Eddy Current Testers
Structure Test Overview

Correct Structure: Tempered Martensite

Wrong Structure: Ferrite, Perlite, Mixed Structure

Eddy Current Instruments/Equipment

Instruments: For testing to verify correct hardness, case depth, case depth location, temper, alloy, etc.

Eddy Current Testing Equipment - Systems - In-Line 100% Testing And Audit Systems

Systems designed and built by ibg NDT Systems are currently testing ball studs, tie rods, steering racks, fasteners, piston pins, rollers, pinion pins, spindles, hubs, races, steering knuckles, camshafts, gears, balls, cylinder liners, pistons, tulips, axle shafts, rocker arms, needles and many, many other components.

  • Structure Test

Structure Test

ID coil for structure test on a contiuous velocity joint for correct case depth and position of hardness zone.

A difficult problem for in process inspection of mass parts is supervision of heat treatment results. Verification of features like correct case depth and hardness are of prime importance for meeting the quality standards of the final product. These features can be monitored by destructive methods like sampling but with concurrent high personnel and financial effort. Moreover, a sampling inspection is sensible for statistically distributed defects only. During heat treat processes in furnaces or induction hardening, accidents involving single parts and that violate statistical distribution laws happen. These "unexpected" defects can be missed by sampling. 100% testing is required to reliably detect them.

Eddy current structure test utilizing the well known Preventive Multi-Frequency Technology (PMFT) and Simultaneous Harmonic Analysis by ibg guarantees maximum test reliability in 100% testing. Heat treatment results in serial testing can be monitored cost efficiently, quickly and reliably, and even unexpected defects are detected with highest reliability. The eight test frequencies of the fundamental wave as well as analysis of two harmonics of the PMFT guarantee highest reliability. The field of application for PMFT with simultaneous harmonic analysis is very wide and includes eddy current testing of surface hardness, case depth, intermediate structure, material mix-up and others.

Setting of the instrument is done with good parts only. 10 to 20 parts are presented to the instrument, and tolerance zones are formed automatically. Eddy current signals from subsequent parts are now compared with these good parts. That means that no artificial defects are required for calibration, only known good parts. Due to the high test speed and the easy setup a new test task is ready to start within shortest time. Another advantage is the multi-dimensional analysis of ibg eddy current test instruments. An individual tolerance zone is formed for each test frequency. If all tolerance zones are satisfied, one can assume that the tested part is okay. Even one single not satisfied tolerance zone classifies the part as not good.

Eddy current tests are always comparative tests with yes/no answer. Very small deviations in the group of parts can be detected. Eddy current cannot answer why there is a deviation. In case of a bad part another test is recommended like metallurgical methods to inspect the structure (cut) or a classic hardness test.

Fixture for eddy current structure test in the laboratory.

NEW: Simultaneous Harmonic Analysis iSHA
The relative permeability is strongly influenced by heat treatment processes. Hardened structures have a lower permeability than soft. Because the ibg Preventive Multi-Frequency Technology tests with small magnetic fields hardened OK parts show no saturation effects. It's within the linear range of the hysteresis curve.

Harmonics occur by each hysteresis of the magnetisation curve of a test part in the coil arrangement. The sinusoidal current in the exciter coil leads to a sinusoidal field strength which induces a magnetic flux in the test part. Its chronological sequence, however, is (depending on formation of permeability) not sinusoidal anymore. Therefore, the sequence of the magnetic flux in the receiver coil induces a distorted voltage,

deviating from the real sine even at small field strengths. The fundamental waves, having been distorted by hysteresis, contain higher frequency harmonics. Fourier analysis of the distorted fundamental waves reveals the higher frequency harmonics, more precisely harmonics of the 3, 5, 7 or 9 times frequency. The analysis of these harmonics supplies a more detailed view of the magnetic features of the part and thus more exact information on the formation of microstructure. In addition, it offers significant advantages when suppressing interferences like batch scattering, part geometry and part temperature or positioning effects. Harmonic signals are very small. That's why immense electronic effort is needed in order to distinguish them from the basic noise. Such small signals can be detected clearly and processed robustly by ibg's unique digital eddyvisor® eddy current instrument family.

