of the current fracture load). Typically about 2000 cycles were applied to mark the crack front. The crack front shapes were measured using an optical microscope with X-Y traveling stages. Three C(T) specimens of width 152 mm were fatigue pre-crack at low stress levels (8 MPa Öm) to an a/W of 0.4. Each specimen was loaded just enough to cause a predetermined amount of crack extension. Then the fracture crack front was marked using fatigue crack growth. For each specimen, some or all the of the following were collected: load, load-line displacement, crack-mouth-opening displacement, d5, unloading compliance, crack extension, and surface field displacements in the vicinity of the original crack tip. The amount of crack extension was based on the desire to characterize the tunneling progression along the crack front for various loading levels. The first specimen was loaded until about 0.25 mm of surface crack extension was visible. Two subsequent specimens were loaded to lower loads based on the unloading compliance load-crack extension curve. Table 1 summarizes the maximum load as well as maximum interior and surface crack extension values. The tunneling magnitude is T = Damax - Das, where Damax is the crack extension on the interior, and Das is the crack extension on the surface. The load corresponding to KIc for this material and configuration is about 10.7 KN. Figure 4 shows the measured fatigue and fracture crack fronts from the multiple crack front specimen test. Also included in Figure 4 are the area-average (dash-dot) crack lengths for each crack front. Unloading compliance crack extension is somewhat less than area average and is omitted here for clarity. During the early stages of growth, for instance, at the lowest load from Table 1, the crack has some extension along nearly 80% of the fatigue crack-front. This extension was attained at only about 25% above the equivalent KIc load for this material, indicating that there may be relatively high constraint on the interior of the specimen encouraging growth at such low loads. The estimated plastic zone radius at this load is about 4 mm -- well beyond the limits of LEFM, but still smaller than the thickness. Additionally, this indicates that the crack extension may be retarded on the surface by plasticity, since surface-crack extension does not initiate until significant tunneling has taken place. Tunneling initially increases as the tensile fracture region develops, then as the shear lips form with increasing plasticity, tunneling decreases to an essentially constant value during fully developed slant fracture. The fatigue crack-front tunneling was about 12% of the plate thickness (B). Tunneling increases to about 40% of B when surface growth starts. After the flat-to-slant transition is complete, tunneling stabilizes at about 20% of B. CRACK EXTENSION CALIBRATION For tunneling cracks, there are, at least, four crack-length measurements: (1) crack length on the free surface, (2) unloading compliance crack length, (3) area-average crack length, and (4) maximum crack length in the interior. Figures 2 (a) and (b) compare experimentally measured surface values of crack extension with straight crack-front FEA results. Since tunneling does occur, it is desirable to account for the tunneling either in the analysis or in the test data. The most desirable approach is to modify the analysis to include crack-front shapes that match the experimental results [5] or use a modeling methodology that allows the tunneling to evolve naturally as part of the analysis [6]. However, these are cumbersome, time consuming, and not currently practical for industry. The alternative is to consider the experimental data and estimate crack extension using either unloading compliance or area average. Herein, the area-average values, based on the 9-point weighted average procedure [7], will be used, unless otherwise noted. Two situations exist requiring separate calibration curves: specimens that remain flat and specimens that exhibit the flat-to-slant crack transition. The process is essentially the same for the two fracture conditions using different calibration curves. Figure 5 shows the tunneling magnitude, T, for the two fracture conditions. The tunneling for the flat-toslant specimens was derived from Figure 4. The tunneling for the flat fracture is adapted from work by Dawicke et al. [5] for a thin sheet 2024-T3 aluminum alloy (scaled from B = 2.3 mm). Dawicke showed through experimental measurements similar to those from Figure 4 that for flat fracture, tunneling increased monotonically until it stabilized at a constant value. The curve for flat fracture in Figure 5 is a curve fit of Dawicke's data. Figure 6 shows the area-average calibration curves derived from the flat and flat-to-slant crack data in Figure 5. The correction ratio, CR, is the ratio of area average crack extension to the surface crack extension. The
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