Volume 2, Number 4 October 2003
TNT FYI:
Practical Contact Ultrasonics - Defining Terms and Principles
by James W. Houf*
This article is the first in an eight-part series detailing practical hands-on contact ultrasonic testing. Most reference material on ultrasonic testing (UT) develops the technical aspects of ultrasonics in great detail with little explanation given to the hands-on application of contact ultrasonic testing being performed daily by Level I and Level II personnel. This series on Practical Contact Ultrasonics includes simple, concise information that operators can use in the performance of their work. The information is not intended to be all-inclusive. Some specific applications will not be found but much of what is presented can be generally applied and may make performing ultrasonic inspections a little easier or may assist in preventing the collection of inaccurate data. Several general definitions are provided first, followed by definitions that follow the progress of the sound path in a typical ultrasonic test. A synopsis of each of the topics to be presented in the rest of the series follows this initial article.
Introduction
The following terms and definitions will be used throughout the Practical Contact Ultrasonic series and their presentation here serves as explanation and review. Where possible, practical examples of the terms being defined have been included as well as the field terms (indicated by parentheses) commonly used by UT practitioners. Explanations of basic ultrasonic principles have also been included.
Contact UT: Contact UT is testing conducted with a handheld ultrasonic search unit that is applied directly to the surface of the material being inspected. This definition applies to thickness testing and both straight and angle beam inspection.
Ultrasound: Sound that is above the range of human hearing (20 KHz) is referred to as ultrasound. For most common contact material inspection applications, the frequencies used are 1.0, 2.25 and 5.0 MHz. The high frequencies of ultrasound do not travel through air as well as through liquids and solids.
Sound beam: Ultrasonic transducers generate a stream of sound energy called a sound beam. This sound energy, in the form of waves, spreads out as it moves farther from the transducer, creating a beam similar to that seen when a flashlight is used in fog. Each sound wave is initially made up of two components, an area of compression, where the sound energy is packed closely together, and an area of rarefaction, where the sound energy is spread farther apart. Wave compression and rarefaction zones can be approximated by throwing a rock into a pond. When the rock hits the water, a single set of waves travels away from the point of impact. The wave crowns represent compression or high-energy zones. Depressions between the wave crowns represent areas of rarefaction or low energy.
Sound path: The path the sound beam follows as it leaves the transducer and enters the test piece is called the sound path.
Longitudinal waves: Longitudinal waves are sound waves generated so that particle movement within the wave is parallel to the direction of the sound beam. Waves in the longitudinal mode travel approximately twice as fast as transverse waves generated in the same material. Longitudinal waves are used to perform straight beam and thickness inspections.
Transverse (shear) waves: Transverse or shear waves are sound waves generated so that particle movement within the wave is perpendicular to the direction of the sound beam. This wave mode travels at approximately half the speed of longitudinal waves generated in the same material. A weak coil spring can give a good demonstration of longitudinal and transverse wave modes. Stretch the spring out until it is almost taut. Then, quickly shove one end slightly towards the other and stop. The coil motion you see moving down the length of the spring is similar to the longitudinal sound wave mode. To demonstrate transverse motion, grasp one end of the spring and quickly move your hand sideways and back to center. The coils move laterally or from side-to-side as the movement travels down the length of the spring. Shear waves are used in angle beam inspections.
Flaw detector (UT machine, or scope): A flaw detector is an electronic device that transmits and receives tiny impulses of electrical energy through a shielded cable to and from a transducer. Electronic signals returned to the flaw detector by the transducer are amplified and converted to a trace on a liquid crystal display or cathode ray tube (CRT) screen. The term scope is derivative of the word oscilloscope, an early UT machine.
Digital thickness testers (D-meters): Digital thickness testers are miniature versions of a flaw detector, typically using a dedicated straight beam transducer and showing only a digital readout of the thickness of the part being tested. Some digital thickness testers also have a miniature screen presentation that shows a waveform like that on a full UT machine. Most D-meters have a data-logging feature that will store thousands of thickness readings that are captured with an accuracy usually within several thousandths of an inch (0.001 in.).
Transducer (probe): A small, hand-held assembly that contains piezoelectric crystals, damping material, wires and a connector for linking the crystal(s) to an electric impulse generator (a thickness tester or flaw detector in this application), and possibly a delay line (Figure 1). Dual element delay line probes are commonly used for thickness checks on thinner materials.
Piezoelectric crystals: Crystals that produce a range of frequencies centered around a chosen frequency when struck by an electric current
and give off an electric current when struck by sound are called piezoelectric crystals.
Damping material: A rubber-like material behind the crystal(s) in a transducer that quickly stops each crystal from ring-down or prolonged vibrating. This damping effect is similar to grabbing a bell immediately after it is struck. The purpose of damping is to keep the ring-down caused by one electrical impulse from interfering with the sound signal returning to the crystal before the next electrical impulse causes the crystal(s) to ring again.
Delay line: A delay line is a sound conductive material that is either placed internally between the crystal and front of the transducer or externally as an attachment to the front of the transducer. A delay line gives the sound beam time to form a single wave train before reaching the part being tested (see Near and Far Fields).
Coaxial (coax) cable: A coaxial cable is shielded electric cable that is used to connect the scope to the transducer. UT coax cables usually have either quick connect (BNC) or microdot connectors to attach the transducer to the scope.
