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The purpose of this study was to create and validate a standardized framework for the evaluation of the ankle stiffness of two designs of 3D printed ankle foot orthoses (AFOs). The creation of four finite element (FE) models allowed patient-specific quantification of the stiffness and stress distribution over their specific range of motion during the second rocker of the gait. Validation was performed by comparing the model outputs with the results obtained from a dedicated experimental setup, which showed an overall good agreement with a maximum relative error of 10.38% in plantarflexion and 10.66% in dorsiflexion. The combination of advanced computer modelling algorithms and 3D printing techniques clearly shows potential to further improve the manufacturing process of AFOs.

Purpose 3D printing technology is investigated for the purpose of patient immobilization during proton therapy. It potentially enables a merge of patient immobilization, bolus range shifting, and other functions into one single patient-specific structure. In this first step, a set of 3D printed materials is characterized in detail, in terms of structural and radiological properties, elemental composition, directional dependence, and structural changes induced by radiation damage. These data will serve as inputs for the design of 3D printed immobilization structure prototypes. Methods Using four different 3D printing techniques, in total eight materials were subjected to testing. Samples with a nominal dimension of 20 × 20 × 80 mm3 were 3D printed. The geometrical printing accuracy of each test sample was measured with a dial gage. To assess the mechanical response of the samples, standardized compression tests were performed to determine the Young's modulus. To investigate the effect of radiation on the mechanical response, the mechanical tests were performed both prior and after the administration of clinically relevant dose levels (70 Gy), multiplied with a safety factor of 1.4. Dual energy computed tomography (DECT) methods were used to calculate the relative electron density to water ρe, the effective atomic number Zeff, and the proton stopping power ratio (SPR) to water SPR. In order to validate the DECT based calculation of radiological properties, beam measurements were performed on the 3D printed samples as well. Photon irradiations were performed to measure the photon linear attenuation coefficients, while proton irradiations were performed to measure the proton range shift of the samples. The directional dependence of these properties was investigated by performing the irradiations for different orientations of the samples. Results The printed test objects showed reduced geometric printing accuracy for 2 materials (deviation > 0.25 mm). Compression tests yielded Young's moduli ranging from 0.6 to 2940 MPa. No deterioration in the mechanical response was observed after exposure of the samples to 100 Gy in a therapeutic MV photon beam. The DECT-based characterization yielded Zeff ranging from 5.91 to 10.43. The SPR and ρe both ranged from 0.6 to 1.22. The measured photon attenuation coefficients at clinical energies scaled linearly with ρe. Good agreement was seen between the DECT estimated SPR and the measured range shift, except for the higher Zeff. As opposed to the photon attenuation, the proton range shifting appeared to be printing orientation dependent for certain materials. Conclusions In this study, the first step toward 3D printed, multifunctional immobilization was performed, by going through a candidate clinical workflow for the first time: from the material printing to DECT characterization with a verification through beam measurements. Besides a proof of concept for beam modification, the mechanical response of printed materials was also investigated to assess their capabilities for positioning functionality. For the studied set of printing techniques and materials, a wide variety of mechanical and radiological properties can be selected from for the intended purpose. Moreover the elaborated hybrid DECT methods aid in performing in-house quality assurance of 3D printed components, as these methods enable the estimation of the radiological properties relevant for use in radiation therapy.

Objective The purpose of this study was the construction of a new semi-automated experimental setup for the evaluation of the stiffness of ankle foot orthoses (AFOs) around an axis aligned to the anatomical ankle joint during the second rocker of the gait. The setup, developed in close collaboration with the orthopedic device company V!GO NV (Wetteren, Belgium), allows measurement of plantarflexion and dorsiflexion in the sagittal plane for a maximal range of motion of 50° (− 25° plantarflexion up to 25° dorsiflexion) in a non-destructive way. Results The mechanical properties of four 3D printed AFOs are investigated, based on the ranges of motion derived from the gait assessment of the patients when they walked with their AFO. The reliability of the stiffness measures was studied by the evaluation of the test–retest repeatability and the intra-tester and inter-tester variability. These studies revealed that the ankle stiffness can be measured with high reliability (ICC = 0.94–1.00). The obtained outcomes indicate that the experimental setup could be applied to measure the ankle stiffness of any topology of AFOs and, in the future, help finding the correlation with the information coming from the gait assessment of the patients. Electronic supplementary material The online version of this article (10.1186/s13104-018-3752-4) contains supplementary material, which is available to authorized users.

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.

