Yu-Hung Li

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.

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.

Bio-Inspired Stretchable Absolute Pressure Sensor Network

A bio-inspired absolute pressure sensor network has been developed. Absolute pressure sensors, distributed on multiple silicon islands, are connected as a network by stretchable polyimide wires. This sensor network, made on a 4’’ wafer, has 77 nodes and can be mounted on various curved surfaces to cover an area up to 0.64 m × 0.64 m, which is 100 times larger than its original size. Due to Micro Electro-Mechanical system (MEMS) surface micromachining technology, ultrathin sensing nodes can be realized with thicknesses of less than 100 µm. Additionally, good linearity and high sensitivity (~14 mV/V/bar) have been achieved. Since the MEMS sensor process has also been well integrated with a flexible polymer substrate process, the entire sensor network can be fabricated in a time-efficient and cost-effective manner. Moreover, an accurate pressure contour can be obtained from the sensor network. Therefore, this absolute pressure sensor network holds significant promise for smart vehicle applications, especially for unmanned aerial vehicles.

Experimental Identification of Structural Dynamics and Aeroelastic Properties of a Self-sensing Smart Composite Wing

Self-sensing intelligent composite materials with state-sensing and awareness capabilities constitute the future of aerospace structures. The objective of this work is to develop technologies that will lead to the next generation of intelligent aerospace structures that can sense the environmental conditions and structural state, effectively interpret the sensing data to achieve real-time state awareness, and employ appropriate self-diagnostics under varying operational environments. In this paper, the design, integration, and experimental identification of the structural dynamics and aeroelastic properties are presented for an intelligent composite UAV wing. Bio-inspired stretchable sensor networks, including integrated piezoelectric, strain, and temperature sensors are monolithically embedded in the composite layup to provide the sensing capabilities. Stochastic signal processing and identification techniques are employed in order to accurately interpret the sensing. The experimental evaluation and assessment is demonstrated via a series of wind tunnel experiment under varying angles of attack and airflow velocities for the identification of the coupled airflowstructural dynamics and strain distribution. The obtained results demonstrate the successful integration of the micro-fabricated stretchable sensor networks with the composite wing, as well as the effectiveness of the stochastic data interpretation approaches. This study constitutes a significant step in proving the integration potential of the approach for the next generation of fly-by-feel UAVs. doi: 10.12783/SHM2015/163

A stochastic global identification framework for aerospace structures operating under varying flight states

Abstract In this work, a novel data-based stochastic “global” identification framework is introduced for aerospace structures operating under varying flight states and uncertainty. In this context, the term “global” refers to the identification of a model that is capable of representing the structure under any admissible flight state based on data recorded from a sample of these states. The proposed framework is based on stochastic time-series models for representing the structural dynamics and aeroelastic response under multiple flight states, with each state characterized by several variables, such as the airspeed, angle of attack, altitude and temperature, forming a flight state vector. The method’s cornerstone lies in the new class of Vector-dependent Functionally Pooled (VFP) models which allow the explicit analytical inclusion of the flight state vector into the model parameters and, hence, system dynamics. This is achieved via the use of functional data pooling techniques for optimally treating – as a single entity – the data records corresponding to the various flight states. In this proof-of-concept study the flight state vector is defined by two variables, namely the airspeed and angle of attack of the vehicle. The experimental evaluation and assessment is based on a prototype bio-inspired self-sensing composite wing that is subjected to a series of wind tunnel experiments under multiple flight states. Distributed micro-sensors in the form of stretchable sensor networks are embedded in the composite layup of the wing in order to provide the sensing capabilities. Experimental data collected from piezoelectric sensors are employed for the identification of a stochastic global VFP model via appropriate parameter estimation and model structure selection methods. The estimated VFP model parameters constitute two-dimensional functions of the flight state vector defined by the airspeed and angle of attack. The identified model is able to successfully represent the wing’s aeroelastic response under the admissible flight states via a minimum number of estimated parameters compared to standard identification approaches. The obtained results demonstrate the high accuracy and effectiveness of the proposed global identification framework, thus constituting a first step towards the next generation of “fly-by-feel” aerospace vehicles with state awareness capabilities.

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.

A self-diagnostic adhesive for monitoring bonded joints in aerospace structures

Bondline integrity is still one of the most critical concerns in the design of aircraft structures up to date. Due to the lack of confidence on the integrity of the bondline both during fabrication and service, the industry standards and regulations still require assembling the composite using conventional fasteners. Furthermore, current state-of-the-art non-destructive evaluation (NDE) and structural health monitoring (SHM) techniques are incapable of offering mature solutions on the issue of bondline integrity monitoring. Therefore, the objective of this work is the development of an intelligent adhesive film with integrated micro-sensors for monitoring the integrity of the bondline interface. The proposed method makes use of an electromechanical-impedance (EMI) based method, which is a rapidly evolving approach within the SHM family. Furthermore, an innovative screen-printing technique to fabricate piezoelectric ceramic sensors with minimal thickness has been developed at Stanford. The approach presented in this study is based on the use of (i) micro screen-printed piezoelectric sensors integrated into adhesive leaving a minimal footprint on the material, (ii) numerical and analytical modeling of the EMI spectrum of the adhesive bondline, (iii) novel diagnostic algorithms for monitoring the bondline integrity based on advanced signal processing techniques, and (iv) the experimental assessment via prototype adhesively bonded structures in static (varying loads) and dynamic (fatigue) environments. The proposed method will provide a huge confidence on the use of bonded joints for aerospace structures and lead to a paradigm change in their design by enabling enormous weight savings while maximizing the economic and performance efficiency.

Epitaxial Growth of Atomically Flat Spin Dependent Tunneling Junctions

Spin dependent tunneling junctions with epitaxially grown underlayers have been investigated to examine the possibility of achieving very flat and uniform barrier layers. Pt/Ni 80 Fe 20 /Fe 50 Mn 50 /Ni 80 Fe 20 layers were deposited on sapphire (0001) substrates at different temperatures and monitored by in-situ reflection high energy electron diffraction (RHEED). The surface morphology has been found to depend strongly on the growth temperature. X-ray diffraction and magnetic hysteresis loop measurements were also performed to characterize the film structures