A Cohesive interface formulation in large displacements

Mechanical interfaces are theoretical and computational tools able to properly reproduce the progressive decohesion along predefined surfaces. Scientific literature is rich of interface models, developed under very different conctitutive framework, but mostly developed in small displacements, whereas a few of them assess the problem in a geometrically nonlinear setting. In the present contribution interface formulation is rigorously developed in the large displacements regime. The relevant cohesive interface constitutive relations are defined in the local reference with normal and tangential axes to the middle surface in the current configuration. The interface is defined as a zero thickness layer with the traction vector acting between the two connected surfaces. Membrane forces are assumed negligible and separation displacement is assumed to remain small, at least up to full debonding

A nonlinear finite element approach with cohesive-frictional interfaces for mode II Transverse Crak Tension test insight

In many circumstances structural failure modes are driven by the formation and propagation of fractures. For instance in composite laminate structures one of the most worrying condition is delamination, which is an interlaminar progressive fracture. Fracture toughness is the material mechanical parameters which ensure fracture safe condition and it is also an essential parameter for performing nonlinear structural analysis, no matter if based on Fracture Mechanics or by means of Interface Cohesive theories. It is then of paramount relevance to evaluate the critical fracture energy by means of simple and reliable laboratory tests. Several tests are available for the direct determination of mode I and mode II fracture energies. If for mode I, fracture energy determination is nowadays well defined and the Double Cantilever Beam (DCB) test is normed and universally adopted, it is not the same for mode II fracture energy. The tests based on bending beams theory such as End Notched Flexure (ENF), End Loaded Split (ELS) test and Four Point End Notched Flexure (4ENF) are all widely used tests, which however for different reasons have not been fully accepted

Finite Displacements and Corrotational Interfaces: Consistent formulation and Symmetry of the Stiffness Matrix

Mechanical interfaces are theoretical and computational tools able to properly reproduce then progressive delamination of composite structures. Scientific literature is rich of interface models, mostly developed in small displacements, whereas a few of them assess the problem in a geometrically nonlinear setting. In the present paper interface formulation is rigorously developed in a geometrically nonlinear setting, and the relevant interface constitutive relations are defined in the local reference with normal and tangential axes to the middle surface in the current configuration. The interface is defined as a zero thickness layer with tractions acting between the two connected surfaces. Membrane forces are assumed negligible and separation displacement is assumed to remain small, at least up to full debonding. Under this “constitutive” hypothesis rotational equilibrium is implicitly verified

A frictional interface model for the propagation of cohesive fracture under cyclic loading

The paper presents an extension of a recent presented mechanical interface model, [1-2], for the description of the smooth cohesive/frictional transition along potentially active cohesive fracture surfaces. The model presented includes the description of internal frictional dissipative mechanisms which are active under combined compressive/sliding loading in either the cohesive process zone, or in the fully fractured interface portion. Moreover, always under compressive/sliding loading conditions, frictional dissipation mechanisms can also develop in the undamaged (or sound) portion of the interface, justified by the circumstance that also at the virgin state in the bonding surface are present initial defects (stable micro-fracures or microvoids) which might generate friction. The main features of the proposed model may be better understood considering deformation mechanisms involved in the interface layer of finite thikness analyzed at the microscale level, i.e. by investigating the microstrural response of an heterogeneous damageable thin layer. Mechanical information passages from the microscale response to the macro interface constitutive relations are investigated in a multiscale point of view. The interface, zero thickness, constitutive model has been implemented in a FE environment and specific nonlinear numerical response are reported for cyclic loading, as well as for monotonic increasing loading. Finally, it will be shown as the model is able to reproduce the cyclic response and the progressive fracture propagation, also in the case of high number of low constant amplitude cyclic loading. The high number cyclic failure is shown to be promoted by the internal friction mechanisms which induces a further low-intensity damage development in the cohesive zone and then fatigue-type failure. The numerical response obtained with the proposed model is also compared with corresponding experimental data. Acknowledgements: A grant from MIUR for PRIN09, 2009XWLFKW project Multi-scale modelling of materials and structures is acknowledged. References [1] Parrinello, F., Failla. B. and Borino, G. \u201cCohesive\u2013frictional interface constitutive model,\u201d Int. J. Solids Strct., 46, 2680-2692 (2009). [2] Marannano G., Pasta, A., Borino, G., Parrinello F. and Terranova, M. \u201cEffetto dell\u2019attrito nell\u2019avanzamento della delaminazione per modo II di frattura, \u201d in: Proc. 40\ub0 Conv. AIAS, Palermo, September 7-10, 2011

