An impact strength test report is a formal technical document that records the energy a material absorbs when fractured under a sudden, high-speed load, providing essential data for material selection, quality control, and regulatory compliance. This guide explains what a professional impact strength test report contains, which international standards govern the testing, how specimens are prepared and tested, and how engineers should interpret the results across plastics, metals, composites, and packaging materials.
Technical Overview
Impact strength describes a material's ability to resist fracture under rapidly applied stress, such as a collision, drop, or sudden shock load. Unlike tensile or flexural testing, which applies load slowly, impact testing subjects a specimen to a high strain rate over milliseconds, revealing brittle failure modes that quasi-static tests cannot detect. An impact strength test report issued by an accredited third-party laboratory documents the complete test chain, from specimen preparation and conditioning to the final statistical results, so that the data can be defended in supplier audits, product certifications, and failure investigations.
A professional report typically includes the following core elements, each of which must be traceable to the referenced standard:
- Complete sample description: material grade, colorant, filler content, supplier, and batch number
- Specimen geometry, thickness, and notch type (notched or unnotched)
- Molding or machining method used to produce the test bars
- Conditioning atmosphere (typically 23°C and 50% relative humidity) and duration
- Test standard, apparatus model, pendulum capacity, and calibration status
- Individual and mean energy values, statistical scatter, and fracture mode classification
- Clear pass/fail conclusion against the customer's specification limit
The report conclusion must distinguish between "complete break," "hinge break," "partial break," and "non-break" results, because a non-break value recorded at the full pendulum capacity is an invalid data point rather than a true material strength. Accredited laboratories operating under ISO/IEC 17025 are required to state this explicitly, along with the measurement uncertainty of the pendulum system.
Testing Samples
Plastics and Engineering Polymers
The largest category of impact testing covers thermoplastics such as ABS, PC, PP, PE, PA (nylon), POM, PBT, and PVC. Specimens are usually injection-molded bars of 80 mm × 10 mm × 4 mm for ISO-based testing, or 63.5 mm × 12.7 mm rectangular bars for ASTM-based testing. Both notched and unnotched configurations are accepted, and glass-fiber-reinforced grades are tested in both the flow and transverse directions because impact anisotropy in reinforced plastics can exceed a factor of two.
Metals and Alloys
Metallic materials, including structural steels, aluminum alloys, cast irons, and pipeline steels, are evaluated with Charpy V-notch specimens of 10 mm × 10 mm × 55 mm per ASTM E23, EN ISO 148-1, and GB/T 229. For weld procedure qualifications, the notch is machined precisely into the weld metal, the fusion line, or the heat-affected zone. Ductile-to-brittle transition behavior is documented by testing at a series of temperatures down to -196°C.
Composites and Advanced Materials
Carbon-fiber and glass-fiber laminates, sheet molding compounds, and sandwich structures require larger specimen energies and are often evaluated with instrumented drop towers rather than pendulums. Fiber orientation, layup sequence, and interlaminar toughness dominate the result, so the report must record laminate orientation codes and cure history.
Films, Sheets, and Packaging Materials
Flexible packaging films are tested with free-falling dart methods (ASTM D1709, ISO 7765-1) in which the mass causing 50% failure is reported. Rigid sheets, thermoformed trays, and injection-molded containers are assessed with falling-weight or Gardner-type impact (ASTM D5420) to simulate drop and handling loads.
Rubbers, Coatings, and Ceramics
Vulcanized elastomers and thermoplastic elastomers are usually characterized by puncture and high-speed tensile methods, while coating systems on metal substrates are checked for impact-induced cracking and adhesion loss. Technical ceramics rely on notched-beam or instrumented methods because their fracture energies are extremely low and scatter is high.
Testing Items
A typical impact strength test report can cover one or more of the following test items, each answering a different engineering question:
- Izod impact strength (notched and unnotched) – pendulum strikes a vertically clamped cantilever specimen; the standard method for plastics per ASTM D256 and ISO 180
- Charpy impact strength – pendulum strikes a horizontally supported beam specimen per ISO 179-1, ASTM D6110, and ASTM E23; the universal method for both plastics and metals
- Tensile impact strength – a... a high-speed pendulum fractures a dumbbell-shaped specimen in tension per ASTM D1822, used for materials too flexible or too tough for flexural impact
- Falling dart impact (film) – staircase or probit method reporting dart mass at 50% failure per ASTM D1709 and ISO 7765-1
- Drop weight and instrumented puncture (multi-axial impact) – a falling tup with force-displacement recording per ASTM D3763, ISO 6603-2, and ASTM D5628, producing full energy-deflection curves
- Gardner / falling weight impact – pass/fail or statistical mean-failure-height testing for painted and coated panels per ASTM D5420 and ASTM D2794
- Low-temperature and thermal-conditioned impact – testing at -40°C, -30°C, or elevated temperatures to verify cold-weather ductility for automotive and outdoor applications
Testing Methods
Pendulum Impact Method
A calibrated pendulum hammer is released from a fixed height; the difference between the strike energy and the residual swing energy equals the energy absorbed by fracturing the specimen. Energy capacity must be selected so that the specimen absorbs between 10% and 80% (per ASTM D256) or 15% and 85% (per ISO 180) of the available energy, otherwise friction and windage corrections dominate the result. Notch radii are strictly controlled: ISO defines notch types A (0.25 mm radius), B (1.0 mm radius), and C (0.10 mm radius), and a dull notch cutter can artificially depress results by 30% or more, which is why laboratories must verify cutter geometry on a scheduled basis.
