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).
Impact strength describes how a material responds to a sudden blow under specified test conditions. For plastics, it is commonly evaluated using Izod or Charpy pendulum tests, which measure the energy absorbed by a specimen during impact. The reported value depends on the test method, specimen dimensions, notch geometry, temperature, and material condition.
ASTM D256 and ASTM D6110 generally report impact resistance in J/m, while ISO 180 and ISO 179-1 express the corresponding impact strength in kJ/m². These values cannot be compared directly. Even when two plastics are tested using the same standard, differences in specimen thickness, conditioning, or manufacturing history can change the results.
For engineers selecting materials for housings, clips, brackets, and protective components, impact strength is a screening property, not a prediction of finished-part durability. A plastic with high impact resistance in a laboratory specimen can still crack at a sharp corner or snap-fit root under service loads.
You need three things before using an impact value in a design decision: a clearly identified test method, comparable material and specimen conditions, and a validation plan that reflects how the actual part will be loaded.
1. What Is Impact Strength in Plastics?
Plastic impact strength is a result from a specified rapid-impact test, not a geometry-independent design allowable.
Unlike tensile strength, which is measured under controlled tensile loading, impact testing subjects a specimen to a rapid blow. The result reflects how the material and specimen geometry respond at the specified loading rate.
A higher reported impact value generally indicates greater resistance under that particular test configuration. It does not automatically mean the material will perform better in every product.
Consider two plastic housings made from different polymer grades. One may have a higher notched Izod value, yet its finished housing could fail earlier because of a thin snap-fit root, a sharp internal corner, or different assembly constraints.
The impact resistance of plastics depends on both material behavior and the way the load is applied. Polymer composition, reinforcement, processing, notch preparation, and temperature can all affect fracture behavior.
Impact properties therefore need to be read alongside other material data, including tensile strength, stiffness, elongation, and environmental performance.
1.1. Impact Strength, Impact Energy, and Toughness
Impact energy is the energy absorbed by the specimen during an impact event. In a pendulum test, the instrument measures an energy loss that must be corrected for applicable machine losses before calculating the reported material result.
Impact strength is a normalized result derived from that corrected energy according to the applicable test standard.
Material toughness, sometimes discussed as polymer toughness, is a broader concept describing the ability to absorb energy before failure. It is not interchangeable with a notched impact value. The comparison of impact strength vs toughness is therefore context-dependent. Fracture toughness is also a different property, determined using its own defined fracture mechanics methods.
| Property | What it describes | Engineering use |
| Impact energy | Corrected energy absorbed during a specified impact test, in joules | Evaluating the tested specimen’s response |
| Impact strength | Absorbed energy normalized by dimensions specified in the method | Comparing plastics under equivalent conditions |
| Material toughness | Capacity to absorb energy and deform before failure | Understanding broader material behavior |
| Fracture toughness | Resistance to crack extension in a defined fracture mechanics test | Assessing crack-driven failure |
For example, a plastic may absorb considerable energy in an unnotched specimen but show much lower resistance once a sharp notch is introduced. The notch creates a local stress concentration that can initiate fracture before substantial plastic deformation develops.
Snap-fit roots, screw bosses and mounting tabs introduce local stress concentrations that an unnotched laboratory bar may not reproduce.
2. How Is Plastic Impact Strength Measured?
Impact testing of plastics commonly uses a calibrated pendulum to strike a prepared impact test specimen once. An impact testing machine measures the energy lost during the blow; the test method specifies corrections, supports, specimen geometry, and how the result is normalized. No single impact test reproduces every dropped, struck, or constrained finished part.
2.1. Izod Impact Test and Izod Impact Strength
An izod impact test clamps a specimen as a vertical cantilever. In the standard notched setup, the striker loads the specimen near the notch. The resulting izod impact strength describes resistance in that arrangement, not a universal fracture property.
ASTM D256-26 commonly reports corrected breaking energy divided by the specimen width at the notch, in J/m. ISO 180:2023 uses a defined cross-sectional area and reports kJ/m². ISO 180 distinguishes unnotched U and notched A/B arrangements. These methods should retain their own identifiers in a material specification.
ASTM D256-26 includes Test Methods A and C and specific provisions for notch sensitivity and reversed notch arrangements. A specimen tested under one arrangement cannot silently be reported as another. For a more detailed treatment of specimen geometry and calculations, see EPOC CRAFTER’s Izod impact strength testing guide
2.2. Charpy Impact Test and Charpy Impact Strength
In a charpy impact test, the bar lies horizontally across two supports. The striker hits near the center, opposite the notch in the standard notched arrangement. This simple-beam loading differs materially from the clamped cantilever used in Izod.
