Design and Integration of Flexible Sensor Matrix for in Situ Monitoring of Polymer Composites.

Sensory polymer composites are highly desirable for applications such as in situ and real-time production processes and structural health monitoring, and for technologies that include human-machine interfaces for the next generation of Internet of Things. However, the development of these materials is still in its infancy: these materials have been reported, but the large-scale fabrication of polymer composites with versatile and customizable sensing capabilities has yet to be demonstrated. Here, we report on a scalable fabrication strategy that enables such materials by designing and integrating PCB technology-inspired large-area flexible sensor matrices into polymer composites. The integrated sensor matrices successfully monitored in situ the production processes and structural health of an industrial polymer composite: from the application of vacuum, resin flow and polymerization, production defects, and temperature distribution. Our results demonstrate that the proposed strategy is a simple and effective solution as a distributed monitoring platform for polymer composites and shows the potential toward next generation of sensory polymer composites.

RTM Production Monitoring of the A380 Hinge Arm Droop Nose Mechanism: A Multi-Sensor Approach.

This research presents a case study of production monitoring on an aerospace composite component: the hinge arm of the droop nose mechanism on the Airbus A380 wing leading edge. A sensor network composed of Fibre Bragg Gratings, capacitive sensors for cure monitoring and thermocouples was embedded in its fibre reinforced lay-up and measurements were acquired throughout its Resin Transfer Moulding production process. Two main challenges had to be overcome: first, the integration of the sensor lines in the existing Resin Transfer Moulding mould without modifying it; second, the demoulding of the component without damaging the sensor lines. The proposed embedding solution has proved successful. The wavelength shifts of the Fibre Bragg Gratings were observed from the initial production stages, over the resin injection, the complete curing of the resin and the cooling-down prior to demoulding. The sensors proved to be sensitive to detecting the resin flow front, vacuum and pressure increase into the mould and the temperature increase caused by the resin curing. Measurements were also acquired during the post-curing cycle. Residual strains during all steps of the process were derived from the sensors' wavelength shift, showing values up to 0.2% in compression. Moreover, the capacitive sensors were able to follow-up the curing degree during the production process. The sensors proved able to detect the resin flow front, whereas thermocouples could not measure an appreciable increase of temperature due to the fact that the resin had the same temperature as the mould.

Dynamic Strain Measurements on Automotive and Aeronautic Composite Components by Means of Embedded Fiber Bragg Grating Sensors.

The measurement of the internal deformations occurring in real-life composite components is a very challenging task, especially for those components that are rather difficult to access. Optical fiber sensors can overcome such a problem, since they can be embedded in the composite materials and serve as in situ sensors. In this article, embedded optical fiber Bragg grating (FBG) sensors are used to analyze the vibration characteristics of two real-life composite components. The first component is a carbon fiber-reinforced polymer automotive control arm; the second is a glass fiber-reinforced polymer aeronautic hinge arm. The modal parameters of both components were estimated by processing the FBG signals with two interrogation techniques: the maximum detection and fast phase correlation algorithms were employed for the demodulation of the FBG signals; the Peak-Picking and PolyMax techniques were instead used for the parameter estimation. To validate the FBG outcomes, reference measurements were performed by means of a laser Doppler vibrometer. Sensors 2015, 15 27175 The analysis of the results showed that the FBG sensing capabilities were enhanced when the recently-introduced fast phase correlation algorithm was combined with the state-of-the-art PolyMax estimator curve fitting method. In this case, the FBGs provided the most accurate results, i.e. it was possible to fully characterize the vibration behavior of both composite components. When using more traditional interrogation algorithms (maximum detection) and modal parameter estimation techniques (Peak-Picking), some of the modes were not successfully identified.

A micro-computed tomography technique to study the quality of fibre optics embedded in composite materials.

Quality of embedment of optical fibre sensors in carbon fibre-reinforced polymers plays an important role in the resultant properties of the composite, as well as for the correct monitoring of the structure. Therefore, availability of a tool able to check the optical fibre sensor-composite interaction becomes essential. High-resolution 3D X-ray Micro-Computed Tomography, or Micro-CT, is a relatively new non-destructive inspection technique which enables investigations of the internal structure of a sample without actually compromising its integrity. In this work the feasibility of inspecting the position, the orientation and, more generally, the quality of the embedment of an optical fibre sensor in a carbon fibre reinforced laminate at unit cell level have been proven.

The quasi-harmonic ultrasonic polar scan for material characterization: experiment and numerical modeling.

