Low Temperature Impact Testing: Principles, Standards, and Applications

Low temperature impact testing is an essential materials evaluation method used to determine how metals, plastics, and composites resist sudden impact loads in cold service environments. By measuring the energy a notched specimen absorbs before fracturing at sub-zero temperatures, this test helps manufacturers prevent catastrophic brittle failures in pipelines, pressure vessels, cryogenic equipment, and cold-climate structures. This technical guide explains the principles, sample requirements, testing procedures, evaluation items, applicable standards, and industrial applications of low temperature impact testing.

Technical Overview

Low temperature impact testing, most commonly performed as a low-temperature Charpy or Izod impact test, measures the ability of a material to absorb energy during fracture at reduced temperatures. Many engineering materials, especially body-centered cubic (BCC) metals such as ferritic and pearlitic steels, undergo a ductile-to-brittle transition as the temperature decreases. Above the transition range, the material deforms plastically and absorbs substantial energy before failure; below it, fracture occurs suddenly by cleavage with very little energy absorption, even at nominal stress levels well below the yield strength.

The test procedure involves machining a notched specimen, cooling it to a specified sub-zero temperature in a controlled medium, and breaking it with a calibrated pendulum impact machine. The absorbed energy, fracture appearance, and lateral expansion are then recorded and compared against the acceptance criteria specified in product standards and design codes. Major design codes such as ASME Section VIII, EN 13445, and API 5L require impact verification at or below the minimum design metal temperature (MDMT) to ensure safe operation in cold service.

• ASTM E23 – Notched Bar Impact Testing of Metallic Materials (Charpy V-notch)
• ISO 148-1 – Metallic Materials, Charpy Pendulum Impact Test
• ASTM D256 / ISO 180 / ISO 179 – Izod and Charpy Impact Testing of Plastics
• ASTM A370 / ASTM A320 / ASTM A350 – Product-Level Impact and Low-Temperature Requirements
• GB/T 229 – Chinese Standard for Metallic Charpy Pendulum Impact Testing

Testing Samples

Low temperature impact testing can be applied to a wide range of engineering materials. Specimen geometry, notch orientation, and sampling location must be strictly controlled because they directly influence the measured toughness.

Metallic Materials

• Carbon and low-alloy steels, including pressure vessel plate (ASTM A516), forgings (ASTM A350 LF2), and castings (ASTM A352 LCC)
• Pipeline steels in accordance with API 5L, tested at the specified minimum service temperature
• Ferritic, martensitic, and duplex stainless steels, which exhibit a measurable ductile-to-brittle transition
• Cast irons, aluminum alloys, titanium alloys, and nickel-based alloys
• Austenitic stainless steels and aluminum alloys, typically tested for qualification even though they retain high toughness at cryogenic temperatures

Welded Joints

Weld metal, fusion line, and heat-affected zone (HAZ) samples are tested during welding procedure qualification to ensure the joining process does not degrade low-temperature toughness. The notch is positioned at the weld centerline, at the fusion line, or at defined offsets (for example, 1 mm and 2 mm from the fusion line) in accordance with EN ISO 9016, ASME Section IX, or AWS D1.1 requirements.

Plastics, Rubbers, and Composites

Thermoplastics such as ABS, polycarbonate, nylon, PVC, polypropylene, and polyethylene, as well as fiber-reinforced composites and elastomeric components, are evaluated using Izod or Charpy configurations after low-temperature conditioning in accordance with ASTM D256, ISO 179, and ISO 180.

Standard Specimen Geometry

The standard Charpy specimen for metals measures 10 mm × 10 mm × 55 mm with a 2 mm deep V-notch having a 45° included angle and a 0.25 mm root radius. Sub-size specimens of 7.5 mm, 5.0 mm, or 2.5 mm thickness are used when full-size bars cannot be extracted, such as from thin-walled pipe or small forgings. U-notch specimens (5 mm deep notch with 1 mm root radius) are used for the KU configuration. Notch orientation relative to the rolling or working direction (transverse or longitudinal) must be reported because it significantly affects the result.

Testing Items

The test delivers several quantitative and qualitative measures of low-temperature toughness:

• Impact absorbed energy (KV2, KV8, or KU2), expressed in joules, representing the total energy consumed during fracture
• Ductile-to-brittle transition temperature (DBTT), the temperature at which toughness drops sharply between upper-shelf and lower-shelf behavior
• Shear fracture appearance (SFA), the percentage of ductile, fibrous fracture area on the broken surface, often used with a 50 percent criterion (FATT50)
• Lateral expansion, measured in millimeters, indicating the plastic deformation capacity adjacent to the notch
• Fracture mode characterization, distinguishing ductile dimpled fracture, brittle cleavage fracture, or mixed modes
• Full transition curve determination, generated by testing replicate specimens at a series of temperatures
• Instrumented test results, separating crack initiation energy from propagation energy via force-displacement records

Testing Methods

The reliability of low temperature impact testing depends on precise temperature control, correct specimen handling, and calibrated impact equipment. The general workflow is: machine and inspect specimens, cool to the target temperature, soak to achieve thermal uniformity, transfer rapidly to the machine, and fracture the specimen while recording absorbed energy.

