Tutorial Example

In this example, a model of a flexible pavement system consisting of three layers (asphalt concrete, base course, and subgrade) subjected to loading by a truck with dual wheels and tandem axles will be created, solution generated, and the results of the simulation interpreted. The example covered here is identical to the solved project "Sample_3Layer_Refined_InfiniteBC_CircLoad_ParBond.evs" that is installed with EverStressFE1.0.

Model Creation and Geometry

Begin by starting EverStressFE. In this example, we will start with the default parameter values and modify them as needed to apply to this example. If another model is already loaded in EverStressFE, the default values can be reset by clicking on File and choosing the New option. You will now be in the Geometry and Layer Properties tab. The default Finite Plan Dimensions will be retained in this example. The Number of Layers will also retain the default value of 3 since this is the number of pavement layers to be modeled in this example. The Thickness (mm) for Layer 1 - Layer 3 should be set to 100, 300, and 1000, respectively. The modulus of elasticity E for Layer 1 - Layer 3 should be set to 3000, 300, and 100, respectively. Poisson's ratios n for Layer 1 - Layer 3 should be set to 0.4, 0.35, and 0.35, respectively. In this example, we will retain the Model Infinite Domain option for the Boundaries, but we will change the Y-Boundary of Layer 1 away from Wheel to Infinite. At this point, save the model using the Save As option under the File menu under the name "Example_1". The EverStressFE window should now appear as shown below in Figure 1.

Figure 1: Geometry and Layer Properties Tab During Model Creation

Loads

We are now ready to define loads. Click on the Loads tab to begin defining the loads. The truck in our example has dual wheels (the default option) and tandem axles, so we must only change the Axle Type to Tandem. We will choose to model the tire contact patch as circular (this could be based on observed tire contact patches of the truck in this example, etc.), so change the Tire Contact to Circular. Note that when a circular load is modeled, the Tire Width (mm) label changes to Contact Radius (mm) when switching to a circular load and the user can no longer input values. The value shown is calculated based on the Load per Tire (kN) and Tire Pressure (kPa). We will retain the default Tire Pressure (kPa), the Load per Tire (kN), and the center-to-center C/C Tire Spacing (mm) values. However, we will reduce the center-to-center C/C Axle Spacing (mm) in this example to 600 for clarity, despite the fact that this may be unrealistically small. Note that the Nonuniform Tire Contact Plotting Options are all disabled since we are using a uniform load in this example. These options are activated when the user chooses the User Defined or Load Custom options for Tire Contact. The EverStressFE window will now appear as shown in Figure 2.

Figure 2: Loads Tab During Model Creation

Meshing

We are now ready to define the mesh. Click on the Meshing tab to begin . For this example, we will retain the Locally Refined option. Note that the refined region of the mesh is currently sized to contain the applied wheel load plus an extra 100 mm in each direction. This is because the size of the refined region is automatically adjusted as the user modifies parameters on the loading tab. The dimesions of the refined region can be modified by the user, but this serves as a safeguard to minimize the chances of using a poor mesh. Generally, it is desireable to refine the mesh in regions where a large stress gradient exists (i.e. near the applied wheel load). For this example, we will retain the current values for the dimension of the refined region (Length, x (mm) = 448, Length, y (mm) = 323). Based on the mesh at this point as shown in the main graphics panel, it appears that the mesh needs to be more refined in the x-direction of the Locally Refined region, so we will increase Elements along Length, x to 12. We will also increase both the Elements along Length, x and Elements along Length, y to 6 in the course region. The Element Divisions for Layer 1 and Layer 3 will be retained, but the Element Divisions for Layer 2 will be decreased to 4 in this example. The Interface Condition at both layer interfaces (Layer 1/Layer 2 and Layer 2/Layer 3 will be changed to Partially Bonded and the Interface Stiffness (N/mm^3) values will be set to 1 and 2, respectively. The values for all of these meshing parameters are arbitrarily chosen in this example. Normally, the user may need to perform a study to determine reasonable values or have reference values on hand. The EverStressFE window will now appear as shown in Figure 3.

Figure 3: Meshing Tab During Model Creation

Solver

We are now ready to solve the model. Click on the Solver tab. Note that the Estimated Memory Required (MB) to Solve Current Model is listed as 158. The user's computer should have at least this amount of memory available to avoid excessively long solution times. In this example, we are only working with the current model, so we will retain the Solve Current Model option. Before pressing the Solve button, note the warning below it. After pressing the Solve button, the main program will be locked-out and the solver command window will pop-up. This command window displays information during the solution process that may be of interest to the user. However, this window can also be ignored and the user can work on other tasks while waiting for the solver to complete. The "Priority" of the solver "Process" is automatically set to "Low" so that other tasks can be performed on the user's computer with miniminal interference. Press the Solve button now and wait for the solution to be obtained. This particular project requires about 60 seconds to run on a machine with adequate memory and AMD Athlon 64 3800+ processor. Once the solver has finished, the command window will close and the user's access to EverStressFE is restored. The EverStressFE window will now appear as shown in Figure 4.

