Impact Toughness Testing: Determination of Impact Toughness Indicators

Impact toughness testing quantifies a material's ability to absorb energy during fracture under dynamic loading, providing critical data for material selection, quality control, and structural safety assessment. This article presents a comprehensive overview of impact toughness indicator determination, covering test principles, international standards, specimen requirements, testing procedures, and industrial applications for metallic materials and engineering plastics.

Technical Overview

Impact toughness is a mechanical property that describes a material's capacity to absorb energy when subjected to a sudden, high-strain-rate load, typically in the presence of a stress-raising notch. Unlike tensile testing, which is performed under quasi-static conditions, impact testing evaluates how a material behaves under dynamic stress states where brittle fracture becomes a significant risk. The most widely used indicator is the impact absorbed energy, expressed in joules (J), which is measured by fracturing a notched specimen with a single blow from a swinging pendulum.

For metallic materials, the standard Charpy V-notch specimen measures 10 mm × 10 mm × 55 mm with a 2 mm deep V-notch, and the absorbed energy is designated KV2 (for a pendulum striker with a 2 mm edge radius) or KV8 (8 mm radius). U-notch specimens yield the KU2 value. Because body-centered cubic (BCC) steels exhibit a ductile-to-brittle transition with decreasing temperature, impact testing is frequently performed across a temperature range to determine the transition temperature, which is a key acceptance criterion in many structural codes. For polymeric materials, the corresponding indicators are Charpy impact strength and Izod impact strength, reported in kJ/m².

Impact toughness data are highly sensitive to notch geometry, specimen orientation, testing temperature, strain rate, and metallurgical condition. Consequently, accredited third-party laboratories follow strict standardized procedures, including machine verification, specimen machining tolerances, and precise temperature conditioning, to ensure repeatable and comparable results.

Testing Samples

Metallic Materials

Impact toughness determination is applicable to virtually all ductile metallic materials, including:

• Carbon steels, low-alloy steels, and high-strength low-alloy (HSLA) steels
• Stainless steels (austenitic, ferritic, martensitic, and duplex grades)
• Aluminum alloys, titanium alloys, nickel-based superalloys, and copper alloys
• Cast steels, cast irons, forgings, and rolled plates, bars, sections, and pipes

Welded Joints

Weld procedure qualification and welder performance testing routinely require Charpy impact specimens taken from the weld metal, fusion line, and heat-affected zone (HAZ). Notch placement in these specimens is critical because the HAZ often represents the region of lowest toughness due to microstructural degradation during welding thermal cycles.

Plastics and Composite Materials

Engineering plastics, including thermoplastics, thermosets, and fiber-reinforced composites, are evaluated using unnotched or notched bar specimens. Notched plastic specimens measure the material's notch sensitivity, which is essential for components containing sharp corners, ribs, or gate marks from injection molding.

Testing Items

Typical impact toughness indicators determined in a professional testing laboratory include:

• Charpy V-notch impact absorbed energy (KV2, KV8) at room temperature
• Charpy U-notch impact absorbed energy (KU2)
• Low-temperature impact testing (e.g., 0 °C, -20 °C, -40 °C, -60 °C, -80 °C, -196 °C)
• Elevated-temperature impact testing
• Ductile-to-brittle transition temperature (DBTT) determination and transition curves
• Shear fracture appearance percentage (SA%) and lateral expansion measurement
• Instrumented impact testing for separation of crack initiation and propagation energy
• Izod impact strength for plastics and small metallic components
• Charpy impact strength for plastics (notched and unnotched)

Testing Methods

Applicable Standards

Impact toughness testing is performed in accordance with internationally recognized standards, including:

• ISO 148-1: Metallic materials — Charpy pendulum impact test — Test method
• ASTM E23: Standard Test Methods for Notched Bar Impact Testing of Metallic Materials
• GB/T 229: Metallic materials — Charpy pendulum impact test method
• EN ISO 148 and JIS Z 2242: European and Japanese equivalents for pendulum impact testing
• ISO 14556: Instrumented Charpy V-notch pendulum impact test
• ISO 179-1 and GB/T 1043: Determination of Charpy impact properties of plastics
• ISO 180, ASTM D256, and ASTM D6110: Izod impact strength of plastics
• ASME BPVC Section IX and AWS D1.1: Impact test requirements for welding qualification

