Mathias Kersemans

Improved accuracy in the determination of flexural rigidity of textile fabrics by the Peirce cantilever test (ASTM D1388)

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.

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.

Towards 3D printed multifunctional immobilization for proton therapy: Initial materials characterization

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 Youngs 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 Youngs 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.

Fast reconstruction of a bounded ultrasonic beam using acoustically induced piezo-luminescence

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.

Calibration and correction procedure for quantitative out-of-plane shearography

Quantitative shearography applications continue to gain practical importance. However, a study of the errors inherent in shearography measurements, related to calibration of the instrument and correction of the results, is most often lacking. This paper proposes a calibration and correction procedure for the out-of-plane shearography with a Michelson interferometer. The calibration is based on the shearography measurement of known rigid-body rotations of a flat plate and accounts for the local variability of the shearing distance. The correction procedure further compensates for the variability of the sensitivity vector and separates the two out-of-plane deformation gradients when they are coupled in the measurement. The correction procedure utilizes two shearography measurements of the same experiment with distinct shearing distances. The effectiveness of the proposed calibration procedure is demonstrated in the case of a static deformation of a centrally loaded plate, where the discrepancy between experimental and finite element analysis results is minimized.

Errors in shearography measurements due to the creep of the PZT shearing actuator

Shearography is a modern optical interferometric measurement technique. It uses the interferometric properties of coherent laser light to measure deformation gradients on the µm m − 1 level. In the most common shearography setups, the ones employing a Michelson interferometer, the deformation gradients in both the x- and y-directions can be identified by setting angles on the shearing mirror. One of the mechanisms for setting the desired shearing angles in the Michelson interferometer is using the PZT actuators. This paper will reveal that the time-dependent creep behaviour of the PZT actuators is a major source of measurement errors. Measurements at long time spans suffer severely from this creep behaviour. Even for short time spans, which are typical for shearographic experiments, the creep behaviour of the PZT shear actuator induces considerable deviation in the measured response. In this paper the mechanism and the effect of PZT creep is explored and demonstrated with measurements. For long time-span measurements in shearography, noise is a limiting factor. Thus, the time-dependent evolution of noise is considered in this paper, with particular interest in the influence of external vibrations. Measurements with and without external vibration isolation are conducted and the difference between the two setups is analyzed. At the end of the paper some recommendations are given for minimizing and correcting the here-studied time-dependent effects.

Six-beam homodyne laser Doppler vibrometry based on silicon photonics technology

This paper describes an integrated six-beam homodyne laser Doppler vibrometry (LDV) system based on a silicon-on-insulator (SOI) full platform technology, with on-chip photo-diodes and phase modulators. Electronics and optics are also implemented around the integrated photonic circuit (PIC) to enable a simultaneous six-beam measurement. Measurement of a propagating guided elastic wave in an aluminum plate (speed ≈ 909 m/s @ 61.5 kHz) is demonstrated.

Characterization of visco-elastic material parameters by means of the ultrasonic polar scan method

The Ultrasonic Polar Scan (UPS) is a non-destructive technique which insonifies a material spot on a sample using ultrasonic pulses from as many oblique incidence angles ψ(φ,θ) as possible. Mapping the transmitted time-of-flight (TOF) and/or amplitudes as function of the incidence angle ψ(φ,θ) in a polar representation yields a UPS image with intriguing patterns that represent a fingerprint of the local visco-elastic properties. The present paper reports on recent advances in the revival of the UPS technique, involving the construction of an automated high-precision UPS scanner, the implementation of advanced simulation models as well as the development of efficient inversion routines. On the experimental level, unprecedented high quality TOF and amplitude UPS landscapes for a range of orthotropic (fiber reinforced) materials have been obtained. Using numerical simulation models, it can be readily demonstrated that the TOF UPS landscapes are directly connected to the elastic properties of the material, wh...

Local Stiffness Identification of Beams Using Shearography and Inverse Methods

Shearography is an interferometric method that produces full-field displacement gradients of the inspected surface. In high-technology industry it is often used qualitatively to detect material defects, but quantitative applications are still rare. The reasons for that are the complicated calibration procedure as well as the denoising, unwrapping, the local sensitivity vector estimation and the local shearing angle estimation needed to get quantitative gradient-maps. To validate the technique and its calibration, results obtained from shearography are compared to results obtained from scanning laser vibrometry. Beams are acoustically excited to vibrate at their first resonant frequency and the mode shape is recorded using both shearography and scanning laser vibrometry. Outputs are compared and their properties discussed. Separate inverse method algorithms are developed to process the data for each method. They use the recorded mode shape information to identify the beam’s local stiffness distribution. The beam’s stiffness is also estimated analytically from the local geometry. The local stiffness distributions computed using these methods are compared and the results discussed.

Simulation of a Circular Phased Array for a Portable Ultrasonic Polar Scan

The development of new composite materials, often anisotropic in nature, requires intricate approaches to characterize these materials and to detect internal defects. The Ultrasonic Polar Scan (UPS) is able to achieve both goals. During an UPS experiment, a material spot is insonified at several angles Psi(theta,phi), after which the reflected or transmitted signal is recorded. While excellent results have been obtained using an in-house developed 5-axis scanner, UPS measurements with the current set-up are too lengthy and cumbersome for in-situ industrial application. Therefore, we propose to replace the complex mechanical steering of the transducers by a hemispherical phased array consisting of small PZT elements. This allows to create a compact and portable setup without compromising the current data quality. By successively activating a specific set of elements of the array and choosing appropriate inter-element time delays, the beam can be electronically steered from any angle to a fixed position on the targeted sample. Consequently, UPS reflection measurements can be performed at this position from a wide range of angles in a timeframe of seconds. Additionally, by using apodization windows, it is possible to efficiently reduce the intensity of unwanted side lobes and to create a phase profile which closely resembles that of a bounded plane wave, leading to an easier interpretation of the recorded data. The appropriate time delays and apodization parameters can be found though a multi-objective inverse problem in which both the phase profile and the side lobe reduction are optimized. This approach enables the creation of an effective beam profile to be used during UPS experiments for the characterization and inspection of composite materials. Our simulation approach is a crucial step towards a next-generation UPS device for industrial applications and in-field measurements.