Mark Melnykowycz

Performance of integrated active fiber composites in fiber reinforced epoxy laminates

Active fiber composite (AFC) composed of lead zirconate titanate (PZT) fibers with interdigitated electrodes (IDEs) has been integrated into orthotropic glass fiber reinforced plastic (GFRP) laminates to characterize the performance of AFC as a smart material component in laminated materials. Monotonic cyclic tensile loading was performed on integrated specimens at different strain levels. The AFC output was monitored to determine the effect of applied strain level on the AFC performance. It was found that the AFC sensitivity degraded beyond strains of 0.20% and approached a minimum at 0.50% strain. The degradation in the AFC performance appears to be attributed to the dominating effect of PZT fiber fragmentation during testing, as opposed to depolarization. Acoustic emission (AE) monitoring was used to detect damage in laminates during testing and was correlated with crack evidence from microscopy observations during testing to characterize damage evolution in response to strain levels.

Piezoelectric Fiber Composites as Sensor Elements for Structural Health Monitoring and Adaptive Material Systems

Piezoelectric Active Fiber Composites (AFC) and Macro Fiber Composites (MFC) have the potential to provide various sensor functions for nondestructive test methods. AFC have been integrated into fiber-reinforced laminates as a first step towards structures with sensing capability. These developments constitute initial stages for developing adaptive composite structures or structures with integrated health monitoring system. So far, the use of AFC and MFC has been explored in selected nondestructive tests for defect detection in model composite systems on laboratory scale with e.g., Acoustic Emission, Acousto-Ultrasonics, and Electromechanical Impedance testing. The present article will focus on limitations and current prospects for structural health monitoring with AFC or MFC and discuss selected concepts and approaches.

Comparison of Piezoresistive Monofilament Polymer Sensors

The development of flexible polymer monofilament fiber strain sensors have many applications in both wearable computing (clothing, gloves, etc.) and robotics design (large deformation control). For example, a high-stretch monofilament sensor could be integrated into robotic arm design, easily stretching over joints or along curved surfaces. As a monofilament, the sensor can be woven into or integrated with textiles for position or physiological monitoring, computer interface control, etc. Commercially available conductive polymer monofilament sensors were tested alongside monofilaments produced from carbon black (CB) mixed with a thermo-plastic elastomer (TPE) and extruded in different diameters. It was found that signal strength, drift, and precision characteristics were better with a 0.3 mm diameter CB/TPE monofilament than thick (∼2 mm diameter) based on the same material or commercial monofilaments based on natural rubber or silicone elastomer (SE) matrices.

Numerical Stress Investigation for Piezoelectric Elements with a Circular Cross Section and Interdigitated Electrodes

The scientific community has put significant effort into the development and optimization of sensors and actuators manufactured as piezoelectric composites with interdigitated electrodes (IDEs), well known as active fiber composite (AFC) and macro fiber composite (MFC). The advantages of these elements are their higher actuation performance and flexibility as compared to monolithic piezoceramic (PZT) elements. In general, their mechanical properties are calculated based on the classical lamination theory and the uniform field model (UFM). These two theories are well suited for predicting the stiffness and piezoelectric strain constants of the AFC or MFC. Although there are a variety of numerical investigations related to their electromechanical properties, there are no appropriate tools for accessing the stresses within these piezoelectric elements (including the inhomogeneous electric field conditions as well as the change in material properties). Explanations are given for this situation indicating the problems in investigating these types of piezoelectric elements with respect to stress states. In this work a finite element modeling approach is presented, which shows the influence of the IDE on the mechanical properties of PZT fibers. Experimental evidence is presented, which affirms the location of critical stress predicted in this model and explains the reported cracking in AFC in past research.

Packaging of active fiber composites for improved sensor performance

Active fiber composites (AFC) composed of lead zirconate titanate (PZT) fibers embedded in an epoxy matrix and sandwiched between two interdigitated electrodes provide a thin and flexible smart material device which can act as a sensor or actuator. The thin profiles of AFC make them ideal for integration in glass or carbon fiber composite laminates. However, due to the low tensile limit of the PZT fibers, AFC can fail at strains below the tensile limit of many composites. This makes their use as a component in an active laminate design somewhat undesirable. In the current work, tensile testing of smart laminates composed of AFC integrated in glass fiber laminates was conducted to assess the effectiveness of different packaging strategies for improving AFC sensor performance at high strains relative to the tensile limit of the AFC. AFC were encased in carbon fiber, silicon, and pre-stressed carbon fiber to improve the tensile limit of the AFC when integrated in glass fiber laminates. By laminating AFC with pre-stressed carbon fiber, the tensile limit and strain sensor ability of the AFC were significantly improved. Acoustic emission monitoring was used and the results show that PZT fiber breakage was reduced due to the pre-stressed packaging process.

Piezoresistive Soft Condensed Matter Sensor for Body-Mounted Vital Function Applications.