An essential innovation is iSHA (ibg's Simultaneous Harmonic Analysis) - combined with ibg's PMFT it guarantees highest possible test reliability. It enables simultaneous calculation of vectors of the fundamental wave at eight frequencies and of two harmonics for each of them without extension of test time. It additionally uses high frequencies and their harmonics for a better solution of even smallest structure differences. Thus iSHA greatly enlarges the possibilities of the well-known Preventive Multi-Frequency Technology.

NEW: Temperature adaptive structure test iTAS
Eddy current testing is based upon the electric conductivity and the magnetic permeability characterising the material to be tested. Different materials and microstructures have different conductivities and permeabilities that are detectible by eddy current. However, both properties also show a temperature dependency. Electric conductivity decreases by 4 % per 10°C. Variations of ambient temperature are compensated by ibg by means of the differential coil principle. Test coil is compensated by a separate compensation coil giving a differential test.
However, test parts are often heated up by production processes and cool down during pauses of production. Thus they have different temperatures when tested. This leads to an erroneous NG sorting.

ibg's new Temperature Adaptive Structure Test (iTAS) considers the influence of temperature fluctuation on the measuring values. If the temperature of the test parts can be captured by an external sensor that is directly connected to the eddyvisor®, the influence of the temperature can almost be entirely compensated. Test parts appear as if tested at the same temperature. Pseudo reject is reduced and the test is more precise. The iTAS also enables quick visualisation of how big part temperature influence is in comparison with other parameter fluctuations (batch and dimension differences etc.) for a specific test application.

New: our new AutoStart allows an even faster and more precise start of the structure test than before. The graphic interface makes it very easy to adjust the AutoStart for quick and also for dynamic testing.

Features of Structure Test

  • Tolerance zones
    During material data recording the eddyvisor® automatically generates elliptic tolerance zones calculated for reliable testing. A tolerance zone editor is integrated for special situations in the structure test field. A skilful operator may freestyle the fields as rectangle, or ellipse, reduced or expanded, rotated or moved, etc.
  • Test speed
    High speed testing within milliseconds. Using eight test frequencies and standard setting (25 Hz - 25 kHz), only 141 milliseconds are needed for the test. In high speed mode only 1 ms is needed for each test frequency > 5 kHz, very useful for very high speed dynamic testing.
  • iSHA harmonics analysis
    Eight test frequencies of the fundamental wave plus two harmonics (2nd to 9th arbitrary) for each of the eight test frequencies can be utilized without extension of test time. The standard setting is evaluation of the 3rd and 5th harmonic on each of the eight fundamental waves.
  • Display
    Display of test results as bargraph diagram, single ellipse or multiple ellipse display selectable.
  • Coils
    A multitude of standard encircling coils up to a diameter of approx. 500 mm and probes for structure test are available for standard applications. Special applications may be solved by custom designed inner diameter I.D. coils or by rectangular coils which are designed and manufactured in house. Monitoring of coil and cable failure as well as 50/60 Hz noise suppression can be activated.
  • Triggering of test
    Start of test can be done manually at touch screen, via PLC or optional start button. There is an auto-start function that detects the part in the test coil and activates testing immediately or after an adjustable delay time.
  • Test frequencies
    Eight frequencies in a range of 5 Hz to 3 MHz are selectable.
  • Classification
    Sorting of different materials and testing of mixed batches can be accomplished with the ibg classification function (OR-conjunction). Up to seven good part types with different characteristic can be taught and during testing compared to the taught types and classified according to their part type.
  • Bar and tube test (option)
    Bars, tubes and wire can be tested in line to verify material, heat treatment and density differences as well as structure changes. Two proximity switches are used to trigger start and end of test. Nearly the entire bar/tube length is tested. An integrated statistical evaluation of the good versus not good tests (absolute and percentage) is automatically updated. A threshold for an allowable percentage (adjustable) of not good tests can be utilized to eliminate rejects caused by irrelevant local material property variations (e.g. from cold working from handling.

ibg NDT Systems - Designers and manufacturers of state-of-the-art NDT eddy current testing equipment, instruments, and systems for automotive, bearing and other metal components including fastener, forged and cast parts, hard metals, precision stamped parts, etc.