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The phenomenon of mechanoluminescence (ML), where phosphors emit light when pressure is applied, is considered to be closely related to the crystallographic structure of those phosphors. In this work we unravel this connection for the anorthite solid solution Ca 1 − x SrxAl2Si2O8, which displays two important phase transitions as a function of strontium content x (denoted as xSr), i.e., the nearly second-order P 1 ¯ -I 1 ¯ transition and the ferroelastic I 1 ¯ -I 2 c transition at ambient temperature and pressure. The spontaneous strains reveal that the ferroelastic transition takes place when xSr ∈ (0.70, 0.75), while other optical methods suggest that the second-order P 1 ¯ − I 1 ¯ transition takes place when xSr is around 0.4. The ML intensity reaches its maximum when the second order transition takes place and drops to zero when the phosphors undergo the ferroelastic transition. The first transition already brings significant changes to electron occupations at traps in this solid solution. The structural phase transitions in the anorthite solid solutions are reflected in specific ML properties, such as the ML intensity and the load threshold. Further analysis suggests this is due to the structural change of the hosts and the trap properties (trap density and electron population function). Analysis of the ML dynamics may therefore serve as a useful tool to investigate phase transitions in ML phosphors.

It is well known that the efficiency of the vibrothermographic non-destructive testing (NDT) technique can be enhanced by taking advantage of local defect resonance (LDR) frequencies. Recently, the classical out-of-plane local defect resonance was extended towards in-plane LDR for enhanced efficiency of vibrometric NDT. This paper further couples the concept of this in-plane LDR to vibrothermography, on the basis of the promising potential of in-plane LDRs to enhance the rubbing (tangential) interaction and viscoelastic damping of defects. Carbon fiber-reinforced polymers (CFRPs) with barely visible impact damage (BVID) are inspected and the significant contribution of in-plane LDRs in vibrational heating is demonstrated. Moreover, it is shown that the defect thermal contrast induced by in-plane LDRs is so high that it allows for easy detection of BVID by live monitoring of infrared thermal images during a single broadband sweep excitation. Thermal and vibrational spectra of the inspected surface are studied and the dominant contribution of in-plane LDR in vibration-induced heating is demonstrated.

Ultrasonic pulse-echo C-scan imaging is a widespread method for detecting and characterizing defects in fiber reinforced polymer composites. However, the accurate assessment of a complex distributed damage cluster, like barely visible impact damage, in multi-layer and heterogeneous composites is not straightforward. For reliably estimating the remaining load carrying capacity and/or remaining useful lifetime of a damaged composite, a proper and complete damage assessment is of utmost importance. In this paper, a statistical time-energy gating approach is proposed in view of obtaining improved ultrasonic pulse-echo imaging of impacted composites. The majority of virgin A-scan signals are first clustered by analyzing their back-wall echoes. Next, using the principle of maximum likelihood, a Rice distribution is matched to the instantaneous amplitude in order to estimate the natural variability in the local energy of the virgin response signals. The resulting time-varying reliability interval provides an effective means to identify signals coming from defects or inhomogeneities, and as such to robustly assess defect parameters. The proposed probabilistic imaging procedure is demonstrated on various carbon fiber reinforced polymer laminates with barely visible impact damage. The obtained results are benchmarked by conventional ultrasonic C-scan imaging in through-transmission mode as well as in pulse-echo mode using the classical time gating approach. In contrast to the classical time gate method, the proposed statistical time-energy gating procedure successfully extracts and quantifies the full extent of the complex impact damage cluster. Further, the good noise resistance of the proposed probabilistic imaging method is demonstrated for a wide range of signal-to-noise ratios.

The mechanical response and damage sequence of composite materials are nowadays still a topic of ongoing research. However, many parameters influencing their overall behavior are still not thoroughly taken into consideration. The effect of multiaxial stresses, the distinction between balanced and unbalanced configurations and the influence of the number of off-axis layers are just a few to mention. Experimental data regarding the effect of all these parameters on the damage progression in composites is of great importance, since it is proven that commonly used failure criteria, neglecting the occurring damage mechanisms, cannot always predict the material response. In this work, a study of the influence of all these parameters is attempted, by testing carbon/epoxy laminates with different off-axis angles to account for different multiaxiality. Both balanced and unbalanced laminates are taken into account, considering the lack of experimental evidence in literature regarding the latter case, and significant differences between the two lay-ups are reported for the first time. Finally, the influence of the number of the off-axis layers on the mechanical response in conjunction with the previous parameters is also studied through elaborate damage observations.

Local wavenumber estimation (LWE) applied to a full wavefield response is a powerful approach for detecting and characterizing defects in a composite structure. However, the narrowband nature of the traditional LWE techniques brings several challenges for application on actual test cases. This study proposes a self-reference broadband version of the LWE technique. The broadband vibrations are injected using low-power piezoelectric actuators (sine sweep signal) or using pulsed laser excitation in the thermoelastic regime. The out-of-plane velocity response of the surface is recorded using an infrared scanning laser Doppler vibrometer. The dispersive Lamb wave behavior, corresponding to the damage-free base material, is identified from the broadband vibrational response. Using the identified dispersion curves (i.e. self-reference approach), a Lamb mode passband filter bank in the wavenumber-frequency domain is constructed. Searching for the maximum bandpower density in function of the assumed material thickness provides a robust estimate of the effective local thickness of the tested component, and as such yields a detection and evaluation of damage. The performance of the self-reference broadband LWE algorithm is demonstrated on aluminum plates with various flat bottom holes, as well as on cross-ply CFRP aircraft components with a stiffener disbond and barely visible impact damage. Compared to the traditional narrowband LWE approaches, the proposed self-reference broadband LWE method allows a higher level of automation, removes the need for a priori knowledge on the material and/or defect properties, and results in an improved characterization of defects.

