Impact toughness testing measures a material's ability to absorb energy during fracture under high-strain-rate loading, providing critical data for material selection, quality control, and brittle fracture prevention. Using standardized methods such as the Charpy pendulum test (ISO 148, ASTM E23, GB/T 229) and the Izod test (ASTM D256, ISO 180), accredited third-party laboratories evaluate the toughness of metals, plastics, composites, and weldments across a wide temperature range. This guide explains the principles, sample requirements, test items, procedures, standards, and industrial applications of impact toughness testing.
Technical Overview
Impact toughness testing quantifies the total energy a notched specimen absorbs when fractured by a single blow from a swinging pendulum or a falling weight. Unlike quasi-static tensile testing, impact testing applies dynamic loading at strain rates of approximately 10² to 10³ s⁻¹, which is far closer to real-world service conditions such as collisions, dropped loads, and explosive or seismic events. The test reveals a material's susceptibility to brittle fracture, a failure mode that cannot be reliably predicted from tensile strength or hardness alone.
Impact toughness is especially important for body-centered cubic (BCC) metals, such as ferritic and low-alloy steels, which exhibit a ductile-to-brittle transition as temperature decreases. Above the transition region, these materials absorb substantial energy through plastic deformation; below it, they may fracture in a brittle manner at very low energy. Impact testing therefore forms a core part of material qualification for pressure vessels, pipelines, offshore structures, and low-temperature equipment.
The most widely used procedures are the Charpy V-notch pendulum test for metallic materials, specified in ISO 148-1, ASTM E23, EN ISO 148-1, GB/T 229, and JIS Z 2242, and the Charpy and Izod tests for plastics, specified in ISO 179-1, ASTM D6110, ISO 180, and ASTM D256. Accredited third-party laboratories perform these tests under controlled temperature conditions and report parameters such as absorbed energy, lateral expansion, and fracture appearance.
Testing Samples
Metallic Materials
The standard Charpy specimen for metals is a bar measuring 10 mm × 10 mm × 55 mm. The most common notch geometry is the V-notch, machined to a depth of 2 mm with a 45° included angle and a root radius of 0.25 mm, leaving an 8 mm ligament. U-notch specimens with a 5 mm deep notch and a 1 mm root radius are used for certain legacy specifications and castings. When plate thickness is insufficient, sub-size specimens of 10 mm × 7.5 mm × 55 mm, 10 mm × 5 mm × 55 mm, or 10 mm × 2.5 mm × 55 mm may be tested, although the absorbed energy values from sub-size bars are not directly comparable to full-size results.
Plastics and Polymers
For plastics, Charpy specimens per ISO 179-1 are typically 80 mm × 10 mm × 4 mm bars, tested unnotched or notched with A-type (0.25 mm radius), B-type (1.0 mm radius), or C-type (0.1 mm radius) notches. Izod specimens per ASTM D256 are commonly 63.5 mm × 12.7 mm bars with thicknesses between 3.2 mm and 12.7 mm, clamped as a cantilever beam. Reinforced and unreinforced thermoplastics, thermosets, and polymer composites are all routinely evaluated.
Weldments and Joints
For welded fabrications, Charpy specimens are extracted with the notch located at the weld centerline, the fusion line, or within the heat-affected zone (HAZ), typically at 1 mm and 5 mm offsets from the fusion boundary. This sampling strategy is mandated in many weld procedure qualification standards and product specifications to confirm that the welding process has not degraded notch toughness in the joint region.
Testing Items
Impact toughness testing programs typically report the following quantities, depending on the material, standard, and client specification:
- Absorbed energy KV2, KV8, KU2, or KU8 in joules, where the letter denotes the notch type and the digit denotes the notch depth in millimeters.
- Impact toughness expressed as absorbed energy per unit cross-sectional area (J/cm²) for sub-size specimens and legacy specifications.
- Lateral expansion of the compressed side of the fractured specimen, measured in millimeters, as an indicator of ductility.
- Percent shear (fibrous) fracture appearance determined visually or by image analysis of the fracture surface.
- Ductile-to-brittle transition temperature (DBTT) curves derived from multi-temperature test series, including upper-shelf and lower-shelf energy.
- Izod impact resistance in J/m for plastics, and Charpy impact strength in kJ/m² per ISO 179-1.
- Instrumented impact results, including total energy, crack initiation energy, crack propagation energy, and characteristic forces from the force–displacement record.
- Nil-ductility transition (NDT) temperature determined by drop-weight testing per ASTM E208.
Testing Methods
Charpy Pendulum Impact Test
In the Charpy test, the specimen is supported as a simple beam against two anvils and struck at mid-span by a pendulum-mounted striker on the side opposite the notch. The absorbed energy is calculated from the difference between the initial and final pendulum heights, or read directly from a calibrated dial or encoder. Pendulum machines for steel testing commonly have energy capacities of 150 J, 300 J, or 450 J, while smaller capacities down to fractions of a joule are used for aluminum alloys, castings, and polymers. Note that ISO 148-1 specifies a 2 mm striker radius while ASTM E23 specifies an 8 mm striker radius, so results obtained to the two standards may differ slightly and must not be interchanged without caution.
