Despite six decades of extensive investigation, the detonation diffraction critical conditions have not yet been predicted from first principles and only semi-empirical models are available to estimate the critical tube diameter. The present study aims at providing a well-defined experimental and numerical framework to help establishing a quantitative theoretical model to predict the critical conditions for diffracting detonation failure and re-initiation. More than 100 diffraction experiments were performed in a narrow channel facility for a 2H2-O2-2Ar mixture at P1=10.3-23.4 kPa and T1=298 K. The diffraction experiments were analyzed in terms of probability of successful transmission as a function of initial pressure. Numerical simulations were performed with a realistic reaction model. Qualitative agreement with the experimental results was demonstrated but the critical pressure for detonation failure could not be quantitatively reproduced. Simplified combustion models were used to estimate the respective effect of shock front curvature and volumetric expansion behind a decaying shock on diffracting detonation failure. It was found that the expansion was the dominant process responsible for the increase of ignition delay-time and possibly for detonation failure.