NDT Ultrasonic Testing
Ultrasonic Testing (UT) uses high frequency sound energy to conduct examinations and make measurements. Ultrasonic inspection can be used for flaw detection/evaluation, thickness measurements, material characterization, and more.
High frequency sound energy is introduced into the material and propagates in the form of waves. These waves are introduced into a material and travel in a predetermined straight line and at a constant speed until they encounter a surface. When there is a discontinuity in the wave path, the energy will be reflected from the flaw’s surface. The reflected wave signal is transformed into an electrical signal by the transducer and is displayed on a screen and evaluated/documented by our technician.
DBI, Inc. utilizes three basic ultrasonic techniques in NDT ultrasonic testing:
Ultrasonic Thickness Examination: Pulse-echo ultrasonic measurements can determine the location of a discontinuity in a part or structure by accurately measuring the time required for a short ultrasonic pulse generated by a transducer to travel through a thickness of material, reflect from the back or the surface of a discontinuity, and be returned to the transducer. Two-way transit time measured is divided by two to account for the down-and-back travel path and multiplied by the velocity of sound in the test material.
Ultrasonic Shear Wave Examination: Angle beam transducers and wedges are typically used to introduce a predetermined angled wave into the test material. An angled sound path allows the sound beam to come in from the side and reflect off the top/bottom surfaces of the part, thereby improving detectability of flaws in and around welded areas.
Automated Ultrasonic Testing (AUT): Ultrasonic scanning systems are used for automated data acquisition and imaging primarily for internal corrosion mapping. They typically integrate an ultrasonic instrumentation, a scanning bridge, and computer controls. The signal strength and/or the time of flight of the signal is measured and recorded for every point in the scan plan. The value of the data is plotted using a colormap to produce detailed images of the surface or internal defects such as corrosion, pitting, or material defects. Systems are usually capable of simultaneously displaying the data in A-, B- and C-scan modes.
- A-Scan Presentation: displays the amount of received ultrasonic energy as a function of time. The relative amount of received energy is plotted along the vertical axis and the elapsed time, which may be related to the sound energy travel time within the material, is displayed along the horizontal axis. Relative discontinuity size can be estimated by comparing the signal amplitude obtained from an unknown reflector to that from a known reflector.
- B-Scan Presentation: is a profile (cross-sectional) view of the test specimen.
- C-Scan Presentation: is a plan-type (top) view of the location and size of test specimen features.
Ultrasonic Testing for Welding Inspection
Ultrasonic weld inspections are typically performed using an ultrasonic thickness examination in conjunction with a shear wave examination. A straight beam transducer, producing a longitudinal wave at zero degrees into the test piece, is first used to locate any laminations in or near the heat-affected zone. This is important because a shear wave transducer may not be able to provide a return signal from a laminar defect.
The second step in the inspection involves using a shear wave transducer to inspect the actual weld. This inspection area will include the root, sidewall, crown, and heat-affected zones of a weld. The process involves scanning the surface of the base material in and around the heat-affected zone of the weldment with a transducer. This sound wave will bounce off a discontinuity in the path of the sound beam. With proper angle beam techniques, returned signals from the weld zone may allow the operator to determine the size, location, and type of discontinuity.
Advantages of NDT Ultrasonic Testing
- It can detect both surface and subsurface discontinuities.
- It has other uses in addition to flaw detection, such as thickness measurement.
- 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.
- Electronic equipment provides instantaneous results.
- Detailed images can be produced with automated systems.
- The depth of penetration for flaw detection or measurement is superior to other NDT methods.
Limitations of NDT Ultrasonic Testing
- Surface must be accessible to transmit ultrasound.
- Skill and training are more extensive than with some other NDT 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.