Ultrasonic Testing (UT)

Use of high-frequency sound to detect internal flaws and measure material thickness — principles, equipment, calibration and common applications.

Introduction History Basic Principles Equipment Calibration Applications Data Presentation Quiz

Introduction

Ultrasonic testing (UT) uses pulses of high-frequency sound (typically 0.5–25 MHz) to detect internal discontinuities and to measure thickness. A transducer converts electrical pulses into sound and receives echoes reflected back from features inside the material.

Basic workflow

1. Couple the transducer to the part (contact or immersion).
2. Send a short ultrasonic pulse into the material.
3. Receive and display returned echoes (A-scan, B-scan, C-scan).
4. Interpret echo amplitude and time-of-flight to size and locate defects.

Common advantages

• Deep penetration in metals; accurate thickness measurement.
• High sensitivity to small cracks and planar flaws (when oriented favorably).
• Fast—suitable for on-site inspection and automated scanning.

Basic Principles

Key physical concepts used in UT:

• Wave propagation: longitudinal and shear waves travel at different velocities in solids.
• Reflection & refraction: occur at boundaries with acoustic impedance mismatch (Snell's law applies).
• Attenuation: energy loss with distance—affects penetration depth and frequency choice.
• Beam profile: near-field and far-field regions influence focusing and resolution.

Common display formats

• A-scan: amplitude vs time (used for basic sizing and TOF thickness).
• B-scan: cross-sectional view along a scan line.
• C-scan: plan-view map of some echo property (amplitude, time).

Equipment & Transducers

Typical UT instrument components: pulser/receiver, transducer, couplant, display/recorder, and sometimes scanning mechanics or immersion tanks for automated inspections.

Transducer types

• Single-element contact probes
• Angle-beam wedges for weld inspection
• Phased array probes — electronic beam steering and focusing
• Immersion probes for bath/scanner systems

Calibration & Reference Blocks

Proper calibration ensures repeatable measurements and correct sizing:

• Velocity calibration with known-thickness standards
• Calibration blocks (IIW block, side-drilled holes, reflectors) for amplitude/time checks
• Delay lines and wedge characterization for angled inspections

Selected Applications

• Weld inspection (detection of cracks, lack of fusion, slag)
• Corrosion and erosion mapping (thickness scanning)
• Bond testing and composite inspection
• Manufacturing quality control and in-service inspections

Data Presentation

Interpreting ultrasonic signals requires understanding the instrument settings (gain, damping), probe characteristics, and expected indications. A-scan analysis combined with scanning modes helps build confidence in sizing and identification.

Non-Destructive Testing (NDT) Quiz — 40 Questions

Ultrasonic Testing (UT)

Q1: What is the purpose of a couplant in UT?

A) To clean the transducer
B) To transmit ultrasonic waves from the transducer to the material
C) To cool the material
D) To increase frequency

Answer: B) To transmit ultrasonic waves from the transducer to the material

Q2: Which wave type is commonly used in UT for flaw detection in metals?

A) Surface wave
B) Rayleigh wave
C) Longitudinal wave
D) Shear wave

Answer: C) Longitudinal wave

Q3: Which display shows amplitude versus time in UT?

A) A-scan
B) B-scan
C) C-scan
D) D-scan

Answer: A) A-scan

Q4: Why are calibration blocks used in UT?

A) To test magnetic properties
B) To measure radiation
C) To apply developer
D) To verify instrument accuracy and sensitivity

Answer: D) To verify instrument accuracy and sensitivity

Q5: Typical ultrasonic frequency range used in UT?

A) 1–10 Hz
B) 0.5–20 MHz
C) 50–100 kHz
D) 200–500 Hz

Answer: B) 0.5–20 MHz

Q6: What is the role of a transducer in UT?

A) To process signals
B) To display results
C) To convert electrical signals to ultrasonic waves
D) To clean the test surface

Answer: C) To convert electrical signals to ultrasonic waves

Q7: Which factor affects sound velocity in a material?

A) Material density and elasticity
B) Operator skill
C) Ambient temperature only
D) Color of material

Answer: A) Material density and elasticity

Q8: Maximum penetration depth in UT depends on?

A) 1–5 mm
B) 5–10 mm
C) 10–50 mm
D) Frequency and material

Answer: D) Frequency and material

Q9: Difference between pulse-echo and through-transmission UT?

A) Pulse-echo uses two transducers; through-transmission uses one
B) Pulse-echo uses one transducer; through-transmission uses two
C) Pulse-echo is for plastics only; through-transmission for metals
D) No difference

Answer: B) Pulse-echo uses one transducer; through-transmission uses two

Q10: Material property affecting ultrasonic attenuation most?

A) Color
B) Shape
C) Grain size and microstructure
D) Surface roughness

Answer: C) Grain size and microstructure

Q11: Which type of probe is used for contact UT?

A) Electromagnetic Acoustic Transducer
B) Piezoelectric contact probe
C) Laser probe
D) Eddy current probe

Answer: B) Piezoelectric contact probe

Q12: In UT, what is the function of a wedge?

A) To measure thickness
B) To cool the transducer
C) To angle ultrasonic waves into the material
D) To apply couplant

Answer: C) To angle ultrasonic waves into the material

Q13: What is the main advantage of UT over radiography?

A) Immediate results and no radiation
B) Lower skill requirement
C) Less expensive equipment
D) Can detect only surface flaws

Answer: A) Immediate results and no radiation

Q14: Shear waves in UT are used mainly for?

