Dewey Wu General Manager & senior mechanical engineer at EPOC CRAFTER, 15 years in design engineering, quality, and metallurgy. Hands-on across CNC machining, metalwork, sheet metal, and prototyping (subtractive + 3D printing).
Charpy impact strength describes how a plastic specimen responds to a single pendulum impact under specified test conditions. In a Charpy impact test, a notched or unnotched specimen rests horizontally on two supports and is struck at its center. The absorbed energy is used to calculate an impact result based on the applicable standard.
For plastics, ASTM D6110-26 and ISO 179-1:2026 use different specimen requirements and reporting conventions. The notch geometry, material condition, test temperature, and fracture behavior all affect how you interpret the result. A higher reported value does not automatically mean a finished plastic part will survive a greater impact load.
1. What Is the Charpy Impact Test?
The Charpy impact test is a pendulum impact testing method used to evaluate how a material behaves when subjected to a sudden bending load.
A specimen is positioned horizontally across two supports. A calibrated pendulum swings down and strikes the specimen at its midpoint. For a single-notched specimen, the pendulum contacts the face opposite the notch.
The machine determines the energy absorbed during the impact. After accounting for applicable machine losses, that energy is used to calculate the reported impact property.
For plastics, two important standards are:
ASTM D6110-26: Determines the Charpy impact resistance of notched plastic specimens. Results are expressed in joules per meter (J/m) of measured specimen width.
ISO 179-1:2026: Determines Charpy impact properties of plastics using a non-instrumented pendulum method. Notched and unnotched impact strengths are expressed in kilojoules per square meter (kJ/m²).
Both methods use a simply supported specimen, but their specimen dimensions, support spans, result calculations, and reporting requirements differ.
The Charpy test is useful during material screening, production quality comparisons, and investigations into notch sensitivity. Engineers can compare how a plastic responds to impact after changes in material formulation, processing conditions, or conditioning, provided the relevant test conditions remain consistent.
For example, two PA66 specimens made from the same commercial grade may produce different results when one has been tested dry and the other after moisture conditioning. The difference cannot automatically be attributed to a change in resin quality.
The test also has a practical limitation. It measures the response of a prescribed specimen, not the impact load that an assembled connector, enclosure, or snap-fit feature can withstand.
2. What Does Charpy Impact Strength Measure?
Charpy impact strength measures the energy absorbed by a plastic specimen during a specified pendulum impact, normalized according to the test method.
The distinction between absorbed energy and normalized impact strength matters.
Absorbed energy is the energy associated with the specimen’s response during the impact, after the required machine corrections. It is reported in joules (J).
Charpy impact strength expresses that corrected energy relative to a prescribed specimen dimension or cross-sectional area. The exact calculation depends on the standard.
Under ASTM D6110-26, the corrected breaking energy is divided by the measured specimen width at the notch. Under ISO 179-1:2026, the corrected energy is divided by the relevant original cross-sectional area, using the remaining section beneath the notch for notched specimens.
Consequently, a numerical result reported in J/m cannot be compared directly with a result reported in kJ/m². Converting the units alone does not establish equivalence between the test methods.
What affects the measured result?
Specimen geometry: Dimensions, notch depth, root radius, and notch orientation influence the stress concentration.
Temperature and moisture: A polymer may respond differently after moisture conditioning or when tested at a lower temperature.
Manufacturing direction: Injection molding flow and 3D printing build orientation can produce direction-dependent behavior.
Failure mode: A complete break, hinge break, partial break, and non-break do not necessarily lead to the same result interpretation.
These influences are particularly important for semi-crystalline polymers such as PA66, whose mechanical response can depend on moisture content and temperature.
A Charpy result can support material comparisons when the specimens have compatible geometries, are prepared and conditioned consistently, and are tested using the same applicable method.
A snap-fit connector, for example, may fail at its root because of local bending strain, a sharp transition, a weld line, or unfavorable material orientation. Those conditions are not fully represented by a standardized Charpy specimen.
3. Charpy Impact Test Specimens and Notch Geometry
A Charpy impact test specimen must match its specified method. Notch tip radius, residual ligament, bar orientation and support span influence the measured fracture response. A Charpy test specimen prepared for ASTM D6110 is not an ISO 179-1 specimen merely because both carry V-notches.
3.1. ASTM D6110-26 Specimen Dimensions and V-Notch
The ASTM D6110-26 simple-beam geometry specifies a specimen length of 124.5 to 127.0 mm, a nominal overall depth of 12.70 ± 0.15 mm and a measured remaining depth beneath the notch of 10.16 ± 0.05 mm. The notch angle is 45° ± 1°, with a 0.25 ± 0.05 mm root radius. Specimen width is selected under the applicable material specification or agreement, within the method provisions. The support span is 101.6 ± 0.5 mm.
The charpy v notch concentrates bending stress at its root. A chipped or heat-damaged root changes notch sensitivity. For a v notch impact test, control cutter condition, inspect the notch contour and measure the remaining ligament instead of relying on nominal dimensions.
In injection-molded materials, notch machining and molded-in weld lines must be considered separately; injection molding materials vary in flow and sensitivity to processing history.
