The low-temperature Charpy impact test is a standardized high strain-rate test that measures the energy absorbed by a notched metallic specimen when fractured by a swinging pendulum at sub-zero temperatures. It is the primary method for evaluating the notch toughness of steels in cold service environments and for determining the ductile-to-brittle transition behavior that governs the fracture-safe design of pressure vessels, pipelines, offshore structures, and cryogenic equipment.
Technical Overview
The Charpy impact test, also known as the Charpy V-notch test, quantifies the resistance of a material to brittle fracture under dynamic loading. A notched bar specimen is struck by a calibrated pendulum hammer, and the absorbed energy, expressed in joules, is calculated from the difference between the initial and final pendulum heights. In the low-temperature variant, the specimen is cooled to a specified sub-zero temperature before fracture, allowing engineers to characterize how toughness degrades as the service temperature drops.
Ferritic and pearlitic steels with a body-centered cubic crystal structure undergo a ductile-to-brittle transition at low temperatures. Above the transition region, these materials absorb large amounts of energy and fail in a ductile mode with a fibrous, dull fracture surface. Below it, they may fracture in a brittle, cleavage mode with a crystalline, bright fracture surface and very low absorbed energy, often below 10 joules. The low-temperature Charpy test is therefore essential for verifying that a material retains sufficient toughness at its minimum design metal temperature.
The test is governed by internationally recognized standards, including:
- ASTM E23 - Standard Test Methods for Notched Bar Impact Testing of Metallic Materials
- ISO 148-1 - Metallic Materials - Charpy Pendulum Impact Test - Part 1: Test Method
- EN ISO 148-1 - European adoption of the ISO standard
- GB/T 229 - Chinese national standard, technically aligned with ISO 148
- JIS Z 2242 - Japanese standard for the Charpy impact test method
Testing Samples
Standard Full-Size V-Notch Specimens
The standard specimen measures 10 mm × 10 mm × 55 mm and contains a V-shaped notch with a 45-degree included angle, a 2 mm depth, and a 0.25 mm root radius machined at mid-length. The V-notch concentrates stress and triaxial constraint at the root, promoting fracture initiation and making the test sensitive to brittle behavior. Absorbed energy from this geometry is designated KV2 when fractured with a 2 mm striker radius.
U-Notch Specimens
Some legacy specifications and product standards call for specimens with a U-shaped notch, typically 2 mm or 5 mm deep with a 1 mm root radius. Results are reported as KU2. U-notch data are not directly interchangeable with V-notch data, so the notch geometry specified by the governing product standard must always be followed.
Sub-Size Specimens
When material thickness or component geometry prevents machining of full-size bars, sub-size specimens of 7.5 mm or 5 mm thickness may be tested. Because absorbed energy scales roughly with cross-sectional area, specifications often compensate by mandating lower test temperatures or proportionally reduced energy acceptance values for sub-size testing.
Welded Joint Specimens
For welded fabrications, Charpy specimens are extracted transverse to the weld with the notch positioned at specific locations, such as the weld metal centerline, the fusion line, or defined offsets into the heat-affected zone (typically fusion line plus 1 mm, 2 mm, and 5 mm). Welding procedure qualification standards such as ISO 15614-1 and ASME Section IX use these results to confirm that the entire welded joint, not just the base metal, meets toughness requirements at the design temperature.
Materials Commonly Tested
- Carbon and carbon-manganese steels for structural and pressure applications
- Low-alloy high-strength steels and quenched-and-tempered grades
- Pipeline steels, including API 5L grades for sour and arctic service
- Austenitic, duplex, and ferritic stainless steels
- Nickel-alloy cryogenic steels such as 9% nickel steel for LNG service
- Aluminum alloys, titanium alloys, castings, forgings, and bolting materials
Testing Items
Low-temperature Charpy testing programs typically report the following results:
- Absorbed energy (KV2 or KV8) in joules for each individual specimen and the average of the set
- Lateral expansion at the compression side of the fractured specimen, in millimeters, as a measure of ductility
- Percent shear fracture appearance, estimated from the proportion of ductile fibrous area on the fracture surface
- Brittle or ductile fracture mode classification for each specimen
- Upper-shelf energy and lower-shelf energy when a full transition curve is generated
- Ductile-to-brittle transition temperature (DBTT) determined at a specified criterion, such as 27 J, half upper-shelf energy, or 50% shear appearance (FATT50)
- Instrumented force-displacement curves separating crack initiation and propagation energy, when instrumented strikers are fitted
Testing Methods
Specimen Preparation and Notching
Specimens are machined with tight tolerances on notch angle, depth, and root radius, since small geometric errors significantly influence absorbed energy. Notches are produced by milling or broaching, and grinding marks perpendicular to the notch root are avoided. For hardenable steels, machining must not alter the material through overheating or work hardening, and orientation relative to the rolling direction (longitudinal or transverse) is recorded because it strongly affects results.