Local defect resonance (LDR) makes use of high frequency vibrations to get a localized resonant activation of the defect. In this study, it is shown for various samples and damage features that the classical out-of-plane local defect resonance can be equally extended towards in-plane local defect resonance. It is found that the in-plane LDR typically occurs at higher frequencies, which is linked to the specific geometry of common defects. This increased frequency allows for a reduction in measurement time. More importantly, the results indicate that the in-plane LDR has an increased sensitivity for defects with dominant out-of-plane defect interfaces (e.g. surface breaking crack). Knowledge of both the out-of-plane and in-plane LDR provides further insight on the internal structure of a defective area. This is explicitly demonstrated on a carbon composite which was subjected to low velocity impact, inducing barely visible impact damage.

A novel approach for creating highly detailed finite element models of wind turbine blades is presented. The approach is implemented as a software tool which handles all the different steps of the model creation process. The novel approach considers the blade to consist of a collection of parametric pre-defined blocks. This allows wind turbine blade models consisting of shell elements, solid elements or combinations to be created. By including the tools to accurately partition the outer mold layer, create the required offset surfaces and calculate accurate element-wise material orientations, a high level of detail and fidelity can be achieved.

Phononic crystal plates (PhPs) have promising application in manipulation of guided waves for design of low-loss acoustic devices and built-in acoustic metamaterial lenses in plate structures. The prominent feature of phononic crystals is the existence of frequency bandgaps over which the waves are stopped, or are resonated and guided within appropriate defects. Therefore, maximized bandgaps of PhPs are desirable to enhance their phononic controllability. Porous PhPs produced through perforation of a uniform background plate, in which the porous interfaces act as strong reflectors of wave energy, are relatively easy to produce. However, the research in optimization of porous PhPs and experimental validation of achieved topologies has been very limited and particularly focused on bandgaps of flexural (asymmetric) wave modes. In this paper, porous PhPs are optimized through an efficient multiobjective genetic algorithm for widest complete bandgap of mixed fundamental guided wave modes (symmetric and asymmetric) and maximized stiffness. The Pareto front of optimization is analyzed and variation of bandgap efficiency with respect to stiffness is presented for various optimized topologies. Selected optimized topologies from the stiff and compliant regimes of Pareto front are manufactured by water-jetting an aluminum plate and their promising bandgap efficiency is experimentally observed. An optimized Pareto topology is also chosen and manufactured by laser cutting a Plexiglas (PMMA) plate, and its performance in self-collimation and focusing of guided waves is verified as compared to calculated dispersion properties. Refereed/Peer-reviewed

For decades, modal analysis has been a tool to gain insights in the vibrational and acoustical behavior of music instruments. This study provides a critical comparison of various experimental modal analysis approaches with the violin as a case study. Both contact and non-contact excitation and different acquisition approaches are considered. The influence of different boundary conditions (clamped, supported, free/free) on the vibrational response of the violin is investigated. The response is analyzed to extract relevant modal parameters (frequency, damping and mode shape) within the frequency range of 0–1000 Hz. The performance of the different approaches is evaluated in terms of (i) signature vibration modes, (ii) experimental reproducibility, (iii) contact requirement and (iv) cost of required equipment. Two optimal approaches are proposed for performing modal analysis on violins: a contact method for use in the violinmaker’s practice, and a non-contact method for use with fragile instruments.

Electrospun fibers have shown great potential for the interlaminar toughening. In this work, electrospun SBS fibers were incorporated into glass epoxy laminates. The mechanical properties of these SBS fibers are tuned using a triazolinedione cross-linker, where a higher amount of cross-linking gives rise to a lower elongation at break and a higher stiffness of the SBS fibers. Consequently, insights are provided into the relation between mechanical properties of electrospun fibers and enhancements in both Mode I and Mode II interlaminar fracture toughness. The SBS fiber's mechanical properties affect the crack path in composites. Non and low cross-linked SBS fibers which have a very low stiffness promote crack growth with SBS fiber bridging under both Mode I and Mode II loading conditions. However, these SBS fibers need to elongate substantially in order to take up a significant amount energy (>300%). As this only occurs under Mode I loading conditions, the Mode I interlaminar fracture toughness can be improved. Nevertheless, the Mode II IFT is negatively affected. Higher cross-linked SBS fibers on the other hand, do take up more energy at much lower levels of elongation. As such, they can improve the Mode II interlaminar fracture toughness substantially.

