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Addition of Plastic Deformation to Fluid-Structure Interaction and Thermal Stress Evolution Models

This article highlights developments to be released in FLOW-3D version 10.1.

The upcoming release of FLOW-3D Version 10.1 includes the option to predict plastic strains in the existing Fluid-Structure Interaction (FSI) and Thermal Stress Evolution (TSE) models. All materials can experience plastic strain when the local stress exceeds a certain limit. In FLOW-3D, this limit is defined as the yield-stress limit, and this defines regions within the material where plastic yielding occurs. This is useful for simulations where stresses are expected to exceed the yield-stress limit of the material; it will provide a better prediction of total deformation and stresses.

The plastic yielding is output as a plastic strain tensor; the individual components of plastic strain can be displayed, as well as the plastic strain magnitude, the second invariant of the plastic strain tensor. The latter measure provides users with an immediate tool to visualize where plastic yielding occurs during a simulation.

 
Solid geometry and initial molten Magnesium-Aluminum alloy
Figure 1. Solid geometry and initial molten Magnesium-Aluminum alloy
Rod temperature at 700 seconds.
Figure 2. Rod temperature at 700 seconds.

A comparison simulation was done with FLOW-3D's TSE model, based on Pokorny et. al 1. Figure 1 shows the setup of the solid geometry and initial molten Magnesium-Aluminum alloy (AZ91). There is a 148mm long horizontal rod with a vertical sprue on one end through which the molten alloy is poured. The simulation begins immediately after pouring is complete; the steel casting mold is initially at 500°C and the alloy is 684°C.

 
Resulting von Mises stress
Figure 3. Resulting von Mises stress
Plastic strain magnitude at the mold-alloy interface
Figure 4. Plastic strain magnitude at the mold-alloy interface

The alloy cools to the mold and the surroundings (at room temperature), and solidification proceeds, with full solidification reached after 380s. Subsequent cooling causes shrinkage of the alloy, and due to the geometry of the alloy within the mold, the shrinkage is frustrated, and large stresses develop around the junction of the sprue and horizontal rod. After 700s, at which point the rod has cooled to around 520K (247°C – see Figure 2), stresses have accrued throughout the alloy, and the maximum stress is constrained by the specified yield stress limit, 500MPa. Figure 3 shows the resulting von Mises stress. Without the prediction of plastic yielding, the predicted maximum stress would be far larger, but with the new capability, plastic deformation is predicted. Figure 4 shows the plastic strain magnitude at the mold-alloy interface. Figure 5 shows the same quantity within a slice along the centerline of the mold. These results compare well to those predicted by Pokorny et. al.

 
Plastic strain magnitude in slice
Figure 5. Plastic strain magnitude in slice
along the centerline of the mold.


Displacement (magnified 20x) at 700s Figure 6. Displacement (magnified 20x) at 700s. Color indicates normal displacement of surface.

Figure 6 shows the normal displacement of the alloy and the overall displacement (magnified by 15×) within the mold. Note the greatest displacement in the vicinity of the sprue-rod junction where plastic deformation occurred.

Although this example focuses on the TSE model, it is also possible to simulation plastic deformations of solid components with the FSI model as well. This enhancement makes FLOW3D's Fluid-Structure Interaction and Thermal Stress Evolution models more useful and powerful. Look for continual enhancements to the model in future releases of FLOW-3D.

Related links:

Development Focus: Fluid Structure Interaction

Material Properties Database (MDPB)

 

Pokorny, M.G., C.A. Monroe, C. Bedermann, Z. Zhen, and N. Hort. Simulation of Stresses during Casting of Binary Magnesium-Aluminum Alloys. Metallurgical and materials trans. A. Vol 41A, 2010.

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