A fibre optic sensor design is proposed for simultaneously measuring the 3D stress (or strain) components and temperature inside thermo hardened composite materials. The sensor is based on two fibre Bragg gratings written in polarisation maintaining fibre. Based on calculations of the condition number, it will be shown that reasonable accuracies are to be expected. First tests on the bare sensors and on the sensors embedded in composite material, which confirm the expected behaviour, will be presented.
Digital image processing methods represent a viable and well acknowledged alternative to strain gauges and
interferometric techniques for determining full-field displacements and strains in materials under stress. This
paper presents an image adaptive technique for dense motion and strain estimation using high-resolution speckle
images that show the analyzed material in its original and deformed states. The algorithm starts by dividing the
speckle image showing the original state into irregular cells taking into consideration both spatial and gradient
image information present. Subsequently the Newton-Raphson digital image correlation technique is applied
to calculate the corresponding motion for each cell. Adaptive spatial regularization in the form of the Geman-
McClure robust spatial estimator is employed to increase the spatial consistency of the motion components
of a cell with respect to the components of neighbouring cells. To obtain the final strain information, local
least-squares fitting using a linear displacement model is performed on the horizontal and vertical displacement
fields. To evaluate the presented image partitioning and strain estimation techniques two numerical and two real
experiments are employed. The numerical experiments simulate the deformation of a specimen with constant
strain across the surface as well as small rigid-body rotations present while real experiments consist specimens
that undergo uniaxial stress. The results indicate very good accuracy of the recovered strains as well as better
rotation insensitivity compared to classical techniques.
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.
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.
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 mechanisms 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 dynamic (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 setup was combined with pattern
projection (moire) technique to study the time history of the out-of-plane 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 high 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 calibration based on previous shape measurements of several different
objects with known dimensions.
In recent years digital image processing techniques have become a very popular way of determining strains and
full-field displacements in the field of experimental mechanics due to advancements in image processing techniques
and also because the actual process of measurement is simpler and not intrusive compared to traditional sensor
based techniques. This paper presents a filtering technique which processes the polar components of the image
displacement fields. First, pyramidal gradient-based optical flow is calculated between blocks of each two frames
of a speckle image sequence while trying to compensate in the calculation small rotations and shears of the
image blocks. The polar components of the resulting motion
vectors - phase and amplitude - are then extracted.
Each of the motion vector angle values is smoothed temporally using a Kalman filter that takes into account
previously calculated angles located at the same spatial position in the motion fields. A subsequent adaptive
spatial filter is used to process both the temporally smoothed angles and amplitudes of the motion field. Finally,
test results of the proposed method being applied to a speckle image sequence that illustrates plastic materials
being subjected to uniaxial stress and to artificial data sets are presented.
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.
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.
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.
Fringes projection and speckle shear interferometry are used for testing of subjected to cycling loading (pressure) composite vessel. As the sensitivity of the applied methods could vary in broad limits in comparison with the other interferometric techniques, the inspection is realized in a wide dynamic range. Two spacing phase stepping fringes projection interferometry is applied for absolute coordinate measurement. Derivatives of in-plane and out-of-the-plane
components of the displacement vector over the object surface are obtained by lateral speckle shear interferometry in static loading (pressure). Non-linear mechanical response and fatigue of composite material are clearly detected after cyclic sinusoidal loading by macro measurement using lateral speckle shear interferometry. Fringes projection and speckle-shear interferometry are suitable for shape and normal displacements measurements in a wider dynamic range. The other advantage of the shown methods is connected with the possibility to realize compact and portable devices for in-situ inspection of investigated objects - machine parts and constructions.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.