Impact Testing Procedures: Charpy, Izod & Drop Weight Methods Explained

Impact testing is one of the most widely performed mechanical tests in materials laboratories — yet it is also one of the most nuanced. Selecting the correct specimen geometry, notch type, testing temperature, and evaluation criteria — and interpreting results in the context of material behavior and application requirements — requires understanding the full range of procedural variables and their effects on measured values. In the materials & quality control industry, mastery of impact testing procedure translates directly to reliable, reproducible toughness data that supports confident engineering decisions.

The Purpose of Impact Testing

Impact testing measures the energy absorbed by a standardized notched specimen when fractured by a single, high-velocity blow from a pendulum hammer. This energy — reported in joules (J) or foot-pounds (ft·lb) — quantifies toughness: the material’s ability to absorb energy during rapid fracture rather than fracturing in a brittle, catastrophic manner with little energy absorption.

The two principal impact test geometries — Charpy and Izod — differ in specimen support and loading geometry, but share the fundamental measurement principle:

Absorbed Energy = Hammer’s Initial Potential Energy − Hammer’s Final Potential Energy = mg(h_initial − h_final)

Where m = hammer mass, g = gravitational acceleration, and h = height above the lowest point.

Specimen Preparation: A Critical Procedural Aspect

Standard Charpy V-Notch Specimen (ASTM E23, ISO 148-1)

The standard Charpy V-notch specimen is:

• 55mm × 10mm × 10mm
• V-notch: 2mm deep, 45° included angle, 0.25mm root radius
• Notch located at mid-length, centered on the 10mm dimension

Specimen preparation must meet tight tolerances:

• Notch root radius: 0.25 ± 0.025mm
• Notch depth: 2.00 ± 0.05mm
• Notch angle: 45° ± 1°
• Overall length: 55 ± 0.60mm
• Square cross-section: 10 × 10 ± 0.075mm

Notch quality is the most critical preparation variable — variations in root radius have disproportionately large effects on absorbed energy. Broaching produces better notch quality than milling for most metals. Notch dimensions must be verified by optical measuring equipment to the required tolerances before testing.

Subsize Specimens

When full-size 10×10mm specimens cannot be extracted from available material (thin plate, small-diameter bar, irradiated material with limited volume), subsize specimens are used:

• 10mm × 7.5mm (3/4 subsize)
• 10mm × 5.0mm (1/2 subsize)
• 10mm × 2.5mm (1/4 subsize)

ASTM E23 Annex A1 provides guidance on subsize specimen impact values and their relationship to full-size values — direct comparison is not valid without correction for the changed section size.

Keyhole Notch and U-Notch Variants

The U-notch specimen (Charpy U-notch, ISO 83; Izod keyhole notch) has a 5mm deep, 2mm radius notch, producing higher absorbed energy values than the V-notch for the same material and testing conditions because the larger notch radius reduces stress concentration. U-notch results are not directly comparable to V-notch results and must be clearly identified.

Temperature Conditioning: A Critical Test Variable

Why Temperature Matters

For body-centered cubic (BCC) metals — particularly carbon and low-alloy steels — impact energy drops sharply over a transition temperature range:

• Above transition temperature: High absorbed energy (ductile fracture)
• Transition range: Intermediate, variable absorbed energy
• Below transition temperature: Low absorbed energy (brittle fracture)

This ductile-to-brittle transition (DBTT) is one of the most important phenomena characterized by impact testing. Steels with low DBTT remain tough at low service temperatures; those with high DBTT may become brittle in cold-service conditions.

Temperature Conditioning Protocol

ASTM E23 specifies:

• Specimens must be held at the test temperature in a temperature-controlled medium until thermal equilibrium is achieved — minimum 5 minutes for specimens in liquid baths
• The transfer from the conditioning medium to the impact machine anvil must be completed within 5 seconds
• The temperature at the time of impact must be within ±1°C of the specified test temperature

Temperature conditioning media:

• Dry ice/alcohol baths — for temperatures to −78°C
• Liquid nitrogen and alcohol mixtures — for temperatures to −196°C
• Silicone oil or glycol baths with heating — for elevated temperatures (23°C to +200°C)
• Liquid nitrogen — for −196°C (0°C with additions for intermediate temperatures)

Temperature measurement must use calibrated thermocouples or resistance thermometers traceable to national standards — temperature errors directly translate to absorbed energy errors, particularly in the transition region where the energy-temperature curve is steep.

