Understanding propagation behavior

Fig. 1 Crack opening and initial propagation behavior in Al-Si dual-phase alloy

@Figure 1 shows a continuous observation of crack propagation in an aluminum cast metal(Paper 3)BThe metal was pure Al-Mg2Si alloy prepared in the laboratory, but it showed very complicated crack properties and large discrepancies, along the front edge of the crack, in both the extent and direction of crack propagation(Paper 5).The damage (shown in red in Fig. 1) at the front of the crack has been believed to be theoretically limited to a range of about twice the crack tip opening displacement according to fracture mechanics, but the damage in the figure extended far beyond that range (paper 6,10,11).The 4D quantifications of the dynamic quantities described below explain the actual, complicated crack propagation behaviors.

Fig. 2 Crack propagation behaviors in Al-Si dual-phase alloy

@We have measured the displacements (Paper 7),stress intensity factors and J-integral values(Paper 2,8,9)of several tens of thousands of microstructure characteristics, and have achieved high density measurement of crack tip opening displacement(Paper 6,8).With the data, we could finally conduct a comparative analysis between experimental and simulation results (Paper 4).Figure 2 shows the crack propagation resistance distribution in an Al-7%Si alloy. The metal had a dual-phase structure. The very tough ƒฟ-Al phase spread near the crack tip in Slices 200 to 500, and the remaining sections consisted of the brittle Al-Si eutectic phase. The fracture resistance was shown to differ by one order of magnitude. Thus, the cracks propagated not at all in the ƒฟ-Al phase but quickly in the Al-Si eutectic phase.

Fig. 3 Fatigue crack opening and closing behaviors in Al-Cu alloy

@The propagation speed of fatigue cracks depends on the opening and closing behaviors of the cracks. However there have been no methods for measuring the local opening and closing of cracks inside materials. Figure 3 plots the precisely measured positions of crack tips (Paper 8)BThe cracks apparently closed in areas (Slices 500 to 600 and 675 to 730) where the positions of the crack tips were markedly different between the bold and thin lines in Figure 3, but almost no closure was observed in the slices between the two areas. Such opening and closing behaviors at intervals of several tens micrometers are the cause for the non-uniformity in propagation speed at the front edge of the crack.

Fig. 4 Closed fatigue cracks (shown in yellow in (a)) in Al-Cu alloy, and local Mode III displacement

@The aforementioned CTOD measurement is relatively sensitive to the microstructure. On the other hand, K and J measurements by tracing microstructure characteristics are insensitive to local microstructures and can measure Mode II and III values with high precision. Figure 4 compares Mode III displacement, and crack opening and closure distributions(Paper 1). Figure 4(b) can be directly converted into KIII(Paper 2), with which the driving force of combined mode propagation can be assessed.

Review paper

  1. H. Toda, T. Kobayashi, T. Ohgaki, Advanced in Visualization Techniques by High-resolution X-ray CT: Application to in-situ Measurement of Internal Local Mechanical Quantities of Materials, Zairyo-shiken-gijyutsu, Vol.48CNo.1C2004C5-10.
  2. H. Toda, M. Kobayashi, Y. Suzuki, A. Takeuchi, K. Uesugi, 3Dฅ4D Materials Science: Its Current State and Prospects, Hihakaikensa, Vol.58CNo.10C2009C433-438
  3. H. Toda, M. Sato, H. Okuda, M. Kobayashi, Observation and analysis of materials with synchrotron radiation, Keikinzoku Vol.61, No.1, 2011

Research paper

  1. H. Toda, I. Sinclair, J.-Y. Buffiere, E.Maire, T. Connolley, M. Joyce, K.H. Khor and P. Gregson, Assessment of fatigue crack closure phenomenon in damage tolerant aluminium alloy by in-situ high-resolution synchrotron X-ray microtomography, Philosophical Magazine A, Vol.83, No.21, 2003, 2429-2448
  2. H. Toda, I. Sinclair, J.-Y. Buffiere, E. Maire, K.H. Khor, P. Gregson and T. Kobayashi, A 3D measurement procedure for internal local crack driving forces via synchrotron X-ray microtomography, Acta Materialia, Vol 52, No.5, 2004, 1305-1317
  3. L. Qian, H. Toda, K. Uesugi, T. Kobayashi, T. Ohgaki, and M. Kobayashi Application of synchrotron X-ray microtomography to investigate ductile fracture in Al alloys, Applied Physics Letters, Vol. 87, 2005, 241907
  4. L. Qian, H. Toda, S. Nishido, T. Kobayashi, Experimental and numerical investigations of the effects of the spatial distribution of ƒฟ phase on fracture behavior in hypoeutectic Al-Si alloys, Acta Materialia, Vol.54, 2006, 4881-4893
  5. L. Qian, H. Toda, K. Uesugi, M. Kobayashi and T. Kobayashi, Direct observation and image-based simulation of three-dimensional tortuous crack evolution inside opaque materials, Physical Review Letters, Vol.100, No.11, 2008, 115505
  6. H. Toda, S. Yamamoto, M. Kobayashi and K. Uesugi, Direct measurement procedure for 3-D local crack driving force using synchrotron X-ray microtomography, Acta Materialia, Vol.56, 2008, 6027-6039
  7. M. Kobayashi, H. Toda, Y. Kawai, T. Ohgaki, K. Uesugi, D.S. Wilkinson, T. Kobayashi, Y. Aoki, M. Nakazawa, High-density three-dimensional mapping of internal strain by tracking microstructural features, Acta Materialia, Vol.56, Issue 10, 2008, 2167-2181
  8. H. Zhang, H. Toda, P.C. Qu, Y. Sakaguchi, M. Kobayashi, K. Uesugi, Y. Suzuki, Three-dimensional fatigue crack growth behavior in an aluminum alloy investigated with in situ high-resolution synchrotron X-ray microtomography, Acta Materialia, Vol.57, No.11, 2009, 3287-3300
  9. P. Qu, H. Toda, H. Zhang, Y. Sakaguchi, L. Qian, M. Kobayashi and K. Uesugi, Local crack driving force analysis of a fatigue crack by a microstructural tracking method, Scripta Materialia, Vol. 60, 2009, 489-492
  10. H. Toda, E. Maire, S. Yamauchi, H. Tsuruta, T. Hiramatsu, M. Kobayashi, In situ observation of ductile fracture using X-ray tomography technique, Acta Materialia, Vo.59, No.5, 2011, 1995-2008.
  11. H. Toda, Y. Sakaguchi, K. Uesugi, Y. Suzuki, M. Kobayash, , Acta Materialia, to be submitted.