Modal curvature-based damage localization in weakly damaged continuous beams

Abstract Modal curvatures have been claimed to contain local information on damage and to be less sensitive to environmental variables than natural frequencies. However, simply using the difference between modal curvatures in the undamaged and damaged states can result into localization errors, due to the complex pattern that this quantity presents when considering broad damages or higher order modes. In this paper, we consider weakly damaged continuous beam and we exploit a perturbative solution of the beam equation of motion to obtain an analytical expression of the modal curvature variations in terms of damage distribution. The solution is then used to introduce a filtering technique of the modal curvature variation and to set up the inverse problem of damage localization based on modal curvatures only. Using numerical examples and experimental tests, we show that modal curvatures can be used for precise damage localization, once properly filtered

Analytical bond model for general type of reinforcements of finite embedment length in cracked cement based materials

In this work, a computational model for simulating the relevant mechanisms governing the pull-out of a discrete reinforcement embedded into cement based materials is described. The model accounts for the material and geometric properties of the reinforcement, which can include an anchored end, the interface between reinforcement and surrounding medium, and the relative inclination of the reinforcement to the crack plane. The reinforcement is modelled as a Timoshenko beam resting on a cohesive-like foundation that allows all the failure modes seen in the experiments to be accounted for, namely: debonding at the interface between the reinforcement and the concrete, cracking and spalling of the concrete matrix, rupture of the reinforcement. A comprehensive comparison with the experimental data available in the literature highlights the good predicting capabilities of the proposed model in terms of both peak force and dissipated energy. Furthermore, since the model is capable of simulating a discrete reinforcement of any direction towards the crack plane, complex mechanisms like micro-spalling of the matrix at the exit point of the reinforcement can be captured conveniently. By carrying out parametric analysis is possible to optimize the geometry of the anchored ends for maximizing the peak force and/or the energy dissipation in the pull-out process. Therefore, the developed model constitutes a relevant numerical tool for the optimization of discrete and continuous reinforcements of concrete structures including Fibre Reinforced Polymer (FRP) systems and Steel Fibre Reinforced Concrete (SFRC).FEDER through the Operational Program for Competitiveness Factors – COMPETE and from the Portuguese Foundation for Science and Technology (FCT) under the project FCOMP-01-0124-FEDER-01484

Pull-out tests with CFRP laminates applied according to the ETS/NSM technique

The retrofitting technique of Near-Surface Mounted (NSM) fiber-reinforced polymer (FRP) strips is receiving more attention recently due to many advantages over the externally bonded technique (EBR). However, in some situation is necessary to increase flexural and shear simultaneous. Therefore, to overcome this drawback, a hybrid strengthening technique that combines the advantages of NSM technique and Embedded Through Section (ETS) technique, is proposed by using an innovative CFRP laminate with rectangular shape in the NSM part and circular shape in the ETS. In order to evaluate the efficiency of the strengthening system, this paper presents the results of a series of pull-out tests using the innovative laminate to quantify the influence of the angle and embedded length. Using the results of this experimental program and developing a numerical strategy, an analytical bond stress–slip relationship was obtained

A multiscale model for optimizing the flexural capacity of FRC structural elements

In the present work, a multiscale model for fibre reinforced concrete (FRC) beams failing in bending is presented. At the microstructural level, the fibre is modelled as a one-dimensional continuum with axial, shear and bending deformability, with cohesive-like interfaces to simulate the interaction with the surrounding concrete. At the macroscopic level, the response of the beam is simulated by discretising the cross-section into layers and by enforcing the proper compatibility conditions between the layers. In the post-cracking stage, the tensile capacity is assured by the fracture energy of the concrete and the fibre resisting mechanisms simulated by the fibre pullout constitutive laws determined at the microstructural level. The model can account for fibre distribution and orientation, controlled by the casting conditions and geometry of the mould. By using experimental data available from the open literature, it is proved that such an integrated approach is able to derive, by inverse analysis, the stress-crack width relationship of FRC, which is the fracture mode I information in the material nonlinear analysis of FRC structures with approaches based on the finite element method.J.O.A. Barros, T. dos Santos Valente and I. G. Costa wish to acknowledge the support by FEDER through the Operational Program for Competitiveness Factors - COMPETE and Internationalization Program (POCI), under the project NG TPfib POCI-01-0247-FEDER-03371

