Unexpected tensile strain at compression face of a buckled pipe section

Hi, 

I am having a problem with strain outputs of a buried pipe in ABAQUS/Standard. the pipe is subjected to axial compression and bending via a displacement controlled procedure. After local buckling of the pipe starts, at a certain point of the analysis, the compressive strain at the compression face starts to decrease and at the end of the analysis this decrease will result in tensile strain at the compression face. This happens with severe distortion and ovalization of the pipe cross section. I have modeled the pipe with S4R shell elements and the soil with C3D8R. I have also introduced a contact algorithm between the pipe and the soil which accounts for friction and seperation. Does anyone have a clue why this is happening. 

Thanks,

Hemen