['Recently', ',', 'research', 'on', 'thermal', '–', 'mechanical', 'analysis', 'of', 'P91', 'has', 'been', 'carried', 'out', 'including', 'the', 'characterisation', 'of', 'the', 'cyclic', 'behaviour', 'of', 'the', 'material', 'using', 'the', 'two', '-', 'layer', 'and', 'unified', 'visco', '-', 'plasticity', 'models', '[', '3,4', ']', '.']