Ultrasonic Testing

Traditional Ultrasonic inspection uses high frequency sound energy to conduct examinations and perform measurements. Considerable information may be gathered during ultrasonic testing such as the presence of discontinuities, material or coating thickness.

The detection and location of discontinuities is enabled by the interpretation of ultrasonic wave reflections generated by a transducer. These waves are introduced into a material and travel in a straight line and at a constant speed until they encounter a surface. The surface interface causes some of the wave energy to be reflected and the rest of it to be transmitted.

The amount of reflected vs. transmitted energy is detected and provides information on the size of the reflector, & therefore the discontinuity encountered.

Ultrasonic inspection

Ultrasonic inspection is a non-destructive method in which high frequency sound waves are introduced into the material being inspected.Most ultrasonic inspection is don't eat frequencies between 0.5 and 20MHz, well above the range of human hearing which is about 20Hz to 20kHz. The sound waves travel through the material with some loss of energy (attenuation)due to material characteristics. The intensity of sound waves is either measured, after reflection (pulseecho) at interfaces (orflaw)or is measured at the opposite surface of the specimen (pulsetransmission).

The reflected beam is detected and analyzed to define the presence and location of flaws. The degree of reflection depends largely on the physical state of matter on the opposite side of the interface, and to a lesser extent on specific physical properties of that matter, forinstance, sound waves are almost completely reflected at metal-gas interfaces. Partial reflection occurs at metal-liquid or metal-solid interfaces.

Ultrasonic testing has a superior penetrating power than radiography and can detect flaws deep in the test specimen.It is quite sensitive to small flaws and allows the precise determination of the location and size of the flaws.

In Ultrasonic Testing high-frequency sound waves, created by a piezoelectric crystal in probe, are transmitted into a material to detect imperfections or to locate changes in material properties. The most commonly used ultrasonic testing technique is pulse echo, whereby sound is introduced into a test object and reflections (echoes) from internal imperfections or the part's geometrical surfaces are returned to a receiver.

It is one of the volumetric NDT methods to detect internal defects of all materials. High frequency sound waves that are introduced into the material being tested to detect surface and sub-surface flaws.

Sound travels through materials under the influence of sound pressure (acoustic impedance). Because molecules or atoms of a solid are bound elastically to one another, the excess pressure results in a wave propagating through the solid.

The sound waves travel through the materials with some attenuation of energy and reflected back. The combined effect of scattering and absorption is called attenuation. Ultrasonic attenuation is the decay rate of the wave as it propagates through material. The reflected beam is detected and analyzed to define the presence and location of flaws.

Ultrasonic Techniques

  • Pulse-echo techniques
  • Through transmission method

Pulse-echo Techniques:

In pulse-echo testing a transducer sends out a pulse of energy and the same or a second transducer listens for reflected energy, also known as an echo. Pulse echo is especially effective when only one side of a material is accessible.

Pulse-echo Techniques

Through transmission method :

This is the very first technique developed initially for testing heavy forgings and castings to detect internal defects making use of two transducers, one as a transmitter other as a receiver. Through transmission is useful in detecting discontinuities that are not good reflectors, and when signal strength is weak. It does not provide depth information. In this technique continuous wave was transmitted from one end to other end, the transmittedenergy was picked up using a receiver. Both end surfaces shall be smooth and parallel, Adequate couplantshall be applied on both sides, Both the transmitter and receiver shall be properly aligned for better inspection.

hrough transmission method

Uses of Ultrasonic testing method

  • Mostly used for detection of flaws in materials.
  • Widely used for thickness measurement.
  • Used for the determination of mechanical properties and grain structure of materials.
  • Used for the evaluation of processing variables on materials.

Ultrasonic Probes

The conversion of electrical pulses to mechanical vibrations and the conversion of returned mechanical vibrations back into electrical energy is the basis for ultrasonic testing. The active element is the Probe. It converts the electrical energy to acoustic energy, and vice versa.

