Ahmad Rafsanjani | McGill University (original) (raw)

Papers by Ahmad Rafsanjani

Advanced materials (Deerfield Beach, Fla.), Jan 21, 2015

A snapping mechanical metamaterial is designed, which exhibits a sequential snap-through behavior... more A snapping mechanical metamaterial is designed, which exhibits a sequential snap-through behavior under tension. The tensile response of this mechanical metamaterial can be altered by tuning the architecture of the snapping segments to achieve a range of nonlinear mechanical responses, including monotonic, S-shaped, plateau, and non-monotonic snap-through behavior.

Advanced materials (Deerfield Beach, Fla.), 2015

By exploiting snap-through instabilities, D. Pasini and co-workers design a damage-tolerant mecha... more By exploiting snap-through instabilities, D. Pasini and co-workers design a damage-tolerant mechanical metamaterial that snaps sequentially under tension, thereby accommodating a very large deformation up to 150%. On page 5931, they describe how the nonlinear mechanical response of the metamaterial can be robustly programmed by tuning the architecture of its unit cell.

Advanced Materials, 2015

Volume 9: Mechanics of Solids, Structures and Fluids, 2013

ABSTRACT Wood, due to its biological origin, has the capacity to interact with water. Sorption/de... more ABSTRACT Wood, due to its biological origin, has the capacity to interact with water. Sorption/desorption of moisture is accompanied with swelling/shrinkage and softening/hardening of its stiffness. The correct prediction of the behavior of wood components undergoing environmental loading or industrial process requires that the hygric, thermal and mechanical (HTM) behavior of wood are considered in a coupled manner. In addition, we propose a comprehensive framework using a fully coupled poromechanical approach, where its multiscale implementation provides the capacity to take into account, directly, the exact geometry of wood cellular structure, using computational homogenization. A hierarchical model is used to take into account the subcellular composite-like organization of the material. Such advanced modeling requires high resolution experimental data for the appropriate determination of inputs and for its validation.

Fluid-Solid Coupling in Porous Media, 2014

Experimental Mechanics, 2014

A new methodology for restraining the swelling of spruce wood samples in the micrometre range is ... more A new methodology for restraining the swelling of spruce wood samples in the micrometre range is developed and presented. We show that the restraining device successfully prevents the free swelling of wood during moisture adsorption, thus modifying significantly the anisotropy of swelling and provoking the intended collapse and large deformations of the wood cells at the edges of the sample in contact with the restraining device. The device consists in a slotted cube designed t o r e s t r a i n s w e l l i n g a n d i s m a d e o f P M M A manufactured by laser ablation. The sample undergoing the restraining experiment is imaged with highresolution synchrotron radiation phase contrast X-Ray Tomographic Microscopy. The deformation of the restraining device itself is only approximately 2 μm with respect to a 500 μm width in cubes containing latewood samples and half of that in the case of cubes containing earlywood.

Scientific Reports, 2015

The spirally arranged stems of the spikemoss Selaginella lepidophylla, an ancient resurrection pl... more The spirally arranged stems of the spikemoss Selaginella lepidophylla, an ancient resurrection plant, compactly curl into a nest-ball shape upon dehydration. Due to its spiral phyllotaxy, older outer stems on the plant interlace and envelope the younger inner stems forming the plant centre. Stem curling is a morphological mechanism that limits photoinhibitory and thermal damages the plant might experience in arid environments. Here, we investigate the distinct conformational changes of outer and inner stems of S. lepidophylla triggered by dehydration. Outer stems bend into circular rings in a relatively short period of desiccation, whereas inner stems curl slowly into spirals due to hydro-actuated strain gradient along their length. This arrangement eases both the tight packing of the plant during desiccation and its fast opening upon rehydration. The insights gained from this work shed light on the hydro-responsive movements in plants and might contribute to the development of deployable structures with remarkable shape transformations in response to environmental stimuli. R esurrection plants are vascular plants tolerant to extreme vegetative desiccation that are able to resume normal growth and metabolic activity upon rehydration 1 . The spikemoss Selaginella lepidophylla is an ancient 2 resurrection plant native to Chihuahuan desert (Mexico and United States) that shows dramatic curling and uncurling with changes in plant hydration 3 . When dehydrated, the spirally arranged stems of S. lepidophylla tightly curl to form a rough sphere. As a result of this morphology, the outer stems serve to substantially reduce solar radiation (.99.7%) exposed to inner stems at the centre of the plant 4 . The morphological and anatomical traits of S. lepidophylla in relation to the curling of its stems were examined at the turn of the twentieth century 5 . At that time, it was elucidated that the movements of the tissues are entirely physicalrather than biophysical-and depend upon the hygroscopic capacities of the tissues 6 .