Cooling Media and Temperature Control
Specimens are conditioned to the target temperature in a controlled bath or chamber using media appropriate to the required temperature:
- Ice and water slush for testing at 0°C
- Various salt-ice mixtures or programmable refrigerated baths for temperatures between 0°C and about -70°C
- Dry ice in ethanol or acetone for approximately -78°C
- Liquid nitrogen for -196°C, used directly or as a coolant for silicone oil or alcohol baths at intermediate temperatures
- Mechanical cryostats with electrically controlled chambers for precise, stable conditioning at any set point
The bath temperature is monitored with a calibrated thermocouple or platinum resistance thermometer placed adjacent to the specimens. Specimens are held at temperature, typically within ±2°C, for a minimum of 5 minutes in liquid media or 30 minutes in gaseous media to guarantee thermal equilibrium throughout the bar.
Specimen Transfer and Break Timing
After removal from the cooling medium, the specimen warms rapidly, so standards require it to be positioned on the anvil and broken within 5 seconds of removal. Pre-cooled tongs or automated specimen extraction and centering devices are used to minimize heat transfer from handling and to guarantee repeatable alignment of the notch with the striker path.
Machine Requirements and Calibration
The pendulum impact machine, with typical capacities of 300 J or 450 J, is verified periodically against reference specimens certified by a recognized standards body, in accordance with ASTM E23 and ISO 148-2. Anvil spacing, striker radius, pendulum center of percussion, and friction losses are all checked, because absorbed energy values are only valid on a machine within its verified energy range.
Transition Curve Determination
To determine the ductile-to-brittle transition temperature, sets of three specimens are tested at a series of temperatures spanning the expected transition region, for example +20°C, 0°C, -20°C, -40°C, -60°C, -80°C, -120°C, and -196°C. Absorbed energy and percent shear are plotted against temperature, and a regression curve is fitted to define the transition temperature at the specified acceptance criterion. Most product specifications and fabrications codes simply require three specimens at a single design temperature to demonstrate a minimum average energy, often 27 J or 40 J depending on the grade and thickness.
Applications
Low-temperature Charpy impact testing is a mandatory acceptance test across many safety-critical industries:
- Pressure vessels and boilers, where ASME Boiler and Pressure Vessel Code impact requirements must be satisfied at the minimum design metal temperature
- Oil and gas pipelines, including API 5L line pipe destined for arctic, offshore, and low-temperature service
- Shipbuilding and offshore structures, where classification societies and standards such as NORSOK require certified impact toughness at service temperatures down to -40°C or lower
- Cryogenic storage and transportation of liquefied gases, including LNG at -162°C and liquid nitrogen at -196°C, using 9% nickel steel, austenitic stainless steel, and aluminum alloys
- Bridge and structural steel governed by specifications such as AASHTO and the Eurocodes, which set toughness classes based on lowest service temperature
- Nuclear power components, where radiation embrittlement surveillance programs track the shift of the transition curve over plant life
- Welding procedure and welder qualification for structures exposed to sub-zero environments
FAQ
Why must Charpy testing be performed at low temperature?
Ferritic steels become progressively less tough as temperature decreases and may shift from ductile to brittle fracture behavior. Testing at the minimum design service temperature proves that the material and its welds will not fail catastrophically under impact or shock loading in cold conditions.
What is the lowest temperature that can be tested?
Liquid nitrogen baths allow testing down to -196°C, which covers the requirements of virtually all cryogenic applications. Intermediate temperatures between ambient and -196°C are achieved with refrigerated baths, dry ice mixtures, or programmable cryostats.
What absorbed energy value is considered acceptable?
Acceptance values depend on the material specification, product form, thickness, and application. Common criteria include a minimum average of 27 J for many structural grades and 40 J or higher for offshore and pipeline steels, together with a limit on the lowest single value, often two-thirds of the specified average.
How is the ductile-to-brittle transition temperature determined?
Multiple specimen sets are fractured at different temperatures, and the absorbed energy values are plotted as a transition curve. The transition temperature is then defined at a chosen criterion, such as the temperature at which the average energy reaches 27 J, half of the upper-shelf energy, or 50% shear fracture appearance.
How many specimens are tested at each temperature?
Most standards and specifications require a set of three specimens per temperature and per notch location. If a single specimen falls below the minimum individual value while the average still passes, additional sets of three may be retested according to the rules of the governing standard.
What is the difference between V-notch and U-notch testing?
The V-notch has a sharper root radius (0.25 mm) and is more sensitive to brittle fracture initiation, making it the modern default geometry designated KV2. The U-notch (KU2) is retained mainly for legacy specifications, and its results are systematically higher and not comparable with V-notch values.