ASNT Level I Ultrasonic Testing

What Is Sound?

Sound is a mechanical disturbance that propagates through a medium by causing particles to vibrate around their equilibrium positions. Unlike electromagnetic radiation (light, radio waves, X-rays), sound cannot travel through a vacuum - it requires a physical medium such as a solid, liquid, or gas.

In ultrasonic testing, we use sound waves at frequencies far above human hearing. The human ear can detect frequencies from approximately 20 Hz to 20,000 Hz (20 kHz). Ultrasonic testing operates at frequencies typically between 500 kHz and 25 MHz - thousands of times higher than audible sound.

How Sound Propagates

When a vibrating source (such as a piezoelectric crystal in a transducer) pushes against a material, it creates a chain reaction of particle displacement:

1. The source pushes particles in the material forward, compressing the region immediately ahead
2. Those compressed particles push their neighbors forward
3. As each particle moves forward, it creates a region of lower density (rarefaction) behind it
4. This alternating pattern of compression and rarefaction moves through the material as a wave

The individual particles do not travel with the wave - they oscillate back and forth around their rest positions. It is the disturbance pattern (the energy) that moves through the material.

Why Solids Are Best for UT

Solid materials are the primary medium for ultrasonic testing because:

• Particles in solids are tightly bound by strong atomic/molecular forces
• This tight coupling means vibrations transfer efficiently from one particle to the next
• Solids support multiple wave modes (longitudinal, shear, surface) while liquids and gases only support longitudinal waves
• Sound velocity in steel (~5,900 m/s longitudinal) is roughly 17 times faster than in air (~343 m/s)

This high velocity and efficient energy transfer make metals ideal candidates for UT. The tighter the atomic structure, the better the material conducts ultrasound.

Acoustic Impedance

Acoustic impedance (Z) is a fundamental property that governs how sound behaves at interfaces between different materials. It is defined as:

Z = ρ × v

Where:

• ρ (rho) = material density in kg/m³
• v = sound velocity in m/s
• Z = acoustic impedance in kg/(m²·s), also called Rayls

Typical acoustic impedance values:

• Steel: ~45.0 × 10⁶ Rayls
• Aluminum: ~17.0 × 10⁶ Rayls
• Water: ~1.48 × 10⁶ Rayls
• Air: ~415 Rayls

The enormous impedance difference between steel and air is why even a tiny air gap (such as a crack) creates a strong reflection - virtually all of the sound energy bounces back when it hits an air interface inside steel. This is the fundamental detection mechanism in UT.

How Sound Energy Behaves in Real Materials

In an ideal material, a sound wave would travel forever without losing energy. In real materials, the wave loses energy as it travels - this is called attenuation. Understanding attenuation is critical because it determines how far your sound beam can travel and still produce a useful signal.

Sources of Attenuation

Absorption - The material converts some of the mechanical wave energy into heat through internal friction. Every material has an inherent absorption rate that increases with frequency. Higher frequencies lose energy faster, which is why low-frequency transducers are needed for thick sections or highly attenuating materials.

Scattering - When the sound wave encounters grain boundaries, inclusions, or other microstructural features, some energy is redirected in random directions. Scattering becomes significant when the grain size approaches the wavelength of the sound. In coarse-grained materials like centrifugally cast stainless steel or some cast irons, scattering can make UT examination very difficult or impossible at conventional frequencies.

Beam Spread - As a sound beam travels away from the transducer, it naturally diverges (spreads out). The energy that started concentrated in a small area becomes spread over a larger area, reducing the intensity at any given point. Beam spread is greater for lower frequencies and smaller transducer diameters.

The Decibel (dB) Scale

UT instruments measure signal amplitude in decibels rather than linear units. The decibel is a logarithmic ratio:

dB = 20 × log₁₀(A₁/A₂)

Where A₁ and A₂ are two amplitude values being compared.

Key dB relationships every UT technician must know:

The 6 dB rule is the most frequently used in UT: every 6 dB represents a doubling (or halving) of amplitude.

Why Decibels Matter in Practice

• Gain adjustment: When you turn the gain knob up by 6 dB, you double the displayed signal height. When you reduce gain by 6 dB, you halve it.
• Distance-amplitude relationship: A reference reflector at twice the distance produces a signal approximately 6 dB lower than the same reflector at the original distance (in the far field).
• DAC curves: Distance-Amplitude Correction curves are built using dB relationships to compensate for the natural decrease in signal amplitude with distance.
• Acceptance criteria: Many codes specify acceptance levels in dB relative to a reference reflector. Understanding dB math is essential for evaluating indications against code requirements.

Longitudinal Waves - The Primary UT Mode

Longitudinal waves (also called compression waves or pressure waves) are the most commonly used wave mode in ultrasonic testing. In a longitudinal wave, particles oscillate back and forth in the same direction the wave travels - parallel to the direction of propagation.

Imagine a line of people standing shoulder to shoulder. If the person at one end pushes the next person, who pushes the next, and so on down the line, the disturbance (the push) travels along the line while each person moves only slightly forward and then back. This is how a longitudinal wave behaves.

Characteristics of Longitudinal Waves

• Particle motion: Parallel to wave propagation direction
• Can propagate in: Solids, liquids, and gases (the only mode that works in all three)
• Velocity in steel: ~5,900 m/s (0.232 in/µs)
• Used for: Straight beam examinations, thickness measurements, most contact testing
• Generated by: Normal incidence transducers (transducer face parallel to test surface)

How Longitudinal Waves Are Generated in UT

A straight beam transducer placed on a test surface with couplant generates a longitudinal wave that travels perpendicular to the surface (straight down into the material). The piezoelectric element in the transducer vibrates back and forth in the thickness direction, creating alternating compression and rarefaction in the material directly beneath it.

Compression and Rarefaction

As the longitudinal wave travels through the material:

• Compression zones: Regions where particles are pushed closer together. The local pressure and density are slightly higher than normal.
• Rarefaction zones: Regions where particles are pulled apart. The local pressure and density are slightly lower than normal.
• These zones alternate as the wave passes, creating the characteristic sinusoidal pressure pattern.

