Nathan Salowitz

A Spider-Web-Like Highly Expandable Sensor Network for Multifunctional Materials

www.MaterialsViews.com C O M M A Spider-Web-Like Highly Expandable Sensor Network for Multifunctional Materials U N IC A By Giulia Lanzara , * Nathan Salowitz , Zhiqiang Guo , and Fu-Kuo Chang IO N The skin of living animals is an inspiration for the next generation of materials, devices, and structures. Human skin is sensitive to pressure, strain, and temperature; [ 1 , 2 ] dolphins or bats drastically reduce drag by sensing fl uid fl ow [ 3–5 ] and adapting either their skin shape [ 6 , 7 ] or rigidity; [ 8 ] and snakes use their skin to detect vibrations. [ 9 ] The fundamental common factor in these examples is that the living tissue is integrated with a network of distributed nanoor microscale sensors and actuators. To mimic such systems, novel materials and devices, such as paperlike displays, [ 10–12 ]

Bio-inspired stretchable network-based intelligent composites

The human skin hosts an array of sensors that are capable of detecting and interpreting many traits important to how we function and survive. The goal of mimicking this capability in composites to create intelligent composite materials has led to the development of a bio-inspired stretchable network composed of numerous micro-fabricated sensors capable of detecting multiple stimuli. The components of the network are small scale and flexible making the network embeddable within complexly shaped composite layups and flexible structures with minimal impact on the host structure. This paper outlines recent progress in ongoing work to develop the bio-inspired network in order to create intelligent composite materials.

Micro-fabricated, expandable temperature sensor network for macro-scale deployment in composite structures

We have developed methods for creating a highly expandable temperature sensor network for distributed temperature measurement. Stresses and strains due to network expansion are minimized through finite element analysis. Through the use of a uniquely patterned polyimide substrate and wire pattern an expansion ratio of 1,000% is achieved and the electrical resistance of components is maintained from pre-expansion to full expansion. Platinum resistance temperature detectors and electrodes are integrated directly in the polyimide-based network through a non-standard micro fabrication process. Calibration and interpolation algorithms have been developed for temperature measurement. Real-time distributed temperature measurement has been achieved through this sensor network, and it has shown great potential to be integrated into composites. 1 2

Microfabricated Expandable Sensor Networks for Intelligent Sensing Materials

Structural health monitoring (SHM) is a technology striving to enable automated evaluation of the health condition of structures. The SHM has recently attracted significant attention in the aerospace and civil infrastructure industries because of its potential to improve operational efficiency, reduce maintenance costs, and enhance the structural reliability in a real-time operation basis. The SHM is developing to include multiple types of sensors and even onboard processing for diagnostics and decision making. Advanced multidisciplinary engineering and manufacturing technologies are being developed enabling integration of sensors, network hardware, and semiconductors into structures with minimal parasitic effects. This is precisely the foundation for developing intelligent structures. This paper highlights recent developments in microfabricated expandable sensor networks for the SHM and intelligent structures at Stanford University. Fabrication and testing of microfabricated ultrasonic and temperature sensing systems in expandable networks are discussed. These advances applied to the SHM and intelligent structures support a paradigm change in design, manufacturing, and maintenance of structures. Successful implementation of the SHM will require a close collaborative effort among academia, government, and industry.

Recent advancements and vision toward stretchable bio-inspired networks for intelligent structures

Significant progress has recently been achieved in structural health monitoring, maturing the technology through quantification, validation, and verification to promote implementation and fielding of SHM. In addition, there is ongoing work seeking to detect damage precursors and to deploy structural health monitoring systems over large areas, moving the technology beyond hot-spot monitoring to global state sensing for full structural coverage. A large number of small sensors of multiple types are necessary in order to accomplish the goals of structural health monitoring, enabling increased sensing capabilities while reducing parasitic effects on host structures. Conventional sensors are large and heavy, adding to the weight of a structure and requiring physical accommodation without adding to and potentially degrading the strength of the overall structure. Increased numbers of sensors must also be deployed to span large areas while maintaining or increasing sensing resolution and capabilities. Traditionally, these sensors are assembled, wired, and installed individually, by hand, making mass deployment prohibitively time consuming and expensive. In order to overcome these limitations, the Structures and Composites Lab at Stanford University has worked to develop bio-inspired microfabricated stretchable sensor networks. Adopting the techniques of complementary metal-oxide semiconductor and microelectromechanical system fabrication, new methods are being developed to create integrated networks of large numbers of various micro-scale sensors, processors, switches, and all wiring in a single fabrication process. Then the networks are stretched to span areas orders of magnitude larger than the original fabrication area and deployed onto host structures. The small-scale components enable interlaminar installation in laminar composites or adhesive layers of built-up structures while simultaneously minimizing parasitic effects on the host structure. Additionally, data processing and interpretation capabilities could be embedded into the network before material integration to make the material truly multifunctional and intelligent once fully deployed. This article reviews the current accomplishments and future vision for these systems in the pursuit of state sensing and intelligent materials for self-diagnostics and health monitoring.