ASTM D6110-26 specifies the notched Charpy approach and reports charpy impact strength as corrected energy per measured specimen width, in J/m. ISO 179-1:2026 describes non-instrumented Charpy configurations, including specified notched and unnotched tests, with results reported in kJ/m². A charpy impact test specimen must match the selected standard and method code.
An ASTM D6110 bar is typically about 125 mm long, with a nominal 12.70 mm depth and a 0.25 mm notch-root radius. A common ISO 179-1 Type 1 specimen is 80 × 10 × 4 mm with a method-dependent support span and notch geometry. These are method-specific dimensions, not interchangeable specimen blanks.
2.3. Izod vs Charpy: Specimen Support, Notch, and Results
For a charpy vs izod test comparison, start with the fixture. The impact point, bending state, constraint, and notch position change the stress field. Even a matching nominal notch radius does not produce an equivalent result. An izod vs charpy comparison belongs in separate columns unless a material-specific correlation has been independently established.
| Question | Izod | Charpy |
| Support | One end clamped as a cantilever | Specimen supported near both ends |
| Strike | Defined position relative to clamp and notch | Near midspan, opposite notch for notched test |
| Primary ASTM plastics standards | ASTM D256-26 | ASTM D6110-26 |
| Primary ISO plastics standards | ISO 180:2023 | ISO 179-1:2026 |
| Interpretation | Method-specific index | Method-specific index; not a conversion of Izod |
2.4. ASTM D256, ASTM D6110, ISO 180, and ISO 179-1
Four standards appear frequently on plastic material data sheets. They address related impact behavior, but their specimen sizes, fixtures, notch details, calculations, and fracture reporting are not identical. The ISO 179 designation in older files should be checked against the specific ISO 179-1:2026 edition and method code.
| Standard and edition | Configuration | Typical reported unit | Critical report detail |
| ASTM D256-26 | Izod pendulum, method identified | J/m | Method A/C, notch geometry, break type and width |
| ASTM D6110-26 | Notched Charpy, supported beam | J/m | Specimen width, support span, complete-break reporting |
| ISO 180:2023 | Izod, A/B/U as applicable | kJ/m² | Notch designation, thickness, remaining ligament and fracture type |
| ISO 179-1:2026 | Non-instrumented Charpy, specified method | kJ/m² | Method code, impact orientation, notch and fracture category |
Notching and conditioning are also part of the test identity. Under ASTM D6110-26, the default conditioning provisions refer to ASTM D618 and include at least 40 hours after notching unless the permitted alternative applies. ISO 179-1:2026 generally uses its specified ISO 291 atmosphere and conditioning provisions, subject to material-specific requirements. The applicable material standard takes precedence where it specifies a different condition.
Record the source and preparation of the bars. Injection-molded flow direction, machined surfaces, fiber orientation, and printing layers can change the answer. The applicable manufacturing tolerances and standards are useful context when defining dimensional and inspection requirements, but they do not replace the selected impact test standard.
2.5. Calculating and Reporting Impact Strength
An impact strength calculation starts with corrected absorbed impact energy, and the impact strength formula must follow the method. not the nominal pendulum capacity. The reporting formula, impact strength units and denominator depend on the standard. For ASTM D256-26 and ASTM D6110-26, the principal energy-per-width expression is net absorbed energy (J) divided by measured specimen width (m), producing J/m. It must not be relabeled as kJ/m².
For a typical ISO notched test, impact strength (kJ/m²) = corrected absorbed energy Wc (J) × 1000 / [specimen thickness h (mm) × remaining width beneath notch bN (mm)]. The unnotched counterpart uses the specified full cross-sectional dimension instead of bN. Consult the individual ISO method definition before using either expression.
For a numerical illustration drawn from the supplied engineering case, a notched ISO 180 specimen absorbed 0.48 J after corrections. With h = 4.0 mm and bN = 8.0 mm, its calculated value is 0.48 × 1000 / (4.0 × 8.0) = 15.0 kJ/m². This is one specimen result, not the batch average or a finished-part energy rating.
A useful supplier report identifies the resin grade, processing history, standard edition and method, actual specimen dimensions, notch preparation, conditioning and impact temperature, pendulum configuration, fracture type, individual results, sample count, and relevant statistics. Charpy impact energy measured in joules is not the same number as its normalized Charpy resistance.
3. What Changes Impact Strength Results?
Reported impact strength changes when the polymer changes, but it can also change when the same polymer is cut, molded, conditioned, or tested differently. Treat notch sensitivity, temperature and moisture, and specimen geometry as controlled variables, not footnotes.