The distance from one compression peak to the next is the wavelength (λ). A shorter wavelength (higher frequency) means the compressions are packed more closely together, which improves the ability to detect small flaws.

Applications of Longitudinal Waves in Level I Work

Thickness Measurement - The most common Level I application. A straight beam transducer sends a longitudinal wave straight through the material. The instrument measures the time between the initial pulse and the back wall echo, then calculates thickness using: t = v × T/2, where T is the round-trip time.

Plate and Forging Examination - Longitudinal waves scan through plates and forgings to detect internal discontinuities like laminations, inclusions, porosity, and voids that lie parallel to the scanning surface.

Corrosion Monitoring - Longitudinal waves measure remaining wall thickness in piping and vessels to assess corrosion damage. This is one of the most common industrial applications of UT.

Shear Waves - The Second Essential Mode

Shear waves (also called transverse waves) are the second major wave mode used in ultrasonic testing. In a shear wave, particles oscillate perpendicular to the direction the wave travels - like a rope being shaken up and down while the wave moves horizontally along the rope.

Key Characteristics of Shear Waves

• Particle motion: Perpendicular to wave propagation direction
• Can propagate in: Solids only (liquids and gases cannot sustain shear stress)
• Velocity in steel: ~3,230 m/s (0.127 in/µs) - approximately 55% of longitudinal velocity
• Used for: Angle beam examinations, weld inspection, detecting cracks and planar flaws oriented at angles to the surface
• Generated by: Angle beam transducers (wedge assemblies that introduce sound at an angle)

Why Shear Waves Cannot Exist in Liquids or Gases

Shear waves require the particles in the medium to resist sideways displacement - this is called shear stiffness or rigidity. Solids have rigid atomic bonds that can transmit shear forces from one atom to the next. Liquids and gases have no fixed structure and cannot resist shear - their molecules simply flow past each other. This is why shear waves propagate only in solids.

Practical implication: When sound travels through couplant (a liquid) between the transducer and the test piece, it can only travel as a longitudinal wave. The shear wave is generated only after the longitudinal wave enters the solid test material at an angle and mode converts.

Shear Wave Velocity Relationship

The shear wave velocity in any material is always slower than the longitudinal velocity in the same material. A useful approximation for most metals:

v_shear ≈ 0.55 × v_longitudinal

Precise values for common materials:

Mode Conversion - How Shear Waves Are Created

Mode conversion occurs when a longitudinal wave strikes an interface at an angle other than 90° (perpendicular). At the interface, part of the energy reflects and part refracts into the second medium. The refracted energy can split into both a longitudinal component and a shear component.

This is the principle behind angle beam transducers:
1. The transducer generates a longitudinal wave in the plastic wedge
2. The longitudinal wave hits the wedge-to-steel interface at a specific angle
3. At the interface, the wave refracts into the steel
4. If the angle is correct, only a shear wave propagates in the steel (the longitudinal component has refracted beyond 90° and does not enter)

The angle at which the longitudinal wave in the steel refracts to exactly 90° (skims along the surface and disappears) is called the first critical angle. Beyond this angle, only shear waves exist in the steel. The angle at which the shear wave also refracts to 90° is the second critical angle. Angle beam testing operates between these two critical angles.

The Fundamental Wave Equation: v = f × λ

The velocity, frequency, and wavelength of a sound wave are locked together by one of the simplest and most important equations in ultrasonics:

v = f × λ

Where:

• v = velocity (determined by the material)
• f = frequency (determined by the transducer)
• λ = wavelength (a consequence of v and f)

What This Equation Tells Us

Velocity is a material property. Once you know the material (steel, aluminum, etc.) and the wave mode (longitudinal or shear), the velocity is fixed. You cannot change it by changing the transducer or instrument settings.

Frequency is a transducer property. The frequency is determined by the thickness of the piezoelectric element in the transducer. A thinner element vibrates faster (higher frequency). Common UT frequencies: 1 MHz, 2.25 MHz, 5 MHz, 10 MHz, 15 MHz.

Wavelength is the result. For a given material and frequency, the wavelength is automatically determined:

λ = v / f

Calculating Wavelength in Practice

Example 1: 5 MHz longitudinal wave in steel
λ = 5,900 m/s ÷ 5,000,000 Hz = 0.00118 m = 1.18 mm

Example 2: 2.25 MHz longitudinal wave in steel
λ = 5,900 m/s ÷ 2,250,000 Hz = 0.00262 m = 2.62 mm

Example 3: 5 MHz shear wave in steel
λ = 3,230 m/s ÷ 5,000,000 Hz = 0.000646 m = 0.646 mm

Notice that the shear wave wavelength is shorter than the longitudinal wave wavelength at the same frequency. This is because the shear wave velocity is lower.

Why Wavelength Matters: Minimum Detectable Flaw Size

A general rule in UT: the minimum detectable flaw size is approximately λ/2 (half the wavelength). Discontinuities significantly smaller than half the wavelength will not produce a usable reflection.

Higher frequency → shorter wavelength → detection of smaller flaws. But remember the trade-off: higher frequency also means more attenuation and less penetration depth.

The Resolution vs. Penetration Trade-Off

This is perhaps the most important practical concept for frequency selection:

• High frequency (5-15 MHz): Short wavelength, excellent resolution, detects small flaws, but limited penetration depth. Best for thin materials, near-surface examination, and fine-grained metals.

• Low frequency (0.5-2.25 MHz): Long wavelength, reduced resolution, cannot detect the smallest flaws, but excellent penetration through thick sections and coarse-grained materials. Best for thick forgings, castings, and austenitic materials.

• Medium frequency (2.25-5 MHz): The most common range for structural steel work. Provides a reasonable balance of resolution and penetration for most applications.

Sound Beam Geometry - Near Field and Far Field

The sound beam generated by a transducer is not a simple, uniform cylinder of energy. It has a complex structure that every UT technician must understand.

Near Field (Fresnel Zone)

Immediately in front of the transducer, the sound beam goes through a region of complex interference patterns. In this region, the beam intensity fluctuates - at some points the pressure waves add together (constructive interference) and at others they cancel out (destructive interference). This creates an unpredictable pattern where a reflector might produce a strong signal at one position and a weak signal at a position just slightly different.