3.2. ISO 179-1:2026 Specimen Types and Notch Types
The common ISO 179-1 Type 1 bar is 80 ± 2 mm long, 10.0 ± 0.2 mm wide, and 4.0 ± 0.2 mm thick, with a support span of 62.0 to 62.5 mm. For the preferred Type A notch, the remaining width beneath the notch is 8.0 ± 0.2 mm. Type A has a root radius of 0.25 ± 0.05 mm; Type B uses 1.00 ± 0.05 mm, and Type C uses 0.10 ± 0.02 mm. These notched impact test geometries cannot be pooled.
ISO 179-1:2026 also defines Types 2 and 3 for materials exhibiting interlaminar shear, using thickness-based lengths and spans. Type 3 dimensions were corrected in the 2026 edition. Do not silently substitute a former edition. For ordinary injection-molded thermoplastics, state the complete designation, such as ISO 179-1/1eA, rather than writing only ISO 179.
4. Charpy Impact Test Procedure and Test Conditions
A charpy impact test procedure starts by recording resin grade, manufacturing route, test method and specimen direction. Inspect each notch and record the dimensions required by the method. Condition specimens, verify supports and pendulum corrections, strike the face opposite the notch, and classify each fracture. Retain only results permitted by the selected method.
4.1. Pendulum, Support Span, Conditioning and Temperature
ASTM D6110-26 specifies a nominal pendulum drop height of 610 ± 2 mm and an approximate impact speed of 3.46 m/s for its configuration. Select a suitable calibrated pendulum; a corrected energy exceeding 85% of nominal capacity is not retained as a valid result. ASTM normally specifies at least 40 hours of post-notching conditioning under ASTM D618 Procedure A unless an applicable material specification or permitted agreement provides otherwise. The standard atmosphere is 23 ± 2 °C and 50 ± 10% relative humidity.
ISO 179-1:2026 uses ISO 13802 for pendulum verification, normally requires at least 16 hours of conditioning after notching at 23 ± 2 °C and 50 ± 10% relative humidity unless otherwise specified, and requires an absorbed energy between 10% and 80% of available pendulum energy. The tested machine configuration and impact test temperature must be recorded. A 23 °C result cannot be substituted for performance at freezing temperatures.
PA66 is moisture-sensitive. The published nylon material properties guide explains why conditioning state matters when reading polymer data. Temperature and relative humidity alone do not establish an individual specimen’s moisture content.
5. Charpy Impact Test Formula, Absorbed Energy and Units
The indicated impact energy needs the loss corrections required by the selected apparatus and standard. Account for friction, windage and any applicable indication loss. A fixed correction is meaningful only for the stated equipment and correction procedure.
5.1. ASTM D6110 Impact Resistance (J/m)
For ASTM D6110-26, charpy impact resistance aC = E_net / b. Here E_net is corrected breaking energy in joules, and b is measured specimen width at the notch in metres. For example, 1.20 J / 0.00600 m = 200 J/m. This demonstrates the arithmetic, not a material test result. ASTM D6110 retains only complete breaks and excludes corrected energies exceeding 85% of selected pendulum capacity.
5.2. ISO 179-1 Notched Impact Strength (kJ/m²)
The ISO 179-1 charpy impact strength formula for a notched specimen is a_cN = 1,000 × Wc / (h × bN). Wc is corrected absorbed energy in J; h and remaining notch width bN are in mm; the result is kJ/m². Using the supplied PA66 specimen C01, 1,000 × 0.571 / (4.00 × 8.00) = 17.84375 kJ/m². The unnotched formula uses the full original specimen width instead of bN.
6. ASTM D6110 vs ISO 179-1: Which Results Are Comparable?
A simple units conversion does not turn an ASTM charpy impact test into an ISO charpy impact test. Comparing unlike geometries or notch conditions can change the crack initiation process. Specify the same standard edition, notch type, impact direction, material state and fracture reporting before ranking candidate materials.
The material properties resource helps identify resin families; it does not override the impact method stated on a supplier test report.
Method comparison table
| Item | ASTM D6110-26 | ISO 179-1:2026 |
| Normalised result | J/m of measured specimen width | kJ/m² of original notch-remaining section |
| Typical geometry | 124.5–127.0 mm length; 101.6 ± 0.5 mm span | Type 1: 80 ± 2 mm length; 62.0–62.5 mm span |
| Notch | 45° ± 1°; 0.25 ± 0.05 mm root | Type A/B/C have different radii |
| Energy selection | Corrected energy no more than 85% of capacity | Corrected absorbed energy 10–80% of capacity |
| Fracture reporting | Complete breaks enter method-specific mean | Report C/H, P and N groups according to ISO rules |
7. How to Interpret Charpy Impact Test Results
Charpy impact test results should include individual corrected energies, measured cross-sectional dimensions, failure categories, and a statistical summary. A charpy impact test graph is useful only when its axes disclose the test method, material state and specimens represented. Do not plot dry and conditioned polymer data on one unlabeled curve or use one number as a universal toughness ranking.