Types of Probe

Ultrasonic transducers are manufactured for a variety of application and can be custom fabricated when necessary. Careful attention must be paid to selecting the proper transducer for the application. It is important to choose transducers that have the desired frequency, bandwidth, and focusing to optimize inspection capability. Most often the transducer is chosen either to enhance sensitivity or resolution of the system

  • Contact transducers
    • T.R probe
    • Dual element transducers
    • Delay line transducers
    • Angle beam transducers
    • Normal incidence shear wave transducers
    • Paint brush transducers
  • Immersion transducers

Contact transducers are available in a variety of configurations to improve their usefulness for a variety of applications. The flat contact transducer shown above is used in normal beam inspections of relatively flat surfaces, and where near surface resolution is not critical. If the surface is curved, a shoe that matches the curvature of the part may need to be added to the face of the transducer. If near surface resolution is important or if an angle beam inspection is needed, one of the special contact transducers described below might be used.

  • Dual element transducers
  • Delay line transducers
  • Angle beam transducers
  • Normal incidence shear wave transducers
  • Paint brush transducers

A straight beam transducer, producing a longitudinal wave at normal incidence into the test piece, is first used to locate any laminations in or near the heat-affected zone. This is important because an angle beam transducer may not be able to provide a return signal from a laminar flaw.

Normal beam testing uses a sound beam that is introduced at 90 degrees to the surface, while angle beam utilizes a beam that is introduced into the specimen at some angle other than 90 degrees. The choice between the two is made based on:

The orientation of the feature of interest so that the sound may produce the largest reflection from the feature

Obstructions on the surface of the specimen that must be avoided. The second step in the inspection involves using an angle beam transducer to inspect the actual weld. This inspection may include the root, sidewall, crown, and heat-affected zones of a weld. The process involves scanning the surface of the material around the weldment with the transducer. This refracted sound wave will bounce off a reflector (discontinuity) in the path of the sound beam

Ultrasonic weld inspections are typically performed using straight beam transducer in conjunction with angle beam transducers.

To get useful levels of sound energy into the material, the air between the transducer and the specimen must be removed. In contact testing, a couplant such as water, oil or a gel is applied between the transducer and the specimen.

The most commonly occurring defects in welded joints are Porosity, Slag inclusions, Lack of side-wall fusion, Lack of intermediate-pass fusion, lack of root penetration, Undercutting, longitudinal or transverse cracks.

Calibration

Calibration is a sequence of instrument control adjustments/instrument responses using known values to verify instrument operating characteristics. Allows determination of unknown quantities from test materials.

Calibration Blocks

Standard blocks are used to calibrate the instrument and to calculate different features of probe and the instrument. These blocks consists accurately cut and fine polished surfaces, holes, angles etc.

  • IIW V1 BLOCK
  • IIW V2 BLOCK

IIW V1 BLOCK

I.I.W.V1 block is used to calibrate the equipment and probe for regular practical purposes and to meet the standard requirements.

IIW V1 BLOCK

IIW V2 BLOCK

I.I.W.V2 block is used for instant calibration of U.T equipment and U.T probes. More convenient for field services.

IIW V2 BLOCK

Advantages

  • It is sensitive to both surface and subsurface discontinuities.
  • The depth of penetration for flaw detection or measurement is superior to other NDT methods.
  • Only single-sided access is needed when the pulse-echo technique is used.
  • It is highly accurate in determining reflector position and estimating size and shape.
  • Minimal part preparation is required.
  • It provides instantaneous results.
  • Detailed images can be produced with automated systems.
  • It is nonhazardous to operators or nearby personnel and does not affect the material being tested.
  • It has other uses, such as thickness measurement, in addition to flaw detection.
  • Its equipment can be highly portable or highly automated.

Disadvantages

  • Surface must be accessible to transmit ultrasound.
  • Skill and training is more extensive than with some other methods.
  • It normally requires a coupling medium to promote the transfer of sound energy into the test specimen.
  • Materials that are rough, irregular in shape, very small, exceptionally thin or not homogeneous are difficult to inspect.
  • Cast iron and other coarse grained materials are difficult to inspect due to low sound transmission and high signal noise.
  • Linear defects oriented parallel to the sound beam may go undetected.
  • Reference standards are required for both equipment calibration and the characterization of flaws.

