Impact strength test standards define the standardized procedures used to measure how materials resist fracture under sudden, high-speed loading, making them essential for quality control, material selection, and product safety across industries. This guide explains the major international standards—including ISO 179, ISO 180, ASTM D256, ASTM D6110, and their GB and JIS equivalents—covering specimen requirements, test procedures, result interpretation, and practical applications.
Technical Overview
Impact strength testing evaluates the ability of a material to absorb energy and resist fracture when subjected to a rapidly applied load. Unlike slow tensile or flexural tests, impact testing reproduces the high strain-rate conditions that materials experience during drops, collisions, and sudden loading, where normally ductile materials may fail in a brittle manner. Because results depend heavily on specimen geometry, notch quality, striker velocity, and energy absorption measurement, international standards organizations have established tightly controlled procedures to ensure repeatability and comparability of data.
The most widely applied impact strength test standards fall into several families, each covering a specific test geometry or material class:
- ISO 179-1 / GB/T 1043.1 / JIS K 7110 – Charpy pendulum impact testing of plastics, covering notched and unnotched specimens.
- ISO 180 / GB/T 1843 / JIS K 7111 – Izod (cantilever beam) impact testing of plastics.
- ASTM D256 – Determination of Izod pendulum impact resistance of plastics; ASTM D6110 covers the Charpy configuration; ASTM D4812 covers unnotched cantilever beam impact.
- ISO 148 / ASTM E23 / GB/T 229 / JIS Z 2242 – Charpy V-notch pendulum impact testing of metallic materials.
- ASTM D1709 / ISO 7765-1 / GB/T 9639.1 – Free-falling dart impact resistance of plastic films and thin sheeting.
- ASTM D5420 (Gardner impact) and ASTM D5628 (dart drop) – Impact resistance of flat, rigid plastic specimens.
- ASTM D3763 / ISO 179-2 – Instrumented impact testing with load and displacement recording.
- ASTM D1822 / ISO 8256 – Tensile-impact energy to break.
ISO-based standards report impact strength in kJ/m², calculated from absorbed energy divided by the cross-sectional area under the notch, while ASTM methods report energy per unit width in J/m or ft·lb/in. Because the specimen dimensions and notch geometries differ between the two systems, results are not directly convertible and should only be compared within the same standard.
Testing Samples
Rigid Thermoplastics and Thermosets
ISO 179 and ISO 180 specify the standard bar specimen of 80 mm × 10 mm × 4 mm, machined or molded directly to shape. ASTM D256 uses a bar of 63.5 mm × 12.7 mm × 3.2 mm (thicknesses up to 12.7 mm may be retained and tested unfiled). Notching is performed with a milling cutter or shaping tool: ISO type A notches have a 0.25 mm radius and 2.0 mm depth, type B notches a 1.0 mm radius, and type C notches a sharp 0.1 mm radius for notch-sensitive materials. ASTM D256 specifies a 2.54 mm notch depth with a 0.25 mm root radius.
Fiber-Reinforced Composites
Long-fiber and short-fiber reinforced plastics are tested in the same bar geometry, with specimens cut along and perpendicular to the fiber direction to characterize anisotropy. Unnotched testing is often preferred for highly notch-sensitive composites.
Plastic Films and Thin Sheeting
For dart impact testing, specimens are typically square sections of film, at least 100 mm × 100 mm, tested in their as-received thickness. Multiple increments of dart mass and a staircase (Bruceton) sequence are used to statistically determine the mass at which 50% of specimens fail.
Pipes, Fittings, and Profiles
Extruded pipes may be tested using cut bar specimens taken from the pipe wall, or in full-section configuration, so that the impact behavior of the finished geometry—including weld lines and orientation effects—is evaluated.
Metallic Materials
Charpy V-notch testing per ASTM E23, ISO 148, and GB/T 229 uses the classic 55 mm × 10 mm × 10 mm bar with a 2 mm deep, 45° V-notch having a 0.25 mm root radius. Sub-size specimens (7.5 mm or 5 mm thick) are permitted when material thickness is limited.
Testing Items
Accredited third-party laboratories typically offer the following impact strength test items under the standards above:
- Notched Charpy impact strength (kJ/m²) per ISO 179-1, GB/T 1043.1, ASTM D6110
- Unnotched Charpy impact strength per ISO 179-1 and ASTM D6110
- Notched and unnotched Izod impact strength (J/m or ft·lb/in) per ISO 180, ASTM D256, GB/T 1843
- Free-falling dart impact resistance of films, reported as F50 failure mass per ASTM D1709, ISO 7765-1, GB/T 9639.1
- Variable-height falling weight (Gardner) impact for rigid sheet and painted or coated panels per ASTM D5420
- Instrumented impact data—peak force, total absorbed energy, and force–displacement curves—per ASTM D3763 and ISO 179-2
- Tensile-impact energy per ASTM D1822 and ISO 8256
- Charpy V-notch absorbed energy (J) for metals, including temperature-series testing to determine the ductile-to-brittle transition temperature
ISO 179 also requires classification of the failure mode: complete break (C), hinged break (H), partial break (P), or non-break (N), because materials exhibiting complete brittle failure cannot be directly ranked against those that merely partially crack.
