Technology

Process

The eddy current testing method is a nondestructive evaluation method. It is widely used for crack detection as cracks cause very large local conductivity changes. However, there are many other applications in which highly sensitive and spatially resolved conductivity analysis can help to solve various inspection tasks. The basic principle is shown below.

Non-contact testing solution for different applications:

• Sheet resistance and thickness measurement of thin films
• Substrate thickness measurement
• Carbon fiber testing 
• Metal, alloy and ceramic testing

Characteristics

• Contactless and nondestructive
• extremely fast / high sample rates (k samples / second)
• good automation abilities
• high sensitivity

EC Density

Since the sensitivity of Eddy Current inspection depends on the Eddy Current density at the defect location, it is important to consider the strength of the induced Eddy Currents at the defect location. Typically, a setup / frequency / sensor is selected which places the expected defect within a one standard penetration depth. This assures that the strength of the Eddy Currents would be sufficient to produce a flaw indication.

Penetration depth depends on to the permeability, conductivity of the material as well as the frequency.

Standard penetration depth

The standard penetration depth is a term that is relevant to select a good testing setup for the characterization bulk materials. The depth that eddy currents penetrate into a material is affected by the frequency of the alternating current, the electrical conductivity and magnetic permeability of the sample. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability. The depth at which eddy current density has decreased to 1/e, or about 37% of the surface density, is called the standard depth of penetration (d or 1d) and is used as criteria of ideal measurement for the investigation of bulk materials. At three standard depth of penetration (3d), the Eddy Current density is down to only 5% of the surface density. So, defects or variation in  deeper depth than this do not add recognizable to the measurement effect and therefore those are hardly detectable. Thus, a setup that achieves a standard penetration depth suitable (1d) to the depth of the characteristics of interest enables the best testing result. SURAGUS offers a wide range of sensors with different frequency ranges for optimum testing of materials with different properties.