The length of the near field (N) depends on:

• Transducer diameter (D): larger diameter → longer near field
• Frequency (f): higher frequency → longer near field
• Material velocity (v): lower velocity → longer near field

N = D² × f / (4 × v) or equivalently N = D² / (4 × λ)

Example: 0.5" diameter, 5 MHz transducer in steel (longitudinal)
N = (0.5")² × 5,000,000 / (4 × 0.232 × 10⁶ in/s) = 0.25 / 0.928 = 0.27 inches

Example: 1.0" diameter, 2.25 MHz transducer in steel (longitudinal)
N = (1.0")² × 2,250,000 / (4 × 0.232 × 10⁶ in/s) = 2.25 / 0.928 = 2.42 inches

Why Near Field Matters

Flaw detection and sizing in the near field is unreliable. The fluctuating beam intensity means a flaw in the near field might produce a signal that varies dramatically with small changes in transducer position. This is why:

• Calibration reflectors should be positioned in the far field when possible
• Examination sensitivity is most reliable in the far field
• Some procedures specify minimum standoff distances to avoid near field effects

Far Field (Fraunhofer Zone)

Beyond the near field, the beam transitions to a well-behaved, gradually diverging cone. In the far field:

• Amplitude decreases smoothly and predictably with distance
• The beam diverges at a consistent angle (beam spread)
• Flaw signals behave predictably - amplitude is proportional to reflector area and inversely related to distance
• DAC curves and sizing techniques are valid

Beam Spread

In the far field, the beam diverges (spreads out) at a half-angle θ defined by:

sin(θ) = 1.22 × λ / D

A narrow beam (small θ) is desirable because it concentrates energy on a small area and provides better lateral resolution. To achieve a narrow beam:

• Use higher frequency (smaller λ)
• Use larger transducer diameter (larger D)

Conversely, a wide beam covers more area per scan pass but with less energy concentration and poorer lateral resolution.

The Piezoelectric Effect - Heart of Every UT Transducer

The entire field of ultrasonic testing depends on one physical phenomenon: the piezoelectric effect. Certain materials generate an electrical charge when mechanically stressed (squeezed or stretched), and conversely, they change shape when an electrical voltage is applied to them.

This bidirectional property makes piezoelectric materials perfect for UT:

• Transmitting: Apply an electrical pulse → crystal vibrates → generates a sound wave in the material
• Receiving: Sound wave returns and hits the crystal → crystal vibrates → generates an electrical signal the instrument can display

Common Piezoelectric Materials

Lead Zirconate Titanate (PZT) - The most widely used piezoelectric material in UT transducers. PZT is actually a ceramic (not a natural crystal) that is manufactured with specific properties. It offers:

• High sensitivity (strong conversion between electrical and mechanical energy)
• Wide frequency range (can be made in any UT frequency)
• Reasonable cost
• Good durability

Lithium Niobate - Used for high-temperature applications (can operate above 300°C where PZT begins to lose its piezoelectric properties). Less sensitive than PZT but more thermally stable.

Polyvinylidene Fluoride (PVDF) - A piezoelectric polymer used for very high-frequency, broadband transducers. Very thin films of PVDF can produce extremely short pulses, improving resolution. Less sensitive than PZT but excellent bandwidth.

Transducer Construction

A typical contact transducer consists of several components:

Piezoelectric Element (Crystal) - The active element that generates and receives sound. Its thickness determines the resonant frequency: a thinner element vibrates faster (higher frequency).

Backing Material (Damping Block) - A heavy, attenuating material bonded to the back of the crystal. It absorbs the backward-directed sound wave and controls the pulse duration. Heavy damping produces a short pulse (better resolution but lower amplitude). Light damping produces a long pulse (higher amplitude but poorer resolution).

Wear Plate (Face Plate) - A thin, hard material bonded to the front of the crystal to protect it from wear during contact scanning. The wear plate material and thickness are designed to optimize energy transfer and protect the fragile crystal.

Matching Layer - An impedance-matching layer between the crystal and the wear plate (or test surface) that improves energy transfer. Without matching, the large impedance difference between the crystal and the test material would cause most of the energy to reflect rather than enter the material.

Housing - The metal or plastic case that protects the internal components, provides electrical connections (BNC or microdot connector), and allows the operator to hold and manipulate the transducer.

Electrical Connections - Electrodes on the front and back faces of the crystal connect to the instrument's pulser (for transmission) and receiver (for detection). The cable connector is typically a BNC or microdot type.

Angle Beam Transducers - Essential for Weld Examination

Angle beam transducers are assemblies that combine a standard contact transducer with an angled wedge (typically Plexiglas/acrylic) to introduce shear waves into the test material at a specific angle. They are the primary tool for weld examination and detecting cracks or planar flaws oriented at angles to the scanning surface.

How an Angle Beam Assembly Works

1. The contact transducer generates a longitudinal wave in the Plexiglas wedge
2. The longitudinal wave in the wedge strikes the wedge-steel interface at an angle
3. At this interface, Snell's Law governs refraction
4. If the incident angle is between the first and second critical angles for the wedge-steel combination, only a shear wave propagates into the steel
5. The refracted shear wave travels through the steel at the specified angle (45°, 60°, or 70° from normal to the surface)

Standard Angle Beam Configurations

45° Shear Wave - The most versatile general-purpose angle. The beam enters at 45° to the surface normal, reaches the opposite surface at an angle, reflects, and returns on a predictable geometric path. Used for:

• General weld scanning
• Detection of flaws at various orientations
• First pass examination before more targeted angles

60° Shear Wave - A steeper angle that is particularly effective for detecting vertically oriented flaws (perpendicular to the surface). The steeper beam path:

• Provides better reflection from vertical cracks
• Reaches the weld root from a greater standoff distance
• Is commonly specified for thick weld examinations

70° Shear Wave - The steepest commonly used angle. Excels at:

• Detecting near-surface flaws
• Examining the weld root area from a close standoff
• Detecting cracks that open to the far surface
• Sometimes used for creeping wave generation at angles very close to the second critical angle

The Index Point (Beam Exit Point)

The index point is the location on the wedge where the refracted beam exits the wedge and enters the test material. It is marked on the wedge by the manufacturer, but it MUST be verified before each examination because:

• Wedge wear changes the actual exit point
• Different wedge-transducer combinations may have slightly different exit points
• An incorrect index point makes every beam path distance measurement wrong

Verification is performed using the IIW (V1) block or DSC (V2) block. The procedure for verifying the index point is covered in Chapter 6 (Calibration).

