Ultrasonic Testing of Welds on Plates

Ultrasonic Testing of Welds on Plates

For testing a weld satisfactorily, the following information should be available
(i) Material, (ii). Details of weld preparation, (iii). Welding process and probable defects expected, (iv) Applica­tion of the part and (v)Acceptance criteria.

Testing of a single 'V'butt joint welded only from one side using transverse waves.
The selection of probe frequency depends on the flaw to be detected and attenuation in the effects of the near zone and beam spread the minimum size of the material. Which varies with frequency should be considered. For mild steel welds usually, a frequency in the range of 1 MHz to 4 MHz will be satisfactory.


Probe Angle: A rule-of-thumb, for determining the angle of the probe is e = 90 - t, where e  =  is the angle of probe, and t,  =  the thickness of material to be tested in mm.
However,. the important factor to be considered is the probable plane of orientation of defects.  The beam should have such an angle so as to strike the defect normally.   To accommodate the variation of echo amplitude with respect to distance,  a  Distance  Amplitude   Correction  (DAC)  curve
should be drawn with a known, reflector.  The first point of the DAC curve should be taken preferably after the beam undergoes one reflection from the bottom surface.
scanning is done from both sides- of the weld with sufficient overlap to ensure complete coverage and with a swivel motion to the. probe.  The distance (minimum and maximum} the probe should travel from the weld axis should be such that the entire volume of the weld is adequately scanned.

The flaws can be located with respect to the probe position and depth from the scanning surface can be determined using the following equations:-

FULL SKIP DISTANCE (S) = 2t Tan ß               HALF SKIP DISTANCE =  B1 Sin ß
                    full-beam path (B) = 2t Sec ß                        half beam path =  B1 Cos ß 

However, an echo must not, at once be taken as evidence of a flaw. On the contrary, the origin of the echo should be sought by a number of tests from as many different directions as possible.  A simple flaw locating rule can be made and tedious calculations can be avoided  On a piece of semi-transparent paper (tracing paper can be used) two parallel lines are drawn, the distance between these two is the thickness of the part being tested. On the  L.H.  side,  a point on the top surface is marked as 'zero'.   Distance   (5 mm marks would do) are marked on the top line increasing towards the R.H. side.  From 'zero' a  line is drawn representing the axis of the sound beam until it touches the bottom surface and returns to the top. Distances are marked at 5 mm intervals along this line.

Now on another paper, a sketch is drawn to scale 1:1 showing the cross-section of the weld.  While scanning, when a  defect echo appears on the screen,  the beam path at which the echo appears and the distance of the probe to the nearest edge of the weld are noted.

Now the semi-transparent paper is placed on the sketch of the weld cross-section in such a way that the distance between 'zero' on the top horizontal line and the weld edge, is the same as the
measured distance between the probe center to the edge of the weld. The position of the flaw can be ascertained by following the slanted line (beam path) to the distance as measured on the CRT.

In addition to this scanning, the weld may also be scanned for the detection of transverse defects. A large angle probe (70 deg) placed alongside the weld edge. and at a small angle to the weld axis will direct the sound beam into the. weld for detection of transverse defects. The scanning is to be done from both sides of the weld.
Some other common weld joint preparations and scanning techniques are given below.

(1). Double 'v' butt-joint  :
If accessible this double 'v' butt joint may be scanned from all four sides. The angle of the prob should be such that the sound beam hits the bevel face at around 90 deg.

(2). 'J'Type joint
This type of edge preparation with 0 deg or very small bevel angle is used for joining large thickness members. As the plane of expected lack-of-fusion between parent metal and weld metal is almost vertical, the weld may also be tested by using the 'tandem' technique. In this tech­nique. two angle probs in tandem (with a fixed distance between them) can be used to detect defects oriented in the vertical plane. Indications will be present on the screen. only when the sound beam is reflected by the defect to the receiving probe.

Different configurations of probe arrangement may b required to completely test the weld.
(3) 'T' type weld joint :
When section thicknesses of this type of full penetration joint are large, it can be tested using a normal and an angle probe. A normal probe can detect  lack-of­ fusion between member A and the weld. Angle probe can detect lack-of-fusion between member B and the weld along the weld bevels.

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