Drop Tower and Instrumented Method
A carriage with an instrumented tup falls onto a clamped disc or plate, and a load cell records the full force-time history at sampling rates of 100 kHz or higher. This yields peak force, deflection at peak force, total absorbed energy, and failure initiation point, which pendulum methods cannot provide. Instrumented data are indispensable for crash-performance studies and for materials that show ductile-to-brittle transitions depending on thickness.
Specimen Preparation and Conditioning
Specimen preparation is the single largest source of impact data scatter. The report must state whether bars were injection molded with gate location recorded, or machined and notched from plate or finished parts, because machined surfaces carry residual stresses and heat damage. All specimens are conditioned to equilibrium at 23°C ± 2°C and 50% ± 10% relative humidity per ISO 291 or ASTM D618 before testing. A minimum of five specimens per condition is required by most standards, and ten or more when a statistical pass/fail decision is needed.
Data Reduction and Units
- ASTM notched Izod: joules per meter of notch width (J/m)
- ISO notched Izod/Charpy: kilojoules per square meter (kJ/m²)
- Metallic Charpy: absorbed energy in joules (J) with lateral expansion in millimeters
- Instrumented puncture: total energy in joules with peak force in newtons
Because ASTM and ISO values are not numerically interchangeable owing to different specimen geometry and notch practice, the report must never convert between systems without a clearly labeled conversion basis.
Report Contents and Interpretation
When reviewing an impact strength test report, engineers should verify the following before accepting the data:
- The tested material and batch match the submitted sample and certificate of analysis
- Notch type, specimen thickness, and direction relative to molding flow are explicitly stated
- Individual specimen values are listed, not only the average, so that outliers and bimodal failure are visible
- Fracture mode classification accompanies every value; "non-break" results must be flagged
- Test temperature and conditioning history are recorded, especially for low-temperature series
- The conclusion quotes the exact specification limit and clause against which judgment is made
Comparative interpretation also matters: a notched result measures notch sensitivity and crack propagation resistance, while an unnotched result reflects the bulk toughness of the material. A material with high unnotched but low notched impact strength is notch-sensitive and should be redesigned with generous fillet radii rather than simply substituted with a tougher grade.
Applications
Impact strength test reports support decisions across nearly every engineered-product industry:
- Automotive: validating bumper fascias, interior trim, battery enclosures, and under-hood components at temperatures from -40°C to 85°C against OEM and GB/T, ISO, and ASTM material specifications
- Construction and infrastructure: confirming cold toughness of structural steel welds, pipeline fittings, window profiles, and roofing sheets against EN, GB, and ASTM product standards
- Consumer goods and appliances: demonstrating drop and knock resistance of housings, connectors, and food containers for safety and market-access compliance
- Packaging: qualifying film dart impact and container drop performance for transport simulation and shelf-life programs
- Electronics and medical devices: screening enclosure materials for drop durability and sterilization-conditioned toughness retention
- Failure analysis and R&D: comparing formulation changes, recycled-content levels, or processing windows through controlled impact comparisons on identical specimen geometry
Because impact strength is highly sensitive to thickness, notch quality, temperature, and processing history, reports are most defensible when the test conditions replicate, as closely as possible, the service conditions of the finished part.
FAQ
How many specimens are needed for a valid impact strength test report?
Most standards require a minimum of five specimens per condition for plastics and three per temperature for metallic Charpy testing. For statistical pass/fail decisions or materials with known scatter, such as composites and recycled grades, ten or more specimens are strongly recommended so the report can present a reliable mean and standard deviation.
What is the difference between notched and unnotched impact strength?
Notched testing introduces a controlled stress concentrator and measures resistance to crack propagation and notch sensitivity, while unnotched testing measures overall ductility. Some polymers, such as super-tough nylon, may not break completely in unnotched tests, and the report must record this as "non-break" rather than inventing a numeric value.
Which standard should I choose: ASTM D256 or ISO 180?
The choice usually follows the market and specification of the end product. North American specifications typically reference ASTM D256 in J/m, while European, Asian, and ISO-aligned specifications reference ISO 180 in kJ/m². Because specimen dimensions, support geometry, and notch practice differ, results are not directly convertible, and the report must state which system was used.
Can the test be performed on finished parts instead of molded bars?
Yes, provided the report clearly states that results are for comparative purposes on that specific part geometry. Cut parts carry machining damage and variable thickness, so data cannot be compared with published material datasheet values generated on standard bars.
How does temperature affect the reported impact strength?
Impact strength can change by an order of magnitude across the ductile-to-brittle transition. Engineering polymers may lose more than half of their room-temperature toughness at -40°C, while metals show a characteristic transition curve. Any report used for outdoor, automotive, or cold-chain applications should include at least one low-temperature test point.
How long does an impact strength test report take?
For standard conditioned specimens, most accredited third-party laboratories complete testing and issue the report within five to seven working days after sample receipt. Low-temperature series, custom machining, or full instrumented force-dispression characterization may require additional time, and expedited services are commonly available.
Does the report have an expiration date?
The report reflects the properties of the specific batch of specimens tested at the stated date and conditions; it does not formally expire. However, many certification schemes and customers accept reports issued within one to three years, or require retesting whenever the material formulation, supplier, or processing conditions change.
What should I do if my sample fails the specification limit?
Review the individual specimen values and fracture modes first. A failure caused by notch-cutting damage, insufficient conditioning, or a single outlier justifies a retest. If the failure is genuine, options include selecting a tougher grade, increasing wall thickness at stress concentration points, adding fillet radii, adjusting processing parameters to reduce molded-in stress, or negotiating a revised specification with the end customer.