The optimization result is not symmetric for L-shaped structure

Hi guys, 

  I obtained the optimal L-shaped structure using free-shape optimization method. Inner nodes can freely move in thickness direction within a certain range (as shown in figure 1). Minimizing the out-of-plane tensile stress (interlaminar tensile stress) is the objective. The constraints are that maximum load point displacement and total weight are less than or equal to that before optimization.  

  However, in the case of shape change, it is not a symmetric result although the input and loadcase are symmetric (In the attached figure 2). Do you have any idea about the reason? If possible, do you have any idea how to obtain symmetric result?  

  I share the model file and results as follows: 

1) Input deck of the free-shape optimization of L-shaped structure: L-shaped_free-shaped optimization.fem

2) Result of the free-shaped optimization: L-shaped_free-shaped optimization_des.h3d 

 

Best regards,

GUO