Understanding propagation behavior
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. 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 ฟ-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 ฟ-Al phase but quickly in the Al-Si eutectic phase.
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.@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.
Fig. 4 Closed fatigue cracks (shown in yellow in (a)) in Al-Cu alloy, and local Mode III displacement
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