Micro-CT based local strain analysis of porous materials: potential for industrial applications

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1 Micro-CT based local strain analysis of porous materials: potential for industrial applications G. Pyka 1, M. Speirs 2, E. Van de Casteele 1, B. Alpert 1, M. Wevers 1, 1 Department of Materials Engineering, KU Leuven, Kasteelpark Arenberg 44 PB2450, 3001 Leuven, Belgium, 2 Department of Mechanical Engineering, Celestijnlaan 300B, 3001 Leuven, Belgium Aims Porous structures hold unique physical properties (mechanical, thermal and electrical) that are related to their low density and architecture. These attributes open a wide variety of potential applications, such as insulation, packaging, filtering, medical implantology, as well as in the automobile, military shipping and aerospace industries 1-5. Beside their dependency on the composition and the microstructure of the raw material, the mechanical and thermal properties of porous structures also depend on the geometrical and morphological properties of the basic cell architecture 1-2. The growing demand for porous structures with highly controlled mechanical and thermal properties, coming from different industrial and scientific applications, has forced researchers to develop novel production techniques to enable the manufacturing of designed structures. For a better understanding and control of the mechanical behaviour and/or thermal conductivity of various porous structures, several studies involving computed tomography (CT) reconstructions and numerical micro-finite element (µfe) models have been conducted 6-8. Typical application of CT in industry covers however, detection of the imperfections such us voids, cracks or in measuring the external as well as the internal geometry of complex parts 9. In this study, microfocus computed tomography (µct) combined with advanced image processing was used to evaluate the temperature gradient induced local deformation of thermal isolator Styrene-acrylonitrile (SAN) polymer foams which might have benefits for the isolation system industry. Additionally, this work attempts to assess and improve the mechanical properties of Ti6Al4V scaffolds by eliminating/modifying the sharp and thin nodes 10, recognized in previous work as the main source of stress concentrations and lowering the mechanical properties 11. This is carried out through a modification of the scaffolds uniform beam thickness design based on the outcome from the µct based local strain analysis. A gradual increase of the beam (strut) thickness around the nodes where corresponding struts meet was designed. The compression performance of these scaffolds was assessed and compared to common examples (unaltered struts) and to scaffolds designed with thicker struts in the centre of the beams (demonstrating the largest contrast). Materials and Method In-situ cooling experiments of the polymeric foam In this experiment, the Bruker Skyscan 1172 µct system and its associated cooling stage were used to change and evaluate the physical characteristics of Styrene-acrylonitrile (SAN) polymer foam cube samples. The samples were mounted on the cooling stage and scanned once at 20 C and once again at -20 C. Analyzing the reconstructed CT models revealed the samples morphologies before and after freezing and provided data like total change in volume, porosity, surface area, and structure thickness at any cross-section. Non-rigid image registration was applied to the data set in order to evaluate the local volumetric deformation 12.

2 In-situ compression of Ti6Al4V scaffolds Selective laser melting was used for manufacturing porous titanium (Ti6Al4V-ELI) using a diamond unit cell with two other variations with respect to the conventional diamond design (Di). One variation introduced a gradual increase in strut thickness from the centre of the beam to the node at a ratio of 2:1. The other variation incorporated the opposite with a gradually increasing thickness from the central node to the centre of the beam at a ratio of 2:1. These two designs will be referred to as reinforced nodes (RN) and centrally thickened beam (CTB) along with the conventional diamond unit cell. The CAD images of the 3 designs are presented Figure 1. All samples were designed with a height of 6.4mm and 6mm diameter with a constant pore size of 1000µm. Applying different designed strut sizes, 200 µm for Di and 140 µm for CTB and RN, allowed to obtain similar volume fraction for each design: vf = 13.2%(+/- 0.6). Each design was created using Magics software [Materialise NV, Haasrode, Belgium]. a) b) c) Figure 1: CAD images of each beam for each design and the pore and strut size for one unit cell: A) conventional diamond, B) reinforced node (RN) and C) centrally thickened beam (CTB) design To evaluate the local volumetric strain in function of the applied displacement a radiotranslucent micro-mechanical compression setup is used to apply and maintain strain during high resolution μct scanning. To evaluate the local strain changes in function of displacement a constant compression rate of 0.2mm/min was applied followed by μct scanning. First, a reference scan of the non-compressed sample was taken using a pre-load of 0.01kN. Afterwards, the sample was compressed to 50% ultimate compressive strain (50% UCS) followed by μct scanning using the nano-focus CT scanner (Phoenix NanoTom S GE Measurement and Control Solutions, Germany). Both μct images taken for each sample were registered to each other non-rigidly using Elastix software 11,12. Results In Situ cooling strain analysis The experiment showed that µct could be applied with consideration of temperature to show the structural changes of an object or sample. Volume and structure thickness alterations were observed in both samples and it can be attributed locally or globally to the temperature decrease. An overall volume decrease was measured in the SAN foam, however, non-rigid image registration allowed to evaluate the structural deformations more locally. The largest local volume changes can be seen at the contact point of the sample with the cooling stage (Figure 2). This in turn reinforces the hypothesis of the structural changes due to cooling and reveals that the base of the stage is colder than its surrounding environment. Therefore, this method can be used to accurately depict and gather data about the morphological properties of materials.

