NDT Back to Basics: Eddy Current Applications
February 1, 2008
Eddy current inspection is a versatile nondestructive testing technique. It allows accurate measurement and evaluation of the conductivity and, therefore, the chemistry, hardness and geometry of any electrically conductive material that is brought into the proximity of a coil of wire. Eddy current can be used to inspect and/or detect cracks, voids, porosity, hardness, materials chemistry, materials density, thread condition, thickness, run-out, and surface condition. A brief review of the technology’s inspection capabilities follows, providing an idea of how it can be used to facilitate inspection of machining operations and related processes.
Eddy current testing is a comparative means of part evaluation. In its most basic form, eddy current testing compares the eddy current characteristics of a known good part sample to the eddy current characteristics of the general part population. During a learn process, the eddy current signature of a number of known good, or master, parts is learned by the system electronics and software and stored in computer memory. Thereafter, as subsequent samples are inspected, the eddy current signature of the parts under test is compared to the learned signatures. If the signatures of the part under inspection match the learned signatures, within limits set by either the operator or the computer, the parts are judged good and pass; otherwise, they are judged bad and fail.
Primary Applications in a Production EnvironmentEddy current testing provides an analog means of evaluating parts. It is not a go/no-go gage, but a proportional means of reviewing the characteristics of machined components. Although a go/no-go capability can be added to this inspection technique through the addition of operator-set acceptance limits, it is fundamentally analog in nature and, therefore, offers a wide variety of inspection applications and potential.
The applications for eddy current testing in a production environment are virtually endless. Because of its extreme versatility, inherent accuracy and ease of implementation, it can be readily adapted to a myriad of uses. A few of these are listed below:
Detection of Threads in Holes or on Studs
Thread detection is a natural application for eddy current inspection, as it offers a noncontact means to evaluate the presence and condition of threads in holes or on studs. In fact, this area of eddy current inspection has been sophisticated to the point where not only the presence of threads can be sensed, but also the number of threads in the hole can be accurately determined and, with properly engineered systems, the portion of the last thread helix present can be determined.
Detection of Cracks
Crack detection is another fertile area for eddy current inspection. In fact, in finished surfaces, dual element probes that evaluate the differences in the eddy current signatures at two adjacent locations on the surface of a part can reliably find cracks that are less than 0.001 inch wide and not visible to the human eye. Also, very small cracks can be reliably discerned in holes. An ideal application for this inspection is in sintered metal parts where pre-cure cracks can be formed that are not easily discernable, yet will severely compromise the integrity of the part after curing. These cracks can be detected either before or after part curing with noncontact techniques that sense in real time by simply bringing a probe into the suspect area of the part.
Detection of Displacement or Run-Out
Eddy current inspection has a characteristic that in some cases can cause problems in the noncontact readings taken from a part, but in other cases represents exactly the parameter that is to be sensed. That parameter is called stand-off, which is defined as the separation between the sensor and the part. In most types of eddy current inspection, this parameter is considered as noise that must be dealt with in the sensed reading, but when very accurate distance measurements are to be made, this parameter provides the information that is the signal. By precisely sensing this parameter, displacements in the micron range can be easily measured. Further, these measurements are noncontact,therefore not subject to wear,and provided in real time by a measurement system that contains no moving parts.
Detection of Hardness
As previously mentioned, eddy current inspection inherently detects hardness along with chemistry, temperature and geometry. If the other three parameters are held constant, or relatively so, differences in materials hardness can be quite accurately discerned. In fact, hardness differences of one Rockwell point can be readily detected.
Detection of Porosity
Materials porosity also is well within the capabilities of eddy current detection. In fact, because the eddy currents themselves penetrate the materials surface, typically to 0.01 inch or so depending on frequency, subsurface porosity also can be detected. This inspection technique works especially well in holes where the eddy currents can inspect the entire 360 degrees of the circumference, but also can be quite effectively used in applications where porosity is to be detected on flat or even irregular surfaces.
Detection of Part Chemistry
Here also, eddy current inspection can be a winner. In parts where individual subcomponents are made of materials of certain chemistry, this noncontact inspection technique can be effective, inexpensive and extremely reliable. For instance, in compression fittings, where the internal ferrules need to be of a specific material, eddy current inspection can be used to inspect the finished assembly for the presence of the correct internal component without disassembling the part.
The Presence and Condition of Seams in Welded Tubing
Since eddy current inspection detects hardness and chemistry changes, the detection of a weld seam is relatively straightforward. Welding operations, whether they are filler or non-filler, equally affect the hardness and chemistry of the base material to the point where seams can be readily discerned with either single element or dual element probes. In addition, weld porosity and other defects also can be detected in many applications.
The State of Eddy Current TechnologyEddy current inspection equipment has evolved to the point where it provides the operator with many of the benefits of this test technology without requiring the due diligence related to understanding the technology that was required only a few years ago. Virtually all the current systems are computer based and offer the operator the capability to not only perform the required inspection, but also perform statistical analysis on the acquired data and then make that data available for SPC or other trend analysis reviews by other systems. Many of the current eddy current systems also are Ethernet compatible and provide easy data transfer to other systems via external serial connectors. The periodic mastering of the systems that was once required is now necessary only when a probe is replaced or an amplifier card changes. Furthermore, modern systems can store the eddy current signatures of many parts in their databases for immediate retrieval, or can archive them to provide a historical record that carries much information related to both the materials characteristics of the part and the machining operation used to create that part.
Perhaps the most significant advance, however, is the concept of using eddy current inspection techniques to profile parts. With this technique the eddy current profile of a group of known good parts is stored in computer memory along with the location of the probe with respect to the surface of the part. The computer then calculates the x bar and ±three sigma profiles for that group of profiles and displays the results of those calculations on the computer screen. Subsequent profiles that fall within those calculated limits are then judged good and passed, while any portion of the part profile that falls outside those limits causes the part to fail. Most up-to-date systems also allow the operator to modify the accept/reject limits so that areas of the part profile that are not of primary interest can be, in effect, de-emphasized during the sort.
Typically, eddy current profiling can be accomplished at rates in excess of two inches per second with good results possible up to several feet per second in well designed systems.
Eddy current inspection can be extremely useful in many applications, the limit of which is only bound by the imagination of the operator. As this technology is moved more aggressively into the realm of the computer-based data acquisition system, it can only become even more useable and capable than it is today.