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

Digital Image Correlation (DIC) is of vital importance in the field of experimental mechanics, yet, producing suitable DIC patterns for demanding in-situ mechanical tests remains challenging, especially for ultra-fine patterns, despite the large number of patterning techniques in the literature. Therefore, we propose a simple, flexible, one-step technique (only requiring a conventional 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 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 10nm to 2um and with control over the feature shape and density by changing the deposition parameters. Pattern optimization, in terms of feature size, density, and contrast, is demonstrated for imaging with atomic force microscopy, scanning electron microscopy (SEM), optical microscopy and profilometry. Moreover, the performance of the InSn DIC patterns and their robustness to large deformations is validated in two challenging case studies of in-situ micro-mechanical testing: (i) 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 aluminum interconnects on a polyimide substrate) and (ii) DIC on SEM images of a much finer InSn pattern allowing quantification of high strains near fracture locations (illustrated for rupture of a Fe foil). As such, the high controllability, performance and scalability of the DIC patterns offers a promising step towards more routine DIC-based in-situ micro-mechanical testing.Comment: Accepted for publication in Strai

Non-contact strain determination of cell traction effects

Irreversible tissue damage leading to organ failure is a common health problem in today's world. Regenerating these damaged tissues with the help of scaffolds is the solution offered by tissue engineering. In cases where the extra-cellular matrix (ECM) is to be replaced by an artificial substrate (scaffold) or matrix, cellular traction forces (CTF) are exerted by the cells on the scaffold surface. An ideal scaffold should exhibit mechanical characteristics similar to those of the ECM it is intended to replace. In other words, the capacity of a scaffold to withstand deformation should be comparable to that of a natural ECM. And with knowledge of those forces and deformations the properties of the scaffolds may be inferred. Digital Image Correlation (DIC), a non-contact image analysis technique enables us to measure point to point deformation of the scaffold by comparing a sequence of images captured during the process of scaffold deformation. This review discusses the methodology involved and implementation of DIC to measure displacements and strain.Irreversible tissue damage leading to organ failure is a common health problem in today's world. Regenerating these damaged tissues with the help of scaffolds is the solution offered by tissue engineering. In cases where the extra-cellular matrix (ECM) is to be replaced by an artificial substrate (scaffold) or matrix, cellular traction forces (CTF) are exerted by the cells on the scaffold surface. An ideal scaffold should exhibit mechanical characteristics similar to those of the ECM it is intended to replace. In other words, the capacity of a scaffold to withstand deformation should be comparable to that of a natural ECM. And with knowledge of those forces and deformations the properties of the scaffolds may be inferred. Digital Image Correlation (DIC), a non-contact image analysis technique enables us to measure point to point deformation of the scaffold by comparing a sequence of images captured during the process of scaffold deformation. This review discusses the methodology involved and implementation of DIC to measure displacements and strain

The use of digital image correlation in the biomechanical area: a review

This paper offers an overview of the potentialities and limitations of digital image correlation (DIC) as a technique for measuring displacements and strain in biomechanical applications. This review is mainly intended for biomechanists who are not yet familiar with DIC. This review includes over 150 papers and covers different dimensional scales, from the microscopic level (tissue level) up to macroscopic one (organ level). As DIC involves a high degree of computation, and of operator- dependent decisions, reliability of displacement and strain measurements by means of DIC cannot be taken for granted. Methodological problems and existing solutions are summarized and compared, whilst open issues are addressed. Topics addressed include: preparation methods for the speckle pattern on different tissues; software settings; systematic and random error associated with DIC measurement. Applications to hard and soft tissues at different dimensional scales are described and analyzed in terms of strengths and limitations. The potentialities and limitations of DIC are highlighted, also in comparison with other experimental techniques (strain gauges, other optical techniques, digital volume correlation) and numerical methods (finite element analysis), where synergies and complementarities are discussed. In order to provide an overview accessible to different scientists working in the field of biomechanics, this paper intentionally does not report details of the algorithms and codes used in the different studies

Fracture of Automotive High Strength Steels

This research is focused on the study of local deformation damage initiation and propagation in DP1000 steels which are good candidate for future generation of cars. The potential of DP1000 for applications in next generation of cars relies on a better understanding of the relationship between its overall mechanical properties and the deformation and damage of its microstructure. Such understanding will in turn favours the advancement in the development of future steels. Damage development and plastic deformation have been studied in a statistically meaningful way by performing a DIC procedure conducted at two different scales simultaneously. Plastic deformation in both ferrite and martensite phase analysed over a large representative microstructure are statistically measured up to the UTS point revealing that the martensite phase in the DP1000 is deforms plastically at very large strain values and showing a very similar strain heterogeneity as observed in the ferrite. A new experimental procedure to study crack propagation in DP1000 steel has been designed for the development of a laboratory scale punch test that generate loading conditions representative of industrial forming operations for the study of damage. Cracks were observed to form from the top outer surface and propagating towards the mid thickness. Void formation is found to take place near the ferrite-martensite boundaries in the ferrite phase. Crack paths are observed to propagate only in the ferrite phase and preferably goes around the martensite phase without crossing or breaking the martensite island. Effect of processing conditions on the macroscopic mechanical properties of DP1000 will also be investigated using the newly developed experimental procedures