Testing Methods
Pendulum Impact Testing (Charpy and Izod)
In the Charpy configuration, the specimen rests horizontally against two anvils and is struck at mid-span, opposite the notch. In the Izod configuration, the specimen is clamped as a vertical cantilever, with the striking edge centered 22 mm above the notch. A calibrated pendulum hammer is released from a fixed height; the difference between the initial and post-impact pendulum height, corrected for friction and windage, gives the absorbed energy. ISO 179 hammer capacities are typically 1 J, 2.75 J, 5.5 J, 11 J, and 22 J, with striker velocities of approximately 2.9 to 3.8 m/s. Standards require the absorbed energy to fall between roughly 10% and 80% of the hammer capacity; if the specimen absorbs too little or too much energy, a different hammer must be selected.
Free-Falling Dart Testing
Film impact resistance is determined with a hemispherical-headed dart dropped onto a clamped film specimen. ASTM D1709 Method A uses a 38 mm dart with a 0.66 m drop height, Method B a 38 mm dart with a 1.52 m height; ISO 7765-1 Method A uses a 38 mm dart at 0.5 m and Method B a 50 mm dart at 1.0 m. Mass is increased or decreased after each pass or failure in a staircase sequence, and the result is reported as the dart mass producing 50% failures, usually based on at least 20 drops.
Instrumented Impact Testing
Instrumented machines fit the striker with a force transducer and the crosshead with displacement sensing, producing a full force–displacement curve at speeds up to several meters per second. This distinguishes ductile from brittle failure, separates initiation and propagation energy, and is invaluable for automotive and packaging applications where failure mode, not just total energy, matters.
Specimen Conditioning and Notching Quality
Plastic specimens are conditioned to equilibrium at 23 °C and 50% relative humidity per ISO 291 or ASTM D618 unless otherwise specified. Notch cutting technique is critical: a dull cutter, excessive feed rate, or machining-induced heat can depress impact strength significantly, so notch radius and surface quality must be verified. Metals for low-temperature testing are soaked at the target temperature for at least 5 minutes before breaking, and broken within seconds of removal from the bath.
Applications
Impact strength test standards support material qualification and product safety across a wide range of industries:
- Automotive: validating interior trims, bumpers, battery housings, and under-hood components against stone impact, low-temperature impact, and crash-related loading.
- Electrical and electronics: verifying enclosure robustness of plugs, switches, and appliances, often referenced alongside IK impact protection codes of IEC 62262 and the polymeric material requirements of UL 746A.
- Packaging: grading blown and cast polyethylene films by dart-drop F50 values for bags, liners, and laminates.
- Pipes and construction: assessing PVC, PE, and PPR pipes and fittings for transport, installation, and service impact; assessing glazing, profiles, and panels for safety compliance under EN-based construction product requirements.
- Metal structures and pressure equipment: Charpy V-notch acceptance testing of structural steels and weldments, and determination of ductile-to-brittle transition temperatures for low-temperature service.
- Medical and consumer products: demonstrating drop and impact durability of device housings, connectors, and consumer goods during design verification.
Manufacturers routinely submit prototypes and production lots to accredited third-party laboratories for testing against these standards to support supplier approvals, regulatory submissions, and comparative material benchmarking.
FAQ
What is the difference between the Charpy and Izod impact tests?
Charpy testing strikes a horizontally supported beam at mid-span opposite the notch, while Izod testing strikes a vertically clamped cantilever specimen above the notch. Charpy is the dominant method for metals and ISO-based plastic testing; Izod remains the dominant method in ASTM-based North American plastic specifications. The two configurations produce different stress states and are not interchangeable.
Why are ISO and ASTM impact results not directly comparable?
The specimen cross-sections (10 mm × 4 mm versus 12.7 mm × 3.2 mm), notch depths (2.0 mm versus 2.54 mm), notch width (10 mm versus full 12.7 mm width in some cases), and energy-normalization conventions all differ. A material may show, for example, approximately 10–20% higher values in one system, but no universal conversion factor exists; data should only be compared within a single standard.
Why are notches machined into impact specimens?
The notch creates a controlled stress concentration and a triaxial stress state that promotes crack initiation, sharpens the measurement of notch sensitivity, and reduces scatter. Unnotched tests are also performed when overall toughness under blunt impact is of interest.
How many specimens are required?
ISO 179 and ISO 180 require a minimum of five specimens per condition, with ten recommended for arithmetically averaged values, plus more when variability is high. Staircase dart-drop methods require roughly twenty drops after the initial failure to establish the 50% failure mass statistically.
What is the ductile-to-brittle transition temperature?
Ferritic steels and some plastics absorb much less energy when cooled below a characteristic temperature. Temperature-series Charpy testing per ASTM E23, ISO 148, and GB/T 229 maps absorbed energy versus temperature so engineers can select materials with a transition temperature safely below the minimum service temperature.
Can impact test data be used directly for product design?
No. Impact strength values are comparative indices influenced by geometry, thickness, strain rate, and notch sharpness. They are used for material ranking, specification acceptance, and quality control; final designs should be validated with component-level drop, crash, or instrumented impact testing at realistic conditions.