Fracture and damage of foamed materials

@Porous metal produced by a casting method has a closed cell structure surrounded by a thin metallic film called a cell wall. Until now, the evaluation of the mechanical properties of porous metal has focused on cell structures several millimeters in size(Review Paper 1).However, with recent advances in production technology, the cell structure has become very fine and homogeneous. For this reason, we believe that the thin film-like metallographic structure comprising the cell wall, especially its uneven spatial distribution, is the key to understanding the mechanical properties, as is the case with conventional metallic materials(Review Paper 2,3). Figure 1 shows an image obtained from an in-situ observation of a cell wall fracture (Paper 1).While the cell walls of a porous metal produced from a pure aluminum matrix display ductile buckling and bending that are insensitive to its internal microstructure, those of an alloy metal matrix produce brittle fractures taking the form of cracks that appear to connect defects such as rough pores. @

Fig. 1 Deformation and fracture behavior of a porous aluminum cell wall. While ductile buckling can be seen with pure aluminum material, with alloyed material, microscopic cracks that emerge from comparatively large micropores are seen to combine and propagate.

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Fig. 2 Result of a 3D measurement of deformation within a cell wall. The measurement was made by using the internal micropores as deformation measurement markers and by tracking these markers. The positional relationship between the internal rough pores (white lines) and deformation is shown by overlaying a transparent image of the cell wall.

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@Figure 2 is an example of the 3D mapping of the deformation inside a cell wall, created by using the micropores inside the cell wall as deformation measurement markers and by tracking the marker points while crushing the porous metal(Paper 1).The deformation was concentrated in the area around the rough pores and clearly showed that these constituted the starting point of fractures. To study this in more detail, compression tests were conducted repeatedly and the behavior of 200,000 micropores (average diameter of 3.6 ƒÊm) was observed with 3D imaging. The result confirmed that while only a meager 81 micropores developed microscopic cracks from among these numerous micropores, the micropores had an average diameter of 70.6 ƒÊm.

@ @Moreover, as in the result of the image-based numerical analysis shown in Figure 3, even relatively small micropores have a large effect when they are aligned(Paper 2).Figure 3 shows both the concentrated deformation near the aligned micropores, and their ability to increase the deformation concentration in areas around the rough pores located slightly apart from them. Additionally, since a much wider area is subjected to high stress when experiencing dynamic deformation, large differences in the damage and fracture behavior were observed compared with those subjected to a static load.

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Fig. 3 Calculation result of the equivalent plastic strain distribution inside porous aluminum derived from an image based numerical analysis. The micropores aligned at gAh caused a change in the strain distribution both at gAh and from gBh to gD,h located slightly apart from gAh.

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@As described above, by studying the relationship between the compositionally and morphologically complex microstructure of porous metal in practical use and its mechanical behavior by using 3D imaging, efficient process control can be realized by taking the localized chemical composition and other factors into consideration (Paper 3).A process control technique is achieved in which even the smallest number of harmful objects is eliminated while allowing the presence of any number of harmless objects.

Review Paper

  1. ‹¤’˜C‘½EŽ¿‹à‘®‚Ì—ÍŠw“I“Á«‚Æ”j‰ó‹““®C‘½E‘̂̐¸–§§Œä‚Æ‹@”\E•¨«•]‰¿C‘æ2Í36ßC2008C284-291
  2. H. Toda, M. Kobayashi, Y. Suzuki, A. Takeuchi, K. Uesugi, 3D¥4D Materials Science: Its Current State and Prospects, Hihakaikensa, Vol.58CNo.10C2009C433-438
  3. H.Toda, M. Kobayashi, Fracture Behavior Analysis in Pours Metals by X-ray Micro-tomography, Materia, Vol.47CNo.4C2008C191-195.

Research Paper

  1. H. Toda, T. Ohgaki, K. Uesugi, M. Kobayashi, N. Kuroda, T. Kobayashi, M. Niinomi, T. Akahori, K. Makii and Y. Aruga, Quantitative assessment of microstructure and its effects on compression behaviour of aluminium foams via high-resolution synchrotron X-ray tomography, Metallurgical and Materials Transactions. A, Vol. 37A, No.4, 2006, 1211-1220
  2. H. Toda, M. Takata, T. Ohgaki, M. Kobayashi, T. Kobayashi, K. Uesugi, K. Makii and Y. Arug, 3-D image-based mechanical simulation of aluminium foams: effects of internal microstructure, Advanced Engineering Materials, Vol.8, No.6, 2006, 459-467
  3. Q. Zhang, H. Toda, Y. Takami, Y. Suzuki, K. Uesugi, M. Kobayashi Assessment of 3D inhomogeneous microstructure of highly alloyed aluminium foam via dual energy K-edge subtraction imaging, Philosophical Magazine, Vol.90, No.14, 2010, 1853-1871