A Self-Learning Manual - Institution of Engineers Mauritius

A Self-Learning Manual
Mastering Different Fields of Civil Engineering Works (VC-Q&A Method) Vincent T. H. CHU
6. In checking the quality of weld, what are the pros and cons of
various non-destructive weld inspection methods i.e. ultrasonic test,
radiographic inspection and magnetic particle flaw detection test?
(DW6)
Currently, there are three common non-destructive testing of weld, namely
radiographic inspection, ultrasonic testing and magnetic flaw detection test.
The method of radiographic approach was used commonly in the past until
the arrival of ultrasonic inspection technique. The major difference between
the two is that ultrasonic testing detects very narrow flaws which can hardly
be detected by radiographic method. Moreover, it is very sensitive to gross
discontinuities. Tiny defects, which characterize welding problems, are
normally not revealed by radiographic inspection.
Moreover, ultrasonic inspection possesses the advantages that it can
accurately and precisely locate a defect as well as figure out its depth,
location and angle of inclination.
In the past, it was expensive to adopt ultrasonic means for inspection.
Nowadays, the rates for both inspection methods are comparable. Most
importantly, the x-ray and gamma ray used in radiographs are radioactive
and pose potential safety hazard to testing technicians on site. Reference
is made to Paul G. Jonas and Dennis L. Scharosch.
Magnetic flaw detection test can only be used for checking flaws in any
metallic objects. This method is commonly used for inspecting surface
cracks and slightly sub-surface cracks. However, surface and sub-surface
cracks can be readily detected by radiographs and ultrasonic inspection.
7. Why is acetylene gas cylinder for gas welding to be erected in
upright position? (DW7)
Acetylene gas is commonly used for gas welding because of its simplicity
in production and transportation and its ability to achieve high temperature
in combustion (e.g. around 5,000 o F). Acetylene is highly unstable and
flammable and would explode in elevated pressure when reacting with
oxygen in air. Storing acetylene gas in cylinders under pressure is very
dangerous. Gas acetylene used for welding purposes is stored in cylinders
of liquid acetone contained in porous material (like firebrick). This is for
cooling purpose in the event of thermal decomposition and to ensure that
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