breakage of all the monofilaments, whereas in the numerical model, the set of monofilaments is modeled by a single thread. Electrical response was recorded as electrical current density (ECD) in Abaqus which was then converted to resistance response using Eqs. (4)‑(6) to validate the experimental piezo-resistive behavior of sensor wire.

Electromechanical behavior of the monofilament is shown in Figure 10.

<sup>J</sup> <sup>¼</sup> <sup>E</sup> ρ

<sup>J</sup> <sup>¼</sup> <sup>α</sup>E with <sup>α</sup> <sup>¼</sup> <sup>1</sup>

<sup>R</sup> <sup>¼</sup> <sup>ρ</sup><sup>L</sup>

where J is the current density, E is the electric field, α is the electrical conductivity, ρ is the resistivity, L is the length, A is the cross-sectional area, and R is the

It was observed that till the plastic region, the electrical resistivity of the yarn changed, but this change in resistance was very small as compared to change in resistance on damage when there was complete breakage in current flow. No gradual increase in the resistance was seen like in experimental results because of the monofilament model. The 3D discrete model of coated monofilament before and

Numerical verification of experimental mechanical behavior of Ag-coated untwisted nylon yarn.

) <sup>J</sup><sup>∝</sup> <sup>1</sup> ρ ρ

<sup>A</sup> (6)

(4)

(5)

the stress due to undamaged response.

Advances in Structural Health Monitoring

7.4 Verification of sensor response

resistance.

Figure 9.

106

after failure is shown in Figure 11.

Figure 12. Electrical response of the sensor with mechanical behavior of composite specimen.

very promising and piezo-resistive flexible sensor was responding very well to the mechanical change in the composite specimen.