3.1. Notched vs Unnotched Impact Strength and Notch Sensitivity
Notched impact strength examines a deliberately weakened section. Notched vs unnotched impact strength is therefore a comparison of different specimen configurations, not a pair of interchangeable material constants. A high unnotched result can coexist with low notched izod impact strength.
Notch sensitivity refers to the degree to which an introduced notch changes fracture response. A smaller notch-root radius generally raises local stress concentration, but the measured response also depends on plastic grade, thickness, test speed and preparation damage. The default Type A notch radius in ISO 180 and ISO 179-1 is 0.25 ± 0.05 mm; those standards also specify other permitted notch arrangements. Do not mix them into one material ranking.
For product engineers, inspect the roots of snap fits and ribs, abrupt section changes, screw bosses, and machined inside corners. They can concentrate load much like a test notch. The DFM design guidelines for plastics provide a natural next step, although a DFM radius suggestion is not an impact test result.
3.2. Temperature, Moisture, and Strain Rate
Low temperature impact strength can fall as a polymer becomes less able to deform before cracking. The relationship between impact strength vs temperature is grade-specific and may be abrupt near a ductile-to-brittle transition. A room-temperature material data sheet is not a low-temperature product qualification.
Moisture effect on plastic strength matters especially for moisture-sensitive polyamides. Conditioning may alter ductility, dimensions and impact behavior; dry-as-molded nylon and humidity-conditioned nylon are different comparison states. Describe the precise conditioning condition rather than saying only “nylon impact strength.”
The strain rate effect is also material dependent. A fast localized hit can cause a different failure mode from a slowly applied assembly force. Service temperature, exposure duration and actual impact speed should be matched in a separate qualification when the finished part is safety- or function-critical.
3.3. Thickness, Orientation, and Manufacturing History
The thickness effect on impact strength is not simply a scaling factor. Thickness changes constraint and fracture mode, while different bar orientations sample different flow, fiber, or layer structures. A vendor’s reported 4 mm specimen does not characterize every thin wall in a molded enclosure.
Molding conditions, weld lines, residual stress, machining cuts and printed interfaces all contribute. A bar machined from a finished panel can behave differently from a separately injection-molded bar. When evaluating CNC machining of engineering plastics, note sheet or rod orientation, stock history and critical internal corners.
3.4. Brittle and Ductile Fracture Modes
Brittle fracture involves relatively little macroscopic deformation before the crack propagates. Ductile fracture involves more deformation and energy dissipation. The brittle vs ductile fracture outcome can shift with temperature, notch geometry, grade and impact speed, so a single label should not be assigned permanently to a polymer family.
ASTM D6110-26 distinguishes complete breakage for its reported resistance calculation; specimens not broken completely are treated differently and should not be silently averaged with complete breaks. ISO methods report their prescribed complete, hinge, partial and non-break categories. A number without its fracture mode can conceal a meaningful change in material behavior.
4. How to Select Impact-Resistant Plastics
Impact resistant materials should be selected for an identifiable part, environment and manufacturing process. No impact resistant plastic can be chosen reliably on the basis of an unspecified headline value alone. Start with plastic material properties that control stiffness, chemical exposure, dimensional stability and fracture response. Then compare impact data using the same method and condition. A high impact plastic claim without grade and test identity is not sufficient for a drawing or purchasing specification.
4.1. ABS, Polycarbonate, Nylon, PP, PLA, and PETG
ABS impact strength depends on rubber modification and grade. Polycarbonate impact strength can be useful for protective covers, but notch and chemical exposure still matter. PC/ABS blends offer a different balance of processing and mechanical behavior from either neat polymer. Polypropylene impact strength varies widely between homopolymer, copolymer and modified formulations.
Nylon impact strength depends strongly on moisture state, reinforcement and grade. PLA impact strength and PETG impact strength should be considered in the context of how the specimen was manufactured, particularly for FDM parts. These labels do not support an unconditional ranking of the six materials.
Instead, request the same test method, specified impact temperature, notch state and specimen thickness for the candidate grades. Cross-check stiffness, environment and manufacturability using the engineering plastic material properties resource and the injection molding material selection guide.
4.2. Injection Molding vs FDM 3D Printing
Injection molding impact strength is influenced by mold filling, fiber or molecular orientation, weld lines, residual stress and post-molding conditioning. EPOC CRAFTER’s rapid injection molding services can support production-representative prototypes, but prototype and production conditions must still be documented.
For 3D printing impact strength, including FDM impact strength, build orientation, raster direction, interfaces, infill and porosity affect the response. A test of 3D printed plastic strength along the deposited roads does not automatically represent impact across weak layer interfaces. In practice, 3D printing anisotropy is a reason to test more than one orientation where the part’s load path warrants it.