To validate the increasingly used numerical models for optimization and verification of the designs subjected to bird strike, initial (calibration) tests are a necessity prior to full scale testing. Bird strike calibration tests on rigid targets specifically, give a valuable insight in the complex behaviour of a bird. This paper presents the results of a series of bird strike tests and simulations on three rigid targets (a plate, a wedge and a splitter) to quantify the forces originating from the change of momentum and splitting of the bird. In this study, momentum transfer is the key parameter to compare birds with different masses, materials, speeds, etc., as proposed in the reference works from the 20th century. The main purpose of this paper is fourfold: (i) to introduce another way to measure Momentum transfer on these kinds of structures and therefore get more consistent results, (ii) to show that gelatine generates similar impact forces.as real birds, (iii) to point out that apart from the change of direction of the momentum, the deviatoric and/or dissipating constitutive behaviour of the bird also plays an important role and (iv) to show that a simple plate structure can be used to measure the residual energy of the bird remainders after an impact event. In a series of numerical simulations, the performance of a SPH bird with an EOS material model is used to validate the analytical models. (C) 2016 Elsevier Ltd. All rights reserved.

International audience; This article provides a simulation methodology based on the concept of virtual fibres and digital elements which can be used to determine the mechanical behaviour of 3D woven fabrics. It takes the fibrous nature of the fabric into account by modelling a yarn as a bundle of virtual fibres. Whilst the digital element method has been typically used as a kinematic method to predict the geometrical behaviour of textile materials, its potential to also capture the mechanical behaviour of fabrics is still under research. Our methodology is able to predict the woven microstructure of a large unit cell 3D woven fabric based on simple input properties (weaving pattern, yarn stiffness) and to virtually asses its behaviour under tensile and shear loading. Hence, both the kinematic and mechanical behaviour of the fabric is taken into account. The main advantage of this methodology is that the simulations are able to predict the mechanical response of the fabric by considering the sub-yarn behaviour without the requirement of complex constitutive laws. Good agreement with experimental data was obtained, indicating the usability of this method to model the mechanical behaviour of a large unit cell 3D woven fabric.

Due to their low self-weight and their inherently high flexibility, technical textiles offer great possibilities for the integration in kinematic structures. Furthermore, the implementation of active bending in a transformable design creates new challenging perspectives. The paper describes an integrated approach for transformable textile hybrids where an improved design is obtained through a parameter study, performing a structural analysis in the different phases of the deployment. The studied parameters include (i) the form-finding position, (ii) the prestress (ratio), (iii) the used materials and sections (including the fibre directions) and (iv) the number of bending-active elements. This research confirms the feasibility of realizing kinematic form-active structures with integrated bending-active elements, where both the membrane and the supporting structure are stable in the different configurations. Due to the high interaction between the bending-active supporting system and the pretensioned membrane, the different parameters influence each other significantly. In a next step, an experimental verification of the designed pringle-shaped textile hybrid is carried out in order to both confirm the possibilities and reveal the remaining challenges.

In this study, a non-destructive testing approach is investigated for finding damages in fiber reinforced polymers using broadband piezoelectric excitation and scanning laser Doppler vibrometer measurements. An efficient broadband filter in wavenumber-frequency domain is considered, in order to calculate a mode-removed broadband weighted root mean square (WRMS) energy map of the guided waves. Compensation of the frequency-dependent wave attenuation is implemented. Because this energy map relates exclusively to abnormalities in the wave field, it shows high sensitivity to all kinds of internal damages. The proposed damage map construction method is automated, and does not require any a priori information on the inspected part or measurement conditions. Further, it is not limited to application on flat coupon samples, but can be equally applied on complex composite parts. The methods are tested on an academic case of a CFRP plate with flat bottom holes and on an industrial case of a large stiffened CFRP aircraft panel with distributed manufacturing defects. It is shown that the broadband mode-removed WRMS energy map is highly sensitive to various kinds of damages, even if they are small and deep inside the material.

Epoxy is a material of choice for demanding applications thanks to its high chemical stability, stiffness, and strength. Yet, its brittle fracture behavior is an important downside for many sectors. Here, we show that the addition of electrospun thermoplastic nanofibers is a viable toughening strategy to design nanofiber reinforced epoxy materials with excellent toughness. Moreover, the use of transparent film-like specimens allowed in-situ imaging during mechanical testing. Optical and scanning electron microscopy, digital image correlation and crack length measurements are used to analyze the toughening mechanisms responsible for high toughening efficiency in detail. The addition of polyamide and polycaprolactone nanofibers resulted in an increased plastic energy uptake up to 100%. In-situ observation of the crack tip showed that the main energy-absorbing mechanism was due to bridging nanofibers. There was a profound decrease in toughening efficiency when nanofibers lacked sufficient adhesion with the matrix only when they were oriented parallel with the crack growth direction. The profound understanding of such underlying mechanisms opens up material design in applications where high toughness is required like adhesives, coatings, and fiber-reinforced composite laminates.

