During angle beam ultrasonic testing, the sound beam travels through the material at a refracted angle rather than moving straight down. As it reaches the back wall, it reflects back toward the scanning surface, creating a repeating V shaped beam path.
The skip distance is the distance measured along the inspection surface between these reflection points. One trip from the probe to the back wall is known as the first leg or half skip, while the complete journey back to the scanning surface is called the full skip. If the beam continues reflecting, additional skips are created.
Understanding this beam path is essential for accurate probe positioning, weld coverage and indication interpretation.
| Half Skip Distance (S½) | The surface distance from the probe index point to the position directly above where the beam first reaches the back wall. This represents the first section of the beam path. |
| Full Skip Distance (S) | The total surface distance covered after the beam has travelled to the back wall and returned to the scanning surface. A full skip is equal to two half skips. |
| Sound Path (SP) | The actual distance travelled by the ultrasonic beam through the material during one leg of its journey. This is measured along the beam itself rather than along the material surface. |
| Index Point | The reference point on the probe wedge where the refracted ultrasonic beam enters the test material. All positioning measurements are taken from this location. |
| Refracted Angle (θ) | The angle at which the ultrasonic shear wave propagates through the material after entering from the wedge. This angle is measured from the surface normal. |
Skip distance is determined using basic trigonometry based on the material thickness and the refracted beam angle.
Where:
If the sound path is known from the ultrasonic instrument, the reflector location can be estimated using:
If the measured sound path exceeds the first leg distance, the indication originates from the second leg of the beam. In this situation, subtract the first leg sound path before calculating the remaining beam geometry.
Material Thickness: 20 mm
Probe Angle: 45°
This means the beam first reaches the back wall beneath a point 20 mm from the probe index, returns to the surface after travelling 40 mm, and follows a total beam path of approximately 28.3 mm during the first leg.
25 mm Steel Plate with a 60° Shear Wave Probe
For a first leg inspection of the weld root, position the probe index approximately 43.3 mm from the weld centreline. Inspections using the second leg require scanning beyond the full skip distance of 86.6 mm.
The most frequently used refracted shear wave angles for steel inspections are 45°, 60° and 70°.
| Angle | Characteristics & Applications |
|---|---|
| 45° | A versatile angle commonly used for butt weld inspections, particularly when examining the weld root. It produces a balanced V shaped beam path with relatively short skip distances. |
| 60° | Provides greater coverage of the upper weld region and is effective for locating many common weld discontinuities. The skip distance is longer than that of a 45° probe. |
| 70° | Produces the longest skip distance and a beam that travels closer to the inspection surface. It is often selected for thinner materials or applications requiring greater surface reach. |
While skip distance calculations provide valuable guidance, inspection results should always be interpreted alongside the applicable inspection procedure, calibration requirements and relevant industry standards. Proper probe selection, calibration and scanning technique remain essential for reliable ultrasonic examinations.