Multiple surface cracking and debonding failure for thin thermal coatings

A mechanical analysis of thin films of quasi-brittle materials used as thermal coatings for superalloy substrate is proposed. The study considers a bi-material element subjected to uniform tension formed by a thin layer of quasi-brittle material (typically a ceramic) bonded on an elastic substrate. The bounding between the coating film and the substrate is realized by a very thin primer which mechanically modeled as a zero thickness cohesive frictional interface. The analysis is developed by a non-linear finite element simulation in which, in order to consider damage size effects, a non-local isotropic damage model is adopted for the quasi-brittle coating. The results of the analysis shows the formation of multiple cracks on the coating surface which propagate up to the interface. At the same time, due to the mismatch between the elastic moduli between the coating and the substrate and the development of the transverse cracks, a competing debonding mechanism along the interface develops. The numerical results show also, for thick coating layers, the development of skew crack bands, which forecast coating spalling

Hybrid equilibrium element with high-order stress fields for accurate elastic dynamic analysis

In the present article the two-dimensional hybrid equilibrium element formulation is initially developed, with quadratic, cubic, and quartic stress fields, for static analysis of compressible and quasi-incompressible elastic solids in the variational framework of the minimum complementary energy principle. Thereafter, the high-order hybrid equilibrium formulation is developed for dynamic analysis of elastic solids in the variational framework of the Toupin principle, which is the complementary form of the Hamilton principle. The Newmark time integration scheme is introduced for discretization of the stress fields in the time domain and dynamic analysis of both the compressible solid and quasi-incompressible ones. The hybrid equilibrium element formulation provides very accurate solutions with a high-order stress field and the results of the static and dynamic analyses are compared with the solution of the classic displacement-based quadratic formulation, showing the convergence of the two formulations to the exact solution and the very satisfying performance of the proposed formulation, especially for analysis of quasi-incompressible elastic solids

An extrinsic interface developed in an equilibrium based finite element formulation

The phenomenon of delamination in composite material is studied in the framework of hybrid equilibrium based formulation with extrinsic cohesive zone model. The hybrid equilibrium formulation is a stress based approaches defined in the class of statically admissible solutions. The formulation is based on the nine-node triangular element with quadratic stress field which implicitly satisfy the homogeneous equilibrium equations. The inter-element equilibrium condition and the boundary equilibrium condition are imposed by considering independent side displacement fields as interfacial Lagrangian variable, in a classical hybrid formulation. The hybrid equilibrium element formulation is coupled with an extrinsic interface, for which the interfacial separation is zero for a sound interface. The extrinsic interface is defined as a rigid-damage cohesive zone model (CZM) in the rigorous thermodynamic framework of damage mechanics and is defined as embedded interface at the hybrid equilibrium element sides

Polyamorphism of ice at low temperatures from constant-pressure simulations

We report results of MD simulations of amorphous ice in the pressure range 0 - 22.5 kbar. The high-density amorphous ice (HDA) prepared by compression of Ih ice at T = 80 K is annealed to T = 170 K at intermediate pressures in order to generate relaxed states. We confirm the existence of recently observed phenomena, the very high-density amorphous ice and a continuum of HDA forms. We suggest that both phenomena have their origin in the evolution of the network topology of the annealed HDA phase with decreasing volume, resulting at low temperatures in the metastability of a range of densities.Comment: 11 pages, 5 postscript figures. To be published in Physical Review Letter

Inducible galectins are expressed in the inflamed pharynx of the ascidian Ciona intestinalis

Although ascidians belong to a key group in chordate phylogenesis, amino acid sequences of Ciona intestinalis galectin-CRDs (CiLgals-a and -b) have been retained too divergent from vertebrate galectins. In the present paper, to contribute in disclosing Bi-CRD galectin evolution a novel attempt was carried out on CiLgals-a and -b CRDs phylogenetic analysis, and their involvement in ascidian inflammatory responses was shown. CiLgals resulted aligned with Bi-CRD galectins from vertebrates (Xenopus tropicalis, Gallus gallus, Mus musculus, Homo sapiens), cephalochordates (Branchiostoma floridae), echinoderms (Strongylocentrotus purpuratus) and a mono-CRD galectin from the ascidian Clavelina picta. The CiLgalsa N-terminal and C-terminal CRDs contain the signature sequence involved in carbohydrate binding, whereas the CiLgals-b C-CRD presents only three out of seven key aminoacids and it could not be suitable as sugar binding motif. Sequence similarity between clusters suggests an evolutionary model based on CRD domain gene duplication and sequence diversification. In particular CiLgals-b N-CRD and C-CRD were similar to each other and both grouped with the ascidian C. picta mono-CRD. Homology modeling process shows a CiLgals molecular structure superimposed to chicken and mouse galectins. The CiLgalsa and CiLgals-b genes were upregulated by LPS inoculation suggesting that they are inducible and expressed in the inflamed pharynx as revealed by real-time PCR analysis. Finally, in situ hybridization and immunohistochemical assays showed their localization in the inflamed tissues, while immunoblotting analysis indicated that CiLgals can form oligomer