Marcias Martinez

Design and verification of a smart wing for an extreme-agility micro-air-vehicle

A special class of fixed-wing micro-air-vehicle (MAV) is currently being designed to fly and hover to provide range superiority as well as being able to hover through a flight maneuver known as prop-hanging to accomplish a variety of surveillance missions. The hover maneuver requires roll control of the wing through differential aileron deflection but a conventional system contributes significantly to the gross weight and complexity of a MAV. Therefore, it is advantageous to use smart structure approaches with active materials to design a lightweight, robust wing for the MAV. The proposed smart wing consists of an active trailing edge flap integrated with bimorph actuators with piezoceramic fibers. Actuation is enhanced by preloading the bimorph actuators with a compressive axial load. The preload is exerted on the actuators through a passive latex or electroactive polymer (EAP) skin that wraps around the airfoil. An EAP skin would further enhance the actuation by providing an electrostatic effect of the dielectric polymer to increase the deflection. Analytical modeling as well as finite element analysis show that the proposed concept could achieve the target bi-directional deflection of 30° in typical flight conditions. Several bimorph actuators were manufactured and an experimental setup was designed to measure the static and dynamic deflections. The experimental results validated the analytical technique and finite element models, which have been further used to predict the performance of the smart wing design for a MAV.

A Hybrid Structural Health Monitoring System for the Detection and Localization of Damage in Composite Structures

A hybrid structural health monitoring (SHM) system, consisting of a piezoelectric transducer and fiber optic sensors (FOS) for generating and monitoring Lamb waves, was investigated to determine their potential for damage detection and localization in composite aerospace structures. As part of this study, the proposed hybrid SHM system, together with an in-house developed algorithm, was evaluated to detect and localize two types of damage: a through thickness damage (hole of 2 mm in diameter) and a surface damage (2 mm diameter bore hole with a depth of 0.65 mm) located on the backside of the plate. The experiments were performed using an aircraft representative composite plate skin, manufactured from carbon fiber reinforced polymer (CFRP).

Single-walled carbon nanotube–modified epoxy thin films for continuous crack monitoring of metallic structures

Cracks are one of the primary forms of damage that can lead to the catastrophic failure of metallic structures. This study focuses on the application of epoxy nanocomposite thin film sensors for continuous monitoring of crack evolution in metallic structures. The core approach was to monitor the current (or resistance) change in these nanocomposite films, as cracks develop and propagate in the metallic host structure. Based on optical, electrical, and mechanical properties of epoxy resins modified with different contents of single-walled carbon nanotubes, two different nanocomposites (with 0.3 and 1.0 wt%) were chosen for the development of a crack sensor. The performance of the nanocomposite sensors was evaluated under tension–tension fatigue tests, on aluminum coupons with centrally located through thickness electrical discharge machining notches. Crack growth in the aluminum was found to transfer to the nanocomposite films in a stable mode. Once the crack was established, a linear correlation was found between the measured current and crack length with a slope of −10−11 and −10−8 A/mm for 0.3 and 1.0 wt% nanocomposites, respectively. Contact between the asperities formed on the crack surfaces in the nanocomposite film while the crack was closed at small loads (<30% of maximum load) was found to be an important limiting factor causing a large variation in measured currents during each fatigue cycle. Hence, a normalized variable based upon current change during each cycle was defined, providing a more accurate measurement of the crack size, with a crack gauge factor of ∼0.04 mm−1. In summary, the nanocomposite thin film sensor developed in this study offers both continuous crack growth monitoring and the possibility of strain sensing. The sensor is also suitable for visual inspection of the host structure due to the transparency of the developed nanocomposite film.

A Novel Approach to a Piezoelectric Sensing Element

Piezoelectric materials have commonly been used in pressure and stress sensors; however, many designs consist of thin plate structures that produce small voltage signals when they are compressed or extended under a pressure field. This study used finite element methods to design a novel piezoelectric pressure sensor with a C-shaped piezoelectric element and determine if the voltage signal obtained during hydrostatic pressure application was enhanced compared to a standard thin plate piezoelectric element. The results of this study demonstrated how small deformations of this C-shaped sensor produced a large electrical signal output. It was also shown that the location of the electrodes for this sensor needs to be carefully chosen and that the electric potential distribution varies depending on the poling of the piezoelectric element. This study indicated that the utilization of piezoelectric materials of different shapes and geometries embedded in a polymer matrix for sensing applications has several advantages over thin plate solid piezoelectric structures.

Finite Element Analysis of Broken Fiber Effects on Hollow Active Fiber Composites

The finite element method was used to study the performance of hollow active fiber composites (HAFC) and the effect of the fiber damage on this performance. The finite element model was developed for the simulation of the PZT-5H hollow fiber-epoxy matrix composite with single and dual electrode ‘bus’ systems. The simulations were performed for the actuation and sensing functions of the composites with healthy and broken fibers. The results of the healthy HAFC models were compared to the experimental data and numerical results found in the literature. The effect of broken fibers in the composite structure on both the actuation and sensing performance was studied. The results demonstrate that a gap in the fiber leads to actuation performance loss. The loss in the performance is directly related to the location of the fiber and its proximity to the electric potential source. The results also demonstrate that this loss could be minimized by applying a dual electrode system at both ends of the composite. In the sensing application utilizing HAFC, it was shown that these types of composites have several advantages over solid active fiber composites.

