Testing and Inspection of Welds
All types of welded structures from jet engines to metal trash cans are expected to perform some function. The joints comprising these structures must possess some service related capabilities. To test that the required function will be met some tests are conducted. The ideal test is the observance of the structure in actual practice. This is usually not possible. Therefore some tests are made on standard specimens to assess the behaviour of the structure in service. Laboratory tests should be used with caution because the size, configuration, environment, type of loading may not be identical to the actual situation. When selecting a test, its function, time and cost factors should be considered.
10.1 TENSILE PROPERTIES
Tension and bend tests are used to evaluate the breaking strength and ductility of a material and to determine that the material meets the specification requirements. Welding causes changes in the metallurgical structure and mechanical properties of a given material. Tension and bend tests are made to assess the suitability of the welded joint for service and are also used to qualify welding procedures for welders according to specific code requirements. In the following paragraphs tension and bend tests according to AWS specifications will be dicussed.
10.1.1 Tension Tests for base metal
Longitudinal or transverse Test. Specimens oriented parallel to the direction of rolling are designated longitudinal, those oriented at right angles to the rolling direction are called transverse. These tests are conducted on the base material.
10.1.2 Weld Tension Test
The tension test for welds is not like that for the base metal because the weld test section is heterogeneous in nature containing base metal, heat affected zone and weld metal. To obtain correct assessment of the strength and ductility several different tests have to be carried out, using different specimens shown in Fig. 10.1. The following tests are commonly carried out.
All Weld-metal tension test. Specimen locations are shown in Fig. 10.1. The details of the specimen dimensions are shown in Fig. 10.2.
depending on t
Fig. 10.1 Typical test specimens for evaluation of welded joints (dimensions in inch units) ■6.4
(a) Tension test specimens with dimensions in mm
(c) All weld metal tension specimen
Fig. 10.2 Tension test specimens with dimension in mm
Transverse butt-weld test. This test shows that the weld metal is stronger than base metal if the failure occurs in the base metal. It fails to give comparative idea about different types of electrodes. When the weld strength is lower than the base metal, the plastic strain occurs in the weld joint. Ultimate strength is thus obtained but no idea about the joint ductility is obtained from this test. Ideally there is no uniform straining within the specified gauge length and therefore, it is not possible to obtain a reliable measure of yield strength across a welded joint.
Longitudinal-butt-weld test. Here the loading is parallel to the weld axis. It differs from all-weld-metal test in that it contains weld, HAZ and base metal along the gauge length. All these zones must strain equally and simultaneously. Weld metal elongates with the base metal until failure occurs. This test thus provides more information about the composite joint than the transverse test specially when base metal and weld-metal strengths differ significantly.
10.1.3 Tension-shear Test
Fillet weld shear test. Tension-shear tests may be used to evaluate the shear properties of fillet welds. Such tests are usually intended to represent completed joints in weldments and so are prepared using similar procedures. Two basic specimen types, transverse and longitudinal, are employed (see Fig. 10.3).
Of the transverse-shear specimens, double lap specimens are preferred because they are more symmetrical and therefore the stress state under load better approaches pure shear. In the single lap joint, pure shear loading requires special test fixtures to align the specimen or prevent bending, particularly for thick plates where eccentric loading becomes significant. Consequently, single lap specimens are generally not used for plates over 6 mm thick. The data obtained from transverse fillet weld tests are the weld shearing strengths, reported as either load per lineal millimetre of weld or megapascals based on the weld throat.
The longitudinal fillet weld shear test measures the strength of the filled weld when the specimen is loaded parallel to the axis of the weld. The weld shearing strength is reported as load per lineal millimetre of weld for welds which fail.
After welding After machining
Fig. 10.3 Various types of tension-shear specimens