Figure 4: Solver Tab after Obtaining Solution

Results -- 2D Plot Option

We are now ready to view the results of the model solution. Click on the Results tab to begin. There are three main types of graphics that can be displayed in the main graphics panel: Standard 2D Plot through Depth, Contour Plot through Plane, and Plot Deformed Mesh. Each of these will be discussed here. The initial default option is to view a Standard 2D Plot through Depth of the Normal Strains (exx, eyy, and ezz) at the plan-view coordinates of the Wheel Centerline (mm) (the intersection of the axes of symmetry). The coordinates of the Wheel Centerline (mm) are also listed on the Results panel for the user's information. In this case, the Wheel Centerline (mm) is located at X = 0, Y = 1000.

The plot on the bottom of the graphics panel is an interactive tool to aid the user in orientation and allow them to select the plan-view coordinates of the 2D plot. The black plus symbol (+) is centered at the plan-view coordinates of the current plot. The user can drag the plus symbol to a new location or simply click another location on the modeling space and the graphic will update for the newly selected coordinates. The user can also manually choose plan-view coordinates for the plot by entering them into the numeric boxes. The plan-view modeling space is always plotted with equal scales on the axes and the tire contact patch is shown for reference. The user can view normal strains (exx, eyy, and ezz) , shear strains (gxy, gyz, and gzx) , or displacements (Ux, Uy, and Uz) . The EverStressFE window will now appear as shown in Figure 5, where the plan-view coordinates have been chosen by clicking near the center of the tire contact area.

Figure 5: Results Tab with 2D Plot Option Selected

Results -- Contour Plot Option

Next, we will examine the results by clicking the Contour Plot through Plane option. You will note that the graphics are updated in the main graphics panel as well as the Graphics tab at the bottom of the Results tab. The plot in the Graphics tab is simply intended to orient the user and is not interactive. The finite layers in the model are outlined in black and the layer interfaces are colored with a transparent gray color for a 3D effect. The tire contact surface is shown in transparent blue and the current plane of the Contour Plot is shown in transparent green.

The main graphics panel displays a shaded contour plot, where the colors represent the magnitude of the Parameter that is being examined. The extents of the contour plot plane can be selected by the user, but the origin is always at the pavement surface and the intersection of the planes of symmetry. The user can choose to plot a plane in any orthogonal direction (X-Z, Y-Z, or X-Y, with X-Z being the default. There are many options for viewing the results with the contour plot:

  • The user can examine 9 different paremeters.
  • The user can specify the coordinate of the plane cut.
  • The user can specify the extents of the plane.
  • The user can specify limits for the contour colors or have them selected automatically.
  • The user can choose to view colors only, outline the layers, or outline the finite elements.
  • The user can choose to show the tire contact area on or near the contours.
  • The user can choose to set axes equal.
  • The user can obtain values from the contour plot based on mouse position.

    • Try playing around with these different options to get familiar with the benefits of each. For example, choose eyy for the Parameter, X-Y for the plane, retain the at Z = 0 option , set the X and Y extents to 750 and 600, respectively, set the contour max/min limits to 40 and -190, respectively, choose the Outline Elements option, Show Tire Contact option, and the Axes Equal option. The EverStressFE window will now appear as shown in Figure 6.

    Figure 6: Results Tab with Contour Plot Option Selected

    Results -- Plot Deformed Mesh

    The final main plotting option can now be examined by clicking on Plot Deformed Mesh. This simply shows the deformed FE mesh in the main graphics panel. The default scale factor for the deformations is 100. The user can modify this parameter to create a visualization that conceptually illustrates where the deformations are occurring in the current model. For this example, a value of 500 appears to be sufficient to show where the deformations are occuring. The user also has the option of overlaying the undeformed mesh in a transparent gray color. It may help to toggle this option on and off. Alternatively, the user can switch between the Results and Solver tabs to compare the meshes. The graphics portion of the EverStressFE window will now appear as shown in Figure 7.

    Figure 7: Results Tab Showing Deformed Mesh and Critical Microstrains

    Lastly, critical results from the analysis can be viewed by clicking on the Critical Microstrains (T-, C+) tab at the bottom of the Results tab. The quantities presented are intended to represent those that might be required as inputs to pavement distress models such as rutting or fatigue. All quantities are obtained from nodal values directly beneath the centerline of the applied loads (i.e. at the intersection at the planes of symmetry). All values can be obtained from either of the graphical plotting options, so these values are only supplementary. The EverStressFE window will now appear as shown in Figure 7