Onorio Iervolino

A spiral passive electromagnetic sensor (SPES) for wireless and wired structural health monitoring

A major goal of structural health monitoring (SHM) in the past decade has been to improve crack detection and monitoring while reducing maintenance and installation costs. This would normally require placing many sensors over a large area, powering and interrogating them. On the other hand, operational aspects such as the temperature effects, battery life, and weight penalties have fundamental roles in the sensor design. In addition, small dimension of the sensors, low cost, and non-contact measurement system for data retrieval are very often required. We present a non-destructive evaluation/structural health monitoring (NDE/SHM) sensor that can be remotely interrogated without any wiring for data transmission or power supply. A spiral passive electromagnetic sensor (SPES) was designed and fabricated. The sensor is a planar 2D inductor circuit of scalable size that resonates at a characteristic frequency when exposed to an electromagnetic field. The specific frequency is dependent on the inductance of the inductor, its parasitic capacitance and resistance, and the electrical properties of the surrounding area. A change in a materials permittivity or permeability due to damage can be sensed through the SPES device. The sensor was tested by using a passive wireless resonant telemetry scheme and a wired interrogation method. Both conductive (i.e. carbon fiber) and non-conductive (i.e. fiber glass) structures were monitored showing very promising capabilities and accuracy in detecting defects/damage in composite structures. The use of the proposed sensor eliminates the need for on-board power and exposed interconnects, reduces the instrumentation mass and volume, increases the reliability due to the continuous operation even in case of a damaged sensor, and increases the life of the device.

Spiral passive electromagnetic sensor (SPES) for smart sensing and de-icing

The objective of this work was to develop a wireless Spiral Passive Electromagnetic Sensor (SPES) to monitor the complex permittivity of a surrounding medium. The sensor is a self-resonating planar pattern of electrically conductive material. Investigation were conducted to demonstrate the capability of the SPES to monitor humidity and temperature gradients, and acting as an ice protection tool. An oscillating signal is used to interrogate remotely the sensor with a single loop antenna or wiring it directly to a spectrum analyser and monitoring the backscattering signal. The excited sensor responds with its own resonant frequency, amplitude and bandwidth that can be correlated to physical quantities to be monitored. Our studies showed the capability of the sensor to monitor temperature and humidity changes in composite materials and uniformly produce induction heating when the conductive path is activated by an external electric power supply that can be used for deicing of aircraft structures.

Design, fabrication, and validation of passive wireless resonant sensors for NDT/SHM

1. Detection of visible crack, delamination etc. in composite structures can be fulfilled by several techniques. However, the problem is of greater complexity in the case of nonvisible defects such as barely visible impact damage and microcracks. The objective of this research work was to create and validate a low cost smart-sensor for NDT and structural health monitoring (SHM) to be used for complex geometries. The smart-sensor presents a dual function, i.e. it determines the presence of delamination and cracks within the cross-section but it also provides information on surface damages due to fatigue or impacts. In the latter case the damage could induce the breakage of the sensor that could still work with a different resonant frequency. The sensor utilizes a passive wireless resonant telemetry scheme based on an inductor capacitor (LC). The use of a passive system eliminates the need for onboard power and exposed interconnects, increasing the life of the device and the reliability due to the continuous operation even in case of damage results from the sensor. The sensor design, the signal processing and the experimental setup that validate the remote interrogation of the antenna sensor are presented. Two different designs were investigated, one for conductive surface and one for nonconductive surface (fiberglass-composite).

A new membrane-type metamaterial for multiple peaks absorption at low frequencies

In the recent years, various membrane-type acoustic metamaterials were developed for low frequency sound absorption. However, a membrane absorber usually requires a large back cavity to achieve low frequency sound absorption and on the other hand, in order to guaranty a multiple peaks absorption decorated membrane resonators or membrane with multiple magnetic negative stiffness cell shall be considered. This research proposes a new concept of membrane-type metamaterial which can achieve multiple peaks and broadband absorption at low frequencies. The basic concept behind the design of the elementary cell is associated to the vibro-acoustic behavior of the structure. In fact, the maximum sound absorption is related to the symmetrical mode of the membrane, so playing with the geometry, the mass and the stiffness of the membrane the eigenfrequencies can be tuned easily in the prescribed frequency range. At same time local increase of strain energy around geometrical discontinuity or around discontinuity associated to the material properties may lead a gain in sound absorption. A mono-layer membrane structure is presented where the geometrical shape and material properties distribution in terms of density and stiffness in the elementary cell are optimized in order to manipulate the vibro-acoustic properties and maximize the absorption at required frequencies. To optimize the geometry and the vibro-acoustic properties of the proposed metamaterial, finite element simulation were carried out. The numerical model was then validated using experimental measurements. A preliminary prototype was tested into an impedance tube test ring and the normal sound absorption was measured following the transfer function approach and compared with the numerical results

Multifunctional Skin based on Spiral Passive Electromagnetic Sensor for Efficient Structural Health Monitoring of Composite Structures