RSC Adv., 2014

Experimental studies reveal that softwoods exhibit different swelling patterns at the cellular sc... more Experimental studies reveal that softwoods exhibit different swelling patterns at the cellular scale depending on the position of the tracheid cells within the growth ring. Thin-walled earlywood cells show anisotropic swelling behavior while the swelling of thick-walled bulky latewood cells is generally isotropic. A poromechanical model is developed to explore the anisotropic swelling behavior of softwoods at the cellular scale. The general description for the macroscopically observable free swelling strain of cellular tissues is derived by upscaling the constitutive equations of a double porosity medium which is found to be dependent on stiffness, Biot coefficient, Biot modulus and the geometry of the cells. The effective poroelastic constants of earlywood and latewood cells are computed from a periodic honeycomb unit cell by means of an efficient finite-element-based computational upscaling scheme. The estimated swelling coefficients compare well with experimental measurements. It is found that the anisotropy in swelling behavior of wood cells can be related to the anisotropy of elastic properties at the cell wall level and the geometry of the cells. The proposed poromechanical model provides a physically relevant description of swelling behavior which originates from the coupled interaction of water and solid phases within the porous cell walls of softwoods.

Philosophical Magazine, 2012

Wood is a hygromorphic material, meaning it responds to changes in environmental humidity by chan... more Wood is a hygromorphic material, meaning it responds to changes in environmental humidity by changing its geometry. Its cellular biological structure swells during wetting and shrinks during drying. The origin of the moisture-induced deformation lies at the sub-cellular scale. The cell wall can be considered a composite material with stiff cellulose fibrils acting as reinforcement embedded in a hemicellulose/lignin matrix. The bulk of the cellulose fibrils, forming 50% of the cell wall, are oriented longitudinally, forming long-pitched helices. Both components of cell wall matrix are displaying swelling. Moisture sorption and, to a lesser degree, swelling/ shrinkage are known to be hysteretic. We quantify the affine strains during the swelling and shrinkage using high resolution images obtained by phase contrast synchrotron X-ray tomography of wood samples of different porosities. The reversibility of the swelling/shrinkage is found for samples with controlled moisture sorption history. The deformation is more hysteretic for high than for low density samples. Swelling/shrinkage due to ad/desorption of water vapour displays also a non-affine component. The reversibility of the swelling/shrinkage indicates that the material has a structural capacity to show a persistent cellular geometry for a given moisture state and a structural composition that allows for moistureinduced transitional states. A collection of qualitative observations of small subsets of cells during swelling/shrinkage is further studied by simulating the observed behaviour. An anisotropic swelling coefficient of the cell wall is found to emerge and its origin is linked to the anisotropy of the cellulose fibrils arrangement in cell wall layers.

Mechanics of Materials, 2012

and sharing with colleagues.

Journal of The Royal Society Interface, 2014

Journal of Building Physics, 2013

Wood, due to its biological origin, has the capacity to interact with water. Sorption/desorption ... more Wood, due to its biological origin, has the capacity to interact with water. Sorption/desorption of moisture is accompanied with swelling/shrinkage and softening/ hardening of its stiffness. The correct prediction of the behavior of wood components undergoing environmental loading requires that the moisture behavior and mechanical behavior of wood are considered in a coupled manner. We propose a comprehensive framework using a fully coupled poromechanical approach, where its multiscale implementation provides the capacity to take into account, directly, the exact geometry of the wood cellular structure, using computational homogenization. A hierarchical model is used to take into account the subcellular composite-like organization of the material. Such advanced modeling requires high-resolution experimental data for the appropriate determination of inputs and for its validation. High-resolution x-ray tomography, digital image correlation, and neutron imaging are presented as valuable methods to provide the required information.

Composites Science and Technology, 2012

The hygro-mechanical behavior of a hierarchical cellular material, i.e. growth rings of softwood ... more The hygro-mechanical behavior of a hierarchical cellular material, i.e. growth rings of softwood is investigated using a two-scale micro-mechanics model based on a computational homogenization technique. The lower scale considers the individual wood cells of varying geometry and dimensions. Honeycomb unit cells with periodic boundary conditions are utilized to calculate the mechanical properties and swelling coefficients of wood cells. Using the cellular scale results, the anisotropy in mechanical and swelling behavior of a growth ring in transverse directions is investigated. Predicted results are found to be comparable to experimental data. It is found that the orthotropic swelling properties of the cell wall in thin-walled earlywood cells produce anisotropic swelling behavior while, in thick latewood cells, this anisotropy vanishes. The proposed approach provides the ability to consider the complex microstructure when predicting the effective mechanical and swelling properties of softwood.