Cooling and Temperature Control

Specimens are cooled to the specified test temperature using one of the following media, selected according to the target temperature:

• Liquid nitrogen, used directly or in programmable spray chambers, for temperatures down to −196 °C
• Dry ice (solid CO2) mixed with ethanol or acetone, achieving approximately −78 °C
• Methanol or ethanol baths circulated through mechanical refrigeration units for precise intermediate temperatures
• Ice, water, and salt mixtures for mild sub-zero temperatures near −5 °C to −20 °C

The bath or chamber temperature must be maintained within approximately ±1 to ±2 °C of the specified value. Specimens are soaked for a minimum of five minutes in agitated liquid media per ASTM E23, and for substantially longer periods in gas-cooled chambers, to guarantee that the material at the notch root has fully reached the test temperature.

Transfer and Fracture

After soaking, the specimen is removed with insulated tongs or an automatic centering device and placed on the anvil supports with the notch positioned away from the striking edge. Per ASTM E23, the specimen must be broken within five seconds of removal from the cooling medium to prevent the notch temperature from rising. Common pendulum capacities for steels are 300 J and 450 J, while low-energy capacities are used for aluminum alloys and plastics. Instrumented strikers with strain gauges can additionally record the force-displacement curve throughout the fracture event.

Transition Temperature Determination

To determine the ductile-to-brittle transition behavior, a series of specimens is tested at stepped temperatures, for example from +20 °C down to −196 °C. Absorbed energy and shear fracture appearance are plotted against temperature to construct the transition curve. Common criteria include the fixed-energy criterion (such as 27 J average absorbed energy widely used for pipeline steels), the 50 percent shear appearance temperature, and the mid-energy transition temperature. For advanced structural integrity assessments, the Master Curve method (ASTM E1921) may be applied to derive a fracture toughness reference temperature.

Applications

Low temperature impact testing is mandatory or highly recommended across many industries where components operate in cold or cryogenic environments:

• Oil and gas pipelines in Arctic and subarctic regions, where API 5L PSL2 grades must demonstrate adequate toughness at design temperatures
• LNG storage tanks, piping, and carriers operating near −162 °C, using materials such as 9% nickel steel, 5083 aluminum, and austenitic stainless steel
• Cryogenic pressure vessels and equipment for liquid nitrogen, oxygen, and argon service at approximately −196 °C
• Offshore platforms and marine structures in cold seas, evaluated under DNV and NORSOK requirements
• Automotive chassis, body, and fastener components exposed to severe winter conditions
• Aerospace hardware, including cryogenic fuel system components and landing gear operating at high-altitude temperatures
• Structural steelwork for bridges, towers, and buildings in cold-climate regions
• Low-temperature bolting and fasteners under ASTM A320, such as grade L7 for −101 °C service
• Welding procedure qualification and routine quality control of incoming plate, forging, and casting lots

FAQ

What is the purpose of low temperature impact testing?

The test verifies that a material, weld, or component possesses sufficient toughness to resist brittle fracture at its lowest expected service temperature. It is a key acceptance requirement in pressure vessel, pipeline, offshore, and structural codes because brittle failures propagate extremely rapidly and can be catastrophic.

Why do materials become brittle at low temperatures?

In BCC metals such as carbon steel, dislocation motion becomes increasingly difficult as temperature drops, causing the yield strength to rise sharply. Once the yield stress exceeds the cleavage fracture stress, the material fails in a brittle manner with little plastic deformation and low energy absorption. Face-centered cubic (FCC) materials such as austenitic stainless steel, aluminum, and copper do not exhibit this transition and retain high toughness down to cryogenic temperatures.

What test temperatures are commonly specified?

Typical specification temperatures include −20 °C, −30 °C, −40 °C, −46 °C, −50 °C, −60 °C, −78 °C, −101 °C, and −196 °C. The selected temperature is normally at or below the minimum design metal temperature defined by the applicable product standard or design code.

How quickly must the specimen be broken after removal from the cooling medium?

ASTM E23 requires the specimen to be broken within five seconds of leaving the cooling medium. Automated specimen transfer and centering devices help ensure compliance and improve repeatability.

What acceptance criteria are typically applied?

Acceptance criteria are defined in product specifications and commonly specify both a minimum average absorbed energy for a set of three specimens and a minimum single-value energy. A frequently encountered pipeline criterion, for example, is a 27 J average, with correspondingly lower minimum single values. Shear fracture appearance and lateral expansion requirements may be added for critical applications.

Can sub-size specimens be used when material thickness is limited?

Yes. When full-size 10 mm × 10 mm bars cannot be prepared, sub-size specimens of 7.5 mm, 5 mm, or 2.5 mm thickness may be tested, and the acceptance energy is reduced in proportion to the cross-sectional area, following the rules of the applicable standard.