7.1 Tensile Testing

What Tensile Testing Measures

Tensile (tension) testing is the most fundamental mechanical test in welding qualification. A specimen is gripped at both ends and pulled apart under a steadily increasing uniaxial (single-direction) load until it fractures, while a machine records the applied force and the specimen's elongation. The governing test method in the United States is ASTM E8/E8M, Standard Test Methods for Tension Testing of Metallic Materials. From the resulting load-versus-extension data the lab derives four properties the Certified Welding Inspector (CWI) must recognize.

Stress is force divided by the original cross-sectional area (engineering stress), and strain is the change in gauge length divided by the original gauge length. A standard round specimen uses a 2 in (50 mm) gauge length; the 0.2% offset yield is located by drawing a line parallel to the elastic slope but offset 0.2% along the strain axis.

The Stress-Strain Curve

The stress-strain curve is the heart of the test, and the CWI should be able to read it:

1. Elastic region — stress and strain are proportional (Hooke's law, stress = E x strain, where E is the modulus of elasticity, ~29,000,000 psi for steel). The specimen springs back if unloaded.
2. Yield point / yield strength — permanent deformation begins; for steels without a sharp yield point the 0.2% offset value is reported.
3. Plastic region (strain hardening) — the metal deforms permanently while still carrying an increasing load up to the peak.
4. Ultimate tensile strength — the curve's maximum; beyond it, localized necking begins and the cross-section shrinks rapidly.
5. Fracture — the specimen breaks. Engineering stress appears to drop after the UTS because it is calculated on the original area while the real area is shrinking.

Elongation and reduction of area are read after fracture by fitting the broken halves together and re-measuring. High values mean a ductile, forgiving material; low values warn of embrittlement, such as a hardened heat-affected zone (HAZ) or hydrogen damage.

Tensile Specimens for Weld Qualification

Weld qualification uses specimens oriented to test different parts of the joint. The CWI must know which specimen tests what.

AWS D1.1 Requirements and Acceptance

For procedure qualification (PQR) of a complete-joint-penetration groove weld, AWS D1.1 (Clause 6, formerly Clause 4) requires two reduced-section transverse tensile specimens. The reinforcement is machined flush so the specimen is uniform thickness. Acceptance is straightforward but heavily tested:

• Each specimen's tensile strength must be not less than the minimum specified UTS of the base metal (or of the weaker base metal when two grades are joined).
• If the specimen breaks in the base metal outside the weld and HAZ, and the value is at or above the minimum specified UTS, the test passes regardless of fracture location — the joint is simply stronger than the base metal.
• A break below the minimum specified UTS is a failure.

Reading fracture location

Exam trap: transverse tensile acceptance is keyed to the base-metal minimum UTS, not the filler-metal classification. A common distractor is "equal to the filler metal UTS." Two specimens are required for a PQR, and a base-metal break at or above the minimum still passes.

Worked Example and the Inspector's Role

Suppose a PQR joins two plates of ASTM A572 Grade 50 steel, which has a minimum specified UTS of 65 ksi. Two reduced-section transverse tensiles are pulled. Specimen 1 fractures in the base metal at 71.4 ksi; specimen 2 fractures across the fusion line at 66.8 ksi. Both values exceed 65 ksi, so both specimens pass even though they broke in different locations. Had specimen 2 broken in the weld metal at 61 ksi, the joint would be understrength and the PQR would fail, forcing the contractor to revise the welding procedure (filler, preheat, or heat input) and re-test.

The CWI rarely runs the tensile machine but must witness, document, and judge results. Practical responsibilities include:

• Confirming the specimen orientation and dimensions match the code (reduced-section width, machined-flush reinforcement, correct gauge marks).
• Verifying the test machine calibration is current and the loading rate is within ASTM E8 limits, since pulling too fast can inflate the reported strength.
• Recording UTS, fracture location, and fracture character (ductile cup-and-cone versus a flat brittle break) on the PQR.
• Cross-checking the result against the WPS/PQR essential variables — a sudden strength change can signal the wrong filler classification or lost preheat.

Strength versus ductility at a glance

A joint can meet the UTS requirement yet show low elongation, a warning of embrittlement that the inspector should flag for metallurgical review even when the strength number alone passes.