Fig. 2 — Fatigue crack growth rate as a function of crack length. A — 316L GMA weld; B — AL6XN GMA weld.
roughness of the welds was determined using optical profilometry. Standard fa- tigue tests with varying DK were then con- ducted on the heat treated base metal sam- ples. Compliance measurements were recorded on both loading and unloading portions of the load-displacement curve. All testing was conducted using constant amplitude loading and a sine waveform at a frequency of 25 Hz at room temperature.
Constant DK Test Results
Figure 2 shows the results of constant DK results for the weld metals. These re- sults show the variation in fatigue crack growth rate as the weld traverses from the base metal, into the base metal plus weld region, and then into the weld. It is inter- esting to note that the fatigue resistance of the weld is better than that of the base metal, i.e., the crack growth rate of the weld is lower than that of the base metal. For each weld, it was observed that the weld metal fracture surface was signifi- cantly rougher than the base metal. This is shown in more detail in Fig. 3 for the 316L weld.
Fig. 3 — Fatigue results from 316L GMA weld constant DK test. A — Fatigue results; B — fracture sur- face; C — surface profile.
The fatigue results from Fig. 2A are re- plotted in Fig. 3A. Figure 3B shows a pho- tograph of the fracture surface, where var- ious locations on the fracture surface are aligned with the corresponding crack growth rates measured in Fig. 3A. The large, columnar grain structure of the weld is readily evident in Fig. 3B. Figure 3C shows results from optical profilometry, which show how the fracture surface roughness changes as the crack propagates from the base metal into the weld metal. As with Fig. 3B, the optical profilometry