Alp Karakoc

A direct simulation method for the effective in-plane stiffness of cellular materials

A simulation model is presented in order to provide the near-exact geometrical description of the actual cellular materials and to determine the effective in-plane stiffness of them. The model consists of a geometric input generator and a micromechanical model. The input generator is used to describe the cellular material geometry including the cell wall thicknesses, cell connectivity, vertex and center coordinates through the in situ specimen images while the micromechanical model is used to incorporate the geometrical description and the mechanical behavior of the cell walls. For model validation, a case study is performed on Nomex honeycomb specimens, the results of which are expressed in terms of the effective in-plane stiffness properties and compared to the measured values provided in the literature.

STATISTICAL STRENGTH ANALYSIS FOR HONEYCOMB MATERIALS

In the present study, a statistical strength model is proposed, which aims at describing how the strength of geometrically irregular honeycomb material is affected by the scale. Hence, the samples are designed based on the selected geometrical irregularity and the number of the cells/scale. Simulation experiments are conducted on these samples under different loading combinations. The experiment results are linked to possible failure mechanisms in order to obtain the critical loads which are expressed in terms of cumulative distribution functions. The discrete distribution data of the critical loads are then fitted to analyze the effect of scale on different strength percentiles by virtue of the least squares function and closed quadric surface fitting. Eventually, the outcome is expressed in terms of ellipsoid surface representing the honeycomb material strength in three-dimensional stress space.

Effects of Morphology and Topology on the Effective Stiffness of Chiral Cellular Materials in the Transverse Plane

The present study investigates the influence of topology and morphology on the effective stiffness of chiral cellular materials in the transverse plane by means of a homogenization method. For this purpose, finite element models of representative volume elements for regular hexagonal and hexagonal-chiral configurations are used and simulations are conducted to quantify how cell topology—that is, chirality inside the cell—and cell wall slenderness affect the effective stiffness. Closed form solutions for regular hexagonal square and triangular RVEs provided in the literature are then taken as a basis for model validation. The results indicate that there are drastic differences between regular hexagonal and hexagonal-chiral configurations, which can be explained in terms of deformation mechanism transformations between bending and stretching. The investigations also reveal the positive impact of cell wall slenderness on stiffness due to volumetric increase in the cell wall material resisting the deformation.

Films based on crosslinked TEMPO-oxidized cellulose and predictive analysis via machine learning

We systematically investigated the effect of film-forming polyvinyl alcohol and crosslinkers, glyoxal and ammonium zirconium carbonate, on the optical and surface properties of films produced from TEMPO-oxidized cellulose nanofibers (TOCNFs). In this regard, UV-light transmittance, surface roughness and wetting behavior of the films were assessed. Optimization was carried out as a function of film composition following the “random forest” machine learning algorithm for regression analysis. As a result, the design of tailor-made TOCNF-based films can be achieved with reduced experimental expenditure. We envision this approach to be useful in facilitating adoption of TOCNF for the design of emerging flexible electronics, and related platforms.

Stochastic fracture of additively manufactured porous composites

Extrusion-based fused deposition modeling (FDM) introduces inter-bead pores into dense materials, which results in part-to-part mechanical property variations, i.e., low mechanical reliability. In addition, the internal structure of FDMed materials can be made porous intentionally to tailor mechanical properties, introduce functionality, reduce material consumption, or decrease production time. Despite these potential benefits, the effects of porosity on the mechanical reliability of FDMed composites are still unclear. Accordingly, we investigated the stochastic fracture of 241 FDMed short-carbon-fiber-reinforced-ABS with porosity ranging from 13 to 53 vol.% under tensile load. Weibull analysis was performed to quantify the variations in mechanical properties. We observed an increase in Weibull modulus of fracture/tensile strength for porosity higher than ~40 vol.% and a decrease in Weibull modulus of fracture strain for an increase in porosity from 25 to 53 vol.%. Micromechanics-based 2D simulations indicated that the mechanical reliability of FDMed composites depends on variations in bead strength and elastic modulus of beads. The change in raster orientation from 45°/−45° to 0° more than doubled the Weibull modulus. We identified five different types of pores via high-resolution X-ray computed tomography. A 22% and 48% decrease in carbon fiber length due to extrusion was revealed for two different regions of the filament.

Sensitivity analysis on the effective stiffness properties of 3-D orthotropic honeycomb cores

ABSTRACT The present study investigates the influences of representative volume element RVE mesh and material parameters, here cell wall elastic moduli, on the effective stiffness properties of three dimensional orthotropic honeycomb cores through strain driven computational homogenization in the finite element framework. For this purpose, case studies were carried out, for which hexagonal cellular RVEs were generated, meshed with eight node linear brick finite elements of varying numbers. Periodic boundary conditions were then implemented on the RVE boundaries by using one-to-one nodal match for the corresponding corners, edges and surfaces for the imposed macroscopic strains. As a novelty, orthotropic material properties were assigned for each cell wall by means of the transformation matrices following the cell wall orientations. Thereafter, simulations were conducted and volume averaged macroscopic stresses were obtained. Eventually, effective stiffness properties were obtained, through which RVE sensitivity analysis was carried out. The investigations indicate that there is a strong relation between number of finite elements and most of the effective stiffness parameters. In addition to this, cell wall elastic moduli also play critical role on the effective properties of the investigated materials.

Shape and cell wall slenderness effects on the stiffness of wood cell aggregates in the transverse plane

The present study investigates a homogenization method in the framework of finite element method to determine the effective stiffness properties of wood cell aggregates in the transverse plane. For this purpose, square and regular hexagonal representative volume elements are chosen to mimic the wood cells. Thereafter, simulation experiments are conducted to understand how different cell shapes and cell wall slenderness, which is cell wall thickness to height ratio, affect the stiffness properties in the transverse plane. The comparison between analytical and computational homogenization results show that square cells have higher elastic moduli than the ones computed for hexagonal cells whereas shear modulus of both cell shapes have more or less the similar values. This can be explained due to the effective deformation mechanisms under different loading conditions. Thus, the present study provides an effective stiffness estimation tool and insight for wood cell aggregates.

A new approach for determining the hidden material parameters of a honeycomb

An optimization method to obtain the cell wall properties of Nomex honeycomb material is presented. There, the outcomes of physical experiments and micromechanical simulations are compared in an effort to identify the geometric or/and material parameters for the best match. Only the cell wall thickness and Young’s modulus, called here as the hidden parameters, are used in the matching as the Young’s modulus is difficult measure reliably. The mean values and standard deviations of the geometric parameters of the cell structure model are obtained through image analysis. In the micromechanical model used, the cell walls are considered as linearly elastic Bernoulli beams. The optimum hidden parameter values for the Nomex case turn out not to be unique but they appear in a combination known as the bending stiffness.