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How does angle beam scanning enable weld inspection?

In this article:

  • Working Range Defines Reliable Flaw Detection: In automated UT, the working range specifies the area and depth where flaws of a certain size (e.g., 6 mm) are detected with less than 6 dB echo loss.
  • Sound Field Behavior and Scan Paths Shape Detection Coverage: The effective working area depends on how the ultrasonic beam propagates and interacts with reflectors across the scanned volume, not just on probe geometry.
  • Normalized Diagrams & Amplitude Drop Define Limits: Using DGS and fixed-threshold diagrams, normalized reflector size (G), probe movement (B), and normalized distance (Z) are combined to determine the precise boundaries of the working range.
  • Waygate’s RotoArray comPAct Enables Field‑Ready Beam Modeling: The Krautkrämer RotoArray comPAct roller probe integrates phased-array beam steering with embedded electronics, allowing real-time visualization and validation of working range criteria directly in the probe—ideal for accurate, automated inspection workflows

How does angle beam scanning enable weld inspection in ultrasonic testing?

Angle beam scanning is a foundational method in ultrasonic weld inspection, enabling the detection of flaws that cannot be accessed by direct sound entry. This article outlines the principles behind shear wave generation through wedges, optimal angle selection, and reflector positioning—key to accurate flaw localization in welded joints.

Sound which travels around corners is an everyday experience in the audible range. Ultrasound in the MHz range shows however a very high directional effect. This is why special testing techniques are necessary if testing ranges of the test specimen are to be covered in which the ultrasonic waves cannot enter directly. 

Weld testing in particular has to cope with these difficulties. It is therefore the main area of application for angle beam probes (fig. 49). 

B

Relationship between sound path s, surface distance a, reduced surface distance a', skip distance p and reflector depth d for the plate thickness t and the beam angle β

Longitudinal waves are transmitted via a wedge to generate a sound beam that runs obliquely to the surface of the test specimen. According to chapter 10 and fig. 29 from the Krautkramer booklet, part of the emitted impulse is reflected at the wedge-coupling medium interface. A damping block in the wedge absorbs this interfering wave. When entering the test piece, refraction occurs, which in general causes both longitudinal and transverse waves to be generated (fig. 29). 

To avoid confusion between the two wave types, shear waves are used almost exclusively for angle beam scanning. For this purpose, the angle of incidence α (wedge angle) must be selected in such a way that no longitudinal wave can occur in the test specimen after refraction, but the shear wave still occurs. According to chapter 10 from the same booklet, equations (33), (34), this means that: 

(44) α' ≤ α ≤ α" 

For the frequently occurring combi- nation of Perspex/steel the following applies:
cp, long = 2730 m/s 

cs, long = 5920 m/s cs, shear = 3255 m/s 

(45) 27,5∘ ≤ α ≤ 57∘

Since the sound beam width must be taken into account with a few degrees of angle, as well as a safety distance, the angles used in testing practice are in a smaller interval than given by Eq. (45). 

The refraction angles β for equation (45) are typically: 

(46) 33,5∘ ≤ β ≤ 90∘

In steel then one should not use a refraction angle of β ≤ 35°. Apart from that already at approx. 70° surface waves occur due to the width of the sound beam (not just at β = 90°). Thus, in steel, refraction angles of above 70° should not be used on any reflecting surface. 

Due to the inclined sound path the locating of a reflector from the screen indication (transit time) and the position of the probe (fig. 49) is only possible by a mathematical means. The lower half of the weld is hit directly, i. e. it is tested via the shortest path and with the narrowest sound beam. There is the following relationship then between the sound path s to the reflector and the depth position d: 

(47) d = s ∙ cos β 

Reflectors in the upper half of the weld are hit by a reflection from the opposite surface of the test specimen by the longer path shown in fig. 49. Here between sound path s, the depth position d and the wall thickness t the following applies: 

(48) d = 2t – s ∙ cos β 

The projection of the full zigzag path in the plate is designated as “skip distance p” 

(49) p = 2t ∙ tan β 

To simplify locating reflectors the reduced projection distance a' has been introduced (also on special DGS diagrams and DGS scales1): 

(50) a' = s ∙ sin β – x
The sound path s to the reflector is projected onto the test piece surface and reduced by the distance X between the front edge of the probe and the sound exit point (→ X-value of the probe). 

Shear (transverse) waves show different type of mechanics with sound attenuation as is shown by longitudinal (compression) waves. Attenuation for longitudinal waves can thus not be simply converted into attenuation for transverse waves. With most of the common types of steel the attenuation coefficient 𝛋 for shear waves is: 

𝛋 ≤ 10 dB/m f ≤ 3 MHz 𝛋 = 40 – 60 dB/m f = 4 MHz always related to the total sound path 2s. 

As the reflectors can often only be detected in a position in which the sound beam does not hit them perpendicularly when testing with angle beam scanning, you must either utilise the mirror effect (tandem method, fig. 36) or select sound beams that are as directionally insensitive as possible, i. e. work with the longest possible wavelength, i. e. the lowest possible frequency. (see chapter 16). 

Conclusion

Effective weld inspection using angle beam scanning depends on precise control of beam angles, understanding of shear wave behavior, and accurate signal interpretation. Mastery of these principles ensures reliable flaw detection, even in geometrically complex or limited-access zones.

Explanations of the terms used for testing with angle beam scanning and the testing procedure are given in [14] and [15] as well as many other recommendations for weld inspection: 

 

[14] International Institute of Welding (NW): List of terms used in ultra- sonic testing in eleven languages with explanations, Publ. Soud. Autog., Paris (1967) 

[15] ISO 17640 (2018): Non-destructive testing of welds—Ultrasonic testing—Techniques, testing levels, and assessment 

1.With the modern, digital flaw detectors, all location variables of an indication are automatically calculated and displayed according to the entered test parameters (t, β, x).