Andrei G. Anisimov

Strain characterization of embedded aerospace smart materials using shearography

The development of smart materials for embedding in aerospace composites provides enhanced functionality for future aircraft structures. Critical flight conditions like icing of the leading edges can affect the aircraft functionality and controllability. Hence, anti-icing and de-icing capabilities are used. In case of leading edges made of fibre metal laminates heater elements can be embedded between composite layers. However this local heating causes strains and stresses in the structure due to the different thermal expansion coefficients of the different laminated materials. In order to characterize the structural behaviour during thermal loading full-field strain and shape measurement can be used. In this research, a shearography instrument with three spatially-distributed shearing cameras is used to measure surface displacement gradients which give a quantitative estimation of the in- and out-of-plane surface strain components. For the experimental part, two GLARE (Glass Laminate Aluminum Reinforced Epoxy) specimens with six different embedded copper heater elements were manufactured: two copper mesh shapes (straight and S-shape), three connection techniques (soldered, spot welded and overlapped) and one straight heater element with delaminations. The surface strain behaviour of the specimens due to thermal loading was measured and analysed. The comparison of the connection techniques of heater element parts showed that the overlapped connection has the smallest effect on the surface strain distribution. Furthermore, the possibility of defect detection and defect depth characterisation close to the heater elements was also investigated.

3D shape shearography with integrated structured light projection for strain inspection of curved objects

Shearography (speckle pattern shearing interferometry) is a non-destructive testing technique that provides full-field surface strain characterization. Although real-life objects especially in aerospace, transport or cultural heritage are not flat (e.g. aircraft leading edges or sculptures), their inspection with shearography is of interest for both hidden defect detection and material characterization. Accurate strain measuring of a highly curved or free form surface needs to be performed by combining inline object shape measuring and processing of shearography data in 3D. Previous research has not provided a general solution. This research is devoted to the practical questions of 3D shape shearography system development for surface strain characterization of curved objects. The complete procedure of calibration and data processing of a 3D shape shearography system with integrated structured light projector is presented. This includes an estimation of the actual shear distance and a sensitivity matrix correction within the system field of view. For the experimental part a 3D shape shearography system prototype was developed. It employs three spatially-distributed shearing cameras, with Michelson interferometers acting as the shearing devices, one illumination laser source and a structured light projector. The developed system performance was evaluated with a previously reported cylinder specimen (length 400 mm, external diameter 190 mmm) loaded by internal pressure. Further steps for the 3D shape shearography prototype and the technique development are also proposed.

EXTREME shearography: development of a high-speed shearography instrument for measurements of the surface strain components during an impact event

This work presents the design and preliminary results of a high-speed shearography instrument in development for surface strain components measurements during an impact event. Composite materials are vulnerable to extreme dynamic loadings such as blade off events or foreign object damage as their mechanical properties are strain rate dependent. The development of new instruments to reconstruct extreme dynamic events will provide important insight into the understanding of the behaviour of composites. Shearography provides a quantitative measurement of the surface strain components with a high sensitivity as it is a non-contact interferometric technique. The current configuration of the shearography instrument realises measurements of the out-of-plane surface strain components during an impact using a double frame approach. The first experimental results reveal phase maps registered during an impact event with μs temporal resolution. Later the experimentally measured surface strain components will be used as input and validation data for new numerical and analytical models of the impact response of composites. The overall set of technical parameters of the developing shearography instrument makes it one of the most extreme applications of shearography for material characterisation.

Epoxy-hBN nanocomposites: A study on space charge behavior and effects upon material

The emergence of nano dielectrics for specialized high voltage applications sparked off a variety of research activities, which proved that nano-fillers are capable of improving the electrical, thermal and mechanical properties of polymers. This paper primarily investigates the effect of addition of hBN (hexagonal boron nitride) nanoparticles into an epoxy polymer base by increasing fill-grade, from 0.2 to 5 % by volume, from two different standpoints: (a) characterizing the electrical space charge (S.C.) accumulation threshold under DC electrical fields, and, (b) demonstrating the alterations in material properties of the modified polymeric materials, from the unfilled polymer. Objective (a) is experimentally investigated by the pulsed electro-acoustic (PEA) technique, well known for determining spatial charge distribution in dielectrics. Objective (b) is investigated by determining the ultrasonic velocity response of the modified composites and unfilled polymer. The obtained results suggest a relation between electrical threshold fields for S.C. accumulation & fill-grades, as well as the fact that incorporating stiff filler materials into brittle polymer bases leads to a tougher composite (capable of withstanding greater breaking stress levels), but with reduced ductility.

High-reflection microprismatic material as a base for passive reference marks in machine vision metrology applications

In this work it is shown that high-intensity microprismatic tapes have a potential to be used as a good substrate for bright and cheap fiducial marks in machine vision metrology applications. The drawback of the tapes is that they have technological netting pattern distributed across the surface. The proposed image processing technique allows good suppression of the parasitic technological netting pattern by a harmonic mean image filtering followed by circle shape recovering based on Fourier descriptors. It was also shown that the combination can provide good results in mark position estimations. In experiments it was shown that subpixel accuracy of position estimation can be achieved after applying proposed image processing, while without filtering the error can exceed 4 pixels in some cases.

Retroreflective microprismatic materials in image-based control applications

This work addresses accurate position measurement of reference marks made of retroreective microprismatic materials by image-based systems. High reflection microprismatic technology implies tiny hermetically sealed pockets, which improve material reflectivity, but result in non-reflective preprinted netting pattern. The mark pattern to be used for measuring can be simply printed on the reflective material as an opaque area with predefined shape. However, the non-reflecting pattern acts as a spatial filter that affects resultant spatial reflectivity of the mark. When an image of the mark is taken, the desired mark shape can be deformed by the netting pattern. This deformational may prevent accurate estimation of the mark position in the image. In this paper experimental comparison of three image filtering approaches (median filtering, morphological close and filtering in a frequency domain) in order to minimize the affection of the netting pattern is provided. These filtering approaches were experimentally evaluated by processing of the images of the mark that was translated in a camera field of view. For that a developed experimental setup including a camera with LED backlight and the mark placed on a translation stage was used. The experiment showed that median filtering provided better netting pattern elimination and higher accuracy of key features position estimation (approximately ±0.1 pix) in the condition of the experiment. The ways of future use of reference marks based on microprismatic material in image-based control applications are discussed.