Already in the early 1980's, it has been conjectured that the pulsed ultrasonic polar scan (P-UPS) provides a unique fingerprint of the underlying mechanical elasticity tensor at the insonified material spot. Until now, that premise has not been thoroughly investigated, nor validated, despite the opportunities this would create for NDT and materials science in general. In this paper, we report on the first-ever implementation of an inverse modeling technique on the basis of a genetic optimization scheme in order to extract quantita- tive information from a P-UPS. We validate the optimization approach for synthetic data, and apply it to experimentally obtained polar scans for annealed aluminum, cold rolled DC-06 steel as well as for carbon fiber reinforced plastics. The investigated samples are plate-like and do not require specific preparation. The inverted material characteristics show good agreement with literature, micro-mechanical models as well as with results obtained through conventional testing procedures.

Within the field of composite manufacturing simulations, it is well known that the bending behavior of fabrics and prepregs has a significant influence on the drapeability and final geometry of a composite part. Due to sliding between reinforcements within a fabric, the bending properties cannot be determined from in-plane properties and a separate test is required. The Peirce cantilever test represents a popular way of determining the flexural rigidity for these materials, and is the preferred method in the ASTM D1388 standard. This work illustrates the severe inaccuracies (up to 72% error) in the current ASTM D1388 standard as well as the original formulation by Peirce, caused by ignoring higher-order effects. A modified approach accounting for higher-order effects and yielding significantly improved accuracy is presented. The method is validated using finite element simulations and experimental testing. Since no independent tests other than the ASTM D1388 standard are available to determine the bending stiffness of fabric materials, experimental validation is performed on an isotropic, homogeneous Upilex-50S foil for which the flexural rigidity and tensile stiffness are related. The flexural rigidity and elastic modulus are determined through both the cantilever test (ASTM D1388) and tensile testing. The results show that the proposed method measures an elastic modulus close to that determined through tensile testing (within 1%), while both the Peirce formulation (+18%) and ASTM standard (+72%) over-estimate the elastic modulus. The proposed methodology allows for a more accurate determination of flexural rigidity, and enables the more accurate simulation of composite forming processes.

This work presents a novel technique known as polarization dependent loss (PDL) towards the accurate detection of small transverse strains using optical fiber Bragg gratings. The fundamentals of PDL are explained and the benefits discussed. A cure cycle of a cross-ply composite with embedded fibers was monitored using this PDL technique and the data from the experiments are discussed. As shown, differences in transverse strain of up to 500 μe will go unnoticed with normal read-out equipment (1 pm resolution), while PDL is capable of accurately detecting much smaller values. Using PDL, it was possible to detect transverse strain build-up due to matrix polymerization, which was found to cause 60 μe of difference in transverse strains. A method was proposed combining the benefits of both PDL and amplitude techniques to achieve superior resolution in FBG read-out.

Several solutions to measure multi-axial strain in composite materials using fibre Bragg gratings have already been presented in the literature. Most of them use Bragg sensors written in highly birefringent fibres. Unless they exhibit a higher transverse strain sensitivity, a major drawback of those highly birefringent Bragg sensors is the necessity to orient them before embedding them into the composite material, because otherwise an underestimation of the present strain field can occur. Therefore, in this paper we describe how the residual strain, which is built up during the cure cycle of a composite laminate, can in-situ create a highly birefringent fibre Bragg sensor making the need of extra manipulations before embedding redundant. Consequently, in the paper this Bragg sensor is used to quantitatively measure multi-axial strain fields of cross-ply laminates loaded along their three principal mechanical axes. Good similarities were found for the measured strains and the reference strains.

This work presents a finite element technique allowing for the accurate prediction of resin pocket geometries surrounding optical fiber sensors in UD prepreg composites. The F.E. implementation solely relies on material models and element formulations readily available in all commercial F.E. codes, rather than requiring specialized user developed element and material formulations. In addition, only a minimal amount of straight-forward experimental characterization is required to determine the necessary material parameters in the model. The F.E. results are validated with microscopic images of several lay-ups with embedded optical fiber samples of different diameters. Very good correspondence is found between F.E. results and microscopic cross-sections, showing the usefulness and applicability of the method to random lay-ups and optical fiber dimensions.

Due to mismatches in size and material properties, optical fiber (OF) sensors act as inclusions when embedded in composite hosts. The resulting stress concentrations surrounding the OF sensor may lead to premature failure of the host structure. In this work, a novel technique is presented to determine optimal coating properties for OF sensors embedded in composite structures in order to minimize stress concentrations surrounding these sensors. The method is validated against methodologies available in literature and is shown to produce identical results under these specific circumstances. Compared to the methods in literature, the proposed method is significantly more flexible as it allows the optimization of the coating for any arbitrary load condition. The results of the computations can be reused for any load case in the given combination of host and coating material, reducing the computations to a one time effort for a specific combination of host and coating.

