The Tensile Test

In this, the second of three articles on mechanical testing methods of welds we are going to look at the simple Tensile Test.

Tensile Test

The tensile test is used to determine a materials yield strength (YS) and ultimate tensile strength (UTS) or the amount of stress and strain the material can take before it yields and breaks.

The tensile test is one of the many mechanical tests performed while certifying a filler metal under CSA W48 Standards and applicable AWS Specifications, for developing a welding procedure, or to check the strength of a material during a failure analysis.

When performing a welding procedure qualification test under various standards, typically CSA W47.1/CSA W47.2 or certifying filler metals from the vast electrode manufacturers around the world for use in Canada, a test assembly is welded in accordance with parameters from a welding procedure data sheet or manufacturers electrode specifications. Once completed there are a series of mechanical testing to perform, one of which is the tensile test.

The test is conducted by removing a section of the test assembly and subjecting the sample to a pull test in a specialized piece of equipment that will pull the piece apart until it fractures. Figure 1 shows a typical hydraulic tensile testing machine.

Figure 1, Typical tensile testing machine

There are many types and sizes of standard tensile test specimens but the two used in the testing of welds are.

1. All-weld-metal tensile test specimen which is used primarily in the classification of welding consumables and is designed to test the tensile properties of weld deposit made under standard conditions.

2. Transverse weld tensile test specimen which is used in the qualification of welding procedures and is designed to test the strength of the welded joint, including the weld metal, heat affected zone and the base material.

The location of the all-weld-metal specimen relative to the weld is shown in Figure 2a. Here the specimen is taken from the centre or the weld in the test assembly as it is the weld metal that is being tested.

An all-weld-metal test specimen is shown in Figure 2b, the diameter is typically 12.5mm as per CSA W 47.1. When determining the size of the tensile test specimen, one must make sure to reference the appropriate Codes or Standards that the test is being performed to. Specimens with either shouldered or threaded ends can be used depending on the grips in the tensile machine.

Figure 2a, Location of all-weld-metal tensile specimen and Figure 2b, All-weld-metal tensile test specimen with shouldered ends.

The location of the transverse tensile test specimen is taken from a cross-section, transverse to the weld and shown in Figure 3a. This means that the total weld joint which includes both the base material, the heat affected zone (HAZ) and weld metal are tested.

A transverse tensile test specimen is shown in Figure 3b, again taken from the CSA W47.1 Standard

Figure 3a, Location of transverse tensile test specimen and Figure 3b, Dimensions of a typical transverse tensile test specimen (CSA W47.1)

Once the tensile specimen is prepared it is then clamped into the tensile testing machine (Figure 1) and a load is applied to the test specimen and it is pulled apart. An example of a traverse tensile test is shown in Figure 4.

Figure 4, Transverse weld test specimen test method

Depending on the specimens, these machines can monitor the loads and record the information such as the yield point, where the steel behaviour changes from elastic to plastic, and then the ultimate tensile strength before the specimen breaks. This information is displayed on a stress-strain curve as shown in Figure 5 which illustrates a curve for a simple steel.

Figure5, Typical stress-strain curve for steel

Elongation is another measurement taken during the tensile test. Elongation is a measure of the ductility of the metal and is determined by measuring the stretching of the metal as it is pulled apart and conveying this as a percentage of the overall length. Before testing two marks are made on the specimen and the distance between these two-gauge marks is carefully measured. The distance is again measured upon the completion of the test which, with increase in length, is calculated to give a % age elongation. Figure 8 shows an example of the marking before and after the tensile test.

Figure 8, Image of gauge marks and percentage of elongation

When the tensile test is used for either a weld procedure qualification test or for the classification of a welding electrode, the procedure for making the weld and carrying out the test are set out in specific Standards. For example, the AWS A5 series and the CSA W48 Standard, for weld consumable testing, specify the joint geometry, initial plate temperature, maximum interpass temperature, number of passes to be deposited and additional significant details that must be followed.

As an example of requirements that need to be met and which will be evaluated in the all-weld metal tensile test, data for two electrodes is shown in Table 1 below.

Table 1. Mechanical Property Requirements Carbon Steel SMAW Electrodes. Single values being a minimum

Regarding the transverse weld test, for weld procedure qualification, the location of the fracture, whether in the plate or weld zone, is recorded and the fractured surface visually examined for any evidence of weld defects if the fracture occurred in the weld metal itself. This will be recorded in the Procedure Qualification Record (PQR).