Full-tube low-temperature tensile strength testing evaluates the tensile performance of complete pipe and tubing sections under sub-zero conditions, providing critical data for cryogenic, refrigeration, and cold-climate service environments. By testing the entire tube cross-section—including wall thickness, weld seams, and as-manufactured geometry—this method delivers a realistic assessment of structural integrity at low temperatures. This article covers the test principle, applicable standards, sample requirements, test procedures, typical results, and industry applications.
Technical Overview
Full-tube low-temperature tensile strength testing is a mechanical evaluation method in which an entire section of pipe or tubing, rather than a machined coupon, is subjected to uniaxial tension inside a cooled environmental chamber until fracture occurs. The purpose of the test is to determine how the material behaves when exposed to sub-zero service temperatures, at which many metals become less ductile and many polymers approach their brittle-ductile transition range.
Testing the whole tube instead of a strip or dog-bone specimen offers several advantages. First, it preserves the as-manufactured condition of the product, including circumferential wall-thickness variation, ovality, surface finish, and longitudinal weld seams in welded tubing. Second, for small-diameter or thin-walled tubes, machining standard flat specimens is often impractical or impossible without destroying the representative geometry. Third, the full-section result reflects the true load-bearing capacity of the tube as installed, making it directly relevant to engineering design and product qualification.
The test is governed by recognized standards for metallic and polymeric materials, including ISO 6892-3 (metallic materials — tensile testing at low temperature), ASTM E8/E8M (tension testing of metallic materials), GB/T 13239 (tensile testing of metallic materials at low temperature), and, for plastic piping, ISO 6259 and ASTM D2290. Product-specific specifications such as ASTM B88, ASTM B280, and EN 1057 for copper tube may also define acceptance criteria for full-section tension testing.
Testing Samples
Metallic Tubes
Copper, brass, carbon steel, stainless steel, aluminum, titanium, and nickel-alloy tubes are commonly tested in full section, particularly when the diameter is too small or the wall too thin for specimen machining. Typical specimen lengths range from 300 to 600 mm, depending on the grip design and available gauge length. Seamless and welded tubes are both suitable, and welded products are often oriented so that the seam lies within the gauge region to evaluate weld integrity under load.
Polymeric Pipes and Tubing
Thermoplastic pipes made from polyethylene (PE), polypropylene (PP), PVC, PEX, polyamide (PA), and fluoropolymers are tested as full tubes to assess ductility retention at low temperature. Because polymers stiffen and lose elongation capacity as temperature drops, whole-tube testing at the rated minimum service temperature is a key qualification step for outdoor, buried, and refrigeration piping systems.
Medical and Specialty Tubing
Catheter shafts, intravascular tubing, and other fine-bore polymer or metal tubes are frequently qualified using full-tube tensile testing, sometimes under cold conditions, to confirm joint strength, material consistency, and resistance to brittle fracture. Sample preparation for these products typically follows the length and grip requirements of the applicable device material standard.
Specimen Preparation
- Cut tube sections to the specified length with square, burr-free ends to ensure uniform load introduction.
- Insert hardened plugs or tapered mandrels into the tube ends to prevent grip crushing and slippage.
- Mark the gauge length on the specimen surface with low-temperature-resistant ink or a light scribe.
- Measure and record outside diameter, wall thickness, and cross-sectional area at multiple locations before conditioning.
- Inspect for shipping damage, dents, or ovality that could act as premature stress concentrators.
Testing Items
The principal parameters determined from a full-tube low-temperature tensile test include:
- Ultimate tensile strength (Rm): the maximum load divided by the original full cross-sectional area of the tube.
- Yield strength (ReH or Rp0.2): the stress at which plastic deformation begins, measured where the material exhibits a defined yield point or via 0.2 percent proof stress.
- Elongation after fracture (A): the percentage of gauge-length extension at break, a direct indicator of low-temperature ductility.
- Fracture location and mode: whether failure occurs in the base material, the weld seam, or the grip region, and whether the fracture is ductile or brittle in appearance.
- Load-displacement curve: the complete record of force versus extension, used to detect anomalies such as serrated yielding or pop-in events at low temperature.
- Comparison data: strength and ductility values compared against room-temperature results to quantify the temperature effect on the material.