Conventionally, the ultrasonic polar scan (UPS) records the amplitude or time-of-flight in transmission using short ultrasonic pulses for a wide range of incidence angles, resulting in a fingerprint of the critical bulk wave angles of the material at the insonified spot. Here, we investigate the use of quasi-harmonic ultrasound (bursts) in a polar scan experiment, both experimentally and numerically. It is shown that the nature of the fingerprint drastically changes, and reveals the positions of the leaky Lamb angles. To compare with experiments, both plane wave and bounded beam simulations have been performed based on the recursive stiffness matrix method. Whereas the plane wave computations yield a pure Lamb wave angle fingerprint, this is no longer valid for the more realistic case of a bounded beam. The experimental recordings are fully supported by the bounded beam simulations. To complement the traditional amplitude measurement, experimental and numerical investigations have been performed to record, predict and analyze the phase of the transmitted ultrasonic beam. This results in the conceptual introduction of the 'phase polar scan', exposing even more intriguing and detailed patterns. In fact, the combination of the amplitude and the phase polar scan provides the complete knowledge about the complex transmission coefficient for every possible angle of incidence. This comprehensive information will be very valuable for inverse modeling of the local elasticity tensor based on a single UPS experiment. Finally, the UPS method has been applied for the detection of an artificial delamination. Compared to the pulsed UPS, the quasi-harmonic UPS (both the amplitude and phase recording) shows a superior sensitivity to the presence of a delamination.

Pitfalls in the experimental recording of ultrasonic (backscatter) polar scans for material characterization.

The ultrasonic polar scan (UPS), either in transmission, reflection or backscatter mode, is a promising non-destructive testing technique for the characterization of composites, providing information about the mechanical anisotropy, the viscoelastic damping, the surface roughness, and more. At present, the technique is merely being used for qualitative purposes. The limited quantitative exploration and use of the technique can be primarily ascribed to limitations of current theoretical models as well as the difficulty to perform accurate, and more importantly, reproducible UPS experiments. Over the last years, we have identified several potential pitfalls in the experimental implementation of the technique which severely deteriorate the accurateness and reproducibility of a UPS. In this paper, we make an inventory of the most important difficulties, illustrate each of them by a real experiment and present a feasible mediation, either numerical or experimental in nature. Once the experimental set-up is fine-tuned to overcome these pitfalls, it is expected that the recording of high-level UPS experiments, in combination with numerical computations, will facilitate the technique to become a fully quantitative non-destructive characterization method.

Scholte-Stoneley waves on an immersed solid dihedral: generation, propagation and scattering effects.

Scholte-Stoneley wave propagation on a dihedral and more precisely the diffraction effects occurring at the corners, has since long been of high importance for nondestructive testing of materials and structures. Experimental investigations have been reported in the past. Simulations based on radiation mode theory have been published before, explaining the only situation for which the model is applicable namely rectangular corners. The current report describes an investigation applying finite element simulations. Results are obtained not only for rectangular corners but for any possible corner angle. The outcome is in agreement with reported experiments. Moreover a critical corner angle is found below and beyond which different diffraction phenomena occur. The study is performed for different isotropic solids.

Flexural mechanical properties of porcine aortic heart valve leaflets.

Freshly excised porcine aortic heart valve cusps were subjected to a uni-axial flexural indentation test, from which the rupture characteristics and a functional stiffness parameter were determined. It was found that the flexural mechanical properties of aortic valve cusps (i) are unaffected by their coronary position and (ii) are sensitive to the effect of mechanical preconditioning. The resulting values of the cusp's flexural mechanical properties are intended as a set of reference properties which scaffolds, meant for the tissue engineering of heart valves, must approximate in order to be considered as a functional replacement.

Microstructured optical fiber sensors embedded in a laminate composite for smart material applications.

Fiber Bragg gratings written in highly birefringent microstructured optical fiber with a dedicated design are embedded in a composite fiber-reinforced polymer. The Bragg peak wavelength shifts are measured under controlled axial and transversal strain and during thermal cycling of the composite sample. We obtain a sensitivity to transversal strain that exceeds values reported earlier in literature by one order of magnitude. Our results evidence the relevance of using microstructured optical fibers for structural integrity monitoring of composite material structures.

Finite element modelling and experimental study of oblique soccer ball bounce.

