X-ray computer tomography (CT) is fast becoming an accepted tool within the materials science community for the acquisition of 3D images. Here the authors review the current state of the art as CT transforms from a qualitative diagnostic tool to a quantitative one. Our review considers first the image acquisition process, including the use of iterative reconstruction strategies suited to specific segmentation tasks and emerging methods that provide more insight (e.g. fast and high resolution imaging, crystallite (grain) imaging) than conventional attenuation based tomography. Methods and shortcomings of CT are examined for the quantification of 3D volumetric data to extract key topological parameters such as phase fractions, phase contiguity, and damage levels as well as density variations. As a non-destructive technique, CT is an ideal means of following structural development over time via time lapse sequences of 3D images (sometimes called 3D movies or 4D imaging). This includes information nee...

https://www.tandfonline.com/doi/pdf/10.1179/1743280413Y.0000000023

Quantitative X-ray tomography

https://researchportal.port.ac.uk/portal/files/4955595/3DImagingCell_ActaBio.pdf

3D imaging of cell interactions with electrospun PLGA nanofiber membranes for bone regeneration

We present a new approach for the numerical homogenization of cellular and heterogeneous materials. The procedure is based on the finite cell method, which is applied to efficiently discretize representative volume elements for which effective material properties are computed. The starting point for our homogenization might be either a computer-aided design of a heterogeneous material or a three-dimensional computer tomography (CT-scan) of the specimen of interest. A fully automatic discretization in terms of finite cells, applying a hierarchic extension process to control the discretization error, is utilized to solve the corresponding boundary value problems arising during the homogenization. Special emphasis is placed on the numerical treatment of boundary conditions. To this end we apply the window method, which can be interpreted as a variant of the self-consistency method. Several numerical examples ranging from porous materials to fiber-reinforced composites will be presented, demonstrating the efficiency of our approach. The homogenization procedure will be also applied to a foam, a CT-scan of which is available.

Numerical homogenization of heterogeneous and cellular materials utilizing the finite cell method

Closed-cell foams are widely used in marine vessel and ground transportation applications due to their compressive energy absorption capabilities, especially as the core material in sandwich composites. In the present work, a set of closed-cell polyvinyl chloride (PVC) foams with different densities is studied for compressive response at a wide range of quasi-static and high strain rates. The results show that the mechanical properties depend on the foam density and are strain rate sensitive. The compressive strength and modulus increase with the foam density. Cell wall buckling is observed prominently in high density foams, whereas low density foams show wrinkling and stretching of cell faces. An extensive literature survey on PVC foams is presented in this work and the mechanical properties reported in published studies are analyzed to understand trends and future directions. It is found that within the quasi-static strain rate regime, the compressive strength of PVC foams can be up to 50% higher at 10−1 s−1 compared to 10−4 s−1. At strain rates of 2000 s−1, the strength can be 200% higher than the quasi-static values noted at 10−4 s−1. Absence of experimentally measured mechanical properties in the intermediate strain rate range of 1–500 s−1 is noticed for PVC foams. Scanning electron micrographs show cell wall buckling followed by folding as the compressive failure mechanism.

Compressive properties of closed-cell polyvinyl chloride foams at low and high strain rates: Experimental investigation and critical review of state of the art

Tuning the conformation and mechanical properties of silk fibroin hydrogels

Electrospinning has proven to be a promising method to produce scaffolds for tissue engineering despite the frequently encountered limitations in 3-dimensional tissue formation due to a lack of cell infiltration To fully unlock the potential of electrospun scaffolds for tissue engineering, the void space within the fibrous network needs to be increased substantially and in a controlled manner Low-temperature electrospinning (LTE) increases the fiber to fiber distance by embedding ice particles as void spacers during fiber deposition Scaffold porosities up to 995% can be reached and in line with the increase in void space, the mechanical properties of the scaffolds shift towards the range for native biological tissue While both the physiological mechanical properties and high porosity were promising for tissue engineering applications, control of the porosity in three dimensions was still limited when using LTE methods Based on a range of LTE spun scaffolds made of poly(lactic acid) and poly(e-caprolactone), we found that changing the ratio between the rate of ice crystal formation and polymer fiber deposition only had a small effect on the 3D-porosity of the final scaffold architecture Varying the fiber stiffness, however, offers considerable control over the scaffold void space

