AbstractThis study describes the evolution of the stress intensity factor KI for fissures in brittle elastic materials subjected to transient thermal actions. The case studied is a ‘penny shaped crack’ located in a rock mass and through which runs a fluid whose temperature falls in time.Computation was carried out using the CASTEM finite element method and its subprograms. The four steps are described: mesh elaboration, calculation of temperature field, thermo‐mechanical computations and estimation of KI by displacements or the J integral method. The results are given for various transient thermal conditions and for a steady state case for reference. Cooling spreads very slowly in the rock mass and there are steep temperature gradients in the first few millimetres.