High Light Induced Disassembly of Photosystem II Supercomplexes in Arabidopsis Requires STN7-Dependent Phosphorylation of CP29

Photosynthetic oxidation of water and production of oxygen by photosystem II (PSII) in thylakoid membranes of plant chloroplasts is highly affected by changes in light intensities. To minimize damage imposed by excessive sunlight and sustain the photosynthetic activity PSII, organized in supercomplexes with its light harvesting antenna, undergoes conformational changes, disassembly and repair via not clearly understood mechanisms. We characterized the phosphoproteome of the thylakoid membranes from Arabidopsis thaliana wild type, stn7, stn8 and stn7stn8 mutant plants exposed to high light. The high light treatment of the wild type and stn8 caused specific increase in phosphorylation of Lhcb4.1 and Lhcb4.2 isoforms of the PSII linker protein CP29 at five different threonine residues. Phosphorylation of CP29 at four of these residues was not found in stn7 and stn7stn8 plants lacking the STN7 protein kinase. Blue native gel electrophoresis followed by immunological and mass spectrometric analyses of the membrane protein complexes revealed that the high light treatment of the wild type caused redistribution of CP29 from PSII supercomplexes to PSII dimers and monomers. A similar high-light-induced disassembly of the PSII supercomplexes occurred in stn8, but not in stn7 and stn7stn8. Transfer of the high-light-treated wild type plants to normal light relocated CP29 back to PSII supercomplexes. We postulate that disassembly of PSII supercomplexes in plants exposed to high light involves STN7-kinase-dependent phosphorylation of the linker protein CP29. Disruption of this adaptive mechanism can explain dramatically retarded growth of the stn7 and stn7stn8 mutants under fluctuating normal/high light conditions, as previously reported

Auxetic orthotropic materials: Numerical determination of a phenomenological spline-based stored density energy and its implementation for finite element analysis

Abstract Auxetic materials, which have negative Poisson’s ratio, show potential to be used in many interesting applications. Finite element analysis (FEA) is an important phase in implementing auxetic materials, but may become computationally expensive because simulation often needs microscale details and a fine mesh. It is also necessary to check that topological aspects of the microscale reflects not only micro but macromechanical behavior. This work presents a phenomenological approach to the problem using data-driven spline-based techniques to properly characterize orthotropic auxetic material requiring neither analytical constraints nor micromechanics, expanding on previous methods for isotropic materials. Hyperelastic energies of auxetic orthotropic material are determined from experimental data by solving the equilibrium differential functional equations directly, so no fitting or analytical estimation is necessary. This offers two advantages; (i) it allows the FEA study of orthotropic auxetic materials without requiring micromechanics considerations, reducing modeling and computational time costs by two to three orders of magnitude; (ii) it adapts the hyperelastic energies to the nature of the material with precision, which could be critical in scenarios where accuracy is essential (e.g. robotic surgery)