The UT Instrument - Understanding Your Primary Tool

A modern ultrasonic testing instrument is a sophisticated electronic device, but its fundamental function is simple: send an electrical pulse to a transducer, receive the returning echo, and display the result. Understanding the instrument's major components and controls is essential for competent Level I operation.

Major Components of a Pulse-Echo System

Clock (Timer/Synchronizer) - The master timing circuit that coordinates all other functions. It triggers the pulser at a set repetition rate (PRF - Pulse Repetition Frequency, typically 100-2000 Hz) and synchronizes the display sweep.

Pulser - Generates a short, high-voltage electrical spike (typically 100-400 volts) that excites the transducer's piezoelectric element. The pulse characteristics affect the sound beam:

• Short pulse → wide bandwidth → better resolution
• High voltage → stronger sound wave → more penetration
• Most instruments allow pulse voltage and damping adjustment

Receiver/Amplifier - Amplifies the weak electrical signals returning from the transducer (echoes from reflectors in the material). The receiver includes:

• Gain (amplification) control - the most frequently adjusted control
• Bandwidth filters - shape the received signal for optimal display
• Rectification - converts the RF (radio frequency) signal to a displayable waveform

A-Scan Display - The standard UT display showing signal amplitude (vertical axis) vs. time/distance (horizontal axis). This is the primary display mode for manual UT.

Time Base Generator - Controls the horizontal sweep speed of the display, which determines the distance range shown on screen.

Reading the A-Scan Display

The A-scan display is your window into the test material. Here is what the horizontal and vertical axes represent:

Horizontal Axis = Time (which represents Distance)
The horizontal position of a signal corresponds to the time it took for the sound to travel from the transducer to the reflector and back. Since velocity is known (you entered it during calibration), the instrument converts this time to distance.

• Left side of screen = near the transducer (near surface)
• Right side of screen = far from the transducer (deeper in the material or far surface)
• The scale is set during calibration to show the desired depth or beam path range

Vertical Axis = Amplitude (Signal Strength)
The height of a signal peak represents the strength of the reflected echo. Amplitude is affected by:

• Size of the reflector (larger reflectors return more energy)
• Orientation of the reflector (perpendicular to the beam returns maximum energy)
• Distance from the transducer (further = weaker signal due to beam spread and attenuation)
• Surface condition of the reflector (smooth, flat surfaces produce stronger reflections than rough or curved ones)

Key Signals on the A-Scan

Initial Pulse (Main Bang) - The large signal at the left edge of the display, representing the transmitted pulse. It obscures the near-surface region (the dead zone).

Back Wall Echo (BWE) - The signal from the far surface of the test piece. Its position confirms correct distance calibration. Its amplitude indicates beam penetration quality.

Flaw Echo - Any signal appearing between the initial pulse and the back wall echo represents a reflector within the material. This could be a discontinuity (crack, void, inclusion) or a geometric feature (hole, step, radius).

Verifying Instrument Performance

Before you can trust the results of any UT examination, you must verify that your instrument and transducer system are performing within acceptable limits. Instrument verification is not the same as calibration - verification confirms that the system is working correctly, while calibration establishes the measurement references for a specific examination.

Key Performance Characteristics

Horizontal Linearity - The distance scale on the display must be accurate and linear. If the display shows 1 inch at the correct position but shows 5 inches at a position that's actually 4.8 inches, the distance readings throughout the range are unreliable.

Verification method: Use a calibration block with multiple known reflectors at different distances. Each reflector should appear at its correct position on the display.

Vertical Linearity (Amplitude Linearity) - The relationship between signal height and actual echo amplitude must be proportional. If doubling the actual echo strength doesn't double the displayed signal height, amplitude-based evaluations (like comparing flaw signals to reference levels) will be inaccurate.

Verification method: ASTM E317 describes methods for checking vertical linearity using controlled attenuation.

Sensitivity - The ability of the system to detect small reflectors at the required examination depth. Sensitivity depends on the transducer, instrument gain, and the overall system noise level.

Verification method: Compare signal-to-noise ratio against a reference reflector to previous verification records.

Resolution - The ability to distinguish two closely spaced reflectors as separate signals. Good resolution requires short pulses and adequate bandwidth.

Types of resolution:

• Axial (depth) resolution: Ability to separate two reflectors that are at different depths but in the same beam path
• Lateral (angular) resolution: Ability to separate two reflectors that are at the same depth but different lateral positions

Verification Intervals

Most codes and procedures require instrument verification:

• At the start of each examination
• At the end of each examination
• After any interruption (cable change, transducer change, battery change, instrument power cycle)
• At specified time intervals during extended examinations (often every 4 hours or less)

If verification at the end of an examination shows the system has drifted beyond acceptable limits, all examination results since the last successful verification may be invalid and the examination may need to be repeated.

Why Calibration Is Critical

Calibration is the process of adjusting your instrument so that the display accurately represents the physical reality inside the test material. Without proper calibration:

• Distance readings are wrong (you can't tell how deep a flaw is)
• Amplitude comparisons are meaningless (you can't tell how significant a flaw is)
• The examination results are unreliable and potentially invalid

Calibration uses reference blocks - pieces of material with precisely known dimensions and reference reflectors - to establish the relationship between what the instrument displays and physical reality.

