TESTING OF WELDS

Purpose

            Testing on welded joints are carried out for the following reasons

            (a)       To ascertain the condition of weld metal.

            (b)       To determine the metals maximum strength.

            (c)        To determine defects on internal and external surfaces of metals.

Methods used

            There are two basic types of tests carried out on welded joints. They are as follows :

(a)       Destructive tests

(b)       Non-destructive test

Destructive tests

In destructive tests, the specimens of the weld material or welded joint are tested until failure occurs in order to determine their maximum strength. The tests are those which are used for any metals.

(a)       A tensile test in which the mean tensile strength must be at least 400

MN/m².

(b)       A bend test in which the specimen must be bent through an angle of

90˚ with an internal radius of 4 times the thickness of the specimen, without cracking at the edges.

(c)        An impact test in which the specimen must absorb at least 47 Joules at about 20˚C.

Any deep penetration electrodes must show the extent of penetration by cutting through welded section and etching the outline of the weld by means of dilute hydrochloride acid. This test may be carried out on any form of welded joint.

Types of electrode, plates and joints may be tested at regular intervals to ensure that they are maintained at the required standard, while new materials may be checked before being issued for general use. The destructive testing of production work is very limited since it simply determines the strength of the joint before it was destroyed by the removal of the test piece.

Non- destructive tests

            Visual inspection of welded joints is most important in order to ensure that there are no obvious surface faults such as cracks and undercut, and to check the leg length and throat thickness of fillet welds.

            For internal inspection of shipyard welds, radiography is used in the form of  x-rays or gamma rays, the former being the most common. Radiographs are taken of important butt welds by passing the rays through the plate on to a photographic plate. Any differences in the density of the plate allow greater exposure of the plate and may be readily seen when developed. Such differences are caused by faults which have the effect of reducing the thickness of the plate. In way of such faults it is necessary to take x-rays at two angles. The resultant films are inserted in a stereoscope which gives the illusion of the third dimension. It is not possible to test fillet welds by means of radiography. It is usual to take 400 to 500 x-rays of welded joints, checking highly stressed members, joints in which cracks are common, and work carried out by different welders on the ship.

            Other non-destructive tests are available but are not common in shipbuilding. Surface cracks which are too fine to see even with the aid of a magnifying glass, may be outlined with the aid of a fluorescent penetrant which enters the crack and may be readily seen with aid of ultra-violet light.

Faults at or near the surface of a weld may be revealed by means of magnetic crack detection. An oil containing particles of iron is poured over the weld. A light electric current is passed through the weld. In way of any surface faults a magnetic field will be set up which will create an accumulation of the iron particles. Since the remainder of the iron remains in the oil which runs off, it is easy to see where such faults occur.

  

A more modern system which is being steadily established is the use of ultrasonics. A high frequency electric current causes a quartz crystal to vibrate at a high pitch. The vibrations are transmitted directly through the material being tested. If the material is homogeneous, the vibration is reflected from the opposite surface, converted to an electrical impulse and indicated on an oscilloscope. Any fault in the material, no matter how small, will cause an intermediate reflection which may be noted on the screen. This method is useful in that it will indicate a lamination in a plate which will not be shown on an X-thickness of plating in repair work and avoiding the necessity of drilling through the plate.

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