Some modern NDT technology has ancient roots. Hundreds of years ago, foundry workers used sound waves to test the integrity of metal castings by tapping them with a hammer and listening to the tone of the ring. But over the past sixty years, the process has become far more sophisticated. Modern digital instruments with small probes called transducers use ultrasonic sound waves to detect hidden cracks, voids, porosity and other internal discontinuities in metals, composites, plastics and ceramics. Ultrasonic testing is completely nondestructive and safe, and it is a well established test method in many basic manufacturing, process and service industries. This article provides a brief overview of flaw detection with conventional ultrasound.

Theory

Sound waves are simply organized vibrations traveling through a solid, liquid or gas. This applies to both the everyday sounds that we hear and the ultrasound used for flaw detection. Sound waves travel through a given material at a specific speed or velocity, in a predictable direction, and when they encounter a boundary with a different material they will be reflected or transmitted according to simple physics rules. This principle underlies ultrasonic flaw detection. Ultrasonic waves will reflect from cracks or other discontinuities in a test piece, so by monitoring the pattern of echoes in a part a trained operator can identify and locate hidden internal flaws.

All sound waves oscillate at a specific frequency, or number of cycles per second, which we experience as pitch in the familiar range of audible sound. Human hearing is no higher than about 20,000 cycles per second (20 KHz), while most ultrasonic flaw detection applications utilize frequencies between 500,000 and 10,000,000 cycles per second (500 KHz to 10 MHz). Lower frequencies penetrate deeper into material, while higher frequencies can resolve smaller flaws due to their shorter wavelength. Ultrasonic waves are much more directional than audible sound and can be aimed and focused.

Sound waves in solids exist in various modes defined by the type of motion involved. Longitudinal waves and shear waves are the most common modes employed in ultrasonic flaw detection. A longitudinal wave is characterized by particle motion in the same direction as wave propagation, as from a piston source. Audible sound exists as longitudinal waves. A shear or transverse wave is characterized by particle motion perpendicular to the direction of wave propagation. Shear waves are used in many weld inspections.

Equipment

Modern ultrasonic flaw detectors are small, portable, microprocessor-based instruments suitable for both shop and field use. They generate and display an ultrasonic waveform that is used by a trained operator, often with the aid of analysis software, to locate and categorize flaws in test pieces. They typically include an ultrasonic pulser/receiver that generates sound pulses and listens for echoes, hardware and software for signal capture and analysis, a waveform display, and a data logging module. Most contemporary instruments use digital signal processing for optimum stability and precision.

Flaw detectors capture a waveform digitally and then perform various measurement and analysis functions. A liquid crystal or electroluminescent display features a screen calibrated in units of depth or distance. Multicolor displays can be used to provide interpretive assistance. An internal clock synchronizes transducer pulses and provides distance calibration. Signal processing may be as simple as generation of a waveform display that shows signal amplitude versus time on a calibrated scale, or as complex as sophisticated digital processing algorithms that incorporate distance/amplitude correction and trigonometric calculations for angled sound paths. Alarm gates are often employed to monitor signal levels at selected points on the display to flag echoes from flaws. Internal data loggers can record full waveform and setup information associated with each test or selected parameters as required. Phased array flaw detectors are advanced instruments that use multi-element probes with swept beams to create cross-sectional and planar images of defects within a part’s volume.

Transducers for ultrasonic flaw detection utilize an active element made of a piezoelectric ceramic, composite or polymer. When this element is excited by a high voltage electrical pulse, it vibrates and generates a burst of sound waves. When it is vibrated by an incoming sound wave, it generates electrical pulses corresponding to echoes. The front surface of the element is usually covered by a wear plate that protects it from damage, and the back surface is bonded to backing material that mechanically dampens vibrations once the sound generation process is complete. Because sound energy at ultrasonic frequencies does not travel efficiently through gasses, a thin layer of coupling liquid or gel is normally used between the transducer and the test piece.

There are five types of ultrasonic transducers commonly used in flaw detection applications:

Contact transducers introduce sound energy perpendicular to the surface, and are typically used for locating voids, porosity and cracks or delaminations parallel to the outside surface of a part, as well as for measuring thickness.

Angle beam transducers are used in conjunction with plastic or epoxy wedges (angle beams) to introduce shear waves or longitudinal waves into a test piece at a designated angle with respect to the surface. They are commonly used in weld inspection.

Delay line transducers incorporate a short plastic waveguide or delay line between the active element and the test piece. They are used to improve near surface resolution and also in high temperature testing, where the delay line protects the active element from thermal damage.

Immersion transducers couple sound energy into the test piece through a water column or water bath. They are used in automated scanning applications and in situations where a sharply focused beam is needed to improve flaw resolution.

Dual Element Transducers utilize separate transmitter and receiver elements in a single assembly. They are often used in applications involving rough surfaces, coarse grained materials, detection of pitting or porosity, and they offer good high temperature tolerance as well.

Procedure

Ultrasonic flaw detection is a comparative technique. Using appropriate reference standards along with knowledge of sound wave propagation and generally accepted test procedures, a trained operator identifies specific echo patterns corresponding to the echo response from good parts and from typical flaws. The echo pattern from a test piece may then be compared to these patterns from calibration standards to determine its condition.

Straight beam testing is generally employed to find cracks or delaminations parallel to the surface of the test piece, as well as voids and porosity. In this type of test, the operator couples the transducer to the test piece and locates the echo returning from the far wall of the test piece, and then looks for any echoes that arrive ahead of that backwall echo, discounting grain scatter noise if present. An acoustically significant echo that precedes the backwall echo implies the presence of a laminar crack or void. Through further analysis, the depth, size and shape of the structure producing the reflection can be determined.

Angle beam testing is used to find cracks or other discontinuities perpendicular to the surface of a test piece, or tilted with respect to that surface, which are usually invisible with straight beam test techniques because of their orientation with respect to the sound beam. Angle beam testing employs wedge-shaped probes to direct sound energy into the test piece at a selected angle that produces an optimum reflection from flaws. Angle beam testing is especially common in weld inspection.

Common applications

The potential uses of ultrasonic flaw detection in manufacturing operations are numerous and diverse. Here are some of the most common ones.

• Stock material – Metal and plastic bars, plates and tubes can be tested for internal cracking, voids or porosity.
• Spot welds – These are commonly for lack of fusion, undersized nuggets or a stick welds using tested with small, high frequency delay line transducers diameter approximates that of the weld.
• Bondlines and braze joints – Adhesive or metallic bonds and brazes can typically be tested as long as one surface is accessible.
• Other manufacturing welds – Laser welds and similar techniques used to join sections of metal can frequently be tested, although geometry must be favorable.
• Molded plastic parts – Voids can be detected and wall thickness measurement is also possible.
• Babbitt bearings – Bonding of Babbitt to outer shell in manufacturing and refurbishing.
• Thermal fusion joints in plastic parts – Can usually be tested with small straight beam transducers as long as the fusion plane is parallel to the outside surface.
• Castings – Voids, porosity, inclusions and cracks produce ultrasonic indications that can be identified by a trained operator. Nodularity in cast iron can also be verified ultrasonically.
• Fiberglass and composites – Delaminations and impact damage can be located.

In short, ultrasonic flaw detection can be a valuable tool in a wide variety of applications.