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In Figure 20 (a)&(b), specimen B-C1, which fractured com- pletely within the weld, is shown. Highlighted in red is a crack initiated adjacent to the failure surface is highlighted in red (Figure 20[b]). Examining this fracture surface was of particular interest regarding any prominent defects in the weld area of this specimen which led to fracture versus the location of the crack. Representative features found on these surfaces are presented in the Fractography section.


Fractography Examination of the fracture surface can lead to a greater un- derstanding of the factors that affect failure initiation. Scan- ning Electron Microscopy (SEM) analysis was performed on all four of the initial bend test samples from locations within the weld and on one sample (B-10_4) away from the weld. Particular attention was paid to any defects along the sample edges which may have caused the variation of frac- ture locations within the central span..


Fractography of the five samples yielded results typical of those which are displayed in Figure 21. The small areas of porosity within the samples likely are caused by the drag of oxide skins into the bulk of the melt during the filling of the plate cavity.


Four-Point Bend Tests - Set 2 In contrast to the Set 1 samples, most of the Set 2 specimens tended to fracture in the center of the span between the top two forces.


Table 8 summarizes the ultimate bend- ing strength (UBS) results obtained from the four-point bend tests. As is shown, values ranging from approximately 226 MPa to 327 MPa were obtained. The


high magnitude of these figures for the untreated A356 alloy reinforces the idea that no major entrainment events (mac- roscopic bubbles) generally took place during the filling of the plates.


Weibull Analysis A Weibull statistical analysis was used to show the distribu- tion of mechanical properties about mean values in relation to failure for each of the gating configurations. One form of the two parameter Weibull distribution is shown in the fol- lowing equation:11


Eqn. 1


where P is the probability of failure at the variable being measured (ultimate bending strength σ), m is the Weibull modulus, and σ0


is the scale parameter, which is a character- istic stress value at which approximately 63.2 percent of the specimens failed.11-12


Figure 17. Area image 3mm confluence weld region (scale shown in microns).


Figure 18. Fracture location of specimen A1-1 between top loads prior to complete fracture monitored using the four point bend test.


International Journal of Metalcasting/Spring 2012


Figure 19. Fractured sample B-B1 after bend testing representing the typical mode of fracture away from the weld (which is encircled in red).


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