A soft condensed matter sensor (SCMS) designed to measure strains on the human body is presented. The hybrid material based on carbon black (CB) and a thermoplastic elastomer (TPE) was bonded to a textile elastic band and used as a sensor on the human wrist to measure hand motion by detecting the movement of tendons in the wrist. Additionally it was able to track the blood pulse wave of a person, allowing for the determination of pulse wave peaks corresponding to the systole and diastole blood pressures in order to calculate the heart rate. Sensor characterization was done using mechanical cycle testing, and the band sensor achieved a gauge factor of 4–6.3 while displaying low signal relaxation when held at a strain levels. Near-linear signal performance was displayed when loading to successively higher strain levels up to 50% strain.

Active fiber composites: optimization of the manufacturing process and their poling behavior

The scientific community has put significant efforts into the manufacturing and optimization of sensors and actuators made of piezoelectric fibres with interdigitated electrodes, well known as Active Fibre Composites (AFC). A great advantage of such AFC is their flexibility and the possibility to integrate them into composite structures. In the current study an approach of optimizing the manufacturing process as well as the polarization of AFCs utilizing piezoelectric Lead-Zirconate-Titanate (PZT) fibres embedded in an epoxy matrix between interdigital Electrodes (IDE) screenprinted on Kapton will be discussed. During the poling process, an electric field is applied over the interdigitated electrodes of the AFC to its piezoelectric fibres along the fibre axis. One of the most important parameters of this polarization is, beside temperature and time, the applied voltage. An increase of the electric field results in an increase of the AFCs performance as shown by free-strain measurements. The manufacturing process developed and used at Empa consists of laminating the piezoelectric fibres in an epoxy matrix between the electrodes. An essential goal of this lamination, carried out in a hot press, is to get a proper contact between piezo fibres and the electrode. By adding soft layers between the Kapton foil and the mould, the interdigitated electrodes are deformed by each single fibre and therefore build up a contact area which in its cross section can be described by a contact angle. This optimization of the manufacturing process is also shown by free strain measurements of the AFC.

Additive Manufacturing of Piezoelectric 3-3 Composite Structures

Fused deposition of ceramics (FDC) is an additive manufacturing technique, were thermoplastic filaments are used for the fabrication of intricate structures that are difficult or impossible to produce with traditional techniques. This processing technique is utilized in the domestic and industrial appliance market. However, this simple and cheap manufacturing method is not widely used for the fabrication of traditional, high performance or functional ceramics. The objective of this contribution is to manufacture grid structures by means of FDC and subsequently investigate their electromechanical properties. Highly loaded ceramic - EVA (ethyl vinyl acetate) based filament is produced, as a feedstock for FDC. After successful printing and sintering of the grid structures, ferroelectric investigations were performed. Moreover, the grid structures are impregnated with a polymer resin resulting in a piezoelectric 3-3 composite that can be used as a hydrophone in under water noise detection.

Piezoresistive Carbon-based Hybrid Sensor for Body-Mounted Biomedical Applications

For body-mounted sensor applications, the evolution of soft condensed matter sensor (SCMS) materials offer conformability andit enables mechanical compliance between the body surface and the sensing mechanism. A piezoresistive hybrid sensor and compliant meta-material sub-structure provided a way to engineer sensor physical designs through modification of the mechanical properties of the compliant design. A piezoresistive fiber sensor was produced by combining a thermoplastic elastomer (TPE) matrix with Carbon Black (CB) particles in 1:1 mass ratio. Feedstock was extruded in monofilament fiber form (diameter of 300 microns), resulting in a highly stretchable sensor (strain sensor range up to 100%) with linear resistance signal response. The soft condensed matter sensor was integrated into a hybrid design including a 3D printed metamaterial structure combined with a soft silicone. An auxetic unit cell was chosen (with negative Poissons Ratio) in the design in order to combine with the soft silicon, which exhibits a high Poissons Ratio. The hybrid sensor design was subjected to mechanical tensile testing up to 50% strain (with gauge factor calculation for sensor performance), and then utilized for strain-based sensing applications on the body including gesture recognition and vital function monitoring including blood pulse-wave and breath monitoring. A 10 gesture Natural User Interface (NUI) test protocol was utilized to show the effectiveness of a single wrist-mounted sensor to identify discrete gestures including finger and hand motions. These hand motions were chosen specifically for Human Computer Interaction (HCI) applications. The blood pulse-wave signal was monitored with the hand at rest, in a wrist-mounted. In addition different breathing patterns were investigated, including normal breathing and coughing, using a belt and chest-mounted configuration.

Soft Condensed Matter Hybrid Fiber Sensors for Vital Function Monitoring

Textile band structures with integrated soft condensed matter sensor (SCMS) can be used as a vital function monitor device to detect pulse wave and breathing on the human body. A textile an elastic band was used as a support material and the U-shaped SCMS fiber sensor was bonded on the surface with elastic band with a liquid rubber bonding material. The sensor signal and gauge factor of the textile sensor structure was investigated using tensile testing experiments. The resistivity of the sensor structure increased linearly within a strain of 10 to 50%, and a slope of 8 (kOhm/% strain) could be detected. The sensor had a gauge factor of 4-5 from 10 to 50% between strain. Using the integrated SCMS sensor textile band around the chest, it was possible to detect talking, normal breathing and coughing. In collaboration with Rice University the textile sensor was tested for proof-of-concept for use in a battery-powered monitor for apnea of premature infants.