ASNT NDT Level III Eddy Current Testing (ET) Practice Quiz

1. What is the primary characteristic of the Standard Depth of Penetration (SDP or δ) in a non-ferromagnetic material?

It is the depth at which the eddy current density is reduced to 50% of the surface value. It is the depth at which the eddy current density is reduced to approximately 37% of the surface value. It is the maximum depth where flaws can be reliably detected. It is directly proportional to the test frequency.

2. How does an increase in the lift-off distance between the probe and the test piece typically affect the impedance plane signal?

The signal amplitude increases and rotates clockwise. The signal amplitude decreases, moving toward the origin along the lift-off vector. The signal phase angle shifts by 90° relative to conductivity changes. The signal remains constant but the frequency component is filtered out.

3. In a test designed to detect subsurface flaws in a thin wall, increasing the test frequency will:

Increase the depth of penetration (δ). Decrease the depth of penetration (δ). Shift the conductivity signal clockwise on the impedance plane. Increase the signal-to-noise ratio, regardless of the δ change.

4. The Primary Factor that governs the shape (phase and amplitude) of the impedance plane locus for a conductivity change is the:

Operating frequency relative to the material’s characteristic frequency. Phase delay introduced by the lift-off angle. Permeability of the test material. Coil winding direction (absolute vs. differential).

5. When testing ferromagnetic materials, ET is primarily complicated by:

The absence of a characteristic lift-off curve. The non-linear, unpredictable variation in magnetic permeability (μ). High electrical conductivity, leading to shallow penetration. Complete shielding of the magnetic field by the material.

6. A Level III must implement Multi-frequency ET for an inspection. The principal reason for this is to:

Achieve simultaneous detection of both surface and very deep flaws. Suppress (or mix out) unwanted signals from variables like tube supports or permeability. Increase the maximum SDP achievable in the test piece. Eliminate the need for calibration standards.

7. In a Phase Array ET (PAET) system, what primary parameter does the instrument control to change the angle of the eddy current field?

The AC drive current amplitude. The excitation frequency. The time delay between excitation pulses to individual coil elements. The DC magnetic saturation bias field.

8. When compensating for end effect in a probe or coil, the Level III would most likely utilize:

A differential probe instead of an absolute probe. A reference end cut-off technique in the signal processing. An extremely low test frequency to reduce edge interference. A conductivity measurement to establish the baseline.

9. What is the fundamental disadvantage of using a differential probe compared to an absolute probe for ET?

Differential probes are more sensitive to slow-developing flaws like corrosion or wall thinning. They cannot be used to measure material conductivity. They are insensitive to gradual material variations over a long area. They require two separate excitation sources instead of one.

10. The primary purpose of the Pre-Amplifier in an ET system is to:

Convert the AC signal to a DC voltage for display. Filter out noise above the Nyquist frequency. Boost the low-level signal from the test coil before significant signal loss in the cable. Provide the AC excitation current to the drive coil.

11. When performing ET on aluminum turbine blades, the Level III uses a conductivity measurement to assess:

Micro-cracking depth. Heat treatment verification (e.g., assessing aging or over-aging). Total metal thickness (wall loss). Material permeability.

12. A Level III must design a calibration standard for tube inspection. The primary purpose of the standard is to provide flaws of:

The maximum allowable size to set the rejection threshold. The smallest detectable size to set the noise floor. Known depth, width, and type to calibrate the instrument’s phase and amplitude. A different material than the component to establish contrast.

13. When establishing the phase angle for a flaw signal in an absolute probe test, the primary control used by the Level III is the:

Gain setting. DC saturation current. Reference phase rotation (or phase angle control). Test frequency.

14. In aircraft maintenance, Eddy Current is the preferred method for detecting:

Subsurface porosity in engine castings. Fatigue cracks under fastener heads without removing the fasteners (bolt-hole scanning). Corrosion in non-metallic honeycomb panels. Lamination in composite wing skins.

