The conventional relations for calculating the fracture stresses consider only elastic deformation but ignore viscoelastic and viscoplastic behaviors. Measuring the joining strength of a composite glass sealant-metallic interconnect specimen at solid oxide cell application relevant at high temperatures is a case where such effects can become significant. In the current study, three-point and four-point bending test results were analyzed using the finite element method (FEM) to assess systematic and random errors. It is shown that plastic deformation of the steel interconnect material at high temperature, although having a large effect on the stress distribution in the steel/glass–ceramic/steel specimen, does not cause a significant difference in the FEM-derived true and the standard equation-based analytically derived flexural stress values. It is also observed that even if the viscous flow of the glass–ceramic is considered in the simulations, the samples’ behavior in a realistic testing condition is not biased significantly by inelastic behavior due to the short testing time.