A criterion to predict crack onset at a sharp V-notch tip in homogeneous brittle materials under a mixed-mode loading was presented and validated by experimental observations in a previous paper by the authors. This criterion slightly underestimates the experimental loads causing failure which is attributed to a small notch tip radius that blunts the sharp corner. This discrepancy is rigorously analyzed mathematically in this paper by means of matched asymptotics involving 2 small parameters: a micro-crack increment length and the notch tip radius. A correction is brought to the initial prediction and a better agreement is obtained with experiments on PMMA notched specimens.Keywords Failure initiation · Mixed-mode fracture · Generalized stress intensity factors · V-notch · U-notchGeneralized stress intensity factors (GSIFs), mode I and mode II, associated with the sharp V-notch A 1c , A blunt 1c Critical mode I GSIF for sharp/blunt V-notch α 1 , α 2 Sharp V-notch singularity exponents λ, GThe two Lamé constants E, ν Young modulus and Poisson ratio G c , σ c , K I c Fracture toughness, stress at brittle failure and plane-strain critical SIF , 0 Crack length and critical crack length at the V-notch tip