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What is the normal behavior in contact property? What does Hard and Soft contact mean?

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See figure 1, the object with mass M wants to move horizontally towards the right side; to refresh your memory, we will have a mass force downwards (Mg) and reaction force (N); so, this reaction force is the “Normal behavior” that you have in the ABAQUS. This is the simplest explanation for understanding normal behavior; however, it is not as simple as it looks in the numerical analysis.

Figure 1: Normal behavior

Now about the Hard and the Soft contact. See figure 1 again; as you see, you have to define contact if you want to analyze this model in the ABAQUS. The bottom of the mass is the slave surface, and the top side of the ground will be the master surface and make tangential behavior for them. If you remember, the slave surface cannot penetrate the master surface. This is the reality in physics and is called the Hard contact (see figure 2); however, you may encounter convergence problems in your modeling because it’s challenging in numerical analysis model the Hard contact. Therefore, you have to allow the mass to penetrate a little bit in the master surface to avoid convergence issues. This is called Soft contact and has several ways to create that in the software (see figure 2).

Figure 2: create Hard contact and ways to create the soft contact

To understand soft contact more and its ways to create, see figure 3. As you can see, when the master and slave surface are in contact, the slave won’t penetrate the master surface (hard contact, reality); but it’s challenging to calculate, so we allow penetration; but how much the slave surface is allowed to penetrate? Imagine a spring under the master surface. It may have Linear, Exponential, Tabular, and Scale Factor stiffness (figure 2); so, depending on the spring stiffness, the slave would be allowed to penetrate the master surface to avoid convergence problems. Therefore, the software will analyze the model more easily and in lower time but with a bit of diversion from accurate results. Also, you can use the keywords “*SURFACE BEHAVIOR, PRESSURE-OVERCLOSE=EXPONENTIAL…” in the input file to insert soft contact.

Figure 3:Hard contact and Soft contact

Figure 4: Soft contact, exponential stiffness

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