Testing Methods
Temperature Conditioning and Environmental Control
Specimens are tested inside an environmental chamber or cryostat attached to a universal testing machine. Cooling is achieved with liquid nitrogen, dry-ice and alcohol baths, or mechanically refrigerated circulators, depending on the target temperature. Common test temperatures include 0 °C, −20 °C, −40 °C, −70 °C, −100 °C, and, for cryogenic applications, −196 °C. The chamber must maintain temperature uniformity within ±2 to ±3 °C, and the specimen is soaked at the set point—typically 15 to 30 minutes for metals and longer for polymers with lower thermal conductivity—before loading begins. Thermocouples are attached directly to the specimen surface to verify the actual specimen temperature rather than the chamber air temperature.
Gripping and Load Application
Full tubes are gripped using wedge grips, collet grips, or specialized clamp fixtures fitted with plugs or mandrels at the tube ends to distribute the clamping force and prevent crushing. After thermal equilibrium is confirmed, the machine applies tension at a controlled crosshead speed or strain rate consistent with the governing standard. For metals, strain rates are usually kept low to avoid adiabatic heating effects; for plastics, the speed specified by ISO 6259 or the product standard is applied. Elongation is measured either by a clip-on extensometer rated for low-temperature use or by crosshead displacement correction, depending on the required accuracy.
Data Acquisition and Evaluation
Force and extension data are recorded continuously until specimen fracture. The software computes tensile strength from the maximum load and the measured cross-sectional area, determines yield properties from the curve, and calculates elongation from the marked gauge length measured after reassembling the fractured pieces. Fractured specimens are examined visually to classify the failure mode and confirm that fracture occurred within the gauge length; failures at the grips are normally rejected and retested. The final report documents the test temperature, conditioning time, cross-sectional dimensions, calculated properties, fracture description, and full load-displacement curves.
Applications
- LNG, gas processing, and cryogenic distribution lines, where tubes must retain strength and toughness at temperatures down to −196 °C.
- Refrigeration and HVAC systems, in which copper and aluminum tubing experiences cyclic low-temperature service.
- Arctic and cold-climate oil and gas facilities, where low-temperature ductility is a code requirement for pressure-containing components.
- Aerospace hydraulic and fuel lines, which are qualified at high-altitude and low-ambient temperature extremes.
- Automotive brake, fuel, and EV thermal-management lines that must withstand winter operating conditions.
- Plastic piping for outdoor, buried, and snow-melt applications, where low-temperature elongation and impact resistance are validated.
- Medical device tubing and catheter shafts, where full-tube tensile qualification supports biocompatibility and performance files.
- Factory quality control and type approval, where full-section testing verifies heat-lot compliance before shipment.
FAQ
Why test the whole tube instead of a cut specimen?
Whole-tube testing preserves the manufactured geometry, wall thickness distribution, and weld condition of the product, and it is often the only feasible option for small-diameter or thin-walled tubing. The result reflects real installed performance more closely than a machined coupon.
What temperatures are typically used?
Common set points range from −20 °C to −70 °C for general cold-service qualification, with −100 °C and −196 °C used for cryogenic and LNG applications. The specific temperature is chosen to match or exceed the minimum design service temperature of the product.
How is the tube prevented from being crushed by the grips?
Hardened plugs, tapered mandrels, or internal supports are inserted into the tube ends so that the gripping force is carried in compression rather than collapsing the wall. Collet-type grips with matching inserts are also used for fine tubing.
Which standards govern the test?
Metallic tubes are tested to ISO 6892-3, ASTM E8/E8M, or GB/T 13239 as modified for full-section specimens; plastic pipes follow ISO 6259 or ASTM D2290; and product specifications such as ASTM B88, ASTM B280, and EN 1057 define additional acceptance requirements.
How long does the specimen need to be held at temperature?
A soak of 15 to 30 minutes after the specimen reaches the set point is typical for metals, while polymers may require longer conditioning because of their lower thermal conductivity. The specimen surface temperature, not the chamber reading, is the acceptance criterion.
What results are reported?
The test report includes tensile strength, yield strength where applicable, elongation after fracture, fracture location and appearance, test temperature and conditioning details, specimen dimensions and cross-sectional area, and the complete load-displacement curve for each specimen tested.