In flash thermography, the maximum inspectable defect depth is limited when only the raw thermographic sequence is analyzed. The introduction of pulsed phase thermography (PPT), in which phase (contrast) images at different thermal wave frequencies are obtained, significantly improved the maximum inspectable depth while reducing the effects of non-uniform heating and non-uniform surface properties. However, in a practical environment, the evaluation of many phase images per inspection is a cumbersome procedure. In this paper, a novel Adaptive Spectral Band Integration (ASBI) procedure is introduced for the post-processing of flash thermographic datasets, which yields a unique damage index map. ASBI integrates the most useful spectral information for each pixel individually, obtaining a maximized defect detectability and an almost zero-reference level. The performance of ASBI with respect to defect detectability as well as defect sizing and depth inversion is evaluated thoroughly with both experimentally and numerically generated datasets. The ASBI procedure is successfully applied on various composite coupons with flat bottom holes and barely visible impact damage, as well as on a stiffened aircraft composite panel with a complex cluster of production defects. The ASBI procedure is compared with existing data-processing techniques in literature, illustrating an enhanced performance.

Injection molded short fiber reinforced composites (SFRC) have different local fiber orientation distribution (FOD) at every point. SN curves of short fiber reinforced composites are known to depend on the fiber orientation distribution. Such materials also suffer from continuous loss of stiffness during cyclic loading. It is not known whether the loss of stiffness is different for SFRC with different FOD. A statistical analysis of the loss of stiffness curves is presented in this paper. Tension-tension fatigue experiments are performed and loss of stiffness is collected for every data point in the SN curve. A systematic method for comparing the loss of stiffness is developed. It is concluded that the difference in loss of stiffness curves for coupons of SFRC with different FOD is not statistically significant. (C) 2016 Elsevier Ltd. All rights reserved.

In this study, static and dynamic compression and crushing tests were conducted on expanded polystyrene (EPS) foam for material characterisation at high strain rates. This was done to obtain the stress strain curve for different temperatures and densities. An influence of the strain rate on the experimental data was shown. The resulting curves for modelling were extracted from the experimental data, which were obtained from high speed drop tower tests. The methodology for the processing of the experimental data for use in the finite element (FE) modelling was presented. The foam material model of LSDyna was used to simulate the dynamic compression process. This model is dedicated to modelling crushable foam with optional damping, tension cut-off, and strain rate effects. The adjustment of the material parameters for successful modelling has been reported. This FE model of EPS foam was validated with experimental data using impact on a "kerbstone" support. This model can be applied for simulation of dynamic loads on a bicycle helmet It is useful for designing a reliable bicycle helmet geometry for different types of accidents. (C) 2016 Elsevier Ltd. All rights reserved.

Today’s architecture is characterized by a growing demand for flexibility and adaptability, allowing to adjust to meet the current needs. Both covering spaces for weather protection and improving energy performance of buildings ask for dynamic architectural solutions. The integration of lightweight technical textiles offers great possibilities for these kinematic structures, due to their inherently high flexibility. Unfortunately, until now, there is a lack of in-depth knowledge on the material properties of technical textiles, their structural behaviour during deformation and the use of available design tools. The inability to keep the fabric properly pretensioned in all deployment stages within the structure’s limitations, obstructs the use of fabric structures for kinematic applications. In order to make a good design and analysis possible, we investigated the material properties of a standard polyester-PVC fabric and implemented these properties in a simple linear elastic computer model of a case study. Afterwards, we performed a parameter study to derive a set of conceptual design considerations for the kinematic prestressed fabric structure. The specified parameters to verify in the design process are (i) the boundary configuration in which form-finding is conducted (i.e. the reference state), (ii) the prestress levels and ratios, (iii) the control of the deployment and (iv) the used material parameters. The paper discusses how the computed model can serve as a design tool. An exhaustive preliminary study is essential to enhance the overall structural behaviour of the membrane structure in all stages of its transformation, within the application range, keeping the membrane properly tensioned and avoiding excessive stress concentrations. In a next step, a large-scale experimental model is set up, measuring the geometry, reaction forces and strains in the membrane. This model will serve as an experimental validation of the numerically obtained results.

Fibre-matrix adhesion affects fibre-reinforced composites' mechanical properties, a process which can be improved by applying appropriate sizing on the fibre. Transverse bending tests and Scanning Electron Microscopy (SEM) can help quantify this effect This paper investigates if modal damping measurements are a reliable alternative for quantifying fibre-matrix adhesion. When a composite sample is vibrating, part of the dissipated energy is due to the internal friction. More internal friction and slipping at the fibre-matrix interface is expected with a weaker fibre-matrix bond, hence increasing the amount of dissipated energy, which in turn is proportional to the modal damping value. This paper researches two different cases to validate this hypothesis. In the first case, we will use two composite samples of flax fibre, one with and one without sizing. In the second case, we will compare flax and carbon fibre laminates. If the only variable is fibre sizing, better adhesion is related to significantly lower damping and higher resonance frequencies. If composite laminates with different fibre and matrix type are compared, lower adhesion is not necessarily related to increased damping and lower resonance frequencies. However, when combining the damping result with SEM microscopy, it is possible to assess the relative contribution to the internal energy dissipation of the fibre, the matrix and the fibre-matrix interface individually. (C) 2016 Elsevier Ltd. All rights reserved.