ISO 179-1 distinguishes C (complete), H (hinge), P (partial) and N (non-break). Its reported grouping combines C and H for the relevant outcome classification and follows the predominant fracture group; a predominant N outcome is reported as N rather than a conventional numeric mean. ASTM D6110 instead excludes specimens that have not completely separated from its ordinary breaking-value mean. These are different reporting systems.
EPOC CRAFTER engineering case CH-PA66-1018-A concerns an injection-molded electrical connector snap-fit made from BASF Ultramid A3K, unreinforced PA66. The supplied ten-specimen series is identified as ISO 179-1:2026/1eA with a 62.0 mm support span, nominal 2.00 J pendulum setting, 2.90 m/s stated striking speed, 23 °C and 50% RH conditioning for 48 hours, and a 0.25 mm Type A notch radius. The report lists individual corrected energies and dimensions.
Across ten specimens, the calculated mean is 18.42 kJ/m² and sample standard deviation 0.47 kJ/m²; the individual results range from 17.69 to 19.17 kJ/m². The case classifies all ten as complete breaks. These results characterize its stated specimen conditions, not connector snap-fit force, assembly cycling or drop-test acceptance. Evaluate weld lines, snap root geometry, moisture state and the actual loading profile on the finished part.
PA66 specimen calculations
| Specimen | Corrected energy (J) | Width h (mm) | Remaining bN (mm) | Impact strength (kJ/m²) |
| C01 | 0.571 | 4.00 | 8.00 | 17.84 |
| C02 | 0.594 | 4.01 | 8.01 | 18.49 |
| C03 | 0.583 | 3.99 | 7.99 | 18.29 |
| C04 | 0.605 | 4.00 | 8.00 | 18.91 |
| C05 | 0.589 | 4.02 | 8.02 | 18.27 |
| C06 | 0.612 | 4.00 | 7.98 | 19.17 |
| C07 | 0.564 | 3.98 | 8.01 | 17.69 |
| C08 | 0.597 | 4.01 | 8.00 | 18.61 |
| C09 | 0.601 | 4.00 | 7.99 | 18.80 |
| C10 | 0.580 | 3.99 | 8.01 | 18.15 |
When component production involves mold tooling, rapid injection molding can reproduce the geometry needed for snap-root and weld-line testing.
8. Charpy vs Izod Impact Test
A charpy and izod impact test comparison starts with the fixture: Charpy rests the bar on two supports and strikes its midpoint, while Izod clamps a cantilever specimen. A charpy vs izod value comparison without full method details is invalid. ASTM D256 and ISO 180 apply to Izod, not the Charpy standards used here.
The published Izod impact strength guide addresses that cantilever configuration, including its separate specimen and reporting requirements.
9. Charpy Impact Testing of 3D-Printed Plastics
For 3d printed impact strength, record build direction, raster orientation, wall configuration and notch preparation. A crack may pass through extruded roads or follow layer interfaces. Printed, milled-from-print and injection-molded bars should not be combined into a single material ranking without evidence of comparable specimen states.
9.1. Build Orientation, Anisotropy and Moisture
3d printing anisotropy affects the fracture path, especially when a notch aligns with layer boundaries. Compare flat and upright specimens only as separate, documented groups. Also record drying, conditioning and test temperature: FDM impact strength may vary with voids and interlayer bonding even when polymer grade is unchanged.
The 3D printing filament types guide discusses feedstock and drying factors relevant to printed specimen preparation.
The additive manufacturing process should also be recorded alongside orientation and infill when the test coupon is printed.
10. FAQs on Charpy Impact Testing
10.1. What Is Charpy Impact Strength?
It is corrected absorbed impact energy normalized under a specified Charpy test method. For notched plastics, ASTM D6110-26 reports J/m and ISO 179-1:2026 reports kJ/m², using different normalization geometries.
10.2. What Does Charpy Impact Test Measure?
It measures the response of a prescribed plastic beam to a single pendulum strike, typically emphasizing fracture behavior at a notch. It does not directly measure an assembled part’s drop resistance or long-term impact fatigue.
10.3. How to Calculate Charpy Impact Strength?
First correct the indicated breaking energy according to the equipment and method. Then divide by the specimen width for ASTM D6110 or by the relevant specimen cross-sectional area for ISO 179-1. Report the applicable units and failure class.
10.4. What Is the Difference Between Charpy and Izod?
Charpy supports the specimen at both ends; Izod clamps one end of a vertical specimen. Their impact positions, methods, and results are not directly interchangeable.
10.5. What Affects Impact Strength?
Polymer grade, reinforcement, notch shape, specimen orientation, humidity, temperature, manufacturing history and fracture class can all change the measured response. Compare matching methods and specimen conditions before interpreting any ranking.
Request the material grade, batch identifier, standard edition, test method designation, specimen geometry, notch preparation, orientation, conditioning, test temperature, corrected individual energies and failure groups. Charpy impact test results help screen materials; finished plastic parts still need functional testing under the intended load and environment.
A rapid prototyping program can reproduce the finished geometry for assembly and drop tests after material screening.
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