Full Tube Tensile Yield Strength Testing: Standards, Methods, and Applications

Full tube tensile yield strength testing is a mechanical evaluation method in which a complete tubular specimen—rather than a machined strip or round bar cut from the pipe wall—is loaded in axial tension to determine its yield behavior, ultimate tensile strength, and ductility. Because the original cross-section, wall thickness, curvature, and (in welded products) the weld seam are preserved, this test provides the most realistic representation of how a tube performs in service. It is widely applied in the oil and gas, boiler, pressure vessel, structural, and precision tube industries for product qualification, mill release, and certification by accredited third-party laboratories.

Technical Overview

Full tube tensile yield strength testing (also called full-section or whole-pipe tension testing) evaluates the mechanical properties of a metallic tube by pulling an unmodified tubular specimen to failure in a universal testing machine. Unlike longitudinal strip tests, which require flattening a section of the pipe wall and can introduce bending strain and property distortion, the full-tube method tests the product in its as-manufactured geometry. This is particularly important for small-diameter and thin-wall tubes where strip extraction is impractical, and for welded tubes where the weld, heat-affected zone, and base metal must be loaded together as they are in service.

The primary output of the test is yield strength, expressed in several forms depending on the material behavior and the governing product specification:

• Upper yield strength (ReH) and lower yield strength (ReL), used for materials that exhibit a distinct yield point, such as many carbon and low-alloy steels.
• Proof strength, plastic extension (Rp0.2), the stress at 0.2% non-proportional plastic extension, used for continuous-yielding materials.
• Proof strength, total extension (Rt0.5), the stress at 0.5% total extension (elastic plus plastic), the standard yield definition for oil country tubular goods (OCTG) under API specifications.

The test also yields ultimate tensile strength (Rm) and elongation after fracture (A). Governing test standards include ISO 6892-1 (metallic materials, tensile testing at room temperature), ISO 8496 (tensile testing of tubes in full-section specimens), ASTM A370 (mechanical testing of steel products, including full-section tube tension testing), ASTM E8/E8M, GB/T 228.1 (the Chinese national standard aligned with ISO 6892-1), and JIS Z 2241. Product standards such as API Spec 5CT, API Spec 5L, ASTM A106, ASTM A519, ASTM A312, EN 10216, and GB/T 8163 define the applicable grades, sampling, and acceptance limits.

Testing Samples

Seamless Carbon and Alloy Steel Tubes

Hot-finished or cold-drawn seamless tubes for pressure service, mechanical applications, and hydraulic cylinders are commonly tested in full section. Typical specimen lengths range from 400 mm to 600 mm depending on grip engagement and machine capacity. The tube ends are plugged or fitted with internal mandrels to prevent crushing and ovalization under the gripping force.

Welded Tubes (ERW, LSAW, SSAW)

For electric-resistance welded and submerged-arc welded tubes, full-section testing loads the weld seam, heat-affected zone, and parent metal simultaneously, providing an integrated measure of joint efficiency. Transverse weld tension tests on smaller tubes are often performed with the weld centered in the gauge length.

Stainless Steel and Non-Ferrous Tubes

Austenitic and duplex stainless tubes, as well as copper, copper alloy, titanium, zirconium, and nickel-alloy tubes used in condensers, heat exchangers, and nuclear or aerospace systems, are tested full-section because their thin walls and small diameters preclude strip specimens.

OCTG and Line Pipe

Casing, tubing, coupling stock, and drill pipe are tested per API Spec 5CT; line pipe per API Spec 5L. These specifications rely heavily on the Rt0.5 total-extension yield definition and require testing at defined frequencies during production and during qualification of new manufacturing routes.

Testing Items

A typical full tube tensile test program reports the following parameters:

• Upper yield strength (ReH) and lower yield strength (ReL), MPa
• 0.2% proof strength, plastic extension (Rp0.2), MPa
• 0.5% proof strength, total extension (Rt0.5), MPa
• Ultimate tensile strength (Rm), MPa
• Elongation after fracture (A) on the specified gauge length, or total elongation to fracture for short specimens
• Percentage total extension at maximum force (Agt)
• Yield-to-tensile ratio (Rt0.5/Rm or Rp0.2/Rm) where required by pipeline and OCTG specifications
• Fracture location and appearance (base metal, weld, or heat-affected zone) for welded tubes

Testing Methods

Testing is performed on a calibrated universal testing machine with a load capacity selected so that the expected maximum force falls within the verified measuring range. Key aspects of the procedure include:

• Specimen preparation: The tube is cut to length with square, burr-free ends. Internal steel plugs or stepped mandrels are inserted into both grip sections so that wedge grips clamp onto a rigid section, preventing wall collapse, flattening, and slippage.
• Cross-sectional area determination: The area is calculated from measured outside diameter and wall thickness, or more precisely by the gravimetric method (area = mass / (density × length)) in accordance with ISO 6892-1 and GB/T 228.1, which is preferred for irregular or oval sections.
• Gripping and alignment: Full-section tube fixtures, V-grooved wedge inserts, or hydraulic wedge grips are used with care to minimize bending; coaxiality of the load train directly affects measured yield values.
• Extensometry: Clip-on averaging extensometers, dual-sided extensometers, or non-contact video extensometers are attached directly to the tube surface. Because the specimen is full section, the elastic-plus-plastic total extension must be measured accurately, especially when determining Rt0.5.
• Speed control: Stress or strain rate is controlled per ISO 6892-1 Method A (closed-loop) or Method B, or per ASTM A370/E8. Yield-point determination requires slow, controlled loading through the yield region, typically at a strain rate of approximately 0.00025 s⁻¹ to 0.0025 s⁻¹.
• Temperature conditions: Room-temperature testing is conducted between 10 °C and 35 °C. Elevated-temperature yield testing is performed per ISO 6892-2 or ASTM E21 with high-temperature extensometry when the product specification requires hot properties.
• Data evaluation: Modern software determines ReH/ReL directly from the force-extension curve, and Rp0.2 or Rt0.5 by construction of the specified extension offset parallel to the elastic slope. Results are compared against the minimum (and, where applicable, maximum) values in the governing material standard.

Accredited third-party laboratories performing this testing operate under ISO/IEC 17025 quality systems with traceable load verification and extensometer calibration, ensuring results are acceptable for mill certification, customer release, and regulatory submission.

Applications

Full tube tensile yield strength testing supports a broad range of industrial uses:

• Mill production certification of seamless and welded steel tubes for pressure, boiler, and structural applications
• Oil and gas: casing, tubing, and line pipe grade verification during manufacturing, qualification runs, and surveillance testing
• Pressure vessel, boiler, and heat exchanger tube qualification to ASME, EN, and GB design codes
• Weld procedure and welder qualification for tubular components, including full-section weld tension testing
• Incoming inspection and lot acceptance verification by fabricators and EPC contractors
• Failure analysis, research and development of new alloys, and evaluation of heat treatment or cold-working processes
• Automotive, shipbuilding, and construction applications where drive shafts, columns, and structural hollow sections require verified yield performance

FAQ

Why test the full tube instead of a strip specimen cut from the wall?

Full-section testing preserves the as-manufactured geometry, wall curvature, residual stress distribution, and weld condition. Flattened strip specimens can show altered yield values due to bending strain introduced during flattening, and thin-wall or small-diameter tubes often cannot yield a valid strip specimen at all. Full-tube results therefore correlate better with actual service performance.

What is the difference between Rp0.2 and Rt0.5?

Rp0.2 is the stress corresponding to 0.2% plastic (permanent) extension, determined by offsetting from the elastic slope. Rt0.5 is the stress at 0.5% total extension, which includes both elastic and plastic strain. Rt0.5 is the standard yield metric for OCTG under API Spec 5CT; because it contains the elastic component, the required plastic offset is verified for each grade.

What specimen length and tube size can be tested?

Specimen length is typically 400 mm to 600 mm plus grip engagement, set by the machine and fixture design. Tubes from a few millimeters up to several hundred millimeters in outside diameter can be tested, provided the machine load capacity covers the expected maximum force. Large-diameter heavy-wall pipe is usually tested with strip or round specimens instead.

Which standards should be specified on a purchase order?

Specify the test method (ISO 6892-1 with ISO 8496, ASTM A370, or GB/T 228.1), the yield definition (ReH, Rp0.2, or Rt0.5), and the applicable product standard (for example, API Spec 5CT, ASTM A106, EN 10216, or GB/T 8163) so that sampling frequency, gauge length, and acceptance limits are unambiguous.

How should results be reported?

A complete report includes specimen identification, material grade and heat number, tube dimensions and calculated cross-sectional area, test temperature, testing standard and speed, extensometer class, measured ReH/ReL, Rp0.2 or Rt0.5, Rm, elongation, fracture location, and a statement of conformity to the specified minimum values.