D.H. Lammlein PhD Dissertation - Vanderbilt University

Figure 28: Top view schematic of tunnel defect initiation, figure from [67].
viscosity:
The authors use the Carreau model for pseudo-plastics to assign material
where λ is the time constant, m is the power law non-Newtonian fluid index, µ ∞ and µ o
are the infinite and zero shear viscosities, γ& is the shear strain-rate, and T o is a reference
temperature. The Carreau viscosity model is found as an embedded function in Fluent.
The required constants have been obtained through experiment by Sheppard and Jackson
[69]. A stick condition is used at the tool interface and a uniform heat flux is assigned to
the interface surface. The heat input is calculated using equation (2.4).
The downstream dynamic pressure along the traversing direction of the tool
is examined at various heights relative to the shoulder for welds at various parameters.
Experimental welds are performed to verify the occurrence of tunnel-like defects and to
record the depth at which the defects occur (on the surface or submerged at some depth,
y). The dynamic pressure trend lines below represent welds and weld heights which do
and do not contain defects.
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