Wedge (shoe): A wedge is typically formed from sound conductive plastic material that can be cut to change the angle at which the sound beam enters the part being tested. Commonly used wedge angles create refracted angles in the test piece of 0 (straight beam), 45, 60 and 70 degrees. Wedges can be built into the transducer or may be removable. Refracted angles are calculated from a vertical line drawn through the thickness of the part being tested. Therefore, a 70 degree refracted angle enters the part 20 degrees down from the plane of the part surface (Figure 2). It should be noted, using detachable wedges can have several advantages. Cost is reduced because the purchase of fewer transducers is required and detachable wedges take up less room and reduce the overall weight of the operator's kit.
Search unit (probe): A search unit is an assembly made up of the transducer and wedge (or external delay line), or a transducer with a built-in wedge angle.
Couplant: Any liquid or gel used to transmit ultrasound between the transducer and shoe or probe and test part is called a couplant. Water is the simplest of all couplants, but cellulose gel, oil and glycerin are other common couplants. For out of position work, a thicker couplant is preferred because it won't run off vertical surfaces as quickly as thinner couplants. [Tip for practitioners - When using thicker couplants, a one inch paint brush is handy for spreading couplant evenly over the scanning surface. On field jobs, to keep from losing the brush and to prevent contamination with dirt, use a small circular magnet like those found at electronics stores and attach it to the handle of the brush with a piece of strong nylon string. This allows the brush to be hung on the part being inspected or on a scaffold post. (It can also save several trips up and down a scaffold).]
Reflector (indication; signal): A reflector is anything within the test object that causes part of the sound beam to reflect back to the probe. Sound returning from a reflector will cause a trace to appear on the screen display. Relevant indications can be foreign materials such as slag or gas pockets, or voids such as cracks, hot tears, shrinkage, etc. Signals returning from geometry changes are considered nonrelevant indications, and signals caused by changes in the type of sound mode (mode conversion) are false indications. Reference reflectors are holes, notches, slots, etc., of a known size that are machined or drilled into reference blocks that can be used as baselines for the comparison of signal sizes.
Screen presentation (display): A screen presentation is the electronic representation of the signal generated by sound returning from a reflector and is shown on a CRT or LCD screen on the front of the scope. The screen width can be calibrated to represent varying sound paths and the screen height can be adjusted so that the amplitude (height) of a signal can be compared to a reference reflector.
Near and Far Fields: The near field is the area in the sound beam immediately in front of the crystal. The length of the near field varies depending on the frequency and cross-sectional area of the crystal surface. The near field effect (Figure 3) occurs because sound is generated from multiple points on the crystal when hit by each electric impulse. As the wave from each piece of crystal travels down the sound path, they eventually merge to form one unified wavetrain called the far field that can be used for inspection. Before individual waves merge, compression and rarefaction areas can overlap, causing null zones or areas that cancel each other out and areas where the sound energy is multiplied. Because of this effect, inspections done in the near field cannot be trusted and are usually prohibited by most codes and specifications. Near and far fields can be demonstrated by throwing several pebbles in a pond at the same time. Each pebble creates its own set of waves immediately around each point of impact, overlapping with the waves formed by the other pebbles. However, as the waves travel outward they eventually merge to form a single wave train (see Sound beam). The area in which the waves travel before they merge represents the near field and the area after that represents the far field.By applying the principles mentioned above and using combinations of the equipment described, an operator can perform accurate ultrasonic inspections. The next article will discuss straight beam inspections, flaw detector screen set-ups and basic straight beam calibrations.
Synopsis of Topics for Practical Contact Ultrasonics Series
Article 2: Straight Beam Inspection is a discussion of the three most commonly used types of straight beam inspection equipment (digital thickness testers, digital thickness testers with wave displays, and straight beam inspection using a flaw detector). General calibration of these instruments and some conditions that affect readings will also be discussed.
Article 3: Basics of Angle Beam Inspection presents fundamentals of angle beam inspection, points to consider when selecting the equipment and setting up screen displays.
Article 4: IIW-based Angle Beam Calibration focuses on angle-beam calibration using IIW-based calibration blocks (basic equipment set-up used for angle beam inspections, why system calibration is required and commonly used basic calibration techniques using IIW, DSC and other IIW-based blocks). Importance of a proper setup for screen presentation, common errors and their prevention and calibration "do's and don'ts" will also be covered.
Article 5: ASME-type Angle Beam Calibration covers angle-beam calibration using ASME-type basic calibration blocks using side-drilled holes and setting up distance amplitude correction (DAC) curves.
Article 6: Angle Beam Scan Patterns and Test Techniques covers basic scan patterns used with angle beam inspection and discusses common test techniques and false indications that can occur during angle beam inspections.
Article 7: Defect Characterization and False Indications relates common defects found while doing angle beam UT inspections to their screen presentations and transducer positions. Common false indications and how to determine them will also be covered.
Article 8: Equipment Maintenance covers general equipment maintenance, wedge angle verification, correcting wedge discrepancies and general cable maintenance. How to tell when maintenance is required and annual flaw detector calibration including vertical linearity checks is also included.
*James W. Houf is Senior Manager of ASNT's Technical Services Department and is responsible for administration of all ASNT certification programs. He's been involved in NDT since 1972 and an ASNT Level NDT Level III since 1984. He currently holds ASNT NDT and ACCP Professional Level III certificates in four NDT test methods. He's also an AWS Senior Welding Inspector and an ASQ Certified Quality Auditor. (800) 222-2768 X212, (614) 274-6899 fax, <jhouf@asnt.org>.
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