Semi-cristalline poymers (SCPs) such as High-Density-Poly-Ethylenes (HDPE) present a spherulitic microstructure. During mechanical loading micro-mechanisms of deformation in spherulites and their effect on the macroscopic response have to be clarified. We intend to use the AMITEX code [1] developed at CEA Saclay, firstly to solve the local mechanical equations on such a complex microstructure, and secondly, to couple it with a modelling approach of phase transformation type to explore the thereby opened simulation paths allowing for a better understanding of the damaging processes which explain the macroscale behaviour. The AMITEX code is based on an FFT method [2] and takes into account behaviour laws in UMAT format. To describe mechanical behaviour of crystal-amorphous two-phase spherulite structure we created a UMAT with a transversal isotropic law based on a DLR model [3]. This model allows predicting the local and overall behaviour between amorphous isotropic viscoelastic regions and orthotropic elastic crystallites accounting for their radial arrangement from spherulites centers. A phase field model was developed in AMTEX source code to describe phase changes in crystal-amorphous two-phase arrangements during loading. First computations of behaviour using the mechanical model coupled with the phase field method reveal a deterioration of crystal phases and crystal/amorphous interactions occurring during strain which can now be investigated more deeply with respect to the model parameters.