Standard Calibration Blocks

IIW Block (Type 1, V1 Block)
The International Institute of Welding block is the most widely used calibration reference in UT. It is a rectangular steel block (200mm × 75mm × 25mm) with several features:

• 100mm radius curved surface - Used to verify beam exit point (index point) for angle beam transducers and to set distance calibration for angle beam
• 50mm radius curved surface - Used for additional angle verification
• 1.5mm diameter side-drilled hole at 91mm depth - Used as a reference reflector for sensitivity setting
• Flat bottom at 25mm and 100mm - Used for straight beam distance calibration
• Scale markings - Graduated markings along the edges for beam exit point measurement

DSC Block (Miniature, V2 Block)
A smaller calibration block (25mm × 37.5mm × 12.5mm) used primarily for:

• Quick beam exit point verification
• Refracted angle verification in the field
• Convenient portability (fits in a pocket)
• Less precise than the IIW block but adequate for field verification

Step Blocks and Custom Reference Blocks
Flat blocks of varying thickness used for:

• Straight beam distance calibration over specific ranges
• Verifying measurement accuracy at specific thicknesses
• Material-specific calibration (using blocks made from the same material as the test piece)

ASME/AWS Reference Blocks
Code-specific blocks containing reference reflectors (side-drilled holes, flat-bottom holes, or notches) at specific depths used for sensitivity calibration per code requirements.

Distance Calibration - Straight Beam

The goal of distance calibration is to make the horizontal axis of the A-scan display accurately represent depth in the material.

Method 1: Using known thickness blocks
1. Place the transducer on a block of known thickness (e.g., 1 inch)
2. Identify the first back wall echo and the first multiple (second back wall at 2 inches)
3. Adjust the DELAY control so the first back wall appears at the correct position
4. Adjust the RANGE control so the second back wall appears at its correct position
5. Verify using additional known thicknesses if available

Method 2: Using the IIW block
1. Place the straight beam transducer on the 25mm face of the IIW block
2. Set the first back wall echo at the 25mm position on screen
3. Verify the second back wall echo (100mm path through the 100mm dimension) appears at the correct position

Calibration is iterative - adjusting range may shift delay, and vice versa. Repeat adjustments until both points are correct simultaneously.

Sensitivity Calibration - Setting the "How Loud" Level

Distance calibration tells the instrument WHERE something is. Sensitivity calibration tells it HOW SIGNIFICANT a signal is by establishing a reference level against which all indications are compared.

The Reference Level Concept

Sensitivity calibration involves:
1. Finding a known reference reflector in a calibration block (side-drilled hole, flat-bottom hole, notch, or back wall)
2. Adjusting gain until that reference reflector's signal reaches a specified screen height (typically 80% or a DAC curve starting point)
3. Using that gain setting as the baseline for the examination

Any indication in the test material is then compared to this reference level:

• A signal at the same height as the reference = same reflective significance
• A signal higher = more significant (potentially larger or more favorably oriented reflector)
• A signal lower = less significant (potentially smaller or less favorably oriented reflector)

DAC Curves (Distance-Amplitude Correction)

A DAC curve compensates for the natural decrease in signal amplitude with distance. A reference reflector at 2 inches will produce a weaker signal than the same reflector at 1 inch, simply because the sound travels further and loses energy to attenuation and beam spread.

The DAC curve is drawn on the screen (or computed electronically) to show the expected amplitude of the reference reflector at every distance in the examination range. Any indication above the DAC curve is more significant than the reference reflector at that distance; any indication below is less significant.

Building a DAC curve:
1. Obtain signals from identical reference reflectors (typically side-drilled holes) at multiple distances in a calibration block
2. Set the gain so the nearest reflector reaches a specified screen height
3. Mark the peak amplitude of each reflector on the screen
4. Connect the marks with a smooth curve - this is the DAC

Transfer Correction

Calibration blocks are typically smooth, machined steel with a specific surface finish. The actual test piece may have a rougher surface, paint, scale, or corrosion that reduces the energy entering the material. This difference in surface condition means that the calibration gain may not produce the same sensitivity on the actual part.

Transfer correction compensates for this difference:
1. Place the transducer on the calibration block and note the back wall amplitude at a specific gain
2. Place the transducer on the actual test piece at a location of known thickness and note the back wall amplitude at the same gain
3. The difference in dB between the two readings is the transfer correction
4. Add this correction to the examination gain

For Level I technicians: your procedure will specify whether transfer correction is required and how to apply it. If the test surface is significantly rougher than the calibration block, the correction can be substantial (6 dB or more).

The Couplant Problem

Without couplant, ultrasonic testing cannot work. The reason is simple physics:

When sound tries to pass from a solid (transducer face) to air and then from air to another solid (test piece), the acoustic impedance mismatch is so extreme that virtually all the sound energy reflects at each interface. The percentage of energy transmitted through a steel-air-steel interface is essentially zero - less than 0.01%.

Couplant eliminates the air gap by filling it with a liquid or semi-liquid material that transmits sound far more efficiently than air. With couplant:

• The transducer-to-couplant-to-steel transmission path is efficient enough for practical examination
• Typical energy transmission through the couplant layer is 5-20% (compared to <0.01% through air)
• The remaining 80-95% reflects back, but the transmitted portion is sufficient for reliable flaw detection

Common Couplant Types

Glycerin - A viscous, transparent liquid. Excellent acoustic properties, low attenuation, stays in place well on vertical and overhead surfaces. Standard couplant for many laboratory and field applications. Water-soluble for easy cleanup.

Ultrasonic Gel - Water-based gel similar to medical ultrasound gel. Good wetting properties, easy to apply, readily available. Works well on smooth surfaces. May dry out in hot conditions.

Light Machine Oil - Thin oil that provides good coupling on smooth surfaces. Flows easily into surface irregularities. Can be messy and may stain. Not suitable for surfaces that will later be painted or coated without cleaning.

Water - The simplest couplant for immersion testing and some contact applications. Inexpensive and readily available. Drains quickly from vertical surfaces. Works well with squirter systems for automated scanning.

Cellulose Paste (Wallpaper Paste) - A thick paste that adheres well to rough and vertical surfaces. Good for overhead work. Can be mixed to various consistencies.

Specialized High-Temperature Couplants - For surfaces above 100°F (38°C), standard couplants may evaporate or degrade. High-temperature couplants are formulated to maintain coupling at elevated temperatures (up to 500°F / 260°C for some formulations).