The pulsed ultrasonic polar scan (P-UPS) technique provides a map with characteristic contours reflecting the critical bulk wave angles, which directly relate to the in-plane elastic properties of the insonified material spot. Besides these contours, additional information is captured in a P-UPS experiment which has particular importance for the inspection of anisotropic materials. By extensive post-processing on a single P-UPS experiment, we successfully extracted the local directional (i) critical bulk wave angles, (ii) phase velocity profiles, (iii) slowness curves, (iv) walk-off angles and (v) energy velocity profiles, for the three different polarization states of bulk waves. The procedure is demonstrated and discussed for an isotropic aluminum sample as well as an autoclave manufactured orthotropic [o](8) carbon/epoxy laminate. In addition, a numerical and experimental investigation of the role of the immersion liquid as a lens for the P-UPS result is performed. This lensing effect permits to zoom in/out on the P-UPS fingerprint, hence broadening the class of materials which can be inspected by the P-UPS technique. (C) 2014 Elsevier B.V. All rights reserved.

The possibility of embedding optical fiber sensors inside carbon fiber reinforced polymer (CFRP) for structural health monitoring purposes has already been demonstrated previously. So far however, these sensors only allowed axial strain measurements because of their low sensitivity for strain in the direction perpendicular to the optical fiber's axis. The design flexibility provided by novel photonic crystal fiber (PCF) technology now allows developing dedicated fibers with substantially enhanced sensitivity to such transverse loads. We exploited that flexibility and we developed a PCF that, when equipped with a fiber Bragg grating (FBG), leads to a sensor that allows measuring transverse strains in reinforced composite materials, with an order of magnitude increase of the sensitivity over the state-of-the-art. In addition it allows shear strain sensing in adhesive bonds, which are used in composite repair patches. This is confirmed both with experiments and finite element simulations on such fibers embedded in CFRP coupons and adhesive bonds. Our sensor brings the achievable transverse strain measurement resolution close to a target value of 1 mu strain and could therefore play an important role for multi-dimensional strain sensing, not only in the domain of structural health monitoring, but also in the field of composite material production monitoring. Our results thereby illustrate the added value that PCFs have to offer for internal strain measurements inside composite materials and structures.

In this paper, the concept of both the 'gradient' and the 'curvature' polar scan is introduced and illustrated for several carbon reinforced composites. While the conventional polar scan concerns the recording of the reflected or transmitted maximum amplitudes or TOFD values for a certain solid angle, here the differences between subsequent scan angles are considered. The quality of a polar scan experiment improves significantly, i.e. more defined and more detailed polar contours are obtained. Even more, several polar contours show up which were hidden in the conventional polar scan experiment. The amplitude and the TOFD recorded experiments suggest that both methods are complementary. For future quantitative modelling, i.e. to extract the elasticity constants Cij of the targeted material, this newly obtained technique can be of great importance.

A very important aspect of a composite manufacturing process is the appearance of residual strains and stresses during the curing cycle. The formation of thermal residual strains arise mainly from the difference in thermal expansion between the reinforcement fibres and the matrix material. In this paper, Fibre Bragg grating sensors are used to monitor the internal strain state of a cross-ply laminate [0(2), 90(2)](2s) during its curing cycle. In a first experiment, it is shown that transverse strain fields can influence the residual strain measurements. Therefore, one should try to isolate the sensor from these transversal strain fields. In the second experiment, this is done by integrating the sensors in a capillary. As such we were able to quantify the longitudinal residual strains of a cross-ply laminate.

This work presents the progress in the development of a finite element model capable of predicting resin pockets occurring in composite structures with embedded sensors. The F.E.- model is built using standard tools in ABAQUS software, avoiding the need of specialized coding. Both progresses in material characterization as well as finite element modeling are shown. The model will eventually be used to optimize the shape of an embedded optical fibre interrogator used within the FP7 ‘SmartFiber’ project

very important aspect of the composite manufacturing process is the appearance of residual strains and stresses during the curing cycle. Composites exhibit large residual strains after curing. Therefore, in this paper, we propose to follow the evolution of the polarization depend loss peaks (amplitude and wavelength) of fibre Bragg gratings during the manufacturing of the composite material to highlight the residual strains appearance

We report on the conversion of ultrasound into light by the process of piezo-luminescence in epoxy with embedded BaSi2O2N2:Eu as active component. We exploit this acoustically induced piezo-luminescence to visualize several cross-sectional slices of the radiation field of an ultrasonic piston transducer (f = 3.3 MHz) in both the near-field and the far-field. Simply combining multiple slices then leads to a fast representation of the 3D spatial radiation field. We have confronted the luminescent results with both scanning hydrophone experiments and digital acoustic holography results, and obtained a good correlation between the different approaches.