15. When using Remote Field Testing (RFT) for tube inspection, the signal relies on the:

Direct magnetic coupling between the coil and the ID surface. Magnetic field that diffuses through the tube wall and back to the receiver coil. AC field being fully contained within the tube wall thickness (δ ≫ wall). Comparison of signals from two coils placed adjacent to each other (differential mode).

16. In ET procedure development, the Level III must specify the Minimum Detectable Flaw Size (MDFS) based on:

The Level II technician’s vision test results. The capability of the calibrated instrument to consistently resolve a signal above the noise floor. The size of the largest discontinuity found in the calibration standard. The wall thickness of the test component.

17. A large, clockwise rotation of the impedance signal on the ET display, primarily affecting the resistive component, typically indicates a change in:

Permeability. Wall thinning (corrosion/wear). Lift-off. Through-wall crack.

18. To verify that an ET system is stable during a long inspection period, the Level III should require the Level II to perform system checks against:

A different calibration standard every 4 hours. The smallest rejection flaw in the calibration standard at the end of the shift. The primary reference defect (e.g., a specific calibration flaw) at a specified time interval. An air point to check for noise only.

19. When writing an ET procedure for a non-ferromagnetic welded component, the Level III must consider that the weld area will exhibit:

A significant change in permeability due to heat input. A slight conductivity change due to microstructural variations. Zero ET signal, as welds are often shielded. A large lift-off signal due to the weld crown only.

20. Which ASTM standard provides the primary guidelines for the ET examination of non-ferromagnetic tubing?

ASTM E164 ASTM E709 ASTM E215 ASTM E1316

21. Pulsed Eddy Current (PEC) Testing is primarily used to overcome which limitation of conventional ET?

Limited sensitivity to surface cracks. Inability to measure thickness through thick insulation or non-conductive coatings. High sensitivity to lift-off variations. Need for highly trained Level II personnel.

22. In a magnetic component, Magnetic Bias is applied to the test area to:

Decrease the test frequency, increasing SDP. Reduce the effects of permeability variations by saturating the material. Increase the lift-off signal to aid in normalization. Convert the test to the Remote Field Testing (RFT) mode.

23. In an automated ET inspection system, which data filtering method is most effective for removing high-frequency electronic noise spikes without distorting the flaw signal?

High-pass filter Low-pass filter Band-reject filter Derivative filter

24. When designing a flaw detection technique, the Level III calculates the Effective Depth of Penetration (δ_e), which is relevant when the wall thickness (t) is:

t < 3δ t ≈ 3δ t = ∞ t > 10δ

25. The principle of Electromagnetic Acoustic Transducers (EMATs) is based on the ET phenomenon interacting with:

Magnetic flux leakage near flaws. The Faraday effect within the tube wall. The Lorentz force that generates ultrasonic waves. The Hall effect in semiconductor probes.

26. A Level III is writing a procedure that requires the ET instrument to be normalized to the air point. This process:

Aligns the crack signal with the horizontal axis. Sets the instrument gain to a fixed value of 40 dB. Establishes the origin (null point) of the impedance plane at the air-coil impedance. Compensates for residual magnetism in the component.

27. When calibrating an ET instrument for wall thickness measurement, the two primary calibration standards required are:

A clean, flaw-free area and the smallest flaw to be detected. A thin reference foil and a magnetic bias shim. Two samples of known, different thicknesses to establish a linear response. A sample with a known crack depth and a pure copper block.

28. According to most codes, the primary responsibility for the final approval of a Level III-written ET procedure lies with:

The ASNT Certificate Holder. The customer’s Quality Assurance Manager. The NDE organization’s Level III manager. The employer or the Certifying Authority designated by the employer.

29. In ET of surface cracks using a hand-held probe, the scanning speed is primarily limited by:

The operator’s ability to maintain constant lift-off. The instrument’s filter settings (filter response time). The magnetic permeability of the test component. The amount of background AC line noise.

30. The most appropriate ET technique for detecting small stress corrosion cracking (SCC) under a non-conductive coating is:

Absolute probe at low frequency. Differential probe at high frequency. Differential probe using a low frequency to minimize lift-off noise. Remote Field Testing (RFT).

ASNT NDT Level III Eddy Current Testing (ET) Practice Quiz