Machine Calibration and Verification

Pendulum Machine Verification — ASTM E23 Annex A2

Impact machines must be verified for:

• Energy accuracy — checked using reference specimens (NIST-certified reference samples with stated energy values and confidence intervals)
• Anvil span — 40.0 ± 0.025mm for Charpy machines
• Striker geometry — 8mm radius for standard Charpy, or as specified for sub-types
• Striking velocity — 5.0–5.5 m/s for standard Charpy
• Friction losses — determined by swinging the pendulum without a specimen and recording the energy loss per swing

Annual verification at a minimum; recertification after any repair or modification.

Reading and Interpreting Impact Test Results

Absorbed Energy Reporting

Absorbed energy is read directly from the pendulum machine’s energy scale at the maximum swing after fracture. Digital instrumented machines record the complete force-time curve, enabling separate reporting of initiation and propagation energy.

For a set of tests at a single temperature, both the individual values and the mean are reported. ASTM E23 requires a minimum of 3 specimens per test condition for a meaningful result; many applications and standards require 5–10 specimens to characterize the transition region adequately 

Fracture Appearance Assessment

Percent shear fracture (ductile fracture area) — the percentage of the fracture surface showing a dull, fibrous appearance (shear fracture) versus the bright, crystalline appearance (cleavage fracture). 100% shear = fully ductile; 0% shear = fully brittle. The temperature at which 50% shear fracture is observed often defines the DBTT.

Fracture appearance is evaluated by:

• Visual estimation — comparing to reference photographs (ASTM E23 Annex A4)
• Grid overlay — dividing the fracture surface into 100 squares and counting ductile vs. brittle squares
• Digital image analysis — computerized discrimination between shear and cleavage zones for improved reproducibility

Lateral Expansion

The widening of the specimen at the notch root in the direction perpendicular to the notch length — called lateral expansion — is measured with a calibrated comparator gauge. Greater lateral expansion indicates greater plastic deformation and, therefore, more ductile behavior. Some nuclear industry applications use 35 mil (0.89mm) lateral expansion as an alternative DBTT criterion.

Instrumented Impact Testing

Modern instrumented Charpy machines measure force vs. time (and displacement vs. time) throughout the impact event using a strain-gauged striker. Integration of the force-displacement curve yields the total absorbed energy consistent with the pendulum energy balance. Additionally, the curve can be partitioned into:

• Initiation energy — energy to initiate crack propagation from the notch root
• Propagation energy — energy to propagate the crack across the specimen thickness

This partitioning provides mechanistic insight that total energy alone cannot — particularly useful for comparing materials with similar total absorbed energy but different damage mechanisms.

Conclusion

Impact testing precision depends on the complete procedural chain — specimen notch geometry within tolerance, accurate temperature conditioning and transfer, calibrated machine verification, and consistent assessment of fracture appearance. Each variable directly influences the absorbed energy value and the ductile-to-brittle transition temperature derived from it. For structural steel qualification, pressure vessel certification, and pipeline material selection, procedural rigor in Charpy testing per ASTM E23 and ISO 148-1 converts raw energy measurements into reliable toughness data that engineers can use with confidence.

Why Choose Infinita Lab for Impact Testing?

Infinita Lab provides comprehensive impact testing services — including Charpy V-notch and U-notch testing per ASTM E23 and ISO 148-1, Izod testing per ASTM D256, instrumented impact testing with force-time recording, multi-temperature DBTT curve characterization (−196°C to +300°C), fracture appearance assessment, and lateral expansion measurement — serving the materials & quality control industry with toughness data for structural steel qualification, weld procedure qualification, low-temperature pressure vessel certification, and polymer impact resistance evaluation. Contact Infinita Lab at infinitalab.com to discuss impact testing for your materials and applications.

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