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m/cycle and 1.00 · 10–9 m/cycle that were used in the analysis. Figure 8 shows the relation between DKth and grain size, where it is evident that large grain size favorably increases the threshold stress in- tensity range. As with the weld metal, the fracture surface roughness of the base metals increased with in- creasing grain size. Figure 9 shows an example of this for the AL6XN base metal. These pho- tomicrographs were each acquired from the sample at points where the crack growth rates were similar at ~ 1.5 · 10–10 m/cycle, and crack growth occurred from left to right in the figures. The base metal tested at a grain size of 21 mm (Fig. 9A) exhibits a fracture surface that is relatively flat compared to the sample with a grain size of 210 mm (Fig. 9B), which exhibits a rougher fracture surface. C Fig. 5 — Fatigue results and slope offset data. A — 316L base metal with a 24-mm grain size tested at an R ratio of R = 0.60; B — 316L base metal with a 103-mm grain size tested at an R ratio of R = 0.60; C — 316L base metal with a 147-mm grain size tested at an R ratio of R = 0.60. The threshold stress intensity factor range, DKth, can be determined from the fatigue data presented in Fig. 7. The value of DKth is defined by ASTM as the DK value corresponding to a crack growth rate of 1.00 · 10–10 m/cycle (Ref. 15). DKth values for the 316L and AL6XN base met- als of varying grain sizes are presented in Table 3. The ASTM Standard E647 re- quires that determination of DKth be com- pleted by conducting a linear regression analysis of the da/dN-DK plot with a min- imum of five data points between 1.00 · Discussion The results presented above demon- strate that the large grain size present in the weld metal provides an increase in the fatigue resistance relative to the base metal, and the improved fatigue resistance is associated with a rough fracture surface. 10–10 m/cycle and 1.00 · 10–9 m/cycle. Therefore, Table 3 also lists the total num- ber of data points between 1.00 · 10–10

The control tests conducted on the base metals of varying grain size confirm that large grain sizes are beneficial for fatigue resistance, particularly at low crack growth rate regimes.

Fracture surfaces that form with signif- icant surface roughness can improve fa- tigue resistance in three ways. First, the high surface roughness presents a tortu- ous crack path that effectively requires formation of a larger fracture surface area for a given length of crack propagation that is orientated perpendicular to the loading direction (compared to relatively flat cracks). In other words, more surface area must be created for a given effective crack length. Second, the crack growth plane can deviate out of the plane that is normal to the applied stress (i.e., out of the Mode I plane). This effectively re- duces the stress intensity range that is available to drive the crack and, thus, re- duces the crack propagation rate. For ex- ample, if Q is designated as the angle be- tween the crack plane and the Mode I plane, then the stress intensity range that drives crack growth is reduced from DK to DKcos (Q) when the crack deviates out of the Mode I plane by an angle of Q. Lastly, rough fracture surfaces can induce crack closure. In this case, large asperities on the mating halves of the fracture surface come into contact with one another during the unloading portion of the curve. This causes the crack to partially close, which shields the crack from part of the applied load and reduces the stress intensity range available for crack growth.

It is evident from Fig. 7 that the bene- ficial effect of large grain size is most op- erable at low levels of applied stress in- tensity range. This is consistent with the three mechanisms described above, since all of these mechanisms occur predomi- nately at low DK levels. First consider the

10 -S JANUARY 2004

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