Tailoring the void space and mechanical properties in electrospun scaffolds towards physiological ranges

A hybrid numerical-experimental approach is used to characterize the macroscopic mechanical behavior of polymeric foams. The method is based on microstructural characterization of foams with X-ray computed tomography (CT) and conver- sion of the data to finite element (FE) models. The 2D models are created from a 3D close-celled foam and subjected to com- pression loads. Since the large strain regime is explored, contact between elements is incorporated. It is shown that, for calculat- ing the effective Young's modulus, a model consisting of at least 11 2 -12 2 cells in the model should be used, whereas for the large strain regime 12 2 -14 2 cells in the model are needed. Discretization had a significant influence on the results, where relatively coarse elements caused loss of connectivity in the cell walls and thicken- ing of the cell walls. It is shown that at least three to four elements should be taken over the thickness of the cell walls for these struc- tures. Finally, a good qualitative agreement is observed between the deformations found with the FE models and in situ com- pression experiments of an open-celled foam during X-ray CT.

http://www.mate.tue.nl/mate/pdfs/11687.pdf

X‐ray computed tomography‐based modeling of polymeric foams: The effect of finite element model size on the large strain response

The timescale at which ductile failure occurs in loaded glassy polymers can be successfully predicted using the engineering approach presented in a previous publication. In this paper the influence of progressive physical ageing on the plastic deformation behaviour of unplasticised poly(vinyl chloride) (uPVC) is characterised and incorporated in the existing approach. With the modification it is possible to quantitatively predict long-term failures which show a so-called endurance limit. The predictions are compared with failure data of uPVC specimens which were subjected to constant or dynamic loads. In dynamic loading conditions a second type of failure mode was observed: fatigue crack growth. A brief study on the influence of the frequency and stress ratio of the applied stress signal shows that crack growth failure is not expected to occur within experimentally reasonable timescales for constant loading conditions.

/pdf/lifetime-assessment-of-load-bearing-polymer-glasses-the-3jv8hlfazy.pdf

Lifetime Assessment of Load-Bearing Polymer Glasses: The Influence of Physical Ageing

A hybrid numerical-experimental approach is proposed to characterize the macroscopic mechanical behavior of structured polymers. The method is based on capturing the details of the material's microstructure using 3D X-ray Computed Tomography (CT). By employing segmentation and voxel- conversion, the reconstructed volume is automatically converted into a finite element model that is subsequently used for mechanical analyses. The approach is demonstrated on a 2D polycarbonate (PC) honeycomb. An ideal representative volume element (RVE), with a volume equivalent to the volume of the real X-ray CT-based model, is used to determine the dependence of the macroscopic response of the structure on intrinsic material behavior, strain rate, and cell wall thickness. A nonlinear elasto-viscoplastic constitutive model is used to describe the intrinsic behavior of the PC base material and a comparison with a hyper- elastic material model reveals that local plastic deformation significantly influences the macroscopic behavior. A cubic relation between the stiffness of the structure and cell wall thickness is found, whereas the strain rate has a minor influence. The ideal RVE shows a different response compared to the real X-ray CT-based model due to local variations of the cell wall thickness in the latter, causing nonhomogeneous deformations. In addition to the geometric imperfections, jagged edges, as a consequence of voxel conversion, contribute to this local variation in cell wall thickness.

http://www.mate.tue.nl/mate/pdfs/9597.pdf

Computed tomography-based modeling of structured polymers

A hybrid numerical-experimental approach is used to characterize the macroscopic mechanical behaviour of polymer foams. The method is based on characterization of foams with X-ray Computed Tomography and conversion of the data to Finite Element (FE) models. Results of FE analyses revealed that plasticity has a large influence on the mechanical response of these structures.

http://www.mate.tue.nl/mate/pdfs/11153.pdf

Jgf Joris Wismans

Papers

Tailoring the void space and mechanical properties in electrospun scaffolds towards physiological ranges

X‐ray computed tomography‐based modeling of polymeric foams: The effect of finite element model size on the large strain response

Lifetime Assessment of Load-Bearing Polymer Glasses: The Influence of Physical Ageing

Computed tomography-based modeling of structured polymers

X-Ray Computed Tomography Based Modelling of Polymeric Foams