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In-depth | SHIPBUILDING TECHNOLOGY In ultrasonic examination a pulse


of sound energy is transmitted from the ultrasonic probe into the material under test. Te elapsed time between the transmission of the sound pulse and the reception of the reflected signal can be used to measure distance, assuming that the velocity of the material is known. A typical ultrasonic flaw detector used for


weld examination during ship construction normally has an ‘A’ scan display which records time or distance on the horizontal scale ‘X’ axis and signal amplitude on the vertical ‘Y’ axis. Before being able to carry out any ultrasonic examination, it is necessary to calibrate the distance on the ‘X’ axis of reflected signals from the zero point using a calibration block. In order to calibrate signal amplitude on the vertical scale, ‘Y’ axis reflectors such as flat bottom holes (FBH), surface notches and side drilled holes (SDH) are used. According to Lloyd’s Register’s Rules [2],


defects are to be assessed on signal amplitude and length, apart from cracks which are to be rejected regardless of signal amplitude. Tis approach is further adopted by an IACS recommendation [6] and most national and international standards. While this is considered to be an acceptable approach it is necessary to recognise that this amount of reflected energy is dependent on the shape of the defect and the type and orientation of the defect relevant to the ultrasonic beam. For optimum defect detection the ultrasonic beam should strike the defect at 90degs. Tis is particularly true for smooth planar defects such as lack of side wall fusion, and can be clearly demonstrated by using a 60degs probe to ultrasonically examine a weld containing lack of sidewall fusion. With the weld preparation having a 60degs included angle, and noting the signal response, the examination is repeated using a 70degs probe. Te signal amplitude from the 70degs probe would be significantly lower aſter allowing for the greater distance travelled by the sound. Tis could result in a defect that was originally rejected using the 60degs probe now being accepted. Advantages of ultrasonic examination are:


• Economics: typical examination costs are significantly less than radiography,


particularly when factoring in loss of production attributed to radiography safety issues


136


• It can be used on multiple weld configurations, and on very thick welds


Figure 2 shows the weld being examined


• Tere are no health hazards compared to radiography


• It enables better detection of planar defects compared to typical radiographic


• It enables quick, real-time analysis to be carried out.


Disadvantages of ultrasonic examination


• Surveyors must rely on reports with little possibility of confirming the results or


are:


• Tere is no permanent record of results except for a report


traceability


• Operator training and competency is required


• It is easy for results to be falsified


• Complex weld geometry can make it difficult to interpret results


• Defects under high compressive load may be forced shut and not detected as the


reflectors


Figure 3: Response to defect from 0degs UT probe.


• Operator fatigue and environmental conditions adversely affect results


ultrasound passes through the defect


• Over sizing and under sizing of defects can occur.


Te following is an example of a difficulty


with ultrasonic examination where the defect is out of plane with respect to the ultrasonic beam. Welds that contain significant amounts of lack of inter-run fusion (see Figure 1) can be difficult to detect during routine ultrasonic examination carried out in accordance with national standards. Such standards oſten specify an assessment of signal amplitude only.


Figure 2: UT Screen showing response to an imperfection using a 70degs probe.


rejectable. Ultrasonic testing is an excellent tool but


the example above highlights the need for operators to be experienced and provided with the correct procedure to allow full interpretation of defects found by the initial scan.


• Portability: the majority of equipment is battery powered


Phased array ultrasonics Although phased array ultrasonics were originally explored in the early 1960s, and initially used for medical diagnostics in the early 1970s, it was not until the late 1980s that it started to be adopted by most major industries; namely, nuclear, oil and gas and petrochemical. Since their introduction, phased arrays have become very widely recognised by national and international codes. Phased arrays are now widely adapted by pipeline users for the inspection of pipeline welds during construction, thus eliminating the need for radiographic inspection. A further benefit of phased arrays is the ability to accurately size defects in the through wall direction (height). Tis is of particular importance when deciding defect size allowances based on fracture


The Naval Architect September 2010


• Defect alignment with ultrasonic beam is particularly important on smooth planar


techniques, and is able to size defects in the length, width and depth


using a 70 degs shear wave probe, with sensitivity in compliance with requirements of the national standard. Te low signal amplitude possibly indicating an area of lack of fusion can just be seen in the centre of the screen. In accordance with the acceptance standard this indication would be disregarded. Figure 3 shows a 0 degs compression probe


placed directly on the cap surface. In this instance the signal from the imperfection at a depth of 14.69mm is large and would be


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