NDT Methods Comparison: IET vs Ultrasonic vs X-Ray

Quick Comparison

Method	Best For	Speed	Cost	Skill Required
IET / Resonance	Elastic properties, bulk defects, QC sorting	Seconds	Low	Low
Ultrasonic	Thickness, laminar defects, bonds	Seconds-Minutes	Medium	High
X-Ray CT	3D defect visualization, metrology	Minutes-Hours	High	High
Eddy Current	Surface cracks, conductivity, sorting	Seconds	Medium	Medium
Dye Penetrant	Surface-breaking cracks	30+ minutes	Low	Low

Impulse Excitation Technique (IET)

Strengths

• Measures elastic moduli directly (E, G, ν)
• Very fast — seconds per part
• 100% inspection capable
• No couplant needed
• Works at high temperature (up to 1600°C)
• Sensitive to bulk porosity
• Low operator skill required

Limitations

• Doesn’t localize defects (bulk measurement)
• Requires standard geometries (bars, discs)
• Not for surface crack detection
• Less effective on complex shapes

Best for: Material characterization, QC sorting, additive manufacturing inspection, incoming material testing, high-temperature studies.

Ultrasonic Testing (UT)

Strengths

• Localizes defects (depth, position)
• Excellent for thickness measurement
• Detects laminar defects, delaminations
• Works on complex geometries
• Portable equipment available

Limitations

• Requires couplant (gel, water)
• High operator skill needed
• Difficult on rough surfaces
• Limited for porous materials
• Temperature limitations

Best for: Weld inspection, thickness gauging, composite laminate inspection, bond testing.

X-Ray Computed Tomography (CT)

Strengths

• Full 3D visualization of defects
• Precise defect location and sizing
• Works on any geometry
• Combines with metrology
• Gold standard for qualification

Limitations

• Slow — minutes to hours per part
• Very expensive equipment
• Not suitable for 100% inspection
• Size limitations
• Radiation safety requirements

Best for: Failure analysis, process development, first-article inspection, complex internal geometries.

Eddy Current Testing (ECT)

Strengths

• Excellent for surface cracks
• No couplant needed
• Fast — high-speed scanning
• Material sorting by conductivity
• Coating thickness measurement

Limitations

• Conductive materials only
• Surface/near-surface only
• Sensitive to lift-off (probe distance)
• Edge effects can mask defects

Best for: Aerospace fastener holes, tube inspection, heat exchanger tubes, surface crack detection on metals.

Which Method Should You Use?

Need elastic modulus values? → IET — Only method that directly measures E, G, and ν

Need 100% production inspection? → IET or Eddy Current — Fast enough for in-line use

Need to see exactly where defects are? → X-Ray CT — Full 3D visualization

Need to detect delaminations in composites? → Ultrasonic — Excellent for laminar defects

Need surface crack detection on metals? → Eddy Current or Dye Penetrant

Need high-temperature measurements? → IET — Works up to 1600°C

Combining Methods

Many applications benefit from combining NDT methods. A typical approach for additive manufacturing:

1. IET screening — Fast 100% inspection, reject obvious failures
2. X-Ray CT sampling — Detailed analysis of borderline or critical parts
3. Eddy current — Surface finish inspection after machining

IET acts as a fast, low-cost filter — reducing the number of parts that need expensive CT scanning.