Damage quantification using smart patch system for hot spot monitoring

Fatigue crack detection and quantification is by far the most challenging task in Structural Health Monitoring (SHM). In the past decade numerous techniques were developed to detect and quantify fatigue damages. Fatigue loading leads to fatigue crack development in metals and delamination growth in composites. It has been found that different techniques are suitable for different damage development. Hence, the selection of the appropriate analysis methodologies pertaining to different problems is crucial. At the same time there has been an effort to reduce the power requirement for data analysis. This in turn triggered the idea of developing low power damage detection algorithms. In this paper a comparison between different damage detection techniques are presented and problems with different materials and structural geometries are considered. Three damage detection techniques were selected and evaluated.

When Conservation Meets Engineering: Predicting the Damaging Effects of Vibrations on Pastel Paintings

Pastel paintings are one of the most fragile types of objects of art. When handling loan requests, conservators lack scientific data to assess the risk for damage in transport, and thus for making decisions whether they can be transported. A research project was initiated in 2014 to investigate the effect of vibrations on the condition of pastel paintings, and to determine under what conditions they can be transported with minimum risk for damage due to vibrations. The initial results of this work indicate that the vibration behaviour of pastel paintings is a cumulative one and can be dealt with as an issue of fatigue. If failure is defined as a given level of unacceptable visual loss of pastel, it has been shown that higher stress amplitudes lead to shorter lives to failure than lower stress amplitudes. The use of a fixative appears to prolong fatigue life. There also appears to be a fatigue limit for freshly drawn pastels without fixative. This study highlights the synergism between typically non associated fields of research, in this, art conservation and the fatigue failure of materials.

Evaluation of mode II fatigue disbonding using Central Cut Plies specimen and distributed strain sensing technology

ABSTRACT The lack of a widely-accepted test standard for characterizing the mode II fatigue disbond growth behavior of adhesively bonded interfaces is a challenge to the research community in terms of producing consistent and repeatable results. Typically, researchers apply the End Notch Flexure specimen, which is already used for static delamination studies. However, the needs for static and fatigue disbond growth characterization are not the same, resulting in some undesirable effects in such specimen. This study looks at a particular mode II test configuration known as the Central Cut Plies (CCP) specimen. A critical evaluation of the suitability of this specimen, including the influence of geometry, disbond measurement approaches and the stability of the disbond growth is carried out through a combination of numerical and experimental investigations. A distributed strain sensing system based on Rayleigh Backscattering provided a surface strain profile from which disbond growth rate data was obtained. A finite element model was used to verify the experimental results and determine the disbond length from the strain profiles. Results of this evaluation have shown that the CCP specimen is a promising specimen configuration for characterizing fatigue disbond growth; however, it also presents several challenges that require consideration in its application.

In-Situ Characterization of Isotropic and Transversely Isotropic Elastic Properties Using Ultrasonic Wave Velocities

In this paper, a one-sided, in situ method based on the time of flight measurement of ultrasonic waves was described. The primary application of this technique was to non-destructively measure the stiffness properties of isotropic and transversely isotropic materials. The method consists of generating and receiving quasi-longitudinal and quasi-shear waves at different through-thickness propagation angles. First, analytical equations were provided to calculate the ultrasonic wave velocities. Then, an inverse method based on non-linear least square technique was used to calculate the stiffness constants using the ultrasonic wave velocities. Sensitivity analysis was performed by randomly perturbing the velocity data, thus observing the effects of perturbations on the calculated stiffness constants. An improved algorithm was proposed and tested to reduce the effects of random errors. Based on the sensitivity analysis, minimum number of angles required to inversely calculate the stiffness constants were suggested for isotropic and transversely isotropic material. The method was experimentally verified on an isotropic 7050-T7451 aluminum with two different thicknesses and a transversely isotropic composite laminate fabricated using 24 plies of CYCOM 977-2 12 k HTA unidirectional carbon fiber reinforced polymer (CFRP) prepregs. The results demonstrated that this technique is able to accurately measure the material properties of isotropic material. As for the transversely isotropic material, this method was able to accurately measure the material properties if the experimental errors can be reduced to less than 1 %.

Maintenance and monitoring of composite helicopter structures and materials

This chapter contains a brief description of current maintenance philosophies and processes that are in use by aircraft original equipment manufacturers (OEMs) and operators/maintainers as a means of reducing maintenance cost and increasing aircraft availability, while maintaining structural integrity. A description of the different types of damage found in composite structures is described. A case study on the maintenance of helicopter floorboards with recommendations regarding the different protection schemes investigated is presented. In addition, structural health monitoring (SHM) and non-destructive inspection (NDI) techniques being considered and currently in use by OEMs and operators in the field are briefly described for their ability to detect and quantify damage in composite structures. Finally, the future trends and technologies being developed by the SHM and NDI communities are discussed.