B(Stress) magneto-mechanical model: dependency between B(H) constitutive law and mechanical stress

Introduction

The magnetic behavior of a material is known to be sensitive to the application of mechanical stress. This may lead to significant effects that modify the performance of electromagnetic devices. For instance, the iron losses in a mechanically stressed magnetic circuit may increase up to 40%. The mechanical stress may be applied in different ways: during the construction of a magnetic device (punching, laser cut, assembly...) or during its operation (magnetostriction, centrifugal forces due to high-speed rotations, etc.).

Flux can model this dependency between the B(H) constitutive relation of an electrical steel and mechanical stress through its B(Stress) analytical multiscale model. This model allows considering the influence of an equivalent mechanical stress on the B(H) magnetic behavior of the material. More specifically, this coupled property allows Flux to account for plastic or elastic deformation effects that modify the B(H) relationship that would be verified in an unconstrained sample of the material. The modified B(H) curves are computed automatically by Flux and used in the FEM computations.

It is important to note that the descriptions of both the B(H) and B(Stress) properties of a material are mandatory to account for magneto-mechanical stress effects. Figure 1 shows the computed B(H) curves that result from different values of mechanical stress for an electrical steel sheet (FeSi alloy):

Figure 1. Example of modified B(H) curves of a FeSi steel sheet that are automatically computed by Flux, in accordance with the coupled B(Stress) property for different mechanical constraint values.

In this model, the influence of mechanical stress on the shape of the B(H) curve is governed by the saturation magnetostriction constant λ. In Figure 1, the unconstrained material may be described by the Isotropic analytic saturation + knee adjustment law with the following parameters:

Creating a material with a magneto-mechanical model and assigning it to a region

The magneto-mechanical model is available only in Flux 2D for Transient Magnetic and Magneto-static applications and may be assigned to a material either during its creation or during its modification.

More specifically, the user must set both the B(H) property of the material to the appropriate subtype (i.e., isotropic analytic saturation + knee adjustment) and the B(Stress) property.

The procedure is detailed below:

• First, create or edit an existing material in a Flux project. Two methods are available to perform these actions:
  1. Using the Physics menu, by selecting the option Material and then New or Edit;
  2. Through the Flux Data Tree located on the left of the main project view, i.e., in its Physics section, by double-clicking on Material to create a new one or by double-clicking on an existing material to edit it.
  Depending on the case, Flux will display either the New Material or the Edit Material dialog box.
• Enable the Magnetic Property option that is available in the B(H) tab.
• Then, choose the following model:
  • Isotropic analytic saturation + knee adjustment and fill the properties of the model: the Initial relative permeability, the Saturation magnetization and the Knee adjusting coefficient.
• Enable the Magneto-mechanical property option that is available in the B(Stress) tab and then choose the following model:
  • Analytic multiscale model and fill the value of the Saturation magnetostriction constant λ.

• Finally, the previously created material must be assigned to a Laminated magnetic non conducting region for a proper consideration of its magneto-mechanical behavior during solving process and post-processing operations (e.g. computation of iron losses). In practice, while creating or editing the chosen laminated magnetic non-conducting region, the user must enable the Mechanical stress dependence option in the main tab of the region dialog box, and then choose between one of the two following constraint types (and thus models):
  1. Uniform over the whole region or
  2. Exponential decay towards region center.

  In both cases, the user must then fill the Equivalent uniaxial stress (MPa) field with the value of the intensity of the mechanical constraint in MPa. For the first case, this value impacts the B(H) law of all nodes of the region, while the second model applies this value only on all boundary nodes of the region (except symmetry and periodicity lines), being the internal nodes constrained by a lower (in absolute value) stress determined by means of an exponential decay equation. To define such a model, the user is also required to fill the Distance-to-boundary defining the stress decay rate (mm) field, which is the inverse value of the exponential decay constant (in other words, the value to fill plays the same role as the "time constant" of RL or RC circuits).

  Note: A positive stress value corresponds to a tension and a negative value to compression (i.e., the typical case for punched laminated sheets employed in electrical machines). Moreover, this stress is said to be equivalent to an uniaxial stress in the sense that the change in the B(H) property in the region corresponds to the change that would take place in a sample of the material subjected to uniaxial stress. The region remains isotropic, i.e., the modified B(H) property is used for all directions in the computations.
  Note: The first approach (Uniform over the whole region) is useful when the user decides to split the original region in sub-regions, which may include parts that are not affected by mechanical stress and others that are impacted by a mechanical constraint.
  Note:

  The second approach (Exponential decay towards region center) may be used as a straightforward way to model a magnetic region that is subjected to a mechanical constraint only in a narrow band lying along its boundaries (e.g. effects of punching operations). Since it does not require splitting the original region into sub-regions, this approach makes the geometrical and the mesh descriptions easier. As a rule of thumb for meshing, it is advised to have in this area a maximum mesh element size equal to the Distance-to-boundary defining the stress decay rate (mm) value.

  In this second approach, all the nodes of the region are subjected to a mechanical stress (and consequently to a modified B(H) property) whose value is highest on boundaries and decays exponentially as the nodes are located inside the region. In such a manner, 95% of the mechanical stress is applied on the narrow band lying along the region boundaries and whose width is three times the distance value filled in the Distance-to-boundary defining the stress decay rate (mm) field.

  A comparison between the two approaches is shown in Figure 2 below where the obtained stress distributions (isovalues) are reported.

  
  

  

  
  Figure 2. Isovalues of the mechanical stress applied on a stator tooth through the two available approaches. In the first (a), the stator is split into two regions: a region (lying along the boundaries) damaged by punching where a uniform stress value is applied and an intact region (in the center). In the second (b), the mechanical constraint decays exponentially from the boundaries towards the center.

Specificities for the solving process

Application example

Figure 3. The geometry of the stator and the adopted meshes for each approach. The mesh used together with the approach based on uniform stress values applied to two regions is shown in (a). The mesh used together with the approach based on the application of a decaying mechanical stress towards the center of the unique region used is displayed in (b).

Several computations were performed with the compressive mechanical stress going from 0 to -210 MPa and with both approaches: in the case of uniform stress a damaged zone width of 0.25 mm has been used, while in the case of exponential decaying stress, the decay rate has been fixed at 0.08 mm, so that 95% of the mechanical stress is applied on the three-times (0.24 mm) width external band. The total Bertotti iron losses were evaluated while in post-processing, after resolution of a Transient Magnetic scenario. A comparison of the total iron losses obtained for each value of mechanical stress is summarized in Figure 4 below:

Figure 4. The iron losses dissipated in the stator as a function of the mechanical stress. The blue line corresponds to the approach based on a stator split in two parts: a boundary region with an active magneto-mechanical property and an inner region without the mechanical stress dependency. The orange line, on the other hand, corresponds to the approach in which the stress decays exponentially from the boundaries towards the center of a single region.

The magnetic flux density distribution may be visualized through an isovalue plot in the stator, as shown in the Figure 5 below. In this figure we can clearly see the impact of the punching process and of the mechanical stress on the stator teeth boundaries. The intensity of the magnetic flux density in a narrow region along the perimeter of a tooth is visibly lower when compared to its innermost parts.

Figure 5. Distribution of the magnetic flux density in the bulk of the stator. Results obtained with approach based on the application of mechanical stress of -60 MPa decaying from the boundaries towards the region center.

The weakening of the magnetic flux density along the boundaries is a consequence of the localized degradation of the magnetic permeability resulting from punching during the fabrication of the stator, as shown in Figure 1.