Charpy Impact Test vs Izod Test: Setup, Procedure and Results Comparison

Charpy and Izod notched-bar tests both quantify a material’s resistance to fracture under sudden, high-strain-rate loading, but they load the specimen in fundamentally different ways and report energy values that are not interchangeable. This article compares specimen geometry, machine setup, energy calculation, and the practical reasons Charpy V-notch testing dominates structural steel qualification while Izod testing dominates polymer qualification.

1. Why the Loading Geometry Matters

Both tests deliver a single, high-velocity blow from a swinging pendulum to a notched bar and measure the energy absorbed in fracturing it. The difference lies entirely in how the specimen is constrained. In the Charpy impact test, the bar rests horizontally across two anvils spaced 40 mm apart, and the striker hits the unnotched face directly behind the notch, producing three-point bending with the notch in pure tension at the moment of fracture initiation. In the Izod test, the bar is clamped vertically in a vice as a cantilever, and the striker hits the notched face itself, a short distance above the clamp, producing a bending moment that increases toward the clamped root.

This changes the effective gauge length, the strain rate gradient ahead of the notch tip, and the degree to which clamping friction and specimen-to-fixture fit influence the measured energy. Izod results are more sensitive to clamping torque and specimen alignment than Charpy results, which is one reason Izod testing requires tighter control of vice closing force in ASTM D256.

1.1 Specimen Geometry

For metals tested under ASTM E23, both the Charpy and Izod specimens share the same 10 mm x 10 mm square cross-section and the same standard 45° V-notch, 2 mm deep with a 0.25 mm root radius, but specimen length differs: 55 mm for Charpy, 75 mm for Izod, to accommodate the clamped length. Sub-size Charpy specimens (7.5 mm, 6.7 mm, 5 mm, 3.3 mm, or 2.5 mm thickness) are permitted when full-size bars cannot be extracted from thin plate or small forgings, but absorbed energy from sub-size bars is not directly comparable to full-size results.

For plastics under ASTM D256, the Izod bar cross-section and notch are defined by the standard separately and the notch is typically cut after moulding using a calibrated notching cutter rather than machined to the metals notch tolerance.

2. Machine Setup and Test Procedure

2.1 Charpy Procedure

The specimen is positioned on the anvils using a centring tongs so the notch sits exactly in the plane of the striker’s swing, with the notch facing away from the point of impact. The pendulum is released from a fixed height, swings through the specimen, fractures it in a single blow, and continues to rise on the far side. The difference between release height and rise height, converted to energy, is the absorbed energy, commonly reported as KV (Charpy V-notch) in joules or ft-lbf.

2.2 Izod Procedure

The specimen is clamped vertically in a vice to a specified depth and torque so that the notch is positioned at a fixed height above the clamp face, with the notch facing the striker. The pendulum swings horizontally and strikes the free end above the notch. As with Charpy, absorbed energy is read from the height differential of the pendulum swing, but because the specimen acts as a cantilever rather than a simply supported beam, the relationship between striking energy and true fracture energy at the notch root differs from the Charpy case.

E = m · g · R · (cosβ − cosα)

where:
  m = effective pendulum mass (kg)
  g = 9.81 m/s²
  R = distance from pivot to centre of percussion (m)
  α = release angle (rise of pendulum before release)
  β = angle to which pendulum rises after fracture

E is read directly in joules (or ft·lbf) from the calibrated dial
or digital encoder; modern machines compute this automatically.

3. Interpreting Charpy Results: The Transition Curve

The principal value of Charpy V-notch testing is not a single energy number but the shape of absorbed energy versus temperature, known as the ductile-brittle transition curve. At high temperatures the material absorbs energy on a ductile upper shelf with extensive shear deformation; at low temperatures it fails on a brittle lower shelf by cleavage with little plastic work. Between the shelves lies the transition region, where scatter is typically largest because the fracture mode is mixed.

