How to Activate Material in Simulations of Manufacturing Processes
Mats Danielsson November 7, 2018
Material deposition is an essential ingredient in certain manufacturing processes, including welding and additive manufacturing. Say that you want to simulate such a manufacturing process. A challenge that you will face during the simulation is depositing material in a way that introduces it in a state of zero stress. Here, we look at the Activation functionality in the COMSOL Multiphysics® software and how it facilitates the simulation of material deposition.
Why Activate or Deactivate Material?
Imagine that you want to simulate a structural material that is in a molten state and then solidifies. Alternatively, the material is initially solid and then melts. This can be the case when you want to simulate manufacturing processes such as arc welding, selective laser melting, and selective laser sintering — the last two being common additive manufacturing methods.
You can use the Activation node to activate or deactivate material in a simple manner during a simulation. As a note, the Activation node is available in the add-on Structural Mechanics Module and MEMS Module as of version 5.4 of COMSOL Multiphysics®.
Activation of Material: The Naive Approach
One approach to emulate that material is structurally absent is to simply reduce its elastic stiffness to a point where it becomes negligible. This way, the rest of the structure is free to deform without “feeling” the structurally weaker material. This is a viable approach as long as we have no desire to actually activate material.
A problem arises if we try to activate the weak material by simply restoring its stiffness to the nominal level at some point during the simulation. When the stiffness is restored, any strains present in the activated material will suddenly produce stresses. In most situations, this is not a desired effect when activating material. Instead, material should be activated in a state of zero stress. This is more physical, as we usually want to emulate that material is deposited or solidified.
Activate Material in a Stress-Free State
The Activation node avoids the problem of the artificially produced stresses described above. This node reduces the stiffness of the inactive material as described before, but, importantly, it also removes any elastic strains that are present at the instant of activation. Simply put, material is activated in a state of zero stress.
The Activation node is located under the Linear Elastic Material node, as shown in the figure below, and it is available for the Solid Mechanics and Membrane interfaces.
The Activation feature and its Settings window.
The Activation panel of the Settings window contains two settings, namely:
- Activation expression
- Activation scale factor
The Activation expression setting is a logical expression that you define. It is used to determine whether the material is active or not, and it is defined per integration point of the mesh elements. For example, an activation expression that reads
T<T_s says that the material is active if the expression is logically true (when the temperature, T, is less than the solidification temperature, T_s) and inactive otherwise.
The Activation scale factor setting defines the factor that multiplies the elastic stiffness to emulate that material is not present. It has a default value of 10-5, but you can change it if you wish. However, a value that is too low could make the stiffness matrix ill-conditioned.
Two built-in variables are provided to describe the active/inactive state, namely:
isactive indicates the current active/inactive state of the material, while the variable
wasactive indicates whether the material has been active at any previous point during the simulation. In the case of a Solid Mechanics interface with the tag
solid, the variable describing the current state of the material is called
wasactive variable can be used to simplify the formulation of the activation expression in some situations, as we will see below.
Note: If a material undergoes multiple activation/deactivation events, the elastic strains are removed at every instant of activation. This means that the material is always activated in a state of zero stress, regardless of its history, including past activations or deactivations. Inelastic strains, such as plastic strains, are not removed.
Let us look at some examples of how to use the Activation node.
Example 1: Pointwise Activation
As a simple 2D example, suppose that you want to gradually activate a material in the y direction as time, t, progresses. The imagined “activation front” travels at a velocity, vel, and the region of active material is therefore given by Y<vel\times t. It is entered as the activation expression, as shown in the following figure.
Activation expression for pointwise material activation.
To illustrate this, consider a solid quadrilateral element with four integration points (Gauss points), as in the figure below. With the activation expression above, each integration point is activated individually by evaluating the activation expression. This means, in practice, that a single mesh element can be partially active if it has more than one integration point.
Activation of individual integration points in a mesh element.
Example 2: Elementwise Activation
Now, consider a case where you want to activate entire mesh elements, and not on an individual integration point basis. To do so, you need to phrase the activation expression so that it evaluates equally for every integration point in each mesh element. This can be done using the centroid operator. The activation expression that was used in the previous example is modified, as shown in the following figure. The coordinate Y is now evaluated at the mesh element centroid, which means that the activation expression will evaluate to the same value for all of the integration points in the mesh element.
Activation expression for elementwise material activation.
Inside the mesh element in the figure below, the activation expression is fulfilled for the element centroid, and therefore all four integration points are active.
Activation of all integration points in a mesh element by using the centroid operator.
Example 3: Using the Previous State of Activation
Suppose that you want to simulate a laser cladding process where a filler material is melted and deposited over time. In this situation, the current position of the laser beam defines the location where material is currently deposited. The region of previously activated material is defined by the entire trajectory of the laser beam from the start of the process. (For details on how to model the movement of a laser beam, you can read this blog post on modeling moving loads and constraints.) The variable
wasactive can be used to avoid having to describe this trajectory mathematically. You can express an activation expression for this situation schematically as:
(logical expression describing the current position of the laser beam) || solid.wasactive
which states that the material is active if the “logical expression describing the current position of the laser beam” is true or if the material has been active at any previous time (or parameter step) during the simulation. If the activation expression is used without the
wasactive variable, the material would become inactive once the laser beam has passed, which is likely contrary to what is intended.
Visualizing the Results
Suppose that you have simulated a time-dependent process where material is deposited over time. It may be interesting to display results only for the active parts of a domain. You can do this by using the variable
isactive as the Logical expression for inclusion in the Filter node, as shown in the figure below. Note that depending on the chosen fulfillment type, the
isactive filtering may show slight differences compared to the underlying
isactive variable defined at the integration points of the mesh elements.
Using the Filter node to display only active parts of a domain.
Concluding Thoughts on the Activation Node
In this blog post, we have described different ways of using the Activation node to activate material during a simulation. The Activation node makes it easy to simulate the deposition of material in simulations of different types of manufacturing processes, such as welding and additive manufacturing. If you want to examine a model that uses the Activation node, click the button below to see the Thermal Initial Stresses in a Layered Plate example in the Application Gallery. Note that you need to have a COMSOL Access account and valid software license to download the related MPH-file.