Technical textiles used in lightweight tensile fabric structures are inherently highly flexible, which makes these materials very suited to, for instance, make lightweight adaptable facade or roof systems. Until now, however, kinematic fabric structures are mostly designed to transform between a prestressed, structural state and a compact state where the fabric becomes untensioned using fixed geometrically determined paths. The goal of this research is to design and validate the structural behaviour of a kinematic fabric structure which remains prestressed in all its possible geometric states by taking advantage of the out-of-plane flexibility of the material rather than the high stretchability. To make the design and the use of such a kinematic fabric structures possible, we investigated the material properties of a standard polyester-PVC fabric. Afterwards, we implemented these properties in a computational model and performed a parameter study to come to a conceptual design of a kinematic prestressed fabric structure where its geometry follows the reorientation of forces rather than restricting its movement to a geometrically determined path. Finally, the designed kinematic structure was built and tested as a prototype, comparing reaction forces and strains to the ones predicted in the computational model. This paper describes this experimental validation by comparing the experimentally obtained results to the values predicted in the computational simulations using a cable-net approximation and a linear elastic orthotropic material model. Although this comparison showed some deviations in the absolute values of the forces and strains, the general behaviour of the prototype was correctly predicted using a standard analysis method. The majority of the deviations could be contributed to the fact that the strains in the computational model do not take into account the compensation applied to the prototype and the high permanent straining of the boundary belts. The investigated prototype thus showed both the potential and the difficulties of using lightweight, highly flexible fabrics as structurally stable, kinematic elements.

Autonomic self-healing materials are materials with built-in (micro-) capsules or vessels, which upon fracturing release healing agents in order to recover the material's physical and mechanical properties. In order to better understand and engineer these materials, a thorough characterization of the material's microstructural behavior is essential and often overlooked. In this context, micro-computed tomography (μCT) can be used to investigate the three dimensional distribution and (de)bonding of (micro-) capsules in their native state in a polymer system with self-healing properties. Furthermore, in-situ μCT experiments in a self-healing polymer and a self-healing concrete system can elucidate the breakage and leakage behavior of (micro-) capsules at the micrometer scale. While challenges related to image resolution and contrast complicate the characterization in specific cases, non-destructive 3D imaging with μCT is shown to contribute to the understanding of the link between the microstructure and the self-healing behavior of these complex materials.

Long-term predictions of material properties such as stiffness and creep resistance are important in many engineering applications and require high reliability and accuracy. This is especially true for polymer materials and their composites as their viscoelastic nature results in time-dependent material behaviour and any measurement uncertainties or errors amplify in long-term predictions. To measure this behaviour at smallest loadings, Dynamic Mechanical Analysis (DMA) is frequently declared as an ideal method. However, the measurement accuracy and repeatability of this method is strongly influenced by (i) the testing fixture and corresponding loading mode, (ii) the sample preparation and (iii) the plotting scale to interpret the test results. In this study, relevant experimental parameters were found for DMA and a proper procedure was designed, which was then applied to measure the viscoelastic behaviour of a highly temperature and creep resistant thermoplastic polymer (polyethersulfone) and of a highly graphite filled polypropylene composite. In combination with finite element simulations and in-situ strain measurements by digital image correlation (DIC), the main influences on measurement accuracy of three-point-bending DMA were identified and subsequently used to determine measurement guidelines. Using these guidelines, DMA measurements allow quantitative determination of the viscoelastic response for rigid polymer and composite materials.

Fibre reinforced composite laminates are key engineering materials allowing to design lightweight components with high mechanical properties. Yet they are prone to delamination between the reinforcing plies, which in turn limits the damage resistance of many applications. This is especially true for the interfaces between dissimilar reinforcing plies that are often encountered in actual components, e.g. differences in fibre orientation, fibre material or ply architecture, where high interlaminar stresses can occur. Nanofibrous toughening veils are known to increase the damage resistance when inserted between similar reinforcing plies, but it is currently unknown how they perform when delamination occurs at dissimilar interfaces. Here, the nanofibre toughening of frequently encountered dissimilar interfaces such as occurring between multidirectionally stacked unidirectional fibre plies (+45 degrees/-45 degrees), multistructural stackings (unidirectional versus fabrics) and multimaterial configurations (glass fibres versus carbon fibres) are analysed. These interfaces largely exert their influence on the crack path during delamination and thus alter the effectiveness of nanofibre toughening. Poly(ether-block-amide) nanofibres of the biosourced polyamide 11 family result in a large increase in mode I and mode II interlaminar fracture toughness for all the tested dissimilar interfaces. We show that their effectiveness however depends on the underlying delamination mechanics present in different dissimilar interfaces. (c) 2020 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Herein, a method is described to weld polytetrafluoroethylene (PTFE), a non-melt processable thermoplast, using a thulium laser. Different settings for laser power and speed were used. The resulting mean lap shear strength per setting ranged from 0.081 N/mm² to 0.297 N/mm². The optimal setting was found to be 12 W irrespective of the welding speed applied. Micro-computed tomography (μ-CT) and optical microscopy was used to show that the welded pattern consisted of tunnel defects. As PTFE is known to be non-melt processable, a physico-chemical characterization was performed to examine the formation of degradation products. Differential scanning calorimetry (DSC) showed a reduction in molecular weight of the PTFE in the weld pattern after welding. Attenuated total reflectance infrared (ATR-IR) and nuclear magnetic resonance (NMR) spectroscopy using hexafluoroisopropanol (HFIP) did not indicate the presence of any new compounds in the respective spectra.

