Ultrasonic flaw detection uses the reflection and transmission of pulsed sound waves at frequencies higher than those that are audible to human ears to look at the hidden internal structure of a test piece. Source: Olympus NDT

Since the middle of the last century, high frequency sound waves have been an important tool for evaluating the quality of welds. Ultrasonic flaw detection, which uses the reflection and transmission of pulsed sound waves at frequencies higher than those that are audible to human ears to look at the hidden internal structure of a test piece, has long been used to check the integrity of welds in pipelines, structural steel, railroad tracks, and other heavy industrial and infrastructure welds, but in many cases the same technology can be adapted to the smaller and lighter welds found in manufacturing environments. In appropriate applications, ultrasonic testing in the hands of a qualified operator is quick and reliable, completely nondestructive, and unlike radiography, has no specific safety hazards or regulatory licensing requirements associated with its use.

Conventional ultrasonic flaw detection using small, handheld instruments with single crystal transducers is well established in many industries such as automotive manufacturing. And the new player on the block is phased array systems that use sophisticated technology based on multi-element probe assemblies to generate steered beams and create cross-sectional pictures similar to those in medical ultrasound imaging. These too are well suited to weld inspections.

Phased arrays units are now portable and functionally convenient as well. Source: Olympus NDT

Conventional Ultrasonic Weld Testing

All ultrasonic flaw detection is based on a simple principle of physics. A sound wave traveling through a medium such as metal will continue to travel in a predictable direction until it encounters a material boundary such as a far wall or a hidden crack. At that point, the sound wave will be reflected and/or diffracted in predictable ways. Ultrasonic flaw detectors generate short bursts of sound energy by means of small, handheld probes that are coupled to the surface of the test piece. The instrument then records the pattern of sound reflections, which will change as material conditions change. Analysis of reflected echo patterns can be used to locate and determine the size of cracks and other flaws, which in the case of weld inspection include lack of full penetration or fusion as well as porosity or slag inclusions.

Here is an example of a phased array screen display performing three angle scans simultaneously, as well as showing an S-scan image.

Spot Weld Testing

Spot welding is a common technique for joining two pieces of metal, usually thin sheets or plates, by means of a series of small circular tack joints. The process is widely used in the automotive industry for body assembly and some chassis joints, and by other types of sheet metal fabricators. Spot welds are made by pressing electrodes against both sides of the parts to be joined and passing a high current through them, which momentarily creates a pool of molten metal that solidifies into a round joint known as a nugget. Spot welds represent the single largest application for ultrasonic weld inspection in manufacturing environments.

A phased array screen display uses multi-group scans with the weld defects clearly visible. Source: Olympus NDT

Phased Arrays

Phased array instruments use more complex hardware and software to provide a higher level of both test and interpretive capability. Instead of a single-element transducer, phased arrays use a multitude of elements, all individually wired, pulsed and time-delayed. Ultrasonic beams are formed by constructive and destructive interference. Phased arrays are sufficiently developed so that costs can be competitive overall with conventional ultrasonics, and software is user-friendly so that everyday operators can use it. Phased arrays are now portable and functionally convenient as well.

Unlike conventional ultrasonics, phased arrays can electronically focus, steer and sweep the beam. Besides reproducing conventional scans, electronic control permits special scans and imaging: S-scans (swept angle scans), electronic raster scans (E-scans) and multiple displays.

For practical purposes, there are two ways to inspect a weld with phased arrays: manually such as conventional ultrasonic testing or using encoded linear scans. Being fully electronic, all setups and data can be saved for convenience. The selected approach will depend on costs, speed required and customer specifications.

In manual mode, the phased array unit closely repeats conventional inspections; this has advantages as it is conceptually easier to understand, while being closer to traditional weld inspection codes. This inspection would fulfill a code such as AWS D1:12 in a single scan, while giving better imaging and some data storage. Manual phased array systems are now quite low cost, at approximately $20,000 to $30,000 each.

The advantages of this type of inspection include:
  • Increased speed of inspection as the operator can perform three scans at once.
  • S-scan display allows better interpretation of data because of 2-D spatial images.
  • Screen displays can be saved for re-analysis and posterity.

A schematic shows electronic scanning (E-scans) of weld at two angles. Source: Olympus NDT

A linear scan is a single-passmechanical scan parallel to the weld to be inspected. The phased array instrument electronically rasters the beam up and down the weld at various angles or scan patterns.

Using appropriate array sizes and scan patterns, the full weld can normally be inspected in a single pass. The linear scanning approach saves a considerable amount of scanning time-it is typically five to 10 times faster-and hence overall cost. Phased array units for linear scanning require more advanced capabilities and software, particularly encoders, multi-group software and sometimes limited scanning hardware. However, costs are still competitive overall-perhaps $60,000 for the complete unit-when the increased productivity is factored in.

Phased array linear scanning offers advanced technology at reasonable prices with portable equipment.
  • All data is stored for different displays or for detailed analysis or advanced processing.
  • The inspection process can be tailored to the weld or component; for example, S-scans, E-scans, time-of-flight diffraction or a combination can be used.
  • The inspections tend to be highly reproducible, unlike conventional manual ultrasonics.
  • The imaging capability is typically much better for correctly characterizing and sizing defects.
  • With electronic setups, flexibility is higher since it is easy to adjust parameters for different weld profiles.

Ultrasonics offers high quality weld inspections with no safety hazards or licensing issues. The technology is well established, codified and widely accepted. Automobile spot welds comprise an important application for conventional ultrasonic flaw detection in a manufacturing enjoinment, but the principles used for testing structural metals can be generally applied to manufactured parts as well.

Phased array systems represent the latest development for weld testing, offering sophisticated imaging capability, and they can be used either manually or encoded for data collection. Thanks to their increased speed of inspection and other features, phased arrays can be cost-competitive for welds. And as with every other area of contemporary inspection technology, ongoing development will undoubtedly lead to further expanded capabilities in the future.