A simple approach for the fabrication of passive, wireless sensing elements and its application in structural health monitoring

Structural health monitoring and non-destructive evaluation of engineering structures in military and civilian applications is vital for ensuring safety and durability. Detection of cracks is an important aspect of health surveillance since catastrophic failure in structures is often preceded by the formation of cracks. Most of the available crack detection techniques are not commercially feasible because of the complex fabrication and interrogation processes and the high costs involved. There is a demand for wireless sensors for structural health monitoring that can be fabricated using low-cost processes. This thesis describes a simple and cost-effective approach for the fabrication of passive, wireless sensing elements that can be applied for the detection of surface-breaking cracks in structures.Passive inductor-capacitor (LC) sensors are well-known and have been used in various wireless telemetry applications. However, large-scale deployment of devices has been a challenge due to the lack of low-cost, high-volume fabrication processes. In this thesis, the fabrication of passive circuits for wireless structural health surveillance using the additive process of screen printing is presented. The viability of the approach has been demonstrated by using the fabricated sensors for the detection of microcracks. These sensors can be wirelessly interrogated even in the absence of a clear line of sight. Screen-printable, conductive silver inks have been specifically formulated for printing the LC elements. The inks can be printed on a variety of flexible (polymers) and rigid (ceramic) substrates. The absence of polymeric/resinous binders in the inks leads to circuits with high quality factors at the frequencies of interrogation (RF) and also enables fabrication on low-cost, temperature-sensitive polymeric substrates due to low-temperature curing. This approach of printing passive, wireless circuits and their application in the detection of cracks paves the way for low-cost structural health monitoring of a variety of substrates including composites and ceramics.