Lightweight SFRC benefitting from a pre-soaking and internal curing process

The presented research program is focused on the design of a structural lightweight fiber-reinforced concrete harnessing an internal curing process. Pre-soaked waste red ceramic fine aggregate and pre-soaked artificial clay expanded coarse aggregate were utilized for the creation of the mix. Copper-coated steel fiber was added to the mix by volume in amounts of 0.0%, 0.5%, 1.0%, and 1.5%. Test specimens in forms of cubes, cylinders, and beams were tested to specify the concrete characteristics. Such properties as consistency, compressive strength, splitting tensile strength, static and dynamic modulus of elasticity, flexural characteristics, and shear strength were of special interest. The achieved concrete can be classified as LC12/13. A strength class, according to fib Model Code, was also assigned to the concretes in question. The proposed method of preparation of concrete mix using only pre-soaked aggregate (with no extra water) proved to be feasible.Web of Science1224art. no. 415

The Effect of Compaction Temperature and Pressure on Mechanical Properties of 3D Printed Short Glass Fiber Composites

Among many thermoplastics that are used in engineering, polyamide 6 (nylon 6) is an extremely versatile engineering thermoplastic. Nylon filled with glass fibers has higher mechanical strength and high wear resistance than general purpose nylon. 3D printed composites, based on fused filament modeling, typically suffer from poor bead-to-bead bonding and relatively high void content, limiting their mechanical properties This thesis explores the effect of compaction pressure and temperature on improving the mechanical properties of 3D printed composites. Engineering moduli in the printing and transverse to printing direction, as well as ultimate strength were measured using the tensile testing with Digital Image Correlation (DIC). Tensile testing is performed on the samples that are compacted at different temperatures with pressure. In addition, microscopic studies were carried out to evaluate the void content for different compaction pressures and temperatures. Fiber orientation state was measured for different sets of samples. Differential scanning calorimetry (DSC) was carried out to calculate the degree of crystallinity and possible changes in crystalline morphology as a result of annealing temperature profile. The results indicate that by selecting appropriate heat treatment profiles both strength and modulus of 3D printed composites can be significantly improved. Strength was improved by over 50% and 100% in printing and transverse directions respectively, and twofold increase of the modulus in printing direction was found. In this respect, the observed mechanical behavior will be explained in terms of various parameters such as degree of compaction, crystalline structure, orientation state and void content

Micro-FEM models based on micro-CT reconstructions for the in vitro characterization of the elastic properties of trabecular bone tissue.

This master’s thesis describes the research done at the Medical Technology Laboratory (LTM) of the Rizzoli Orthopedic Institute (IOR, Bologna, Italy), which focused on the characterization of the elastic properties of the trabecular bone tissue, starting from october 2012 to present. The approach uses computed microtomography to characterize the architecture of trabecular bone specimens. With the information obtained from the scanner, specimen-specific models of trabecular bone are generated for the solution with the Finite Element Method (FEM). Along with the FEM modelling, mechanical tests are performed over the same reconstructed bone portions. From the linear-elastic stage of mechanical tests presented by experimental results, it is possible to estimate the mechanical properties of the trabecular bone tissue. After a brief introduction on the biomechanics of the trabecular bone (chapter 1) and on the characterization of the mechanics of its tissue using FEM models (chapter 2), the reliability analysis of an experimental procedure is explained (chapter 3), based on the high-scalable numerical solver ParFE. In chapter 4, the sensitivity analyses on two different parameters for micro-FEM model’s reconstruction are presented. Once the reliability of the modeling strategy has been shown, a recent layout for experimental test, developed in LTM, is presented (chapter 5). Moreover, the results of the application of the new layout are discussed, with a stress on the difficulties connected to it and observed during the tests. Finally, a prototype experimental layout for the measure of deformations in trabecular bone specimens is presented (chapter 6). This procedure is based on the Digital Image Correlation method and is currently under development in LTM

Damage and failure modelling of carbon and glass 2D braided composites

Composite materials have been increasingly used in the past two decades since they offer significant potential weight reduction, part design flexibility and improved specific mechanical performance compared to traditional metals. For specific applications, braid reinforced composites offer better near net shape part and manufacturing flexibility than conventional unidirectional laminates, albeit at the expense of slightly lower in-plane stiffness and strength. Furthermore, for impact and crash applications, which is the emphasis of this thesis, their tow waviness and interlocking can offer excellent damage tolerance and energy absorption. In this work, heavy tow (24k) biaxial carbon and glass braided preforms were used to manufacture coupons and beam structures to undertake an extensive testing campaign to characterise different damage and failure mechanisms occurring in braided composites. Due to large shear deformation and surface degradation, non conventional measurement techniques based on marker tracking and Digital Image Correlation were successfully used to measure strains in the damaging material. The modelling of braided composites was conducted using the meso-scale damage approach first proposed by P. Ladevèze for unidirectional composites. The calibration of an equivalent braid unidirectional ply was achieved using the experimental results obtained for different braided coupons. Furthermore, failure mechanisms observed experimentally, such as tow stretching and fibre re-orientation occurring during loading history, were integrated into the model. A new unidirectional ply formulation was subsequently implemented into the explicit finite element code PAM-CRASHTM. Validation of the new model using single element, coupons and beams were conducted that provided a satisfying correlation between experimental tests and numerical predictions