3 Figure 2: a) Bruker in-situ cooling stage holding polymer sample, b) 3D and c) 2D strain visualization showing local deformation in SAN polymer sample In Situ compressive strain analysis The RN scaffolds showed a similar stiffness to the conventional diamond design, which was higher than CTB scaffolds. Analysis of the mean strain indicated differences in the level of the local deformations of the CTB in comparison with RN and Di (5.9%, 3.2% and 2.7% of mean compressive strain for CTB, RN and Di respectively). Measurements of the most frequent (dominant) strain revealed a similar pattern (7.2%, 3.0% and 2.7% of compressive strain for CTB, RN and Di respectively). Comparison of the strain distribution (Figure 3) obtained for Di and RN did not show significant differences. Figure 3: Frequency of local distribution strain for CTB and RN at 50% of ultimate compressive strain (50% UCS) However, as presented in Figure 4 the locations of the largest compressive strains are different for each design. It can be shown that a more uniform strain is observed for Di and RN (Figure 4). CTB shows large sections with high compressive strain (indicated in pink colour in Figure 4c). This confirms visually the differences in the strain histograms (Figure 3). Additionally, for RN the largest compressive strains are typically observed at the centre of the beams (indicated with the white arrows in Figure 4). This is in contrast to Di where high compressive deformations were observed mainly in the connection between beams and nodes. Perhaps, thicker size of struts at central zones have led to a localized strain by further

4 thickening of those areas in comparison with nodal joints where the struts have bended over and produced a tensile strain. In contrast, the gradual increase in strut thickness from the centre of the beam eliminated the amount of critical points on the scaffolds. Finally, both Di and RN revealed a more homogenous strain distribution across the scaffold structure in comparison to CTB (Figure 4), which could be advantageous during implant loading triggering osteoinduction uniformly throughout the scaffold. Figure 4: 2D visualization in the coronal slices of the strain computed for each scaffold design a) Di, b) RN and c) CTB. White arrows indicate typical locations of the high compressive deformations Conclusion This study showed that combination of the µct imaging with non-rigid image registration based local strain analysis can be applied for evaluation of the thermal gradient and/or mechanical deformation induced morphological changes of various porous materials. In that way, production methods can be further used to create an iterative process optimizing the components for their requirements in industry from isolation to mechanical loading and biocompatibility. For example this method can be used to improve the scaffold design by a gradual strut thickness (in a comparable volume fraction) for an improved bio-mechanical performance. Additionally, proposed experimental evaluation of the local deformation distribution can be applied to validate the foam geometry based effective thermal conductivity model of porous structures for the isolation system industry.

5 References: 1. Gibson L.J et al. Cellular solids: Structure and Properties. Cambridge University Press, Cambridge, Ohgaki T. et al. In situ observations of compressive behaviour of aluminium foams by local tomography using high-resolution X-rays. Philosophical Magazine, , Hollister S.J. Porous scaffold design for tissue engineering. Nature Materials, , Salgado A.J. et al. Bone tissue engineering: state of the art and future trends. Macromol Biosci, , Emmelmann C. et al. Laser additive manufacturing of modified implant surfaces with osseointegrative characteristics. Physics Procedia, , Baas E. et al. A numerical model of heterogenous surface strains in polymer scaffolds. J Biomech, , Jacques S.V.N. et al. Individualised, μct-based finite element modelling as a tool for biomechanical analysis related to tissue engineering of bone. Biomaterials, , Xu Y. et al. Automatic FEM model generation for evaluating thermal conductivity of composite with random materials arrangement. Computational Materials Science, , De Chiffre L. et al. Industrial applications of computed tomography. CIRP Annals, , Speirs M. et al. Design enhancement of biomedical scaffolds made by selective laser melting. Proceedings of the solid freeform fabrication symposium, Austin, TX, Pyka G et al. Evaluation of credibility and limitations of the non-rigid registration of micro-ct images as a tool for local strain analysis. Micro-CT User Meeting Klein S, et al. Elastix: a toolbox for intensity-based medical image registration. IEEE Trans Med Imaging, 29(1): p , 2010.