The plate surface is heated from outside so that the temperature gradients of the surface have been constant on steady state. However, if a crack exits on that surface, the surface temperature is disordered by the crack with the air layer, and the temperature gradients are changed.
  In this paper, using the plate specimens of alminum castings with a surface crack, the estimation of the surface crack depth is tried by measuring the surface temperature. The temperatures obtained from the experiments were changed to the temperature gradients, and then the relationship between the surface crack depth and the dimensionless temperature gradient was obtained. In addition, the results were compared with the numerical calculation obtained by the finite element method.
  The results show that the surface crack depth can be estimated by using the dimensionless temperature gradient. On account of the fact that the dimensionless temperature gradient decreases at the vicinity of the surface crack and shows the maximum value at position of that, this method is also applicable for the detection surface cracks.