Abstracts:Crystal plasticity finite element simulations of tensile tests on thin polycrystalline samples with grain orientations representative of the microstructure in Al–Cu–Li rolled sheets are carried out to study the influence of grain-to-grain interactions on plastic strain localization. Anisotropic work-hardening and rate-sensitivity of the material behavior are assumed. The grains are modeled as thin platelets in the through-thickness direction, elongated in the rolling and transverse directions. The only distinctive feature of the present simulations with respect to standard crystal plasticity calculations is the enforcement of tangential continuity conditions on the elastic and plastic distortion rates along grain boundaries, which introduces grain-to-grain interactions and renders the simulations nonlocal. Whereas standard crystal plasticity calculations do not predict any significant plastic strain pattern, slanted shear bands spontaneously emerge throughout the sample in the simulations involving tangential continuity, in agreement with experimental observations. Also in agreement with experimental data, shear banding is delayed when the tensile axis shifts from rolling to transverse direction, and the trend to shear banding is enhanced when grain thickness is decreased, particularly in loading along the rolling direction.

Abstracts:Hyperelastic constitutive models for anisotropic biological materials are frequently based on orthotropic incompressible stored energy functions. The material parameters of these models are then obtained through an optimization procedure as to fit some stress-strain experimental data. For example, in arterial wall mechanics the material data usually employed for the Holzapfel-Gasser-Ogden and the Gasser-Ogden-Holzapfel models are two uniaxial tension curves from circumferential and axial specimens. The transverse strains from these specimens are frequently not taken into consideration. In this paper we analyze the evolution of those strains, showing that an unrealistic behaviour may be predicted. We then show how transverse strains may be prescribed using our What-You-Prescribe-Is-What-You-Get (WYPIWYG) model in a very intuitive way, still capturing the longitudinal stress-strain behavior in an exact manner without employing any constitutive parameter. This is possible because, in contrast to what it is usually done, we exactly solve the equilibrium and compatibility equations without imposing the shape of the stored energy function. Furthermore, we show that the small strains formulation is naturally recovered and that the physical insight from the infinitesimal theory is preserved. In fact, for incompressible materials, the present approach can be considered as a natural extension of the infinitesimal continuum elastic framework to large strains. This new physical insight clearly shows that if some subclasses of orthotropic incompressible material models are determined with just two uniaxial curves, then the transverse behavior should be contrasted with additional experimental observations.

Abstracts:The displacement discontinuity method is developed to analyze an arbitrarily shaped planar interfacial crack in a three-dimensional isotropic thermal elastic bi-material under combined thermo-mechanical loadings. The fundamental solutions for uniformly distributed displacement and temperature discontinuities applied over a triangular element are obtained via the displacement and temperature discontinuity- boundary integral-differential equation method. In order to eliminate the oscillatory singularity near the crack front, the Delta function in the fundamental solutions is approximated by the Gaussian distribution function, and accordingly, the unit Heaviside step function is replaced by the Error function. The stress and heat flux intensity factors without the oscillatory singularity as well as the Energy release rate are presented. An elliptical interface crack is analyzed as an example to validate the fundamental solutions and the proposed numerical method. The influences of thermal loading and material-mismatch on the thermo-mechanical response are studied, the difference between the homogeneous material and bi-material is also analyzed.

Abstracts:Micro- and nano-scale elements are more prone to the effects of adhesion than are their macro-scale counterparts. At the same time these structural elements are important in a variety of modern applications. For example carbon nanotubes and single layer graphene have excellent mechanical and electrical properties which make them attractive elements for many of these small scale applications. Adhesive contact and the resulting morphology of micro-/nano-scale beams/plates on a sinusoidal/grooved surface are the subject of this investigation. A non-dimensional adhesion energy is shown to play a key role in the transition between different configurations. There are three possible morphologies – point/line contact at the surface peaks, partially conformal contact over a finite length, and fully conformal contact. The objective of this study is to determine the conditions under which these elements will follow the topology of the surface, when separation will occur, and the extent of the separation.

Abstracts:Buildings, railway tracks, drill strings and off-shore pipelines are all treated as structures on elastic foundations in order to study their response behavior in many engineering applications. Also, flexible polyurethane foams used for cushioning in furniture, foot-ware, and automotive industries serve as foundations, and exhibit complex nonlinear viscoelastic behavior. It is challenging to develop models of systems that include these foam-like materials and are able to predict the behaviour over a wide range of loading conditions. Even when using the simpler models commonly utilized in the literature, it is computationally expensive to predict the steady-state response of these structures to static and harmonic loads. In this work a pinned-pinned beam interacting with a viscoelastic foundation which can react both in tension and compression, or in compression alone is considered. The model developed here is capable of predicting the response to static as well as dynamic forces, whether they are concentrated or distributed. If the foundation reacts only in compression, the contact region changes with beam motion and the estimation of the unknown contact region is embedded into the iterative solution procedure. The steady-state solution is expressed as the sum of an arbitrary number of modes of an undamped pinned-pinned beam and Galerkin method is used to derive equations for the modal amplitudes. Incremental harmonic balance is used to make the steady-state frequency response predictions more efficient and a pseudo arc-length continuation technique is used to track both stable and unstable solution branches. By using these computationally efficient solution approaches, it is possible to explore a much wider variety of loading conditions and also quickly determine the number of modes required for convergence of the periodic solution. By using this solution method, the influence of various system parameters on the response of the beam is studied.

Abstracts:In this study, a theoretical basis of thermomechanical instability during the solidification of pure metals on a coated planar mold is presented. This study extends the previous works by taking into account the presence of a deformable coating layer on the inner mold surface. The thermal and mechanical problems are assumed to be coupled through the pressure dependent thermal contact resistances at the shell/coating and coating/mold interfaces. On the other hands, the thermal capacitance of solidified shell, coating layer and mold materials are neglected for the sake of simplicity. A linear perturbation method is used to reduce the spatial dimension of the problem. The model leads to two coupled differential equations for the shell thickness perturbation and residual stress which are solved numerically. The results document the variation of the perturbed solidification front as a function of ratio between the thermal conductivities of the shell and coating materials for combinations of other process parameters such as the combination of shell and mold materials, the values of coating thickness and coupling rates. In case of weak coupling, the maximum magnitude of this perturbation decreases regardless of the value of coating thickness and shell-mold material combinations when the thermal conductivity of the coating material is decreased. In case of strong couplings however, a critical thermal conductivity ratio between shell and coating materials which leads to a formation of greatest or lowest perturbation in the solidification front depending on the values of other process parameter has been found.

Abstracts:Temperature-sensitive hydrogels have recently been implemented vastly for biomedical and microfluidic sensors and actuators. The accurate and efficient design of the bilayer sensors and actuators made of temperature sensitive hydrogels are of crucial importance. In this work, we develop an analytical method to solve the swelling induced bending of temperature responsive hydrogel bilayer under plane strain condition. The bilayer consists of a neutral incompressible elastomer layer attached to a temperature sensitive hydrogel layer. An analytical approach is developed to predict the thermomechanical response of these bilayers. At the other hand, the finite bending of the bilayer is simulated applying the finite element method. Several cases are solved to demonstrate the validity and performance of the proposed analytical method. The deformation and the stresses inside the layers are presented for various material parameters employing both the developed analytical formulation as well as the finite element method. A good correspondence between the presented method and the finite element method is observed. Finally, the effect of material and geometrical parameters on curvature are also investigated.