Local Defect Resonance (LDR) is exploited for non-destructive testing (NDT) by using ultrasonic vibrations to get a localized resonant activation of defected zones. The LDR technique relies on the local stiffness difference between the defect and the sound material. Analyzing the structure’s displacement field at this localized resonance frequency reveals the defect’s location and provides information about the defect’s characteristics, i.e. geometry, size and depth. In this study, the opportunities and limitations of linear LDR for NDT of materials are investigated in a parametric way. Both finite element simulations and experiments (using scanning laser Doppler vibrometry) are performed for aluminum alloy and carbon fiber reinforced polymer coupons with flat bottom holes and delaminations ranging in both depth and diameter. The resonance frequencies as well as the associated defect-to-background ratios are parametrically evaluated. For shallow defects, a clear LDR is observed caused by the strong local stiffness reduction at the defect. On the contrary, deep defects are associated with a limited stiffness decrease that results in the absence of LDR behavior. The local stiffness reduction at damages is further exploited using a weighted band power calculation. It is shown that using this technique, deep defects can be detected for which no LDR behavior was observed.

In this work an innovative electrospinning system is proposed that simultaneously has an adequate temperature resistance, a high increase in mode I (+51%) and mode II (+96%) delamination performance and can be commercially produced. Interleaving nanofibrous veils can potentially solve the issue of the limited delamination resistance encountered in composite laminates, but industrial upscaling has always been impeded by one or more critical factors. These constraining factors include a limited temperature stability of the nanofibers, a lack in simultaneous mode I and II delamination performance increase and the complexity of the electrospinning system because non-commercial polymers or specialty nanofibers (e.g. coaxial) are required. In this paper, a robust electrospinning system is proposed that is the first to overcome all major hurdles to make nanofiber toughening industrially viable. A new class of nanofibers based on biosourced polyamide 11 and its poly(ether-block-amide) co-polymers is used to deal with those shortcomings. The nanofibers have tuneable diameters down to 50 nm and cross-section morphologies ranging from circular to ribbon-shaped. The key to this work is the fundamental underpinning of the toughening effect using a broad range of interleaves with different mechanical and thermal properties, fiber diameters and fiber morphologies, all produced from the same bio-based base polymer. Generally, round and thin nanofibers performed better than larger and ribbon-like fibers. The relationship between the fiber morphology and the delamination performance is further underpinned using detailed analysis of the fracture surface. Ultimately, this results in a range of optimized nanofibrous veils capable of improving the delamination resistance considerably without suffering from the aforementioned drawbacks.

Local defect resonance (LDR) employs a specific high frequency, the LDR frequency, to get a localized strong resonant activation of the defect. However, one of the major difficulties for applying LDR as a non-destructive testing technique, is the proper identification of the required LDR frequency, and the subsequent LDR localization. In this study, post-processing methods in both time and frequency domain are applied to low-power broadband vibration data in view of automated extraction of LDR parameters, i.e. LDR frequency and LDR location. In order to reduce the computational effort for large datasets (>1 GB), various data compression methods have been considered: power spectral density (PSD), principal component analysis (PCA) and operational modal analysis (OMA). The actual LDR parameter extraction from the (compressed) data is based on an iterative procedure to threshold the vibrational amplitudes. The LDR parameter extraction procedure is demonstrated on different carbon fiber reinforced polymers with various defect types: flat bottom holes, inserts and low velocity impact damage. It is further demonstrated that the procedure can equally handle multiple defects. A comparison of the performance of the various data compression methods is provided.

This study analyses the damage tolerance of nanofibre interleaved composites when subjected to low velocity impact. Cross-ply glass/epoxy composite laminates are produced. Drop-weight impact and residual compressive strength measurements are performed on these laminates according to the ASTM D7136 and ASTM D7137 standards for a range of impact energies around the Barely Visible Impact Damage energy limit. Polyamide 6, polyamide 6.9 and polycaprolactone nanofibrous veils with two different veil densities are selected to assess their effect on the damage tolerance. The low velocity impact resistance of nanofibre interleaved laminates increases considerably compared to the virgin material. The (projected) damage area decreases up to 50–60%, especially at higher impact energies where the virgin material shows widespread delamination. As more energy is absorbed in the interleaved laminates by the nanofibres, less damage to reinforcing fibres and matrix resin is produced. Analysis of fracture surfaces shows that the development of nanofibre bridging zones is the main reason for the improved impact damage tolerance. Keywords: Nano particles, Damage tolerance, Electrospinning, Toughening, Compression after impact, CAI

Inclusions in short fiber reinforced composites (SFRC) suffer from debonding and cannot be directly modeled using Eshelby based mean field methods. This paper proposes a method of treatment of inclusions with debonded interface by replacing them with a fictitious “equivalent bonded inclusion” (EqBI) whose properties are calculated based on the reduced load bearing capacity of the inclusion due to the debonded interface. Approximate expressions are derived for stress redistribution in an inclusion due to the presence of debonded interface for the six elementary loading cases and the corresponding terms in the stiffness tensor are estimated as a function of the reduced average stress in the inclusion. Mechanical equivalence of the EqBI is confirmed by comparison with finite element models having inclusions with debonded interface and the overall stress strain response of a SFRC composite is validated against experimental data.

