This example illustrate how to perform a fatigue assessment of a component made using anisotropic material, where the fatigue strength is significantly dependent on the direction of the material relative to a reference.

This feature has been developed in collaboration with Amin Shahrestani Azar, Even Wilberg Hovig and Magnus Reiersen (SINTEF / NTNU).

A recent, relevant case necessitating this is that of metal additive manufacturing. Here the fatigue strength is governed by one of two phenomena; either surface roughness or internal defects. Both of these compete to destroy the component. For machined surfaces, the internal defects are usually dominating, whereas for as-built surfaces, crack initiation usually starts at the surface.

The internal defects, such as lack of fusion illustrated above have the most impact when they are perpendicular to the loading direction. For the surface, on the other hand, it is highly dependent on the location around the cross section. On the bottom (underneath a print) it is very coarse due to the removed support. On the top is very fine, on the sides it is intermediate and at inclinations there may occur a staircase shape with high roughness due to the layers.

The expected influence on the fatigue strength is visualized below for the two cases, named defect and surface sensitive, respectively.

Since version 2.034, Fatlab can include this effect in the fatigue assessment. The SN curve setup has been extended with an input section to describe the scaling of fatigue strength relative to a reference direction.

Of course, the user is left with the difficult task of establishing the anisotropic relationship. Help may be found here.

During the fatigue analysis, Fatlab will determine the angle of the surface of each node and scale down the fatigue strength according to the input given above. The angle is calculated between the surface normal vector and the reference direction vector (input). The angle may be visualized under Contour plot > Result = Angle, as exemplified below. The magenta arrow indicates the reference direction.

The resulting utilization is shown below. Since the utilization is now dependent on the orientation of the component relative to the reference direction, there will be some orientations that are more favorable than others and an optimization is called for. This has been implemented under Run Analysis > Advanced > Optimize anisotropy. Pressing this button requires some patience, but will optimize the reference direction (printing direction) in order to minimize utilization. It requires the Global Optimization Toolbox of Matlab.

The example is setup using a variety of unsupported formats; Abaqus, Gmsh and hence, all the necessary inputs have been manually converted to the custom .csv format in order to import it into Fatlab.

Download