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influence of crack closure. As the loads are reduced in order to reduce DK during development of a full fatigue curve, the crack opens a proportionally lesser amount and the mating surfaces come closer to each other during the unloading portion of the fatigue cycle. Eventually, the loads can be reduced to the point where the asperities of mating fracture surfaces can come into contact if the frac- ture surface exhibits enough roughness. This shields the crack from a portion of the applied load and effectively reduces the stress intensity range available to drive crack growth. In fact, the closure mea- surements made during fatigue testing of the base metals (Figs. 5 and 6) provide an indication of the DK level at which crack closure contributes to the improved fa- tigue resistance. These conditions were described in the Results section and are summarized in Table 4. The DK values in Table 4 indicate the stress intensity range at which crack closure occurs as DK is re- duced. Note that the values shown in Table 4 increase as the grain size increases, which indicates that closure effects occur at higher DK levels as the grain size in- creases. This is in direct response to the rougher fracture surface that forms with increasing grain size. Thus, crack closure is at least one cause of the improved fa- tigue resistance for crack growth rates below the DK values summarized in Table 4. It should be noted that crack closure can also occur due to residual stress effects as discussed in previous work (Ref. 12). However, these tests were conducted on samples that were slowly heated and cooled during the grain growth treat- ments, so significant residual stress is not expected in these samples. In addition, previous work (Ref. 12) has demonstrated that residual effects in these alloys are overcome with the R value of 0.6 that is

used here. ( r c y = 0.033 DK s ys The remaining two mechanisms that account for improved fatigue resistance (devia- tion of the crack plane out of the Mode I plane and creation of more surface area for a given crack length) can be attributed to crystallographic ef- fects on the fatigue crack growth. It has been established (Ref. 16) that fa- tigue crack growth occurs on preferred crystallographic planes when the plastic zone size that develops dur- ing growth is ap- proximately equal to or less than the grain size. The plastic zone size c ) , i n t u r n , i s c o n t r o l l e d b y t h e y i e l d s t r e n g t h ( s y s ) a n d a p p l i e d D K a n d c a n b e e s t i m a t e d b y ( R e f s . r c y c 16–18) For a given material, the plastic zone size decreases with decreasing DK. Thus, as DK is reduced during the fatigue test, a point is eventually reached when the plas- tic zone size approaches (and eventually becomes smaller then) the grain size. C 2 (1) Fig. 6 — Fatigue results and slope offset data. A — AL6XN base metal with a 21-mm grain size tested at an R ratio of R = 0.60; B — AL6XN base metal with a 211-mm grain size tested at an R ratio of R = 0.60; C — AL6XN base metal with a 280-mm grain size tested at an R ratio of R = 0.60. Under this condition, crack growth occurs along preferred crystallographic planes, even though that direction may not be ori- ented within the Mode I plane. In austenitic stainless steels, growth is fa- vored predominantly on {111} planes (Refs. 19, 20). Crack growth occurs along such favorable planes until reaching a neighboring grain of different crystallo- graphic orientation. The crack is then forced to find the favorably oriented plane for continued propagation into the neigh- boring grain. When this process occurs in large grained materials, the cracks may ex- tend farther distances along the favorable planes and out of the Mode I plane prior


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