Impact Test Calculator - Charpy and Izod
Use this calculator to analyse Charpy and Izod pendulum impact tests. Enter the pendulum mass and either initial and final heights or release and rebound angles, and instantly get the energy absorbed, impact strength at the notch, capacity utilization, and impact velocity. Switch between Charpy and Izod test types, choose metric or imperial units, and optionally enter specimen dimensions for a full toughness breakdown.
What are Charpy and Izod impact tests?
The Charpy and Izod tests are standardised pendulum impact tests that measure the energy a material absorbs when fractured by a single blow from a swinging hammer. Both methods use the same physical principle: a heavy pendulum is released from a known height, strikes a notched specimen, and rises to a lower height on the other side. The difference in potential energy before and after impact, less any friction and air drag losses, equals the energy absorbed by the specimen. This absorbed energy is the primary measure of the material's notch toughness. Charpy tests (ASTM E23, ISO 148-1) position the specimen horizontally as a simply-supported beam with the notch facing away from the striker. Izod tests (ASTM D256, BS 131) clamp the specimen vertically as a cantilever with the notch facing the striker. In practice, Charpy is preferred for metals at engineering temperatures and Izod is more common for plastics.
Impact test formulas
The energy absorbed during an impact test is calculated from the height difference of the pendulum: E = m x g x (h1 - h2) - E_loss Where m is the pendulum mass in kg, g is the standard acceleration of gravity (9.80665 m/s^2), h1 is the initial height of the striker above the specimen plane, h2 is the height reached after impact, and E_loss is the energy lost to friction and air resistance (obtained from a free-swing calibration with no specimen). When a machine reads angles rather than heights, the heights are recovered with h = S x (1 - cos theta), where S is the arm length and theta is the angle from vertical. Impact velocity at the moment of impact is V = sqrt(2 x g x h1), showing that a taller drop produces a faster, higher-energy strike. Impact strength (also called specific impact energy or aK) normalises the absorbed energy to the net cross-section area at the notch: aK = E / (b x (t - a)), where b is the specimen width, t is the thickness, and a is the notch depth.
Capacity utilization and test validity
A key quality check is capacity utilization, which is the absorbed energy divided by the machine rated capacity expressed as a percentage. Most standards and calibration guides recommend keeping utilization between 20% and 80% for reliable results. Below 20%, the reading is a small fraction of the machine range and small systematic errors (friction, instrument resolution) represent a large percentage of the result. Above 80%, there is a risk of bottoming out the machine and the pointer reading may be less precise at the top of the scale. If utilization falls outside this range, the test is often repeated with a machine of a different energy range. This calculator shows capacity utilization as a gauge so you can assess test validity at a glance.
Ductile-to-brittle transition and temperature testing
One of the most important applications of impact testing is mapping the ductile-to-brittle transition (DBT) temperature of body-centred cubic (BCC) metals, especially structural and pressure-vessel steels. At elevated temperatures a steel may absorb 150 J or more in a Charpy test and show a ductile, fibrous fracture. Below the transition temperature the same steel can drop to under 5 J and fracture in a brittle, crystalline manner. Standards such as EN 10025 and ASTM A673 specify minimum Charpy energies at defined temperatures (often -20 deg C or -40 deg C) to ensure structural safety in cold environments. By running tests at a series of temperatures and plotting the absorbed energy curve, engineers can identify the transition temperature and select a material whose curve stays above the required threshold across the intended service range.
Typical Charpy V-notch absorbed energy by material (room temperature)
| Material | Typical energy (J) | Toughness category |
|---|---|---|
| Mild steel (A36/S275) | 80-150 | High (ductile) |
| High-strength steel (A572 Gr50) | 40-100 | Medium-high |
| Structural steel (Charpy min. 27 J) | 27-80 | Medium |
| Grey cast iron | 2-10 | Low (brittle) |
| Aluminium alloy 6061-T6 | 15-35 | Medium-low |
| Polycarbonate (PC) | 50-200 | High (ductile polymer) |
| Acrylic (PMMA) | 2-5 | Very low (brittle) |
| Nylon 66 (dry) | 30-80 | Medium |
| Rubber-toughened ABS | 30-80 | Medium |
| Uniaxial CFRP (0 deg) | 50-200 | High (fiber direction) |
Representative values from published literature. Actual results depend on specimen geometry, notch geometry, temperature, and heat treatment. All values are for standard 10 mm x 10 mm specimens.
