Showing papers in "Strain in 2019"

One‐step deposition of nano‐to‐micron‐scalable, high‐quality digital image correlation patterns for high‐strain in‐situ multi‐microscopy testing

Abstract: Digital image correlation (DIC) is of vital importance in the field of experimental mechanics, yet producing suitable DIC patterns for demanding in-situ (micro)mechanical tests remains challenging, especially for ultrafine patterns, despite the large number of patterning techniques reported in the literature. Therefore, we propose a simple, flexible, one-step technique (only requiring a conventional physical vapour deposition machine) to obtain scalable, high-quality, robust DIC patterns, suitable for a range of microscopic techniques, by deposition of a low-melting temperature solder alloy in the so-called island growth mode, without elevating the substrate temperature. Proof of principle is shown by (near-)room temperature deposition of InSn patterns, yielding highly dense, homogeneous DIC patterns over large areas with a feature size that can be tuned from as small as ~10 nm to ~2 μm and with control over the feature shape and density by changing the deposition parameters. Pattern optimisation, in terms of feature size, density, and contrast, is demonstrated for imaging with atomic force microscopy, scanning electron microscopy, optical profilometry, and optical microscopy. Moreover, the performance of the InSn DIC patterns and their robustness to large deformations is validated in two challenging case studies of in-situ micromechanical testing: (a) self-adaptive isogeometric digital height correlation of optical surface height profiles of a coarse, bimodal InSn pattern providing microscopic 3D deformation fields (illustrated for delamination of Al stretchable interconnects on a PI substrate) and (b) DIC on scanning electron microscopy images of a much finer InSn pattern allowing quantification of high strains near fracture locations (illustrated for rupture of a polycrystalline Fe foil). As such, the high controllability, performance, and scalability of the DIC patterns, created by island growth of a solder alloy, offer a promising step towards more routine DIC-based in-situ micromechanical testing.

Ultrasonic Lamb wave-based debonding monitoring of advanced honeycomb sandwich composite structures

Abstract: Honeycomb sandwich composites are extensively used in military shelters, aerospace structures, ground transportation structures, auto-racing bodies, ship panels, and other special purpose structures requiring lightweight construction materials. But debondings at the face-sheet-to-core junctions frequently occur due to the variable operating and loading conditions, which may menace the safety and overall integrity of the structural assembly. This paper aims to effectively identify such hidden debonding regions in these advanced structures, using Lamb wave-based monitoring technique. A semianalytical analysis of Lamb wave dispersion in a healthy sandwich structure is carried out to identify various Lamb modes and to study their propagation phenomenon. A combined finite element-based simulation and experimental analysis of Lamb wave propagation in sandwich panels (healthy and with debonding) are then carried out using piezoelectric transducer networks. It is observed that the presence of debonding significantly reduces the propagating Lamb wave mode amplitudes. A debonding detection algorithm, which uses the differential changes in Lamb mode amplitudes, is applied to efficiently identify single and multiple debonding regions in the structure.

Nonlinear modal analysis via non‐parametric machine learning tools

Abstract: Modal analysis is an important tool in the structural dynamics community; it is widely utilised to understand and investigate the dynamical characteristics of linear structures. Many methods have been proposed in recent years regarding the extension to nonlinear analysis, such as nonlinear normal modes or the method of normal forms, with the main objective being to formulate a mathematical model of a nonlinear dynamical structure based on observations of input/output data from the dynamical system. In fact, for the majority of structures where the effect of nonlinearity becomes significant, nonlinear modal analysis is a necessity. The objective of the current paper is to demonstrate a machine learning approach to output‐only nonlinear modal decomposition using kernel independent component analysis and locally linear‐embedding analysis. The key element is to demonstrate a pattern recognition approach which exploits the idea of independence of principal components from the linear theory by learning the nonlinear manifold between the variables. In this work, the importance of output‐only modal analysis via “blind source” separation tools is highlighted as the excitation input/force is not needed and the method can be implemented directly via experimental data signals without worrying about the presence or not of specific nonlinearities in the structure.