Specimen Preparation

Standard full-size Charpy specimens measure 10 mm × 10 mm × 55 mm. The V-notch angle is 45° with a 2 mm depth and a notch root radius of 0.25 mm; the U-notch is typically 2 mm or 5 mm deep with a 1 mm root radius. When material thickness does not permit full-size specimens, sub-size specimens (7.5 mm, 5 mm, or 2.5 mm thick) may be tested with proportionally reduced acceptance energy values. Notches must be machined carefully to avoid introducing residual stress, overheating, or grinding marks that could distort the results. Specimen orientation (longitudinal or transverse) and notch position must be documented, as toughness is strongly anisotropic in rolled products.

Equipment and Machine Verification

Testing is conducted on pendulum impact machines with nominal energies of 150 J, 300 J, or 450 J, verified according to ISO 148-2, ASTM E23, or JJG 145. Machine direct and indirect verification, centering of the specimen on the anvils, and pendulum alignment are prerequisites for valid results. Instrumented systems equipped with strain-gauged strikers record force–displacement curves, allowing detailed analysis of yield behavior, maximum force, and the partition of energy between crack initiation and propagation.

Temperature Conditioning and Test Procedure

For tests at other than room temperature, specimens are conditioned in temperature-controlled media: a mixture of liquid nitrogen and anhydrous ethanol for subzero temperatures, ice water for 0 °C, or heated oil or air furnaces for elevated temperatures. The specimen must be positioned on the anvil and broken within 5 seconds of removal from the conditioning medium to prevent temperature drift. The pendulum is released from the latched position, the specimen is fractured in a single blow, and the absorbed energy is read from the dial, digital display, or data acquisition system. When the absorbed energy exceeds 80% of machine capacity, the result is considered invalid due to insufficient reserve energy, and a higher-capacity machine must be used. A minimum of three specimens per condition is standard practice, and individual values together with the average are reported.

Applications

Impact toughness data serve as a fundamental acceptance criterion and design safeguard across many industries:

• Pressure vessels, boilers, and heat exchangers, where codes such as ASME VIII and EN 13445 mandate minimum impact energy to prevent brittle fracture
• Oil and gas pipelines, including line pipe steels for Arctic and deep-sea service requiring verified low-temperature toughness
• Bridges, high-rise structures, and offshore platforms exposed to low service temperatures and dynamic loads
• Shipbuilding and marine engineering, including hull structural steels graded A, B, D, and E
• Automotive and rail industries for crash-relevant components, axles, couplings, and suspension parts
• Aerospace and nuclear power, where neutron embrittlement surveillance programs rely on periodic impact testing of reactor vessel materials
• Fabrication quality control, including heat treatment verification, hydrogen embrittlement screening, and evaluation of aging or in-service degradation

FAQ

What is the difference between Charpy and Izod impact tests?

The Charpy test strikes a horizontally supported, notched specimen at the middle, with the notch facing away from the striker, while the Izod test strikes a vertically clamped cantilever specimen with the notch facing the striker. Charpy testing dominates metallic material specifications; Izod testing is more common for plastics and small components in certain markets.

Why is impact testing performed at low temperatures?

Steels and other BCC metals lose ductility as temperature decreases and may fracture in a brittle manner below their transition temperature. Low-temperature impact testing verifies that the material retains sufficient toughness at the minimum design metal temperature, which is a primary safeguard against catastrophic brittle fracture.

What absorbed energy value is considered acceptable?

Acceptance values depend on the product standard, material grade, specimen orientation, and thickness. A common requirement for structural steels, such as grade S355, is a minimum average KV2 of 27 J at the specified test temperature, with limits on the permitted single-value scatter. Always verify the applicable specification for the exact criteria.

How many specimens are required for one test condition?

A set of three specimens is the standard minimum for each temperature and location. Transition temperature determination typically requires testing at four to six temperatures spanning the fully ductile upper shelf to the fully brittle lower shelf.

Can thin materials be tested for impact toughness?

Yes. Sub-size Charpy specimens of 7.5 mm, 5 mm, or 2.5 mm thickness are used, with reduced energy acceptance criteria proportional to the cross-sectional area, as permitted by standards such as ASTM A20 and GB/T 229. Alternatively, small-scale techniques such as instrumented miniaturized testing may be considered for material surveillance programs.