Methods to determine the joint strength of C/C to copper joints

Abstract

The plasma facing high heat flux components to be used in the divertor of long pulsed nuclear fusion experiments like Tore Supra, Wendelstein 7-X, ITER, EAST or NCT require joints between carbon–carbon-composites (C/C) and copper. Beside residual stresses resulting from the manufacture these joints have to sustain thermo-mechanical stresses that are superimposed during the application. Up to now different shear tests of the joint are in use for quality assurance during the manufacture. In addition, thermal fatigue tests of components are employed to qualify this crucial joint in a condition close to the final application. Recent results from high heat flux tests and finite element calculations have suggested that the shear strength level determined after manufacture is not sufficient to predict the performance of the C/C to copper joint under high heat flux. Therefore, different approaches are currently employed by the authors for a better judgement of this particular joint. Firstly, the joint is characterized by a shear test after repeated thermal quenching. It is assumed that such a quenching generates at least microcracks, which reduce the integral capability of the joint to transfer mechanical loads. A comparison of the shear values obtained on quenched samples to the shear value obtained directly after manufacture gives a measure to determine the thermal shock resistance of the joints. In addition, tensile test and shear test samples are employed at different temperatures to characterize the joint. Finally, possibilities have been investigated to characterize the performance of the joint in the presence of a crack. For that purpose, a method based on notched three-point bending tests has been applied to the C/C-copper joints. This allows to determine the fracture toughness of the joint. Besides the description of the different test methods the paper also presents results obtained for selected C/C to copper joints.