A) Surface cleaning
B) Developer application
C) Direct viewing
D) Detecting flaws at angles to the surface

Answer: D) Detecting flaws at angles to the surface

Q15: Which factor affects UT resolution most?

A) Couplant type
B) Frequency of the ultrasonic wave
C) Surface color
D) Test operator height

Answer: B) Frequency of the ultrasonic wave

Q16: What is the primary purpose of a flaw detector in UT?

A) Generate couplant
B) Measure density
C) Detect and display internal flaws
D) Magnetize material

Answer: C) Detect and display internal flaws

Q17: What is a dead zone in UT?

A) Area near the surface where small flaws are not detected
B) Area of high sensitivity
C) Calibration block
D) Wedge placement area

Answer: A) Area near the surface where small flaws are not detected

Q18: What is a dual-element transducer used for?

A) Only thickness measurement
B) Only flaw detection
C) Visual inspection
D) Separate transmitting and receiving for near-surface detection

Answer: D) Separate transmitting and receiving for near-surface detection

Q19: Which calibration method is used for thickness measurement?

A) Flaw calibration
B) Known thickness calibration block
C) Visual calibration
D) Eddy current calibration

Answer: B) Known thickness calibration block

Q20: What is the purpose of gating in UT?

A) Apply couplant
B) Angle the probe
C) Select a portion of the signal for analysis
D) Measure surface roughness

Answer: C) Select a portion of the signal for analysis

Q21: What is the primary use of pulse-echo UT?

A) Detect internal flaws and measure thickness
B) Apply couplant
C) Surface cleaning
D) Visual inspection

Answer: A) Detect internal flaws and measure thickness

Q22: What is the function of a delay line in UT?

A) Increase transducer frequency
B) Allow detection of near-surface flaws
C) Reduce couplant thickness
D) Measure radiation

Answer: B) Allow detection of near-surface flaws

Q23: Which factor affects sound velocity in UT?

A) Surface roughness only
B) Color of material
C) Material density and elasticity
D) Operator height

Answer: C) Material density and elasticity

Q24: Which scan displays a cross-sectional view of the test object?

A) A-scan
B) B-scan (time vs amplitude)
C) D-scan
D) B-scan (cross-sectional view)

Answer: D) B-scan (cross-sectional view)

Q25: What is the main limitation of UT?

A) Cannot detect internal flaws
B) Surface must be smooth and accessible
C) Always requires radiation
D) Cannot measure thickness

Answer: B) Surface must be smooth and accessible

Q26: Which couplant is preferred for high-temperature inspection?

A) High-temperature gel
B) Water
C) Oil
D) Alcohol

Answer: A) High-temperature gel

Q27: What is the primary advantage of angled-beam UT?

A) Easier couplant application
B) Lower equipment cost
C) Detect flaws at angles to the surface
D) Works without calibration

Answer: C) Detect flaws at angles to the surface

Q28: Which factor reduces near-surface resolution in UT?

A) High frequency
B) Couplant
C) Calibration block
D) Dead zone effect

Answer: D) Dead zone effect

Q29: Which parameter is measured in UT thickness testing?

A) Surface roughness
B) Time of flight of ultrasonic pulse
C) Material color
D) Magnetic field

Answer: B) Time of flight of ultrasonic pulse

Q30: Which material property affects reflection coefficient in UT?

A) Color
B) Operator height
C) Acoustic impedance mismatch
D) Temperature only

Answer: C) Acoustic impedance mismatch

Q31: What is the purpose of a raster scan in UT?

A) Apply couplant evenly
B) Angle the probe
C) Calibrate the instrument
D) Inspect a large area systematically

Answer: D) Inspect a large area systematically

Q32: Which transducer is best for high-resolution flaw detection?

A) High-frequency piezoelectric
B) Low-frequency probe
C) Shear wave probe
D) Delay line probe

Answer: A) High-frequency piezoelectric

Q33: Which flaw type reflects strongest UT signal?

A) Porosity
B) Planar cracks
C) Rounded inclusions
D) Voids

Answer: B) Planar cracks

Q34: Which scan produces a top-down 2D image of flaw locations?

A) A-scan
B) B-scan
C) C-scan
D) D-scan

Answer: C) C-scan

Q35: Which factor reduces UT sensitivity?

A) High frequency
B) Proper calibration
C) Smooth surface
D) Coarse surface and poor coupling

Answer: D) Coarse surface and poor coupling

Q36: What is a reference reflector in UT?

A) Known feature used for calibration
B) Surface crack
C) Wedge
D) Couplant layer

Answer: A) Known feature used for calibration

Q37: What type of UT probe is used for curved surfaces?

A) Straight-beam probe
B) Flexible or conformable probe
C) Shear wave probe
D) High-frequency probe

Answer: B) Flexible or conformable probe

Q38: Which UT mode is used for detecting laminar defects?

A) Longitudinal normal beam
B) Delay line pulse-echo
C) Shear wave at angle
D) Surface wave

Answer: C) Shear wave at angle

Q39: In immersion UT, the material is inspected using?

A) Air gap
B) Oil film
C) Couplant gel only
D) Water or liquid medium

Answer: D) Water or liquid medium

Q40: What is the main advantage of dual-element transducers?

A) Detect only surface flaws
B) Near-surface flaw detection with reduced dead zone
C) Use without calibration
D) Measure only thickness

Answer: B) Near-surface flaw detection with reduced dead zone

Quick links

Introduction Principles Equipment Calibration Applications

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