Couplant Selection Criteria

Straight Beam Examination - The Foundation of UT

Straight beam examination uses a longitudinal wave transducer placed directly on the test surface to send sound perpendicular (normal) to the surface. This is the simplest and most commonly performed UT technique, used for:

• Thickness measurement
• Lamination detection in plate and forgings
• Internal void and porosity detection
• Inclusion detection
• Corrosion assessment

Scanning Patterns

Systematic scanning ensures complete coverage of the examination area. The procedure will specify the scanning pattern, but the most common patterns are:

Grid Pattern (Raster Scan) - The most thorough pattern. The transducer is moved in parallel lines across the surface, like mowing a lawn. Each pass overlaps the previous one by a specified amount (typically 10-25% of the transducer diameter) to ensure no gaps in coverage.

The overlap ensures that even if a flaw is at the edge of the beam during one pass, it will be near the center of the beam during an adjacent pass, where sensitivity is highest.

Spot Check Pattern - Individual measurements taken at specific locations, typically on a predetermined grid. Used for thickness surveys where complete area coverage is not required (e.g., corrosion monitoring at specific grid points).

Spiral Pattern - Used on cylindrical components. The transducer follows a helical path around the circumference while advancing along the length.

Scanning Speed

Scanning speed affects detection reliability:

• Too fast: The transducer may not maintain consistent coupling, and the screen update rate may not be fast enough to display brief signals from small flaws
• Too slow: Examination takes unnecessarily long, increasing fatigue and cost
• Typical scanning speed: 2-6 inches per second for manual contact examination
• The maximum scanning speed depends on the instrument's PRF (Pulse Repetition Frequency) and the size of flaws you need to detect

Surface Preparation Requirements

The scanning surface must be prepared to allow consistent coupling:

• Remove loose scale, rust, and debris
• Remove or thin excessive paint (>6 mils may significantly attenuate the beam)
• Grinding may be required for heavily corroded or irregular surfaces
• Weld spatter, tack welds, and protrusions should be removed from the scanning area
• The required surface finish is typically specified in the procedure (usually ≤250 µin Ra or equivalent)

Thickness Measurement Technique

Thickness measurement is the most common Level I straight beam application:

1. Set the instrument to the correct material velocity
2. Calibrate the distance range on a reference block of known thickness
3. Apply couplant to the measurement location
4. Place the transducer firmly on the surface
5. Read the thickness from the display (digital readout or A-scan first back wall position)
6. Take multiple readings at each location to verify consistency
7. Record the minimum reading at each grid location

Angle Beam Examination - Finding Flaws That Straight Beam Cannot

Straight beam examination sends sound perpendicular to the surface, making it excellent for finding flaws parallel to the surface (laminations, porosity). But many critical flaws - especially cracks - are oriented at angles to the surface or buried within weld fusion zones where straight beam cannot reach.

Angle beam examination solves this by sending a shear wave into the material at a known angle (typically 45°, 60°, or 70° from the surface normal). This angled beam can:

• Reach the interior of welds from the adjacent base material
• Detect cracks perpendicular to the surface (which reflect poorly with straight beam)
• Interrogate weld root areas, fusion boundaries, and heat-affected zones
• Find flaws at various orientations by using multiple angles

Beam Path Geometry

When a shear wave enters the material at an angle, it travels in a straight line until it hits a surface (typically the opposite side of the material). At that surface, it reflects at the same angle and continues through the material. This reflection creates a zigzag pattern called the skip path.

Key Terminology:

• Beam path (S): The total distance the sound travels from the index point to a reflector, measured along the zigzag path
• Skip distance: The surface distance between the point where the beam enters the material and the point where it hits the back surface and reflects back to the scanning surface
• Half skip: Half the skip distance - the surface distance from the beam entry point to the point directly above where the beam hits the back surface
• Full skip: The complete zigzag cycle from entry to return to the scanning surface
• Leg: One segment of the zigzag path. "First leg" = entry to first reflection. "Second leg" = first reflection to return.

Skip Distance Calculation

Skip distance depends on the refracted angle and the material thickness:

Half skip distance = thickness × tan(refracted angle)

Example: For a 0.75-inch thick plate with a 45° transducer:

• Half skip = 0.75" × tan(45°) = 0.75" × 1.000 = 0.75"
• Full skip = 0.75" × 2 × tan(45°) = 1.50"
• The beam enters the material, travels 0.75" horizontally while going 0.75" deep, hits the back surface, reflects, and returns to the scanning surface 1.50" from the entry point

Why Skip Distance Matters

Skip distance tells you where the beam is in the material relative to your transducer position. This is critical for:
1. Positioning the transducer to examine specific weld volumes
2. Determining where to place the transducer to examine the weld root
3. Calculating the actual location of a detected flaw within the weld
4. Ensuring complete coverage of the weld volume

Reading the A-Scan - Your Window Into the Material

The A-scan display is the primary tool for ultrasonic examination. Every signal on the screen represents sound energy returning to the transducer from somewhere in the material (or from the system itself). Learning to identify what each signal represents is a core Level I competency.

Categories of A-Scan Signals

1. Initial Pulse (Main Bang)
The large signal at the left side of the display. This is the transmitted pulse - not a reflection from inside the material. Its width determines the dead zone (the minimum depth at which reflectors can be detected). Important characteristics:

• Always present when the instrument is operating
• Width depends on pulse damping and frequency
• Signals within the initial pulse cannot be reliably interpreted

2. Back Wall Echo (BWE)
The signal from the far surface of the test piece. This is your most important reference signal:

• Its position confirms distance calibration is correct
• Its amplitude indicates beam penetration quality
• Changes in amplitude indicate material or coupling changes
• Loss of BWE is itself a significant finding that must be reported

3. Flaw Echoes
Signals appearing between the initial pulse and back wall that represent reflectors within the material. Characteristics of flaw echoes:

• Appear at depths less than material thickness
• Back wall echo may be reduced when a flaw echo is present (the flaw blocks energy)
• Position on screen indicates depth of the reflector
• Amplitude depends on reflector size, orientation, and surface condition

4. Multiple Echoes (Reverberations)
In thin material or low-attenuation material, the sound bounces back and forth multiple times between the front and back surfaces. You'll see echoes at 1×, 2×, 3×, 4× the material thickness, each progressively weaker. These are normal and expected - they confirm good penetration.