Evaluating the use of digital image correlation for strain measurement in historic tapestries using representative deformation fields

Abstract: An analysis technique to assess the viability of digital image correlation (DIC) in tracking the full‐field strains across the surface of hanging historic tapestries is presented. Measurement uncertainty related to the use of the inherent tapestry image in tracking displacements is investigated through use of “synthetic” deformation fields. The latter are generated by mapping the details of a given tapestry image into finite element analyses. The combination of self‐weight loading, material non‐linearity, and image specific heterogeneity (related to slit stitching, damage, and patch‐restorations) serve to generate a bespoke deformation field complex enough to assess the reliability of DIC measurements. Accuracy is evaluated by comparing measured results with the original known deformations. The technique demonstrates that the optimum imaging settings and the choice of subset size for DIC analysis are strongly influenced by the tapestry image and the goal of the measurement, they are found using a compromise between conflicting objectives: minimising measurement error while maximising resolution.

Virtual image correlation of magnetic resonance images for 3D geometric modelling of pelvic organs

Abstract: Numerical simulation of pelvic system could lead to a better understanding of common pathology, through objective and reliable analyses of pelvic mobility, according to mechanical principles. In clinical context, patient-specific simulation has the potential for a proper patient-personalized cure. For this purpose, a simulable 3D geometrical model, well suited to patient anatomy, is required. However, the geometric modelling of pelvic system from medical images (MRI) is a complex operator-dependent and time-consuming process, not adapted to patient-specific applications. This paper is addressing this challenging computational problem. The objective is to develop a technique providing a smooth, consistent and readily usable 3D geometrical model, seamlessly from image to simulation. In this paper, we use a generic topologically-simplified B-Spline model to represent pelvic organs. The presented paper develops a Virtual Image Correlation (VIC) method to find the best correlation between the geometry and the image. The final reconstructed geometrical model is to be compatible with meshing and Finite Element (FE) simulation. Then, a variety of tests are performed to prove the concept, through both prototypical and pelvic models. Finally, since the pelvic system is complex, including structures hardly identifiable in MRI, some feasible solutions to introduce more complex pelvic models are also discussed.

Time-domain imaging system in the terahertz range for immovable cultural heritage materials

Abstract: In the field of cultural heritage science, the use of non‐destructive and contact‐free techniques has increased sharply over the past 10 years. Compared with conventional spectroscopic and imaging techniques such as X‐ray, ultraviolet, infrared, and laser spectroscopy, terahertz time‐domain imaging (THz‐TDI) is an innovative, non‐invasive, and safe technique, which provides good penetration depth (~1 cm) and broad spectral bandwidth (0.1–10 THz). This paper sets out the protocol and methodology for the application of THz‐TDI to immovable cultural heritage, illustrated by a series of case studies. The case studies demonstrate the efficacy of the technique in providing structural and material information for conservators.

Tearing behaviour of two types of leather: A comparative study carried out at the local scale using the full kinematic and thermal field measurement techniques

Abstract: Leather materials undergo various strain and stress states during their elaboration process and their application in numerous different functions. Among the key properties required for such materials, tearing resistance appears as one of the most important. In this paper, the tearing behaviour of two types of leather, a grain pigskin leather and a grain calf leather, was investigated at the local scale using the full‐field techniques. During the tests, thermal fields were measured at the surface of the two leathers by means of an infrared camera. Measurements of the displacement and deformation fields were also performed at the surface of the pigskin sample using the digital image correlation technique, which was not possible for the calf sample due to surface wrinkling. The results obtained enable us to discuss and compare the tearing resistance of both leathers in terms of the thermal activity in the zone of influence of the crack. The best tearing resistance was obtained for the grain calf leather that has undergone a retanning operation and whose matrix contained a plasticiser.