There are two main categories of tests: destructive tests in which the parent material or representative samples of the product are tested, and non-destructive tests performed on the finished product.
This is probably the most revealing of the mechanical tests that can be performed upon a specimen of pipe or tubular product material. A longitudinal specimen 1 of known cross sectional area is taken from the material and gripped at each end, and then pulled apart until fracture occurs. By recording the gradually increasing load applied and the extension during loading a Stress-Strain Graph can be plotted.
Brinell Hardness Test. A standard size hardened steel ball is indented into the surface of material by an applied standard load. The diameter of the impression is measured accurately by microscope and converted to a hardness value using tables.
Vickers Diamond Hardness Test. This determines hardness by measuring the impression left in material by a diamond pyramid under a standard load. The impression is accurately measured, and its area calculated. The Vickers Hardness Number is calculated by dividing the load (kg) by the area of impression (mm2).
Rockwell Hardness Test. This determines hardness by measuring the depth to which a diamond cone or hardened steel ball, under specific load, penetrates the material. Two loads are used, a minor load (10 kgf) and then a major load (100 or 150 kgf), the difference in indentation being used by the machine to determine the Rockwell number. The number increases with increasing hardness and is displayed or printed by the machine. Two scales are most frequently used, a B scale with a 100 kgf load and 1.588mm steel ball, and a C scale with a 150 kgf load and diamond cone. A Rockwell superficial hardness machine is used for testing very thin wall thicknesses, the minor load used being 3 kgf and the major load being 15, 30 or 45 kgf. The superficial hardness scales used are then 15T, 30T or 45T with a 1.588mm steel ball, or 15N, 30N or 45N with a diamond cone.
For heavy wall fittings, Hardness Tests across the wall thickness are available on request.
In this type of test, a sample is subjected to sudden force to measure its toughness or resistance to shock.
Charpy Impact Test. In this test a specimen is supported at both ends and subjected to a blow by a Pendulum immediately behind a prepared notch, either ‘U’ or ‘V’ shape in cross section. The energy absorbed in fracturing the specimen is measured by the height to which the pendulum rises after breaking the test piece. These tests can be carried out at various temperatures to determine the performance of material at either elevated or cryogenic temperatures.
At higher temperature specimens fracture by a ductile mechanism, absorbing much energy. At low temperatures they fracture in a brittle manner absorbing less energy. Within the transition range a mixture of ductile and brittle fracture is observed. Minimum test results for absorbed energy, fracture appearance, lateral expansion or a combination of these, may be specified.
These tests prove the ductility of certain tubular products and confirm the soundness of welds.
Bend Tests. A bend test involves bending a sufficient length of full size pipe through 90° or 180° degrees around a mandrel having 12 or 8 times the nominal pipe diameter. This checks the ductility and weld soundness of pipe (2 in and under) used for coiling. Transverse guided bend tests may also be specified to check the ductility of fusion welds. These involve bending the root or face of the weld in a specimen against a plunger.
Flange Test. This tests the ability of boiler tubes to withstand bending into a tube sheet. It involves the tube having a flange turned over a right angles to the tube body.
Flattening Test. This is usually applied to tube and involves flattening a sample of tube between two parallel faces without the tube showing flaws or cracks. The length of the test piece and degree to which it is to be flattened (i.e. the distance between the parallel faces) are specified.
Flare or Drift Test. This is an alternative to the flange test for certain types of pressure tube. A cone is forced into the end of the tube. The end of the tube is expanded by a specified increase in diameter without splits or cracks. The included angle of drift is also specified.
Various corrosion tests are available using different corrosive environments to indicate the performance of material under heavy duty applications.
Weld Decay Test. This test detects inter-crystalline corrosion and involves the use of boiling copper sulphate/ sulphuric acid solution. Test samples are first sensitised and then immersed in the solution for 72 hours. After the immersion the samples are bent through 90 degrees and are considered satisfactory if no cracks are present.
Strauss Test. This test detects inter-crystalline corrosion and involves the use of boiling copper sulphate / sulphuric acid solution which must contain solid electrolytic copper. The test samples are immersed in the solution for 15 hours. After immersion the samples are bent through 90 degrees and are considered resistant to inter-crystalline corrosion if they bend without cracking.
Huey Test. This test detects the susceptibility of a material to intergranular attack and involves the use of boiling nitric acid. The test samples are immersed in the solution at a concentration of 65% by weight for five 48 hour periods. The effect of the acid on the material is measured by the loss in weight after each period and the corrosion rate assessed as a thickness loss in a given time.
Potentiostat Test This is a method of determining the corrosion properties of stainless steel by producing polarisation curves which relate electrode potentials and a current flow. The shapes of the curves, which are very sensitive to microstructure and composition, provide a critical method of assessing the corrosion properties of stainless steel.
Other tests are available to meet Shell, BP, Exxon Mobil, Norsk and many other companies’ regulations
Non-destructive tests do not damage the material or product being tested. Frequently they are built into production processes, as is the case with pipe tested using eddy current equipment.
Ultrasonic Testing. This test involves ultrasonic sound waves being aimed, via a coupling medium, at the material to be tested. A proportion are bounced back at the interface but the remainder enter the material and bounce from the internal surface, to the external surface, where a transducer converts them into electrical energy. This is the monitored on a cathode ray tube where results are compared with those from a calibration standard. Any deviations from the standard are visible, thus indicating cracks or internal defects.
Eddy-Current Testing. This involves inducing eddy currents into the material by exciting a coil which surmounts two narrow search coils surrounding the material. Any discontinuities in material are found by comparing the electrical conditions that exist in the two search coils. The fault signals are amplified and can be shown on a cathode ray tube or as an audible signal.
Hydrostatic Testing. This is used to test the manufactured items under a pressure equivalent to or greater than pressure to be encountered in service. It involves filling the tube with water, which cannot be compressed, and increasing the pressure inside the tube to that specified.
Magnetic Particle Testing. This method of testing is used when trying to detect discontinuities in material of ferromagnetic structure. The method is based on the principle that an imperfection will cause a distortion in the magnetic field pattern of a magnetised component. The imperfection can be revealed by applying magnetic particles to the component during or after magnetisation.
Radiographic (X-Ray) Testing. This is usually used to determine whether a weld is sound. It involves subjecting a weld or weld area to an X-Ray source with an X-Ray sensitive film plate on the under side of the weld. The results are shown on the developed film (a photomicrograph) and interpreted according to specification.
Dye-Penetrant Test. This is used to detect cracks and involves spraying a dye on the area to be tested. After allowing time for penetration the surplus dye is removed and the area is then sprayed with a white developer. Any faults are revealed as coloured lines or spots caused by the developer absorbing the dye seeping from the cracks. If more sensitive results are required, a fluorescent dye is used and the same process is followed. When viewed under ultraviolet light any defects show as a highly fluorescent line or spot.