Mechanical Testing of Welds

• Module 12 is about the mechanical testing of welds.
• Mechanical testing is performed to verify connection performance, certify welding consumables, and qualify welders and welding procedures.
• Topics covered include stress and strain, tensile tests, code requirements for tensile and bend tests, nick-break and fillet weld break tests, hardness tests, impact tests, fracture mechanics tests, proof tests, and fatigue tests.
• Standardized methods are important for meaningful data in mechanical testing, and reference should be made to the standard to ensure proper procedure.

Stress and Strain

• Stress is the intensity of internal forces within a metal resisting an external force (load).
• Stress is calculated as the load divided by the cross-sectional area.
• In the metric system, load is measured in newtons (N) and area in square meters (m²), stress is measured in pascals (Pa) or megapascals (MPa). 1 MPa = 1 N/mm².
• In the Imperial system, load is measured in pounds (lb) or thousands of pounds (kips), area in square inches, and stress in pounds per square inch (psi) or thousands of pounds per square inch (ksi). To convert ksi to MPa, multiply by 6.895.
• There are three ways in which a load can act on a cross-sectional area in a body: tension, compression, and shear.
• Strain is the measurement of the change in dimensions due to an applied load. It is calculated as the extension divided by the original length.
• Strain is dimensionless but often expressed in units like "mm per mm" or as a percentage.
• Stress and strain both occur in the axial direction when a bar is loaded in tension. The specimen also becomes thinner and suffers a strain in the transverse direction.
• Plane stress is a condition with stress in only one direction but strains in other directions, typical in thin sections. Plane strain is encountered when no thinning occurs and is typical in thick sections.
• Elastic deformation occurs when a metal deforms under stress but returns to its original dimensions when the stress is removed. The stress at which permanent deformation begins is known as the elastic limit.
• The Modulus of Elasticity (E) or Young's modulus characterizes the stiffness of a material and is the slope of the stress-strain line in the elastic region.
• Hooke's Law: σ = Eε, relates stress, strain, and the Modulus of Elasticity.
• Plastic deformation is permanent deformation that remains after the load is removed.

Yield Point and Yield Strength

• The permanent set is the strain that remains after a bar has been stressed in tension and then unloaded.
• For some metals, the transition from elastic to plastic behavior is gradual, while some steels show an abrupt change.
• In some steels, there is an abrupt change from elastic to plastic behaviour and the stress-strain curve displays an upper yield point followed by a drop in stress and a yield strain.
• In metals with a smooth change, the yield strength is defined as the stress at which a given permanent set is recorded (usually 0.2%).
• The yield strength is determined from a stress-strain curve by drawing a line parallel to the elastic line offset by 0.2% strain.

Tensile Test

• If loading continues, the specimen will eventually break, stress rises to a maximum value known as Ultimate Tensile Strength (UTS).
• Specimen begins to become thinner at some location, known as necking. This starts when the UTS is exceeded. From there deformation concentrates in the necked region until it fractures.
• Stress is the load divided by the original cross-sectional area, but the true stress at the neck is higher because the cross-sectional area decreases.
• Ultimate tensile strength (UTS) is the maximum load applied divided by the original cross-sectional area, and yield strength (YS) is the load at the yield point divided by the area. These are expressed mathematically as: UTS = Pu / A and YS = Py / A.
• Elongation measures the stretch of the metal expressed as a percentage of the original length.
• % Elongation = (L₂ - L₁) / L₁ x 100 expressed as a percentage.
• The deformation in a tensile specimen is localized, make the elongation value meaningless unless the gauge length is indicated
• Reduction of area measures the decrease in cross-sectional area after necking during the tensile test.
• The reduction of area, R, is calculated as: R = (A₁ - A₂) / A₁ x 100.
• Tensile tests require special machines to exert force in a controlled manner. These are either mechanical or hydraulic, both with one fixed and one moving crosshead.
• Mechanical machines apply force via a screw driven by a motor. Hydraulic machines apply force via a hydraulic ram.
• Modern machines use load cells to record load and provide an electrical output.
• If it is desired to measure the strain during a test rather after completion, an extensometer is placed on the specimen.
• The temperature at which tensile testing is performed will affect the outcome of the test.
• Two types of specimens are all-weld-metal tensile test specimens and transverse weld test specimens.
• All-weld-metal tensile test specimens are primarily used in the classification of welding consumables and are designed to test the tensile properties of a weld deposit.
• The transverse weld test specimen is used in the qualification of welding procedures and is designed to test the strength of the welded joint as a whole.

All-Weld-Metal Specimens

• All required data is obtained from the load and the initial and final dimensions of the specimen.

Transverse Weld Test Specimens

• Specimens have a square or rectangular cross-section and are prepared transverse to the weld to evaluate both the base material and weld metal.
• The specimen thickness is equal to that of the plates being welded, and only the weld reinforcement is removed.
• CSA W47.1 specifies single full thickness specimens for plates up to 38 mm thick and multiple specimens for thicker plates.
• ASME Section IX also allows multiple specimens but, instead of taking the average of the test results, each specimen's result is considered individually.

Soundness Tests

• Soundness tests are used to determine the quality of a weld.

Bend Test

• The bending of higher strength steels requires a larger radius to reduce applied strain, and the strain applied to the specimen depends on the spacing of rollers and the radius of the plunger.