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In-depth | SHIPBUILDING TECHNOLOGY


mechanics, and has allowed pipeline engineers and designers to significantly increase the size of weld defects compared to what would normally be permitted in quality standards; thereby reducing the numbers of welds requiring repair. One area where phased array has not been


widely adopted is ship new construction. Tis could be due to several factors, the main ones being economics, reluctance to adopt new technology and relatively low inspection requirements. Phased arrays can be defined as “A mosaic of


elements in which the timing of the elements’ excitation can be individually controlled to produce effects such as beam focusing and beam axis steering”. Ultrasonic phased arrays consist of a series of individual elements, acoustically insulated from each other with their own individual connection to a time delay circuit and A/D (analogue to digital converter). To achieve phasing, the elements are pulsed in groups using pre-calculated timings for each element. By phasing the elements, it is therefore


possible to focus and steer the ultrasonic beam, and by utilising a sectoral scan during weld inspection it is possible to inspect the weld fully. Tis will ensure that all relevant beam angles are used and will dramatically improve defect detection and allow for more accurate sizing of the defects found. Phased arrays have several major advantages


over manual pulse echo techniques for the volumetric examination of welds:


• High inspection speed


• One phased array probe array is capable of producing multiple probe angles


• Advanced signal processing


• Improved visualisation of signals through colour pallets and ability to display data as


• Increased probability of detection of defects


A- scan, B–scan (E-Scan), C-scan, S-Scan


• Accurate and reliable sizing of defects • A permanent record of results • Reduced probe size


• Auditable and reproducible data


It also has a few disadvantages: • It is complex to initially set up


• Repeatable inspection as calibration data files can be stored.


• Operator training is required and there is a lack of skilled operators


• Tere can be unwillingness to adopt new technology


138


Figure 4: Radiograph of weld in 20mm plate.


• Cost: it is more expensive than conventional manual ultrasonic equipment.


Figures 4 – 6 show the benefits of phased


array over both standard ultrasonics and radiography. Tey compare the results of a 20mm carbon steel plate welded using the FCAW process. Figure 4 shows the radiographic image


which clearly identifies lack of fusion, slag, wormholes and porosity, but provides no indication of through thickness depth of the defects. Figure 5 shows the screen of a standard ultrasonic examination of the same area, which shows an indication at a depth of approximately 12.4mm from the plate surface, considered by the operator to be lack of fusion. Finally, Figure 6 shows the phased array image of the same area: the greater detail and ease of interpretation are easy to see.


Weld defects A perfect butt weld joint, when subjected to an external force, provides a distribution of stress throughout its volume which is not significantly greater than that within the parent metal. Tis is achieved as long as the following features apply:


• Welds should consist of solid metal throughout a cross section at least equal to


• All parts of a weld should be fully fused to the parent metal


that of the parent metal


• Welds should have smoothly blended surfaces


• Welding is carried out in accordance with the welding procedure specification.


If any of these requirements are not fulfilled


then the weld is imperfect and the stress distribution through the joint is disrupted. A weld imperfection (or discontinuity) is therefore any object or shape which is capable of creating a stress concentration within a welded construction. It should be remembered that, in practice, all welds contain small imperfections. However, the majority of these are so small that they have no significant effect on the performance of


• lower the load bearing capacity of the joint


• initiate brittle fracture • nucleate a fatigue crack, and


the joint. If an imperfection is considered of sufficient size to be detrimental to the structure (as defined by an acceptance standard), it is classified as a defect. Stress concentrations are detrimental to the performance of a structure as they can:


• initiate stress corrosion cracking. The greater the stress concentration


produced by an imperfection, the more likely it is to cause failure of the weld in service. Terefore, imperfections need to be assessed according to the severity of their stress concentration. Tis is achieved mainly by the geometric descriptions ‘planar’ or ‘non-planar’. Planar imperfections (those which are


essentially two-dimensional) are effective stress raisers and are almost invariably treated as unacceptable. Non-planar imperfections (those which


have three significant dimensions) are less effective stress raisers and are usually accepted in limited quantity and size. It is important that an imperfection is


• Solidification cracking. • HAZ hydrogen cracking.


correctly identified to allow the welding procedure to be suitably modified to prevent their re-occurrence. Weld imperfections can be grouped into five distinct types according to their nature and shape: a. Cracks


• Weld metal hydrogen cracking.


• Lamellar tearing. Cracks are more significant than other types


of imperfection as their geometry produces a very large stress concentration at the crack tip, making them more likely to cause fracture. Note that this section is concerned only with cracks produced at the time of welding, not subsequent service cracking, such as fatigue or stress corrosion cracking. Cracks can occur in the weld metal or heat affected zone. Due to


The Naval Architect September 2010


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