In life assessment the question of what is the effect of a pre-existing stress field on the behaviour of a crack at high temperatures needs to be addressed. This paper proposes a laboratory test method for inducing a superimposed residual stress in a C-ring specimen by plastic loading at room temperature. X-ray diffraction and neutron diffraction techniques are then compared in measuring the induced residual stresses across the crack plane of the C-ring specimen on three steels (P22, P91 and 316LN) taken from the 'HIDA' project [1]. The lack of standards and practices for both neutron and X-ray diffraction techniques makes any set of independent measurements difficult to validate. Therefore both analytical and numerical FE methods were used to apply, calculate and validate the residual stress profiles. Good agreement has been found between X-ray and neutron diffraction methods and the predicted FE results for all the three materials.