Experimental Testing and Nonlinear Viscoelastic Modeling of Filled Rubber

Owing to its unique physical properties, rubber plays a keyrole in countless industrial applications. Tires, vibration absorbers and shoe soles are only but a few of the myriad uses of natural and synthetic rubber in an industry which in 2009 had an estimated market value of 2 billion euro. Despite a peculiar internal structure, the macroscopic behavior of filled-rubber is reminiscent of several biological soft tissues. While rubber is internally constituted by flexible long chain molecules that intertwine with each other, a similar role is played, in soft-tissues, by collagen fiber bundles. As a consequence, both classes of materials are able to sustain large strains and exhibit the characteristics of a viscous fluid and an elastic solid. In industry, the requirement to model complex geometrical structures made of materials exhibiting a nonlinear constitutive behavior is a compelling reason to use Finite Element Analysis (FEA) software. The predictive capabilities of these numerical tools strongly rely upon the capabilities of the underlying model to describe the material’s rheological properties. The possibility of simulating accurately the material behavior over the entire working range avoids the use of excessive number of prototypes, thereby reducing the need for expensive and difficult experimental tests; consequently, development costs can be drastically reduced. The theory of viscoelasticity is crucial in describing materials, such as filled rubber, which exhibit time dependent stress-strain behavior. In many engineering applications, such as the estimate of the rolling resistance of tires and hysteretic losses in soft biological tissues, the energy dissipation is a primary feature to be predicted. In addition, in the usual operative range, tires, shock absorbers and other rubber components bear finite dynamic deformations. Therefore, a reliable constitutive equation must be assessed within the theory of nonlinear viscoelasticity. A review of the literature revealed significantly more well-established studies dealing with hyperelastic constitutive models, than those dealing with finite viscoelasticity. Over the years, many hyperelastic models able to describe all the relevant aspects of the quasi-static response have been introduced. Furthermore, the American norms (ASTM D412, ASTM D575, ASTM D945, ASTM D6147, ASTM D1456) establish all the experimental techniques to identify the material constitutive parameters. In this context, many authors have recently addressed the problem of finite amplitude wave propagation or focused their interest upon particular boundary value problems. On the other hand, there is a lack of well-established nonlinear viscoelastic models capable of describing all the relevant effects in the material response. Moreover, a standardization similar to that concerning the static norms is yet to be achieved. The usual methodology provides for small harmonic deformations superimposed on a large static displacement. However, such a prescription does not allow the capture of many of the relevant nonlinear phenomena. In the literature, experimental evidence concerning finite dynamic deformations is rarely reported

Magneto-induced remodelling of fibre-reinforced elastomers

Fibre-reinforced elastomers are a class of elastomeric materials with peculiar me- chanical properties. If the fibres are metallic or metallic-coated their orientation can be controlled by applying a magnetic field during the curing phase of the elastomer so to tailor the material anisotropy. Once the elastomer is cured, the application of a magnetic field may be used to control the deformation of the solid. In this paper, we aim at modelling both the pre-curing and post-curing phases within a unifying framework compatible with nonlinear elasticity and growth theory. The coupling between elasticity and magnetic field is obtained by considering a proper form of the free energy density which accounts for the mutual orientation between the magnetic field and the fibres. By considering some asymptotic limits, it is shown that the proposed approach incorporates both the usual magneto-elastic model of solids as well as those models used to study the magnetic reorientation of fibres in viscous solutions

A form-finding strategy for magneto-elastic actuators

We study the inverse problem which arises when designing thin magneto-elastic actuators with bespoken deformation modes. By using the nonlinear model of magneto-elastic rods which we have recently proposed, we formulate the design problem as a PDE-constrained minimization whose solution gives to the optimal distribution of the magnetization profile necessary to achieve the desired shape. The same problem is extended to control multiple deformed configurations which would allow a controlled motion of the actuator to be realized

Multiscale modelling nano-platelet reinforced composites at large strain

We study the behaviour of an incompressible particle-reinforced neo-Hookean (IPRNC) material when subjected to large plain strain deformation. The peculiarity of the model consists in the rectangular shape of the particle which yields the macroscopic response of the composites non isotropic. This is indeed the case for many reinforcements currently used in composites at all length scales: short-fibres, clays, graphene. The consequence of the anisotropic reinforcement in this model at short strain is evident in the stiffness that is observed to depend strongly on the platelet orientation; a transverse stiffening effect when the platelet is oriented perpendicular to the loading direction proves to be almost as significant as the longitudinal stiffness contribution usually considered for anisotropic reinforcements. The large strain effects of orientation are also significant and an understanding of them is relevant to a number of applications that can take advantage of the large strain non-linear response