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Passing examinations In-depth | SHIPBUILDING TECHNOLOGY


Following is part two of a three part paper written by Lloyd’s Register’s David J Howarth and John Durkin (part one appeared in the July/August issue and part three will appear in the October issue of Te Naval Architect). Part two looks at methods of examining welding work and defect detection.


R


adiography is normally preferred for the examination of butt welds of 10mm or less. Ultrasonic testing is


acceptable for welds of 10mm or above and is always used for the examination of full penetration of tee butt or cruciform welds. Radiography works on the principal that


radiation passing through an object will be differentially absorbed based on the differing density of the object under test. Te resulting image can then be recorded on a radiographic film. In the eyes of many, the largest advantage


for the selection of radiography is based on the availability of a permanent record of the examination and the ability to compare the results against the radiographic report. While radiography is capable of producing a permanent record, it suffers from several major limitations/disadvantages, these being:


• Ionising radiation presents a serious health hazard and its use is strictly governed by


a. Health risks


• Before carrying out radiography, it is essential to ensure that the area where


both national and international law, which in most cases state that radiography must only be used when no other form of non-destructive testing is available


radiography is to be carried out is clearly marked with visible and audible barriers to prevent personnel entering the controlled area. A typical controlled area for an unshielded 10 curie source of Iridium 192 would be a radius of 80m from the source. Tese large controlled areas therefore have a direct effect on production schedules.


b. Defect detection The detection of volumetric defects such as slag, porosity, metallic and non-metallic inclusions are relatively easy to detect using the standard radiographic techniques employed in ship construction. On the other hand, planar defects such as lack of fusion and


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• defect alignment relative to the direction of the beam of radiation


cracks are sometimes very difficult to detect. Trials have shown that cracks are readily detectable when the source of radiation is perpendicular (+/- 20degs) to the vertical axis of the crack. Detection of Lack of Fusion (LOF) type defects, particularly lack of side wall fusion, is nearly impossible unless the radiation beam is placed normal to its axis. Te factors directly affecting defect detection are:


• the ratio of component thickness to relative defect thickness


• radiographic sensitivity • the skill of the interpreter.


c. Interpretation Radiographic interpretation requires the surveyor to have a good technical understanding of the welding process, weld geometry, radiographic technique, material type and thickness and the reference application code and acceptance standard. Interpretation is divided into two stages, the first being to assess the quality of the radiograph itself, and the second to assess the weld quality.


• Film density: Te degree of darkening or film density should be measured


d. Radiograph quality


• Sensitivity: Radiographic sensitivity can be measured using image quality


indicators (IQI) or penetrameters. Tese indicate sensitivity by specifying, for IQIs, an essential wire thickness, and, for penetrameters, an essential hole size,


using a densitometer, or compared against a calibrated film density strip, to confirm that the film density is in accordance with the referencing code or standard. Lloyd’s Register recommends a minimum film density of 2.0, based on the fact that defects are more readily detected at higher film densities and high film contrast


Figure 1: Weld containing lack of sidewall and inter run fusion.


Ultrasonic examination Ultrasonic examination of welds during new construction has become the method of choice in most shipyards for carrying out volumetric examination of welds. In terms of quality control, the technique is problematic as they have to rely solely on the operator’s report, which is unlike radiography where the actual radiograph can be viewed against the actual report.


The Naval Architect September 2010


• Identification: Te identification should clearly identify the weld being tested


for a given material thickness


• Artefacts or processing and handling damage: Films should be free from


processing and handling damage such as scratches, water marks, fogging, mechanical damage or any other blemish which will directly interfere with film interpretation.


e. Weld quality


• Interpretation of weld defects is reliant on the inspector’s technical knowledge


• Identified defects should be assessed against current classification society


and experience


rules, or alternatively, national standard acceptance criteria, provided the latter is considered equivalent and has been agreed with the classification society.


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