The objective of this research work was to create and validate a low-cost smart-sensor for non-destructive evaluation (NDE) and structural health monitoring (SHM) to be easily embedded or surface mounted on complex aerospace composite skin structures. A Spiral Passive Electromagnetic Sensor (SPES) was designed and fabricated for damage detection. The sensor is a planar two-dimensional inductor circuit of scalable size that resonates at a characteristic frequency when exposed to an electromagnetic field. A change in a materials permittivity or permeability due to the presence of flaws within the structure can be sensed with the SPES by monitoring its resonant response. The devices described herein has already demonstrated, in previous works, the ability to detect flaws within conductive (i.e. carbon fibre) and non-conductive (i.e. fibre glass) structures[1, 2], [3, 4]. Moreover, the SPES can act as de-icing device and detect temperature and humidity variation [5, 6]. A study was conducted to investigate the capability of sensor to give information on the location and on the size of delamination. Multiple sensors were embedded in a smart-layer together with the antenna to create a sensing skin able to monitor composite structures. The sensor use a passive wireless resonant telemetry scheme and/or a wired interrogation method. The use of a passive system eliminates the need for on-board power and exposed interconnects, reduce the instrumentation mass and volume, increasing the life of the device and the reliability due to the continuous operation even in case the damage affect the sensor itself. The sensor presents a great potential to monitor defects in composite structure, acting as SHM sensor. The technique illustrated is able to locate and quantify the presence of damage, giving information on the defect before it causes a failure of the structure.

Wearable Spiral Passive Electromagnetic Sensor (SPES) glove for sign language recognition of alphabet letters and numbers: A preliminary study

Sign language is a method of communication for deaf-mute people with articulated gestures and postures of hands and fingers to represent alphabet letters or complete words. Recognizing gestures is a difficult task, due to intrapersonal and interpersonal variations in performing them. This paper investigates the use of Spiral Passive Electromagnetic Sensor (SPES) as a motion recognition tool. An instrumented glove integrated with wearable multi-SPES sensors was developed to encode data and provide a unique response for each hand gesture. The device can be used for recognition of gestures; motion control and well-defined gesture sets such as sign languages. Each specific gesture was associated to a unique sensor response. The gloves encode data regarding the gesture directly in the frequency spectrum response of the SPES. The absence of chip or complex electronic circuit make the gloves light and comfortable to wear. Results showed encouraging data to use SPES in wearable applications.

Bioinspired twisted composites based on Bouligand structures

The coupling between structural support and protection makes biological systems an important source of inspiration for the development of advanced smart composite structures. In particular, some particular material configurations can be implemented into traditional composites in order to improve their impact resistance and the out-of-plane properties, which represents one of the major weakness of commercial carbon fibres reinforced polymers (CFRP) structures. Based on this premise, a three-dimensional twisted arrangement shown in a vast multitude of biological systems (such as the armoured cuticles of Scarabei, the scales of Arapaima Gigas and the smashing club of Odontodactylus Scyllarus) has been replicated to develop an improved structural material characterised by a high level of in-plane isotropy and a higher interfacial strength generated by the smooth stiffness transition between each layer of fibrils. Indeed, due to their intrinsic layered nature, interlaminar stresses are one of the major causes of failure of traditional CFRP and are generated by the mismatch of the elastic properties between plies in a traditional laminate. Since the energy required to open a crack or a delamination between two adjacent plies is due to the difference between their orientations, the gradual angle variation obtained by mimicking the Bouligand Structures could improve energy absorption and the residual properties of carbon laminates when they are subjected to low velocity impact event. Two different bioinspired laminates were manufactured following a double helicoidal approach and a rotational one and were subjected to a complete test campaign including low velocity impact loading and compared to a traditional quasi-isotropic panel. Fractography analysis via X-Ray tomography was used to understand the mechanical behaviour of the different laminates and the residual properties were evaluated via Compression After Impact (CAI) tests. Results confirmed that the biological twisted structures can be replicated into traditional layered composites and are able to enhance the out-of-plane properties without a dangerous degradation of the in-plane properties.

Spiral Passive Electromagnetic Sensor (SPES) for composite structural changes in aircraft structures

A major goal of structural health monitoring (SHM) is to provide accurate and responsive detection and monitoring of flaws. This research work reports an investigation of SPES sensors for damage detection, investigating different sensor sizes and how they affect the sensor’s signal. A sensor able to monitor structural change that can be remotely interrogated and does not need a power supply is presented in this work. The SPES-sensor presents the great advantage of monitoring conductive and non-conductive structures such as fiberglass-reinforced composites (FRC) and carbon fiber-reinforced polymers (CFRP). Any phenomena that affect the magnetic field of the SPES can be detected and monitored. A study was conducted to investigate the capability of sensor to give information on structural changes, simulated by the presence of an external mass placed in the proximity of sensor. Effect of different positions of the SPES within the sample, and how to extend the area of inspection using multiple sensors was investigated. The sensor was tested embedded in the samples, simulating the structural change on both sides of the sample. In both configurations the sensor described herein demonstrated a great potential to monitor structural changes.