Patent ID: 7519487

Claim:
An improvement to a method for non destructive testing of an object by determination of depth of surface penetrating crack in said object that comprises applying alternating current to a sample; measuring voltage drop V 1 (f) between two probes separated by a gap of distant Δ and making electrical contact with the surface of the sample for at least one frequency f, wherein said probes are located remove from the surface penetrating crack; measuring voltage drop V 2 (f) between two probes separated by a gap of distance Δ and making electrical contact with said surface of said sample, for at least same frequency f, wherein said probes are located at different sides of said surface penetrating crack; and calculating δ based on electrical parameters of said samplers material, the improvement comprising the steps of: determining the depth d of said surface penetrating crack from the equation: V 2 ⁡ ( f ) V 1 ⁡ ( f ) = Δ + 2 ⁢ d - F 1 ⁡ ( Δ , δ ) - F 2 ⁡ ( d , δ ) Δ , wherein F 1 ⁡ ( Δ , δ ) = Δ · exp ⁡ [ - Δ / 2 + δ δ ] , and F 2 ⁡ ( d , δ ) = 2 · d · exp ⁡ [ - d + δ δ ] , and wherein: δ = 1 ( π ⁢ ⁢ μ r ⁢ μ 0 ⁢ σ ⁢ ⁢ f ) 1 / 2 , where μ r is the relative magnetic permeability; μ 0 is the magnetic permeability of free space; σ is the electric conductivity; f is the frequency; and rejecting said object based on said determined depth.