Structural Health Monitoring of High Temperature Composites

High-temperature polymer-matrix composites (PMCs) are necessary and critical for the development of supersonic aircraft and orbital re-entry vehicles because of the need for light-weight design, high strength-to-weight ratios and high thermal stability in structures. Damage detection is a primary concern in composite structures because they are prone to multiple damage forms that can be hidden within the structure. Damage can include matrix cracking, fiber breakage, and delamination which can be caused by impacts, fatigue, or overloading. To overcome these shortfalls highly damage tolerant structures are employed to improve the safety of structures. Unfortunately this requires additional, potentially unnecessary, structural weight which is detrimental to aerospace structures. Acoustic ultrasound based structural health monitoring (SHM) has demonstrated the ability to overcome these problems by using arrays of Lead Zirconate Titanate piezoelectric transducers typically mounted on a flex circuit all of which is permanently affixed to, or embedded within, a structure [1] [2] [3] [4]. These transducers can excite and detect ultrasonic wave propagation in the structure and diagnostic algorithms, interpreting the signals, have been developed enabling real time inspection for damage. However, modern SHM systems are not capable of surviving the high temperatures experienced in the fabrication and service of High-temperature polymer matrix composites. In particular the Lead Zirconate Titanate piezoelectric elements typically depolarize and lose their functionality at around 200°C [5] [6]. Additionally, current SHM diagnostic algorithms are dependent on baseline data to compare signals to. These signals change with temperature and even just a few degree change can be detrimental to the system’s abilities. The current method for enabling functionality over a range of temperatures is to take numerous sets of baseline data at very high resolution across a range of temperatures. In order to adapt SHM for high temperature composites new piezoelectric materials must be developed capable of surviving elevated fabrication and operational temperatures. Small scale network components must be integrated to reduce detrimental effects of embedding SHM systems within the composite layup [7] [8] [9]. Additionally, methods for reducing the number of baseline data sets in the diagnostic algorithms must be developed. This paper presents development and testing of Bismuth Scandium Lead Titanate piezo ceramic transducers for high temperature SHM. These transducers are incorporated into a stretchable network system and mounted on a glass backing. Functionality is tested using a commercially available data acquisition system designed for SHM and intended for use with PZT transducers. Ongoing development of temperature compensation algorithms is also presented herein.© 2011 ASME

Design and analysis of radially polarized screen-printed piezoelectric transducers

Piezoelectric transducers have applications from ultrasonic structural health monitoring to micro-electromechanical systems. Small physical size coupled with large actuation is desirable in many applications, requiring unique transducer designs to take advantage of the material properties. Screen-printed piezoceramics were developed as a means of mass producing mezzo-scale transducers that are geometrically small and light weight, but large enough to generate significant actuation. Screen-printed piezoceramic transducers display significantly different properties than chemically identical bulk ceramic elements, largely attributed to high void fraction of screen-printed piezoceramic materials and detrimental to the functionality of traditional transducer designs. This article presents analysis, simulation, and initial testing of new designs for screen-printed piezoceramic transducers with concentric through-thickness electrodes. Analytical models were developed enabling analysis across material properties and design parameters. Analytical results were verified against finite element models for some designs. Prototypes were created and underwent initial testing to assess the properties of the design.

Functionalization of stretchable networks with sensors and switches for composite materials

An investigation was performed to develop appropriate techniques to design and fabricate (using complementary metal-oxide semiconductor/micro-electro-mechanical systems technologies) highly stretchable networks of distributed sensors and organic diodes that could be stretched, and surface-mounted or embedded into polymeric materials to cover an area several orders of magnitude larger than its original size. Both analysis and experiments were performed to validate the design and fabrication methods. The techniques sought to reduce stresses due to network expansion, and a new spin-coated fabrication process was developed to enable high-resolution features in the network. Networks with temperature sensors and piezoelectric sensors were fabricated and tested to demonstrate functionality in advanced composite materials that are common in aircraft.