The 3D printing and additive manufacturing capability page explains the process context. Existing articles on ABS vs PETG strength and PETG vs PLA impact resistance address specific printing material choices.
5. Using Impact Data in Plastic Part Design
In plastic part design, a material impact index can reject clearly unsuitable choices or identify candidates for further testing. It cannot tell you how much impact load a thin wall, clip or mounted enclosure survives. For that question you need geometry, support stiffness, striker contact, impact location, rate and a functional acceptance criterion.
5.1. Impact Loads, Drop Testing, and Part-Level Verification
A drop impact test applies kinetic energy to an assembled component under a defined event. Unlike pendulum-bar testing, a drop test plastic part can transfer energy into screws, ribs, adjoining pieces and the backing structure. A specimen impact number and an incident drop energy in joules are not equivalent measures.
EPOC CRAFTER’s anonymized engineering case concerns an electrical connector protective housing molded from unreinforced Covestro Bayblend T65 XF PC/ABS. The housing is 120 × 75 × 28 mm, with 2.5 mm nominal wall thickness, a 2.0 mm snap-fit root and a 0.8 mm root radius. Four screws hold it against a rigid backplate. The intended impact concern is a tool strike during maintenance.
Five machined Type A notched Izod bars from the specified resin had dimensions 80 × 10 × 4 mm, with an 8.0 mm remaining ligament. The recorded ISO 180:2023 test used 23 °C and corrected absorbed energies of 0.48, 0.51, 0.46, 0.50 and 0.49 J. The calculated notched impact strengths are 15.00, 15.94, 14.38, 15.63 and 15.31 kJ/m², respectively; mean 15.25 kJ/m² and sample standard deviation about 0.60 kJ/m². All five bars were classified as complete breaks.
The five recorded bars are reported as one engineering dataset. ISO 180:2023 generally specifies at least ten specimens per relevant direction unless a permitted exception applies; this distinction affects how broadly the sample statistics can be interpreted.
For the housing-level event, a 0.250 kg hemispherical striker falls 0.800 m onto the front face, 15 mm from a fixed corner. The calculated incident energy is 0.250 × 9.80665 × 0.800 = 1.961 J, and ideal contact speed is about 3.96 m/s. The striker contact radius is 12.5 mm; the assembled snap fits are closed and the backing plate is rigid.
The specified acceptance conditions are no through-crack, no released snap fit, permanent indentation no greater than 0.5 mm, joint gap increase no greater than 0.3 mm, and continued protective function. The available case dataset records these limits and the impact setup, but does not give the individual measured housing outcomes for the five assemblies. No finished-part pass or fail conclusion is therefore stated.
This is exactly why the two measurements belong in separate parts of the engineering decision. The 15.25 kJ/m² bar result is a method-specific material comparison index. The 1.961 J figure describes nominal input energy at the housing. Neither value alone establishes its post-impact geometry or snap-fit function. A proposed increase in root radius from 0.8 to 1.2 mm is a documented design option, not an observed improvement in the five-bar results.
For a consumer electronics housing manufacturing project, document the strike position, actual assembly and inspection limits before selecting resin. Rapid prototyping for impact validation can help establish a representative geometry and test sequence, but do not transfer a prototype pass to a different production process without checking the differences.
6. Frequently Asked Questions
6.1. How Do You Measure Impact Strength?
To answer how to measure impact strength, use a specified impact strength test such as ASTM D256-26, ASTM D6110-26, ISO 180:2023 or ISO 179-1:2026. Prepare the correct specimen, condition it, measure the relevant dimensions, strike it with the prescribed machine, apply energy corrections and report the result with its method, units, fracture category and sample statistics.
6.2. How Is Impact Strength Different from Tensile Strength?
Impact strength vs tensile strength compares different test responses. A tensile test pulls a specimen under a defined loading schedule and reports quantities such as strength and elongation. An impact test applies a rapid blow to a specified bar geometry. A high tensile-strength value does not establish resistance to a notched impact or assembled-part drop.
6.3. Can You Compare Izod and Charpy Values Directly?
No. The fixtures, specimen geometry, notch and strike locations, and normalization rules differ. Matching units are not evidence of equivalence. A validated material-specific correlation may support a limited comparison, but there is no universal conversion factor between Izod and Charpy results.
6.4. How Can You Improve Plastic Impact Strength?
How to improve impact strength depends on the failure. For material test performance, consider a different verified grade, modification, conditioning or processing state. For an actual part, reduce sharp stress raisers, examine wall transitions and attachment constraints, and test at the temperature and speed the part will experience. Improvements to finished-part survival do not necessarily increase a standardized specimen’s reported value.
For supplier comparisons, specify the resin grade, preparation, notch, orientation, temperature and reporting method before comparing results.
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