3.1 Transition Temperature Criteria

Several criteria are used to define a single transition temperature from the curve: the energy criterion (temperature at a fixed absorbed-energy value, such as 20 J or 27 J), the 50 percent shear-area criterion (temperature at which the fracture surface is half cleavage, half shear), and the average of upper- and lower-shelf energy criterion. Different criteria can give different transition temperatures for the same data set, so the criterion used must always be stated alongside the reported value.

3.2 Why Microstructure Controls the Curve

Grain refinement, lower carbon content, and the absence of coarse grain boundary carbide films all shift the transition curve to lower temperatures. Bainitic and fine-grained quenched-and-tempered microstructures generally show lower transition temperatures than coarse-grained normalised ferrite-pearlite structures of the same composition, which is why heat treatment specification is as important as chemistry for low-temperature service steels.

4. Interpreting Izod Results

For plastics, Izod results under ASTM D256 are typically reported at room temperature as a single notched or unnotched impact energy, often normalised by specimen width as J/m, rather than as a transition curve. Because polymer impact behaviour is strongly dependent on test temperature, moisture content, and moulding-induced orientation, comparing Izod values across different suppliers or batches requires identical conditioning, not just identical notch geometry.

4.1 Notched vs Unnotched Izod

An unnotched Izod test is sometimes run alongside the notched test to assess a material’s general toughness without the stress concentration of a machined notch. The ratio between notched and unnotched energy, called the notch sensitivity, indicates how strongly a material’s impact resistance depends on surface defects and machining quality, a consideration directly relevant to moulded part design.

5. Specification and Code Usage

Structural and pressure-equipment codes such as ASME Section VIII, API 5L line pipe, and EN 10025 structural steel specify minimum Charpy V-notch absorbed energy at a defined test temperature, often the lowest anticipated metal temperature in service, as a practical screen against unstable brittle fracture. Weld and heat-affected-zone qualification in ASME Section IX and API 1104 likewise relies on Charpy testing of weld metal and HAZ locations, not Izod testing, because the transition-curve behaviour is what code committees have historically correlated with fracture safety margins.

Izod testing dominates polymer and electrical-insulation specifications, including many UL 746 flammability and mechanical property listings for moulded plastic components, where the cantilever geometry better represents the loading a moulded part edge or rib might see in service.

6. Significance for Material Selection and Quality Control

In a steel mill or fabrication shop, Charpy V-notch testing is the routine acceptance test for plate, pipe, forgings, and weld procedure qualification records, run at the minimum design temperature specified by the purchaser. A failed Charpy test at the qualification temperature is grounds for rejecting a heat, a weld procedure, or a heat-treatment lot, and re-test provisions in most codes require additional specimens rather than simple averaging. Hardness testing is often run alongside Charpy testing as a faster, non-destructive screen, but hardness alone does not predict transition temperature reliably across different microstructures.

For polymer component design, Izod notched impact strength is one of several toughness metrics engineers compare against expected service temperature and impact loading, alongside tensile elongation and flexural modulus, when selecting a resin grade for an application such as a housing, bracket, or fastener boss.

7. Industrial Applications and Practical Notes

7.1 Pressure Vessel and Pipeline Steel

Charpy V-notch testing at the minimum design metal temperature is mandatory for many pressure vessel and pipeline materials. Cryogenic and arctic-service vessels often require Charpy testing at temperatures as low as -46°C or lower, driving material selection toward fine-grained, low-carbon, micro-alloyed steels.

7.2 Weld Procedure Qualification

Charpy specimens are extracted transverse to the weld, with the notch located at the weld centreline, fusion line, or HAZ as required by the applicable code, to confirm that the weldment as a whole, not just the base metal, meets the specified toughness.

7.3 Polymer Part Design

Izod notched impact data sheets for engineering plastics are generated under standardised laboratory conditioning and may not directly predict field performance of a moulded part with different wall thickness, gate location, or fibre orientation, so design verification testing on the actual part geometry is recommended for impact-critical applications.

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FAQ: Charpy vs Izod Impact Testing

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