Some of the most common Ultrasonic application :

  • Flaw detection (cracks, inclusions, porosity, delaminations etc.
  • Erosion/Corrosion thickness gauging
  • Assessment of bond integrity
  • Estimation of grain size in metals
  • Estimation of void content in composites and plastics

Dowload Sample Report and Procedure of Ultrasonic Testing

Ultrasonic Testing NDT Sample Procedure
Ultrasonic Testing NDT Sample Test Report Format
  • Perform Specific Calibrations
  • Specific NDT
  • Interpretation of Codes
  • Evaluations for Accept or Reject Determinations according to written Instructions
  • Record Results

Responsibilities of ASNT Level II Ultrasonic Testing (UT) Personnel

  • A certified ASNT Level II Ultrasonic Testing (UT) personnel is qualified to Select proper ultrasonic testing (UT) technique and probe.
  • A certified ASNT Level II Ultrasonic Testing (UT) personnel is qualified to Calibrate the equipment and probe.
  • A certified ASNT Level II Ultrasonic Testing (UT) personnel is qualified to Perform ultrasonic testing based on the standard procedure followed by ASNT.
  • A certified ASNT Level II Ultrasonic Testing (UT) personnel is qualified to Interpret the ultrasonic test results as per applicable standards.
  • A certified ASNT Level II Ultrasonic Testing (UT) personnel is qualified to Know about the merits and demerits of ultrasonic testing (UT) and other common NDT methods.
  • A certified ASNT Level II Ultrasonic Testing (UT) personnel is qualified to Prepare ultrasonic test report based on the acceptance/rejection Criteria.
  • A certified ASNT Level II Ultrasonic Testing (UT) personnel is qualified to Prepare written instruction for Level I personnel.
  • A certified ASNT Level II Ultrasonic Testing (UT) personnel is qualified to Maintain and handle the ultrasonic Testing (UT) equipment carefully.

Module 1: Manufacturing Discontinuities

Module 2: Wave Modes

Module 3: Acoustic Impedence

Module 4: Refraction and Reflection

  • Reflection and Refraction
  • Snell’s Law
  • Mode Conversion
  • First and Second Critical Angle
  • Creeping Waves
  • Problems on Mode Conversion

Module 5: Piezoelectric Transducer

Module 6: Pulser Receiver

  • Calibration of Ultrasonic Equipment - Time and Amplitude Linearity
  • All other Topics Covered in Level I Training

Module 7: Attenuation

Module 8: Thickness Measurement

Module 9: Immersion Testing

Module 10: Flaw detection – Normal Probe

Module 11: Calibration Blocks

  • IIW Block
  • Miniature Angle Beam
  • DAC Block
  • Step Wedge
  • Area- Amplitude Block
  • Distance- Amplitude Block

Module 12: Angle Beam Inspection

Module 13: Writting an Ultrasonic procedure

  • Selection of Screen Range
  • Measurement of Beam Exit Point
  • Measurement of Actual Refracted Angle
  • Calibration using IIW, and DAC Block
  • Sensitivity and Resolution
  • Reference Amplitude
  • Distance Amplitude Correction Curve
  • Discontinuity Length Sizing using 6 dB and 20 dB drop method
  • Discontinuity Evaluation
  • Angle Selection
  • Surface Distance, Skip Distance, Depth, Full V Path
  • Plotting of Discontinuities like Crack, Lack of Fusion, Lack of Penetration, Slag,
  • Porosity in welds
  • Worksheet: Plotting of discontinuities for butt welds

Advance Techniques

  • Time of Flight Diffraction
  • Phased Arrays
  • ASME Section V, Article 4, 2004 Edition
  • ASME Section VIII
  • SA 609 Castings
  • SA 388 Heavy Steel Forging
  • SA 578 Straight Beam Inspection of plain and Clad Steel Plates
  • Additional Codes Standards as per student’s requirements (please discuss this at the time of registration)