Total Energy Decreasing During the Dynamic Front Crash Analysis of Chassis

Hello everyone,

I am performing the dynamic front crash analysis of the chassis of a vehicle in Radioss. During the post-processing of the results obtained, I have plotted the total energy, kinetic energy, and internal energy graph. Theoretically, the total energy of the system should remain constant during the whole analysis but contrary to that it remains constant initially for some time but then starts decreasing gradually. For your information, I have used the M2_PLAS_JOHN_ZERIL as the material card image. (Please refer to the image attached for the material parameters used)

Does anyone know the reason behind it and how to correct it? Any help would be appreciated. (I have attached the image of the respective energy graphs with stress plots for reference) 

Thanks in Advance