5. Mode Conversion Echoes
When sound reflects from an angled internal surface, it can convert from longitudinal to shear (or vice versa). These converted signals arrive at unexpected screen positions because they've traveled at different velocities during part of their path. Mode conversion signals can be confusing - they don't correspond to any physical reflector at the indicated depth.

6. Geometric Echoes (Structural Signals)
Signals from intentional features of the test piece - edges, holes, steps, threads, backing bars, fit-up geometry. These are real reflections from real features, but they are NOT flaws. Recognizing geometric signals prevents false calls.

7. Noise (Grass, Hash)
Low-level signals scattered across the baseline of the display. Caused by:

• Grain boundary scattering
• Surface roughness
• Electronic noise
• Couplant inconsistencies

Noise is normal at low levels. It becomes problematic when it's high enough to mask real flaw signals.

Dynamic Signal Analysis - How Signals Move

A critical aspect of A-scan interpretation is understanding how signals behave when you move the transducer. This dynamic behavior gives important clues about the nature and orientation of the reflector.

Echo Dynamics - Flaw vs. Geometric

Flaw signals (cracks, porosity, inclusions) typically show:

• Amplitude changes as the beam sweeps across the flaw
• Maximum amplitude when the beam is optimally positioned on the flaw
• The signal "rises and falls" as the transducer approaches and passes the flaw location
• The signal may appear at slightly different depths as the beam angle changes relative to the flaw surface

Geometric signals (backing bars, root geometry, counterbores) typically show:

• Consistent amplitude along the length of the feature
• The signal appears at the same beam path distance continuously
• The signal does not rise and fall - it remains fairly constant as long as the beam intersects the geometric feature
• Multiple geometric signals may appear together (e.g., root geometry produces several echoes from different reflection paths)

Planar vs. Volumetric Reflectors

Planar reflectors (cracks, lack of fusion, laminations):

• Produce strong, sharp signals when oriented perpendicular to the beam
• Signal amplitude drops rapidly as the transducer moves off-axis
• The signal has a narrow "peak" when scanning across it
• Can produce very high amplitudes from relatively small physical flaws
• Most dangerous type of flaw in structural applications

Volumetric reflectors (porosity, slag inclusions, voids):

• Produce broader, less sharply defined signals
• Signal amplitude changes gradually as the transducer scans across
• May produce multiple echoes from different parts of the reflector
• Generally less structurally significant than planar flaws of the same size
• Porosity clusters may produce a "hash" of low-amplitude signals rather than a single peak

Using Back Wall Echo for Interpretation

The back wall echo provides valuable interpretive information:

• BWE drops when flaw signal appears → The flaw is blocking significant beam energy. The flaw is directly in the beam path.
• BWE remains normal despite flaw signal → The flaw is small relative to the beam, only intercepting a small portion of the energy. Alternatively, the flaw is off the beam centerline.
• BWE drops but no flaw signal visible → The flaw may be oriented such that it deflects the beam away from the transducer (e.g., an angled surface). Or severe attenuation is present.

Common Discontinuities and How They Appear on the A-Scan

As a Level I technician, you need to recognize the common types of discontinuities found during ultrasonic examination. While detailed flaw characterization is a Level II responsibility, understanding the basic signal patterns helps you provide accurate reports.

Weld Discontinuities

Cracks - The most critical discontinuity type. Cracks are planar (flat) discontinuities that produce:

• Strong, sharp signals when the beam is perpendicular to the crack face
• Signal amplitude drops rapidly as the transducer moves off the optimal position
• May be found at the weld root (fatigue, hydrogen cracking), toe (fatigue, cold cracking), or within the weld (hot cracking, solidification cracking)
• Often associated with a significant drop in back wall echo
• The signal "tracks" along the crack length as you scan parallel to the weld

Incomplete Fusion (Lack of Fusion, LOF) - Occurs when weld metal fails to fuse with the base metal or with a previous weld pass. UT characteristics:

• Planar reflector oriented along the fusion boundary
• Strong signal at the expected depth of the fusion zone
• Signal typically appears at consistent depth as you scan along the weld
• Can be difficult to distinguish from a crack at the same location
• Most commonly detected with angle beam examination

Incomplete Penetration (Lack of Penetration, LOP) - The weld root does not fully penetrate through the joint thickness. UT characteristics:

• Detected with angle beam from the scanning surface
• Signal appears at a beam path distance corresponding to the root area
• May produce a signal similar to the root geometry echo - differentiation requires experience (Level II evaluation)
• Often continuous along the weld length

Porosity - Gas pores trapped in the solidified weld metal. UT characteristics:

• Volumetric (spherical or near-spherical) reflectors
• Produce relatively low-amplitude, broad signals compared to cracks
• Scattered porosity appears as multiple small signals across a range of depths
• Clustered porosity produces a localized hash of signals
• Piping porosity (elongated pores) can produce stronger signals similar to small cracks

Slag Inclusions - Non-metallic material (flux, oxide) trapped in the weld. UT characteristics:

• Volumetric reflectors, often irregular in shape
• Moderate amplitude signals, broader than crack echoes
• May appear at depths corresponding to weld pass boundaries
• Can be linear (wagon tracks) or isolated
• Generally produce less dramatic back wall loss than cracks

Undercut - A groove melted into the base metal at the weld toe, not filled by weld metal. UT characteristics:

• Detectable by straight beam at the weld toe location
• Creates a localized thickness reduction
• Typically identified visually before UT, but may be detected during angle beam scanning as a near-surface signal near the weld toe

Base Metal Discontinuities

Not all discontinuities are in welds. The base material itself can contain discontinuities from manufacturing processes:

Laminations

Laminations are planar separations within rolled plate or forging material, typically parallel to the rolling direction and the plate surface. They originate from:

• Pipe or shrinkage cavities in the original ingot that get elongated during rolling
• Non-metallic inclusions that flatten and extend during rolling
• Hydrogen flaking

UT Detection:

• Straight beam is the primary detection method for laminations
• A lamination perpendicular to the beam produces a very strong reflection (the steel-air interface)
• Complete loss of back wall echo indicates a large lamination blocking the entire beam
• Partial back wall loss with a mid-wall signal suggests a partial-width lamination
• Laminations are typically at mid-thickness (center of the plate) where the original ingot defect was located

Inclusions

• Non-metallic particles (slag, oxide, sulfide) trapped in the base metal
• Produce smaller, less dramatic signals than laminations
• Often detected as scattered low-level signals during plate examination
• May be elongated in the rolling direction (stringer inclusions)

Forging Discontinuities

Forging Bursts - Internal tears caused by excessive forging forces
Forging Laps - Surface material folded over during forging, creating a near-surface discontinuity with trapped oxide between the lapped surfaces

Casting Discontinuities

Shrinkage - Voids formed during solidification as the metal contracts
Porosity - Gas pores trapped during casting
Hot Tears - Cracks formed during cooling due to constrained contraction

Reporting Requirements

As a Level I technician, your reporting responsibilities include:

1. Location: Mark and record the position on the test piece (surface coordinates and depth)
2. Depth: Record the depth of the indication from the scanning surface
3. Amplitude: Record the signal amplitude relative to the reference level (in dB)
4. Extent: Scan around the indication and record the area or length over which it is detectable
5. Back wall behavior: Note whether the back wall echo is affected (and how)
6. Reproducibility: Confirm the signal reproduces when the transducer is removed and replaced

You do NOT make accept/reject decisions. Your job is to find, characterize (to the extent possible), and document indications. Your Level II supervisor evaluates them against the applicable acceptance criteria.

Why Documentation Matters

UT examination results are only as valuable as the documentation that supports them. Without proper records:

• Results cannot be verified or reproduced
• There is no audit trail for quality assurance
• Liability in case of failure cannot be properly assessed
• The examination has no legal or contractual standing

As a Level I technician, documentation is one of your most important responsibilities. Even though you don't make acceptance decisions, you are the person who records the raw data that everything else depends on.

What to Document

Pre-Examination Information:

• Date, time, and location of examination
• Identification of the component or weld being examined (drawing number, weld ID, serial number)
• Material type and thickness
• Examination procedure number and revision
• Applicable code or specification
• Your name, certification level, and certification number
• Name of supervising Level II or III

Equipment Information:

• Instrument manufacturer, model, and serial number
• Transducer manufacturer, model, serial number, frequency, diameter, and angle
• Calibration block identification
• Couplant type
• Cable type and length (cable length can affect signal quality)

Calibration Records:

• Velocity setting
• Range and delay settings
• Gain settings (reference level, scanning level, recording level)
• Calibration verification results (start, during, and end of examination)
• Time of each verification
• Any drift noted and action taken

Examination Results:

• All indications above the reporting level (location, depth, amplitude, extent)
• Areas where back wall loss was observed
• Areas where examination could not be performed (access limitations, surface condition, geometry)
• Sketch or diagram showing indication locations on the component

Post-Examination Records:

• End-of-examination calibration verification
• Summary of findings
• Disposition (if made by Level II/III at the time): accept, reject, additional testing required
• Your signature and date

Data Recording Best Practices

• Write legibly - your records may be reviewed years later during a failure investigation
• Record data as you go - don't rely on memory to fill in details later
• Use the correct units - don't mix metric and imperial without clear labeling
• Include sketches when they help clarify locations
• Never alter records without a documented reason - cross out errors with a single line, initial and date the correction
• If using electronic data recording, ensure files are properly saved and backed up

Safety Considerations in Ultrasonic Testing

UT is one of the safest NDT methods - it does not involve ionizing radiation, hazardous chemicals (beyond couplants), or high-pressure systems. However, there are real safety hazards that Level I technicians must recognize and manage.

Electrical Safety

• UT instruments operate on AC power or rechargeable batteries. AC-powered instruments create electrical shock risk, especially in wet or conductive environments.
• Never use an AC-powered instrument in wet conditions without a Ground Fault Circuit Interrupter (GFCI)
• Inspect power cords and cables for damage before each use
• Do not use instruments with cracked housings or exposed wiring

Ergonomic Hazards

• Manual UT scanning involves repetitive arm and wrist motions that can cause repetitive strain injuries (carpal tunnel, tendinitis)
• Overhead scanning is particularly fatiguing - take frequent breaks
• Use ergonomic scanning tools and transducer holders when available
• Rotate between different tasks when possible to vary the physical demands

Working Environment Hazards

• UT is often performed at elevation (scaffolding, aerial lifts), in confined spaces, or in industrial environments with multiple hazards
• Always follow site-specific safety requirements (PPE, fall protection, confined space entry procedures)
• Be aware of nearby activities (welding, grinding, crane operations) that may create hazards
• Couplant on walking surfaces creates slip hazards - clean up spills promptly

Chemical Safety

• Most UT couplants are low-toxicity, but some may cause skin irritation with prolonged contact
• Use gloves when handling couplants for extended periods
• Some high-temperature couplants contain chemicals requiring specific PPE
• Read the Safety Data Sheet (SDS) for any couplant you use
• Ensure adequate ventilation when using solvent-based cleaning agents

Hot Surface Hazards

• UT is frequently performed on components at elevated temperatures (in-service piping, vessels near operating temperature)
• Standard transducers can be damaged by prolonged contact with hot surfaces
• Burns are a real risk when touching hot metal - use heat-resistant gloves and test surface temperature before touching
• Plan your work to minimize contact time on hot surfaces

Method Limitations

Every Level I technician must understand what UT cannot do:

• UT cannot detect discontinuities oriented parallel to the beam (the beam passes along the flaw without reflection)
• UT is difficult or impossible in highly attenuating materials (some castings, coarse-grained materials)
• Surface-breaking flaws that are very shallow may be in the dead zone
• UT requires a coupling medium - it cannot work through an air gap
• Rough surfaces reduce sensitivity and may prevent examination
• Complex geometries can create confusing signals that complicate interpretation
• UT results depend on operator skill - it is not fully automated in most applications