Fabrizia Ghezzo

Acoustic cloaking transformations from attainable material properties

We propose a general methodology and a set of practical recipes for the construction of ultra-broadband acoustic cloaks—structures that can render themselves and a concealed object undetectable by means of acoustic scattering. The acoustic cloaks presented here are designed and function analogously to electromagnetic cloaks. However, acoustic cloaks in a fluid medium do not suffer the bandwidth limitations imposed on their electromagnetic counterparts by the finite speed of light in vacuum. In the absence of specific metamaterials having arbitrary combinations of quasi-static speed of sound and mass density, we explore the flexibility of continuum transformations that produce approximate cloaking solutions. We show that an imperfect, eikonal acoustic cloak (that is, one which is not impedance matched but is valid in the geometrical optics regime) with negligible dispersion can be designed using a simple layered geometry. Since a practical cloaking device will probably be composed of combinations of solid materials rather than fluids, it is necessary to consider the full elastic properties of such media, which support shear waves in addition to the compression waves associated with the acoustic regime. We perform a systematic theoretical and numerical investigation of the role of shear waves in elastic cloaking devices. We find that for elastic metamaterials with Poissons ratio > 0.49, shear waves do not alter the cloaking effect. Such metamaterials can be built from nearly incompressible rubbers (with 0.499) and fluids. We expect this finding to have applications in other acoustic devices based on the form-invariance of the scalar acoustic wave equation.

Onset of Resin Micro-Cracks in Unidirectional Glass Fiber Laminates with Integrated SHM Sensors: Experimental Results:

This article presents the results of experiments conducted in order to identify and locate the failure initiation in glass fiber/epoxy laminates with integrated structural health monitoring sensors (SHM) and electronics. Recent advances in health monitoring technologies have resulted in the development of micro-dimensional devices that can be embedded into composite laminates. Notwithstanding their small size, such inclusions may affect the response of the composite. Damage induced by the peak values of stress concentration around the embedded inclusion is, in fact, one of the main concerns in smart structures technology. To address this specific issue, unidirectional S2 glass fiber/epoxy laminated composites are fabricated with embedded small implants that mimic potential sensors and microprocessors. Quasi-static tensile tests are then performed on those samples while monitoring them by the acoustic emission (AE) technique. Additionally, the microstructure of the material with and without implants is explored. The AE results show that early low-medium amplitude events are detected at the implant location and the micrographic inspections reveal that micro-cracks initiate at the device-composite matrix interface and grow around the implant causing the debond of the external component from the surrounding resin system.

Development and Characterization of Healable Carbon Fiber Composites with a Reversibly Cross Linked Polymer

Carbon fiber reinforced polymer (CFRP) laminates with remendable cross-linked polymeric matrices were fabricated using a modified resin transfer mold (RTM) technique. The healable composite resin, bis-maleimide tetrafuran (2MEP4F), was synthesized by mixing two monomers, furan (4F) and maleimide (2MEP), at elevated temperatures. The fast kinetic rate of the reaction of polymer constituents requires a fast injection of the healable resin into the carbon fiber preform. The polymer viscosity as a function of time and temperature was experimentally quantified in order to optimize the fabrication of the composite material and to guarantee a uniform flow of the resin through the reinforcement. The method was validated by characterizing the thermo-mechanical properties of the polymerized 2MEP4F. Additionally, the thermo-mechanical properties of the remendable CFRP material were studied.

Integration of Networks of Sensors and Electronics for Structural Health Monitoring of Composite Materials

The low-cost, widespread availability and robust nature of current electronic devices suggest the feasibility of creating a composite structure with integrated networked sensors to monitor in real time the life of civil and aerospace structures while in service conditions. For structures that need to survive to high number of life cycles under varying load-environmental conditions, it is of crucial importance that the strength, stiffness, endurance, and general load-bearing capabilities of the composite not to be severely degraded by the integrated networked components. Therefore, design tools must be developed to achieve optimized, safe, and reliable structures. High values of stress concentrations due to the presence of a rigid device within a highly anisotropic material can trigger the initiation of microcracks in the resin matrix. To quantify these effects, the acoustic emission technique is used to characterize the initiation of microfailures within laminated composites with integrated electronics.

Mechanical properties of composite materials with integrated embedded sensor networks

We present efforts to develop structural composite materials which include networks of embedded sensors with decision-making capabilities that extend the functionality of the composite materials to be information-aware. The next generation of structural systems will include the capability to acquire, process, and if necessary respond to structural or other types of information. We present work related to the development of embedded arrays of miniature electronic-based microsensors within a structural composite materials, such as GFRP. Although the scale and power consumption of such devices continues to decrease while increasing the functionality, the size of these devices remain large relative the typical scale of the reinforcing fibers and the interlayer spacing. Therefore, the question of the impact of those devices on the various mechanical properties is relevant and important. We present work on characterizing some of those effects in specific systems where sensors, or suitable dummy sensors, are arrayed with ~1 cm spacing between elements. The typical size of the microelectronic sensing element is ~1 mm, and here is orthorhombic. Of particular importance are the effects of inclusion of such devices on strength or fatigue properties of the base composite. Our work seeks to characterize these effects for 1 and 2 dimensional arrays lying in planes normal to the thickness direction in laminated composites. We also seek to isolate the effects due to the sensing elements and the required interconnections that represent the power-carrying and data communications capabilities of the embedded network.

