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Satin woven composites can be seen as a series of connected unit cells with a definite length and width. Along the length and width of one unit cell, local strain fields can vary significantly due to the yarn interlacing pattern. Embedded Type I FBGs with a 80 μm- and 125 μm cladding diameter are used to study the internal transversal strain variations in unloaded and tensile loaded thermo-plastic 5-harness satin weave composite test specimens. The residual strains are examined at different (axial) load levels up to 300MPa and 1.2million cycles. Differential transversal strains up to 0.07% are presented. Results show that the embedded FBGs are capable of measuring long term the transversal strain distributions. It is found that the yarn interlacing pattern of the satin weave composite causes complex and random local strain fields during loading and that, even though axial strain measurements are matching very well with external strain measurements, a large scatter exists in local transversal strain states between the different composite test-coupons. more...

Protection of buildings and critical public infrastructure against blast load has been recently improved by retrofitting glass windows with a safety film. As the exact physical mechanis ms of the interaction between glass and safety film are not quite well understood, intensive research is conducted on the properties of this assembly. The loadings on the glass/film assembly are typically dynami c (blast, wind pressure, impact), so the lab tests are done on a drop weight set-up, where a mass is falling on a retrofitted glass plate. In this work, the drop weight s etup was combined with pattern projection (moire) technique to study the time history of the out-of-p lane deformations of the glass/film assembly. The fringe pattern, projected on the back side of the specimen, was generated by means of a sinusoidal phase grating under divergent high intensity infrared illumination. The whole process was recorded with a hi gh speed camera. Local routines based on Fast Fourier Transform were used to process the captured images, and to extract the phase. The exact out-of-plane displacements were calculated by means of calibratio n based on previous shape measurements of several different objects with known dimensions. Keywords: drop weight, pattern proj ection, Fourier analysis more...

The goal of this study is to apply photoelasticity for analysis of the mechanical behavior of thermoplastic polyester window security films. More specifically, the change of the photoelastic pattern for film samples with mechanical stress concentrators (holes and cracks) under tensile load is observed. For the purpose, the samples are covered with photoelastic birefringent PhotoStress® coatings. Being subjected to a tensile external load, the film transfers the strains over its surface to the coating. The latter are observed as interference fringes - isochromatic fringes, which exhibit the difference of the principal strains and isoclinic fringes, which characterize principal strains orientation. The photoelastic measurements were performed with a circular reflection polariscope with a white light and monochromatic illumination (575 nm). In the second case, two-loads phase-shifting technique is used for digital retrieval of isochromatics and isoclinics by successive acquisition of two pairs of four fringe patterns obtained at four different configurations of the optical elements of the polariscope. Since the specifics of the studied thermoplastic materials requires simultaneous recording of the phase-shifted patterns in the non-linear part of their loading curve, we propose an optical arrangement for real-time recording as a future development for solution of non-linear dynamic tasks. more...

We present a novel type of optical strain gage. The strain gage consists of a thin polyimide foil with an integrated optical circuit. The strain sensing elements are optical microresonators. The optical response to strain of these microresonators is a wavelength shift of the resonance wavelength. The optical circuit includes several of these resonators to measure strain in different directions. The strain sensor is read-out using a single-mode optical fiber. Because the different microresonators in the optical circuit have different resonance wavelengths, they can be read out using the same fiber. Our strain sensor is some kind of a cross between electrical resistance foil gages and fiber Bragg grating (FBG) sensors. It is a thin foil device, with a thickness of a few tens of micrometers, but it is an optical device and can be read out in a similar way as FBG sensors. We present the working principle, fabrication and first experimental results. more...

For the new generation aircraft families, the use of fibre-reinforced plastics is considered for the leading edge of the wings. However, this leading edge is very prone to bird strike impact. This paper presents the use of the projection moire technique to measure the out-of-plane deflections of composite plates subject to bird strike. Very strict constraints with regard to: (i) high speed image acquisition, (ii) vibrations of the impact chamber, and (iii) projection and observation angles - complicated substantially the development of the set-up. Moreover, the high frame rates (12000 fps) required a very intensive illumination. In the optimized configuration, a specially designed grating with gradually changing period is projected by means of special Metal Hydride lamps through one of the side windows of the impact chamber onto the composite plate riveted in a steel frame. The digital high speed camera is mounted on the roof of the impact chamber and records through a mirror the object surface with the projected fringe pattern on it. Numerical routines based on Local Fourier Transform were developed to process the digital images, to extract the phase and the out-of-plane displacements. The phase evaluation is possible due to the carrier frequency nature of the projected moire pattern. This carrier frequency allows separation of the unwanted additive and multiplicative fringe pattern components in the frequency domain via the application of a proper mask. The numerical calculations were calibrated for the bird strike of an aluminium plate, where the plastic deformation could be checked after the test. more...

This paper discusses a new sensor design based on optical fibre Bragg gratings which is being developed in the framework of the MASSFOS-project ('Multi-Axial Stress and Strain sensing of thermo hardened composite elements using Fibre Optic Sensors'-project). The objective of this ESA co-funded project is to develop a monitoring system which measures dynamically the multi-axial stress and strain plus temperature in thermo hardened composite elements. The sensor consists of a 'High Birefringence'-fibre in which two gratings have been inscribed; each grating yields two distinct Bragg peaks. The first grating is sensitive to the total stress field in the material, while the second one is isolated from transverse stress components. By measuring the four Bragg peaks of the sensor, it is theoretically possible to determine the total strain field, plus the temperature inside a composite material. Static experiments (a tensile test and a compressive test) have already been carried out to prove the feasibility of the sensor embedded in a composite laminate. more...