Electrospun thermoplastic nanofibres have a large potential for the interlaminar toughening of composite laminates. They can easily be placed in resin rich interlayers between reinforcing plies prior to laminate production and require no dispersion into the matrix resin. Although there are many expected benefits, the research on composite laminates enhanced with electrospun thermoplastic nanofibres is still very limited and a thorough understanding of the toughening mechanism is still missing. This article provides thorough insights into the micromechanisms that lead to the interlaminar toughening of carbon/epoxy composite laminates interleaved with electrospun polyamide nanofibrous veils. The main mechanism leading to a higher interlaminar fracture toughness, both under Mode I and Mode II loading conditions, was the bridging of (micro)cracks by PA nanofibres. The effectiveness of the nanofibre bridging toughening mechanism is dependent on a good load transfer to the nanofibres. Crack propagation under Mode II loading conditions resulted in much higher improvements than under Mode I loading due to an optimal loading of the nanofibres along their fibre direction in the plane of the nanofibrous veil. In Mode I crack propagation, however, the loading of the nanofibres is less optimal and was shown to be dependent on both the primary reinforcement fabric architecture, as well as on the presence of a carbon fibre bridging zone.

The presented literature review of cruciform shapes used for biaxial characterization of materials indicates that the majority of shapes can be divided into two large groups when the following selection criteria are taken into consideration: (i) the shape of the outer boundaries and (ii) the load capacity needed to achieve failure in the biaxial region. Manipulation of the outer shape boundaries appears to be essential to bundle the applied loads to the central zone where failure is intended to be built up. For each group, one particular cruciform design is reported whereby the outer boundaries are based on a single curved shape. Although the use of discontinuous double radii edges should be avoided according to earlier reports [1,2], it is shown here through the construction of an optimization algorithm, that the use of a single curve for the outer boundaries leads to strains in the arms that are strongly dependent on these single curved edges. Numerical simulations based on the finite element method as well as experiments performed on polymeric test pieces in combination with DIC measurements, show good agreement on this matter and demonstrate this sensitivity very clearly.

Dynamic measurements on bicycles are implemented for assessing the vibration comfort of the cyclist or for the measuring the load distribution at bicycle components. Accelerometers are typically used for comfort evaluation, force sensors are selected for comfort and load measurements. For cycling purposes, typically custom made, strain gauge based, are designed. These sensors are implemented for static and dynamic load measurements. When force transducers are used for dynamic measurements, it is important to have detailed knowledge of the dynamic properties of the force transducer and the corresponding electronic measuring equipment, as considerable errors can occur under dynamic conditions. Moreover, the arrangement of the force transducer, the mounting conditions and the whole mechanical structure of the measuring arrangement may significantly influence the uncertainty of dynamic force measurement. This is investigated for a strain gauge based handlebar force sensor. The dynamic calibration procedure assesses the amplitude sensitivity, the phase response and the seismic mass. It is observed that (i) in-situ boundary conditions significantly reduce the expected behavior calculated from idealized clamped boundary conditions and (ii) the dynamic force sensor properties are different from the DC properties. As a result from the dynamic calibration procedure, the developed handlebar force sensor can be used in the frequency range from DC to 35 Hz.

Retrofitting existing windows with a safety window film, to improve impact resistance, has been increasing along with the awareness of potential attacks on strategic buildings. However, current classification of glass panels is based solely on discrete outcomes of standardised tests without any instrumentation. In this paper, a versatile small-scale drop weight test set-up is conceived which is widely instrumented (accelerometer, force sensor, displacement sensor, high-speed visual observation) in order to gain more insight in the mechanical impact response of glass fitted with a safety window film. The elastic response, determined by the much higher stiffness of the glass, was very reproducible. The impacted surface (glass vs. film side) had a large influence, whereas the tensioned surface and laminated surface (air vs. tin side), do not. The film thickness only had an influence when the film was tested separately (without glass substrate) or in case of a soft impact of glass.

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.

The methods for determining the bicycle frame stiffness exist in many forms. Because the measuring method is not standardized, each bicycle magazine or bicycle constructer uses his own test setup. This leads to a wide variety of setups; they differ in many aspects such as applied load, boundary conditions and frame deflection measurement. To clarify some misunderstandings in frame testing, a multi-directional rating test method for bicycle frame stiffness has been developed. Prior to testing the stiffness of different frames it is important to assess the confidence limits of the stiffness result. This includes (i) the contribution of the test bench due to its non-zero compliance, (ii) the influence of mounting the frame in the test bench with a certain preload, (iii) the errors related to the force-and displacement measurement and finally (iv) estimating the influence of experimenter This sensitivity analysis on the test bench already led to a better understanding of frame stiffness testing, and which minor modifications can lead to major differences in stiffness values. (C) 2014 Elsevier Ltd.