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.

Embedded optical fibre sensors are considered in numerous applications for structural health monitoring purposes. Since the optical fibre and the host material in which it is embedded, have different material properties, strain in both materials will not be equal when external load is applied. In this paper, an experimental evaluation of the response of uni-axial fibre Bragg grating sensors embedded in small cross-ply composite laminates subjected to out-of-plane transverse loading is discussed.

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.

This work highlights some of the achievements obtained within the EU FP7 SmartFiber project, aiming to develop a fully embeddable optical fiber sensor system including the interrogator chip. The focus is on resolving issues holding back the industrial uptake of optical sensing technology. In a first section, the development of a placement head for automated lay-down of an optical sensor line (including the SmartFiber interrogator system) during composite manufacturing is discussed. In a second section, the attention is shifted to the occurrence of resin pockets surrounding inclusions such as the SmartFiber interrogator. A computationally efficient F.E. approach is presented capable of accurately predicting resin pocket geometries. Both small (i.e. optical fiber sensors) and large (i.e. the SmartFiber interrogator) inclusions are considered, and the F.E. predictions are validated with experimental observations. doi: 10.12783/SHM2015/123

Digital Image Correlation or DIC is an already widespread and commonly used technique to perform full field strain measurements. Usually, a so called 'speckle pattern' is put on the specimen and then, pictures are taken first in the undeformed stage, which is the reference and then in various deformed states. The technique itself consists of comparing the image of a deformed pattern with a reference image and determining the displacements of the so called 'subsets'. The mathematical algorithm eliminates rigid rotations of the subset and is able to determine the strain field. This technique was originally designed for large-strain measurements and as such, it works very well when large strains are present, but when determining (very) small strain fields, especially in combination with large (rigid body) deformations or large strain gradients, this technique becomes a lot more sensitive to the boundary conditions of the experimental setup. This manuscript will illustrate the errors induced by experimental factors such as in-plane rotation, in-plane rigid body translation, out-of-plane rigid body rotation, which are in fact all related to how the specimen is gripped in the tensile machine, on the eventual derived strain field. Furthermore, processing parameters such as subset and step size and the used strain window on numerically and experimentally induced transformations will be assessed.

SMARTFIBER is a EU funded FP7 research project, aiming to resolve technological and practi- cal issues holding back the industrial uptake of optical fiber sensing technology. In this paper, a broad overview of the most significant achievements – from a structural health monitoring perspective – within the project are discussed. In a first section, the development of an automated, robotic lay-down system for the optical fiber system is discussed. Secondly, a finite element method is presented capable of predicting the structural behavior of a composite structure with the embedded health monitoring system. It is shown to be capable of accurately modelling the resin pocket formation surrounding the inclusion, and predicting the resulting mechanical re- sponse of the structure. Thirdly, a numerical technique for coating optimization is presented, allowing the coat- ing properties to be tuned to reduce the impact of the sensor network on the host performance. Finally, a novel interrogation technique known as PDL is demonstrated, leading to an increased resolution in transverse strain sensing with traditional FBG sensors, enabling (amongst others) very accurate residual strain measurements.

We developed a highly birefringent micro-structured optical fiber that, in combination with a fiber Bragg grating sensor, allows measuring transverse strains in reinforced composites. The first generation of this dedicated fiber sensor featured a hydrostatic pressure sensitivity of −15 pm/MPa and yielded a transverse strain sensitivity of −0.16 pm/µe when embedded in a carbon fiber reinforced polymer. The second generation of this sensor has now been fabricated and hydrostatic pressure experiments and FEM simulations show that this generation returns a sensitivity of more than twice that of the first generation. FEM simulations additionally show an increased sensitivity when this sensor is embedded in a reinforced composite, achieving an unprecedented transverse strain sensitivity of 0.29 pm/µe. We explain how the optimized micro-structure yields this record-high sensitivity. In addition we demonstrate the selectivity of the bare fiber sensor, which remains insensitive to temperature changes or axial strain. This sensor can therefore play an important role in the domain of structural health monitoring.