Frequently asked questions
What is the difference between Charpy and Izod impact tests?
Both tests use a swinging pendulum to break a notched specimen, but the specimen orientation and clamping differ. In the Charpy test the specimen lies horizontally on two supports and is struck from behind the notch. In the Izod test the specimen stands vertically, is clamped at the bottom, and is struck on the same side as the notch. The Charpy test is more common for metals and allows low-temperature testing more easily; the Izod test is widely used for plastics. The energy formula is the same for both: absorbed energy equals the change in pendulum potential energy.
What units are Charpy impact results reported in?
In SI (metric) units, absorbed energy is reported in joules (J). Impact strength is reported in kJ/m^2 or J/m^2 depending on the standard. In the US customary system, energy is in foot-pounds-force (ft-lbf) and impact strength in ft-lbf/in^2. Older literature occasionally uses inch-pound-force (in-lbf) or kg-cm. This calculator shows both metric and imperial outputs and converts automatically.
What is impact strength (aK) and how does it differ from absorbed energy?
Absorbed energy (in joules or ft-lbf) is the total energy the specimen consumes during fracture. Impact strength, often denoted aK, divides that energy by the net cross-section area at the notch, giving energy per unit area (kJ/m^2 or J/m^2). This normalisation makes it easier to compare specimens of different sizes. Standard Charpy and Izod specimens have fixed dimensions, so absorbed energy and impact strength are directly proportional for standard specimens, but the area-normalised value becomes important for sub-size specimens.
Why does the absorbed energy formula subtract a friction loss term?
A real pendulum machine loses a small amount of energy to bearing friction, air resistance, and vibration even when no specimen is present. This is measured by running the machine with no specimen in place and noting the height the hammer reaches (the free-swing test). The resulting energy is subtracted from subsequent impact readings to isolate the energy that went into breaking the specimen. Many modern digital machines correct for this automatically; on older dial-pointer machines it is entered manually. If your machine certificate does not list a friction correction, entering zero is acceptable.
What is a good absorbed energy value for steel?
It depends on the grade and temperature. A common requirement for structural steels is 27 J minimum at 0 deg C or -20 deg C (for example, EN 10025 S355J2 requires 27 J at -20 deg C). Tougher grades such as S460NL may require 40 J at -50 deg C. Pressure vessel codes (ASME, EN 13445) specify impact energy as a function of thickness, temperature, and material group. Absorbed energies above 100 J generally indicate a ductile steel with good toughness; values below 10 J indicate brittle behaviour.
How do I convert Charpy energy to fracture toughness (K_IC)?
There is no exact conversion, but several empirical correlations exist. A widely cited relationship is K_IC (MPa sqrt(m)) approximately equal to sqrt(E_J x sigma_y / 0.0007), where E_J is the Charpy energy in joules and sigma_y is the yield strength in MPa. This is only an estimate and not a substitute for a proper fracture mechanics test (ASTM E399). The correlation works best for steels in the transition regime and can be significantly off for highly ductile or very brittle materials.
What is the ductile-to-brittle transition temperature?
Many BCC metals, particularly ferritic steels, shift from ductile to brittle fracture as temperature decreases. The transition temperature is typically defined as the temperature at which the absorbed energy falls below a specified threshold (often 27 J for structural steels) or where the fracture surface changes from primarily fibrous to predominantly crystalline. Testing at a series of temperatures and plotting the energy-temperature curve allows engineers to find this transition point and ensure the material remains tough in service.