Izod Impact Test
In the Izod configuration, the notched specimen is clamped vertically as a cantilever beam and struck on the notched side near the free end at a standard velocity of approximately 3.46 m/s per ASTM D256. The Izod method is used predominantly for plastics and is reported as energy per unit width (J/m). Both notched and unnotched Izod tests are available, and reversed-notch testing can be applied to laminate materials.
Instrumented Impact Test
Instrumented testing, covered by ISO 14556 for metals and ASTM E2298, equips the striker with a force sensor and optical displacement measurement to record the full force–time and force–displacement history. This separates the energy consumed by crack initiation from the energy consumed by crack propagation, providing far richer fracture characterization than a single absorbed-energy value. It is particularly valuable for advanced high-strength steels, pipeline steels, and research programs.
Temperature Conditioning
Because toughness is strongly temperature-dependent, specimens are conditioned to the test temperature before fracture. Liquid baths using liquid nitrogen, dry ice with alcohol, ice water, or heated oils, together with furnaces, cover a typical range from −196 °C to +300 °C. Standards such as ASTM E23 prescribe minimum soak times and require the specimen to be fractured within a short window, typically five seconds, after removal from the conditioning medium to prevent temperature drift. A calibrated reference thermometer or thermocouple verifies bath temperature.
Machine Verification and Quality Control
Pendulum impact machines are verified periodically using certified reference specimens and direct verification of geometry, velocity, and energy losses, as required by ISO 148-2 and ASTM E23 annexes. Accredited third-party laboratories maintain these verification programs, traceable calibration of dimensional measurement instruments, and qualified technicians to ensure repeatable, defensible results.
Applications
Impact toughness testing supports design, qualification, and failure analysis across virtually every heavy industry. Typical applications include:
- Pressure vessels, boilers, and piping, where product standards impose minimum absorbed-energy requirements at the minimum design metal temperature.
- Oil and gas pipelines and storage tanks, including weld seam and heat-affected-zone toughness verification.
- Shipbuilding and offshore structures operating in cold climates, where fracture arrest and weld toughness are critical.
- Structural steel for bridges and buildings, including seismic performance qualification.
- Automotive and rail components subjected to crash and impact loading, including advanced high-strength steels and composites.
- Aerospace forgings and castings, fasteners, and landing gear materials.
- Power generation, including turbine and generator rotors, and reactor vessel surveillance programs that monitor irradiation embrittlement.
- Quality control of castings, forgings, and heat-treated parts to detect temper embrittlement, hydrogen embrittlement, brittle phases, and improper heat treatment.
- Failure analysis, where low impact energy at service temperature helps confirm the root cause of brittle fractures.
FAQ
What is the difference between the Charpy test and the Izod test?
In the Charpy test the specimen is supported at both ends and struck opposite the notch, while in the Izod test the specimen is clamped as a cantilever and struck on the notched side. Charpy testing dominates for metallic materials and is specified in ISO 148-1, ASTM E23, and GB/T 229, whereas Izod testing, specified in ASTM D256 and ISO 180, is used mainly for plastics.
How many specimens are required for an impact toughness test?
Most specifications require a minimum of three specimens per material condition and temperature. A full ductile-to-brittle transition curve typically requires fifteen to thirty specimens tested at several temperatures spaced across the transition range.
What temperature range can be tested?
Standard pendulum testing covers approximately −196 °C to +300 °C using liquid nitrogen baths, dry-ice baths, ice water, heated media, and furnaces. Elevated-temperature testing above this range is possible with specialized equipment.
What if my material is too thin for a full-size specimen?
Sub-size Charpy specimens of 10 mm × 7.5 mm, 10 mm × 5 mm, or 10 mm × 2.5 mm cross-section can be used, and many product standards define reduced minimum energy values for these sizes. Because absorbed energy scales with ligament area, sub-size results must be compared only against the corresponding sub-size criteria.
How is the ductile-to-brittle transition temperature determined?
Specimens are tested at a series of temperatures and the absorbed energy, lateral expansion, and shear fracture appearance are plotted against temperature. The transition temperature may be defined at a fixed energy criterion such as 27 J, at a fixed percent shear criterion such as 50% (fracture appearance transition temperature), or at the midpoint between the upper and lower shelf energies.
What does a low impact energy result indicate?
Low absorbed energy at the service temperature indicates a tendency toward brittle fracture. Possible causes include low service temperature, high strain-rate loading, grain coarsening, brittle second phases, hydrogen embrittlement, temper embrittlement, strain aging, or deficient weld heat input. Follow-up examinations such as fractography and metallography are often recommended.
Are ISO 148 and ASTM E23 results interchangeable?
Not automatically. The two standards use different striker radii, 2 mm for ISO and 8 mm for ASTM, and slightly different support and verification requirements, which can shift measured energies. Test reports must clearly state the standard applied, and comparison between datasets generated to different standards should be done with expert judgment.