Onset of Resin Micro-cracks in Unidirectional Glass Fiber Laminates with Integrated SHM Sensors: Numerical Analysis

The embedment of micro-sensors and micro-devices into composite laminates for structural health monitoring systems leads to stress/strain concentrations due to geometrical and material discontinuities around such embedded inclusions, with high potential to initiate premature failures. This article presents the efforts to estimate the effects of these stress/strain concentrations induced by the integration of rectangular-shape sensors within unidirectional fiber-glass composites. The micro-crack initiation sites and the failure load are predicted using finite-element simulations. Good agreement has been found between the numerical results and the experimental findings presented in an accompanying paper.

Integration of sensing networks into laminated composites

We summarize the methodology that we have used to address integrating sensing network into composite materials for structural self diagnosis. First, we have examined the effect of stress concentration that arises due to the embedment of sensors and external devices on the strength and endurance of laminated glass fiber composites. To analyze the mechanical response of the composite material under study subjected to in-plane or impact loads, we have fabricated a series of samples, with and without embedded (dummy) sensors/micro-processors, using S2 glass fiber/epoxy, and have characterized their response by acoustic emission. Guided by the corresponding results, we can select sensors and other necessary components in such way as to minimize the impact of the embedded electronics on the material integrity and, at the same time, to implement acoustic sensing monitoring functionalities within the material. A 4-tree hierarchical network of PVDF sensors capable of acquiring signals typically related to resin micro cracking phenomena has been developed and partially integrated into a cross ply laminate. The achieved results and ongoing research will be discussed.

Optimization of the mechanical properties of composite materials with integrated embedded sensor networks

The increasing demand for in-service structural health monitoring has stimulated efforts to integrate self and environmental sensing capabilities into materials and structures. To sense damage within composite materials, we are developing a compact network microsensor array to be integrated into the material. These structurally-integrated embedded microsensors render the composite information-based, so that it can monitor and report on the local structural environment, on request or in real-time as necessary. Here we present efforts to characterize the structural effects of embedding these sensors. Quasi-static three-point bending (short beam shear) and fatigue three-point bending (short beam shear) tests are conducted in order to characterize the effects of introducing sensors, or suitable dummy sensors in the form of chip resistors, and commonly used circuit board material, namely G-10/FR4 Garolite on the various mechanical properties of the host structural composite material. Furthermore, various methods and geometries of embedding the microsensors are examined in order to determine the technique that optimizes the mechanical properties of the host composite material. The work described here is part of an ongoing effort to understand the structural effects of integrating microsensor networks into a host composite material.

Passive damage detection in composite laminates with integrated sensing networks

The initiation and propagation of damage in composite laminates generate Acoustic Emission. The use of real time AE monitoring has been quite extensive for in-service composite structures. In the present work, experimental and numerical studies were performed to characterize the acoustic wave propagation in thin glass/epoxy composite plates. Experimentally obtained and simulated emission signals were used to identify and locate the source of the acoustic wave. Signal processing algorithms and a passive damage diagnosis system based on AE techniques were proposed for continuously monitoring and assessing the structural health of composite laminates. The local sensing and distributed processing features of the sensor system result in a decreased demand for bandwidth and lower computational power needed at each node.

Self-Healing Materials as New Biologically Inspired Materials

Lightweight, high strength fibre-reinforced polymeric composites are leading materials in many advanced applications including biomedical components. These materials offer the feasibility to incorporate multi functionalities due to their internal architecture, heterogeneity of materials and the flexibility of combining them using currently available fabrication methods. In spite of the excellent properties of these materials, their failure is still a questionable and not well predicted event. Delamination, debonding and micro-cracks are only some of the failure mechanisms that affect the matrices of polymer based composites. More complex cases exist with the combination of multiple failure mechanisms. In such cases a self-repairing mechanism that can be auto-triggered in the matrix material once the crack has been formed, would be very beneficial for all the applications of these materials, reducing maintenance costs and increasing their safety and reliability. Self-healing materials have been studied for more than a decade by now, with the specific objective of reducing the risks and costs of cracking and damage in a wide range of materials. Different approaches have been taken to create such materials, depending on the kind of material that needs to be repaired. The most popular methods developed for polymers and polymer reinforced composites are considered in this review. These methods include materials with micro-capsules containing a healing agent, and composites with matrices that can self-heal the cracks by repairing the broken molecular links upon external heating. While the first approach to healing has been widely used and studied in the past decade, in this review we focus on the second approach since less is reported in the literature and more difficult is the development of the materials based on such a method.