Composites Part A: Applied Science and Manufacturing, 2013

The swelling of the hierarchical cellular structure of wood can be properly predicted when both t... more The swelling of the hierarchical cellular structure of wood can be properly predicted when both the cellular and the growth ring scales are taken into account. In this study, a multiscale computational upscaling finite element model is utilized for the estimation of the free swelling behavior of Norway spruce softwood. The microstructural information, e.g. the geometry of the wood cells, the local density and the microfibril angle across the growth rings is the input of the lower scale cellular model. The elastic properties and the swelling coefficients within the growth ring are estimated using a periodic honeycomb unit cell model. Based on this model, the transverse anisotropy in the swelling behavior of softwood at timber or growth ring level is then predicted. Comparison of simulation results with experimental measurements obtained using digital image correlation shows very good agreement.

Composite Structures, 2013

In this paper, the hygro-elastic behavior of two-dimensional periodic honeycombs composed of mult... more In this paper, the hygro-elastic behavior of two-dimensional periodic honeycombs composed of multilayered cell walls is investigated using a computational micromechanics approach. Detailed numerical results for the effective hygro-expansion coefficients and the elastic moduli of honeycombs are obtained. The influence of the arrangement of the cell wall layers and the geometrical parameters of the honeycombs on the effective hygro-elastic properties is examined. Limiting cases are considered, and the validity of the model is established by comparison with the analytical solutions available in the existing literature. The obtained results suggest that the layered architecture of the cell wall enhances the anisotropy in swelling behavior of honeycombs with irregular configuration which is reflected in their transverse hygro-expansion coefficients while regular honeycombs show isotropic behavior. The proposed model explains the complex thermo-hygro-mechanical behavior of natural cellular materials and provides a predictive tool for bio-mimetic material design. (A. Rafsanjani), dominique.derome@empa.ch (D. Derome), carmeliet@arch.ethz.ch (J. Carmeliet). Composite Structures 95 (2013) 607-611

Applied Physics Letters, 2013

We find that, in two-dimensional periodic cellular solids, the hygro-expansion properties of the ... more We find that, in two-dimensional periodic cellular solids, the hygro-expansion properties of the cell wall and the geometrical configurations of the lattice determine the effective swelling behavior of the medium. In this letter, we present the associated phase diagram for the swelling anisotropy of conventional and re-entrant honeycomb morphologies. The presented results are obtained numerically from a finite element based computational upscaling scheme. We show how the pattern of anisotropy in swelling behavior of cellular materials reverses when swelling is more important across or along the cell walls. V C 2013 AIP Publishing LLC. [http://dx.

Proceedings of the Institution of Mechanical Engineers, Part K: Journal of Multi-body Dynamics, 2008

The current paper focuses on the non-linear torsional vibration of a one-stage transmission gear ... more The current paper focuses on the non-linear torsional vibration of a one-stage transmission gear system. Four different methods have been applied for solution of the equation of motion; the discretization method, the perturbation method, the Ritz method, and the stepwise time integration of the equation of motion. The time and frequency results from these analyses have been compared with each other, as well as those reported in literatures. Although all of these methods are accurate and computationally effective for finding the main spectral contribution, however, only the discretization method and the step-wise time-integration model are able to identify the other frequency components.

Acta Mechanica, 2011

A semi-analytical analysis for the transient elastodynamic response of an arbitrarily thick simpl... more A semi-analytical analysis for the transient elastodynamic response of an arbitrarily thick simply supported beam due to the action of an arbitrary moving transverse load is presented, based on the linear theory of elasticity. The solution of the problem is derived by means of the powerful state space technique in conjunction with the Laplace transformation with respect to the time coordinate. The inversion of Laplace transform has been carried out numerically using Durbin’s approach based on Fourier series expansion. Special convergence enhancement techniques are invoked to completely eradicate spurious oscillations and obtain uniformly convergent solutions. Detailed numerical results for the transient vibratory responses of concrete beams of selected thickness parameters are obtained and compared for three types of harmonic moving concentrated loads: accelerated, decelerated and uniform. The effects of the load velocity, pulsation frequency and beam aspect ratio on the dynamic response are examined. Also, comparisons are made against solutions based on Euler–Bernoulli and Timoshenko beam models. Limiting cases are considered, and the validity of the model is established by comparison with the solutions available in the existing literature as well as with the aid of a commercial finite element package.