In this study, we develop a finite element model to examine the oblique soccer ball bounce. A careful simulation of the interaction between the ball membrane and air pressure in the ball makes the model more realistic than analytical models, and helps us to conduct an accurate study on the effect of different parameters on a bouncing ball. This finite element model includes a surface-based fluid cavity to model the mechanical response between the ball carcass and the internal air of the ball. An experimental set-up was devised to study the bounce of the ball in game-relevant impact conditions. Ball speed, angle, and spin were measured before and after the bounce, as well as ball deformation and the forces during the impact. The finite element model has been validated with three different sets of data, and the results demonstrate that the finite element model reported here is a valuable tool for the study of ball bounce. After validation of the model, the effect of the friction coefficient on soccer ball bounce was studied numerically. Simulation results show that increasing the friction coefficient may result in reversal of the horizontal impact force.

Strain measurements of composite laminates with embedded fibre bragg gratings: criticism and opportunities for research.

Embedded optical fibre sensors are considered for structural health monitoring purposes in numerous applications. In fibre reinforced plastics, embedded fibre Bragg gratings are found to be one of the most popular and reliable solutions for strain monitoring. Despite of their growing popularity, users should keep in mind their shortcomings, many of which are associated with the embedding process. This review paper starts with an overview of some of the technical issues to be considered when embedding fibre optics in fibrous composite materials. Next, a monitoring scheme is introduced which shows the different steps necessary to relate the output of an embedded FBG to the strain of the structure in which it is embedded. Each step of the process has already been addressed separately in literature without considering the complete cycle, from embedding of the sensor to the internal strain measurement of the structure. This review paper summarizes the work reported in literature and tries to fit it into the big picture of internal strain measurements with embedded fibre Bragg gratings. The last part of the paper focuses on temperature compensation methods which should not be ignored in terms of in-situ measurement of strains with fibre Bragg gratings. Throughout the paper criticism is given where appropriate, which should be regarded as opportunities for future research.

Machine design and processing considerations for the 3D plotting of thermoplastic scaffolds.

3D plotting by micro-extrusion is a promising layer-wise fabrication method for the production of scaffolds in thermoplastic polymers. It is a solvent-free direct technique which permits extensive control over geometry and porosity. This paper highlights the complications that arise when using this technique for the processing of thermally sensitive polymers. It has been noted that the material is subject to extensive thermal load during processing, which may result in degradation by chain scission. This negatively affects scaffold (mechanical) properties as well as predictability and repeatability of the fabrication technique. A rationale is offered as to the main causes of this thermally induced degradation during processing and tentative ideas towards a solution are equally put forward.

Ultrasonic polar scans: numerical simulation on generally anisotropic media.

Ultrasonic polar scans are based on the recording of the reflected or transmitted amplitude of sound, impinging a fiber reinforced composite from every possible angle of incidence. The mechanical anisotropy of such materials makes the reflection coefficient direction dependent, whence an ultrasonic polar scan forms a fingerprint of the investigated material. Such scans have already proved to be very valuable in the characterization of composites. Simulations have been performed for single layered and multi-layered systems, for pulsed and harmonic waves. Fiber reinforced composites are mostly orthotropic. The current report presents simulations not only on orthotropic materials but on materials of any kind of anisotropy. These extended numerical simulations are not only valuable in the characterization of highly sophisticated composites, but may also be used to characterize thin slices of crystals and even layered crystals.

Fringed sound fields and their interaction with liquid-solid interfaces.

On the one hand, it is well known that Gaussian beams possess the ability to stimulate Rayleigh waves, resulting in the Schoch effect, a lateral beam displacement. This effect, often characterized by a reflected sound pattern consisting of two anti-phase beams, is due to the re-radiation of sound because of the stimulation of leaky Rayleigh waves. On the other hand, fringed sound beams are characterized by the fact that they consist of a number of neighboring anti-phase narrow beams. They are a first approximation of a sound field originating from a phased array of harmonic vibrating crystals in which each crystal vibrates in anti-phase compared to its neighbor. The individual lobes within the fringed sound pattern diverge much less than standard Gaussian beams of the same size. The current study investigates the interaction of fringed beams with a liquid-solid interface. It is found that under certain conditions, a fringed beam, incident at the Rayleigh angle, produces a reflected sound pattern that contains a wide lobe that is not fringed. It is also shown that under other conditions, contrary to the famous forward displacement of the reflected sound for incident Gaussian beams, a strong backward displacement occurs for fringed beams.

Enhanced anisotropy in Paratellurite for inhomogeneous waves and its possible importance in the future development of acousto-optic devices.