Following the developments in pulse compression techniques for increased range resolution and higher signal to noise ratio of radio wave radar systems, the concept of thermal wave radar (TWR) was introduced for enhanced depth resolvability in optical infrared thermography. However, considering the highly dispersive and overly damped behavior of heat wave, it is essential to systematically address both the opportunities and the limitations of the approach. In this regard, this paper is dedicated to a detailed analysis of the performance of TWR in inspection of carbon fiber reinforced polymers (CFRPs) through frequency and/or phase modulation of the excitation waveform. In addition to analogue frequency modulated (sweep) and discrete phase modulated (Barker binary coded) waveforms, a new discrete frequency-phase modulated (FPM) excitation waveform is introduced. All waveforms are formulated based on a central frequency so that their performance can be fairly compared to each other and to lock-in thermography at the same frequency. Depth resolvability of the waveforms, in terms of phase and lag of TWR, is firstly analyzed by an analytical solution to the 1D heat wave problem, and further by 3D finite element analysis which takes into account the anisotropic heat diffusivity of CFRPs, the non-uniform heating induced by the optical source and the measurement noise. The spectrum of the defect-induced phase contrast is calculated and, in view of that, the critical influence of the chosen central frequency and the laminate’s thickness on the performance of TWR is discussed. Various central frequencies are examined and the outstanding performance of TWR at relatively high excitation frequencies is highlighted, particularly when approaching the so-called blind frequency of a defect.

Both effective properties of composite and the stresses in the individual inclusions and in the matrix are necessary for modelling damage in short fibre composites. Mean field theorems are usually used to calculate the effective properties of composite materials, most common among them is the Mori–Tanaka formulation. Owing to occasional mathematical and physical admissibility problems with the Mori–Tanaka formulation, a pseudo-grain discretized Mori–Tanaka formulation (PGMT) was proposed in literature. This paper looks at the predictive capabilities for stresses in individual inclusions and matrix as well as the average stresses in inclusion phase for full Mori–Tanaka and PGMT formulation for 2D planar distribution of orientation of inclusions. The average stresses inside inclusions and the matrix are compared to solutions of full-scale finite element (FE) models for a wide range of configurations. It was seen that the Mori–Tanaka formulation gave excellent predictions of average stresses in individual inclusions, even when the basic assumptions of Mori–Tanaka were reported to be too simplistic, while the predictions of PGMT were off significantly in all the cases. The predictions of the matrix stresses by the two methods were found to be very similar to each other. The average value of stress averaged over the entire inclusion phase was also very close to each other. The Mori–Tanaka formulation must be used as the first choice homogenization scheme.

The susceptibility to delamination is one of the main concerns in many advanced laminated composite applications. Laminates interleaved with electrospun nanofibrous veils provide a potential solution in order to increase the material's resistance to interlaminar fracture. Previous studies have shown that nanofibres are able to bridge microcracks in the laminates resulting in an increased interlaminar fracture toughness (IFT). However, the exact micromechanisms resulting in these nanofibre bridging zones are still unclear. In this article, aligned nanofibrous structures are used to identify and study the different micromechanisms which take place during Mode II crack propagation. Three nanofibrous veil morphologies with a distinct orientation of the nanofibres are used: (1) a random deposition of nanofibres, (2) nanofibres oriented parallel to the crack growth direction, and (3) nanofibres oriented perpendicular to the crack growth direction. A thorough analysis of the fracture surface of tested specimens and crack path behaviour is performed in order to determine the micromechanisms associated with the development of nanofibre bridging zones. A strong effect of the nanofibre orientation distribution on the Mode II IFT and the underlying toughening mechanisms was observed: different micromechanisms were observed depending on the nanofibre orientation.

This article employs an adaptive wear modeling method to study the wear progress in radial sliding bearings contacting with a rotary shaft. Mixed Lagrangian–Eulerian formulation has been used to simulate the contact condition between the bearing and the shaft, and the local wear evolution is modeled using the Archard equation. In the developed wear processor algorithm, not only remeshing is performed on the contact elements, but also is executed for their proximity elements. In this way the wear simulation becomes independent of the size of the contact elements. Validation was done for a laminated polymeric composite bearing. The composite has been modeled as a linear orthotropic material. The wear coefficients were obtained from flat-on-flat experiments and were applied as pressure and velocity dependent parameters in the wear processor. Finally, the effect of the clearance on the wear of the radial bearings has been studied numerically. The simulations also demonstrate how the contact pressure evolves during the wear process, and how the clearance influences this evolution.

Digital image correlation (DIC) has seen widespread acceptance and usage as a non-contact method for the determination of full-field displacements and strains in experimental mechanics. The advances of imaging hardware in the last decades led to high resolution and speed cameras being more affordable than in the past making large amounts of data image available for typical DIC experimental scenarios. The work presented in this paper is aimed at maximizing both the accuracy and speed of DIC methods when employed with such images. A low-level framework for speckle image partitioning which replaces regularly shaped blocks with image-adaptive cells in the displacement calculation is introduced. The Newton–Raphson DIC method is modified to use the image pixels of the cells and to perform adaptive regularization to increase the spatial consistency of the displacements. Furthermore, a novel robust framework for strain calculation based also on the Newton–Raphson algorithm is introduced. The proposed methods are evaluated in five experimental scenarios, out of which four use numerically deformed images and one uses real experimental data. Results indicate that, as the desired strain density increases, significant computational gains can be obtained while maintaining or improving accuracy and rigid-body rotation sensitivity.