The anisotropic feature of most crystals, involves a direction dependent wave velocity for each of the possible modes. Paratellurite (Tellurium dioxide) is extraordinary because, for one of the propagation modes, i.e. the quasi shear horizontal (QSH) mode, the anisotropy is exceptional. This results, on the one hand in a very strong directional dependent sound velocity and on the other hand, in a low wave velocity in certain directions, resulting in a high figure of merit for the acousto-optical interaction. In the case of inhomogeneous waves, the slowness surfaces change their shape and magnitude, for all crystals. However, for paratellurite, this effect is again extraordinary. As soon as a relatively small inhomogeneity is considered, the sound velocity for the QSH mode becomes really exceptionally anisotropic, resulting in a slowness surface that is almost spherical, covered by pins. The velocity corresponding to those 'pins', is much lower than in the case of homogeneous plane waves, which is very promising for the future development of acousto-optic cells involving an even higher figure of merit.

The history and properties of ultrasonic inhomogeneous waves.

This paper gives a historical survey of the development of the inhomogeneous wave theory, and its applications, in the field of ultrasonics. The references are listed predominantly chronologically and are as good as complete. Along the historical description, several scientific features of inhomogeneous waves are described. All topics of inhomogeneous wave research are taken into account, such as waves in viscoelastic solids and liquids, thermoviscous liquids and solids, and anisotropic viscoelastic materials. Also inhomogeneous waves having complex frequency are described. Furthermore, the formation of bounded beams by means of inhomogeneous waves is given and the diffraction of inhomogeneous waves on periodically corrugated surfaces. The experimental generation of inhomogeneous waves is considered as well.

The radiation mode theory in ultrasonics.

This paper describes the history and the state of the art in radiation mode theory (RMT) in ultrasonics. The RMT originates from electromagnetism in which it has proved to be very efficient in the field of wave guides and discontinuities. In ultrasonics, the RMT made its entrance only a decade ago and has already proved to be very efficient in describing the interaction of sound with discontinuities such as a step on a plate, a liquid wedge, the extremity of a plate and much more. It is likely that the development of the RMT for two-dimensional (2-D) isotropic media has come almost to an end. This paper lists the results obtained so far. Further extensions to more complicated media are to be expected in the coming decade.

Schlieren photography to study sound interaction with highly absorbing materials.

Strong absorption of sound is often caused by the conversion of sound energy into heat. When this happens, it is not possible to study the interaction of sound with the absorbing material by means of reflected sound characteristics, because there is no reflected sound. Detecting for example the distance that sound travels in a strongly absorbing material, can be done by heat detection systems. However, the presence of temperature detectors in such materials interferes with the sound field and is therefore not really suitable. Infrared measurements are a possible option. Another option is the use of Schlieren photography for simultaneous visualization of sound and heat. This technique is briefly outlined with a 3 MHz sound beam incident on a highly absorbing sponge.

On the theoretical possibility to apply an acoustic diffraction grating as a complex frequency filter device for electronic signals.

In electronics, it is well known that filtering devices can be made that are able to decompose a signal instantaneously into a number of real frequency components. This procedure is equivalent to a numerical real time Fourier transform. However, it is also known that an electronic signal can be decomposed not just in real frequency components, but also in complex frequency components. The current paper shows that it is theoretically possible to create a device, made of a periodically rough surface and a system that transforms the electronic signal into acoustic waves, that can be used to measure the amplitude attributed to considered complex frequency components of an electronic signal, in real time. This 'thought device' is mainly based on the directivity of diffracted sound and the complex frequency dependence of this directivity.

Diffraction of homogeneous and inhomogeneous plane waves on a doubly corrugated liquid/solid interface.

This paper extends the theory of the diffraction of sound on 1D corrugated surfaces to 2D corrugated surfaces. Such surfaces, that are egg crate shaped, diffract incoming sound into all polar directions, which is fundamentally different from 1D corrugated surfaces. A theoretical justification is given for extending the classical grating equation to the case of incident inhomogeneous waves, for 1D corrugated surfaces as well as for 2D corrugated surfaces. Even though the present paper presents a theory which is valid for all angles of incidence, special attention is given to the particular case of the stimulation of surface waves by normal incident sound. The most interesting conclusion is that, depending on the frequency and the incident inhomogeneity, Scholte-Stoneley waves and leaky Rayleigh waves can be generated in different directions. This effect might be of particular interest in the development of surface acoustic wave devices and the basic idea of this steering effect can be of importance for planar actuators.