Impact Strength Testing of Plastic Sheets and Panels: Methods, Standards, and Technical Requirements

Impact strength testing of plastic sheets and panels is a critical mechanical evaluation used to determine how well sheet materials resist fracture under sudden, high-speed loading conditions. This testing is essential for quality control, material certification, and product development across industries such as construction, automotive, glazing, signage, and packaging. This article provides a comprehensive technical overview of impact strength testing methods, applicable international standards, specimen requirements, and practical applications for plastic sheet materials.

Technical Overview

Impact strength is a measure of a material's ability to absorb energy during rapid deformation and resist brittle fracture when subjected to sudden shock loads. For plastic sheets and panels, this property is one of the most important mechanical performance indicators because sheet products are frequently exposed to impact events during transportation, installation, and service life, including hail impact, wind-borne debris, tool drops, vandalism, and dynamic loading in vehicles.

Unlike metals, plastics exhibit pronounced viscoelastic behavior, meaning their impact response is strongly dependent on temperature, strain rate, molecular orientation, and stress concentration geometry. A plastic sheet that performs well at room temperature may become brittle at low temperatures, and a smooth surface may fail prematurely when scratched or notched. For this reason, impact strength testing of plastic sheets must be conducted under carefully controlled conditions specified in recognized international standards.

There are two principal families of impact test methods for plastic sheets: pendulum impact tests (Charpy and Izod), which measure the energy absorbed by a swinging hammer as it breaks a clamped or supported specimen, and falling-weight tests (falling dart, Gardner, and instrumented puncture impact), which drop a striker with a defined geometry onto a clamped sheet specimen. Results are typically expressed in kJ/m² (energy per unit cross-sectional area) for pendulum tests and in joules or as a statistical F50 failure energy for falling-weight tests.

Testing Samples

Thermoplastic Engineering Sheets

Common engineering sheet materials submitted for impact testing include polycarbonate (PC) solid and multiwall sheets, polymethyl methacrylate (PMMA/acrylic) cast and extruded sheets, and acrylonitrile butadiene styrene (ABS) sheets. These materials are widely used in glazing, safety guards, machine covers, and display applications where impact resistance is a primary selection criterion.

Commodity Plastic Sheets

Rigid polyvinyl chloride (PVC-U) sheets, foamed PVC panels, polypropylene (PP) solid and corrugated sheets, polyethylene (HDPE and LDPE) sheets, and polyethylene terephthalate (PET) sheets are routinely tested for impact performance. These materials serve in chemical liners, wall cladding, packaging, hygiene panels, and general fabrication.

Reinforced and Composite Panels

Glass-fiber reinforced plastic (FRP/GRP) panels, aluminum composite panels with plastic cores, and glass-mat thermoplastic sheets require impact evaluation because their failure behavior is governed by the interaction between the polymer matrix and reinforcement architecture. Testing is often performed on both faces and in both principal directions to reveal anisotropy introduced by the reinforcement.

Specimen Orientation and Thickness

Because extruded, calendered, and laminated sheets are anisotropic, specimens should be cut in both the machine (extrusion) direction and the transverse direction. For pendulum tests, the preferred specimen thickness is 4 mm; specimens from thicker sheets may be machined on one face (with the tested surface noted), while thin sheets may be tested at original thickness or evaluated by falling-weight methods that better represent in-service sheet geometry.

Testing Items

Typical impact strength test items for plastic sheets and panels include:

• Charpy impact strength, notched and unnotched, in accordance with ISO 179-1, ASTM D6110, and GB/T 1043.1
• Izod impact strength, notched and unnotched, in accordance with ISO 180, ASTM D256, and GB/T 1843
• Falling dart and falling weight impact resistance (F50 failure energy) in accordance with ASTM D5628, ASTM D5420, ISO 7765, and GB/T 11548
• Instrumented puncture impact (peak force, deflection at peak force, and energy to failure) in accordance with ISO 6603-2 and ASTM D3763
• Low-temperature impact strength at specified service temperatures (for example -20 °C, -30 °C, or -40 °C)
• Notch sensitivity evaluation comparing notched versus unnotched performance
• Directional comparison of machine-direction versus transverse-direction impact resistance

Testing Methods

Specimen Preparation and Conditioning

Test specimens are cut from representative sheet samples, free of visible defects, warp, and surface damage. Specimens are then conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 5 % relative humidity for a minimum of 16 hours (48 hours or longer is recommended for hygroscopic materials such as nylon). Where notched testing is required, notches are machined with a dedicated notching cutter: the Type A notch (2.0 mm depth, 0.25 mm tip radius) is the standard reference notch, while Type B (1.0 mm radius) and Type C (0.10 mm radius) notches are used for specific evaluations. The notch tip radius must be verified microscopically because it strongly influences the measured result.

Charpy Impact Testing (ISO 179-1 / ASTM D6110 / GB/T 1043.1)

In the Charpy configuration, a rectangular specimen (typically 80 mm × 10 mm × 4 mm per ISO) is supported horizontally on anvils with a defined span (62 mm per ISO) and struck by a pendulum on the face opposite the notch. The pendulum hammer energy is selected so that the specimen absorbs between approximately 10 % and 80 % of the available energy, and friction and windage corrections are applied. The impact strength is calculated as the absorbed energy divided by the cross-sectional area under the notch (notched) or the full cross-section (unnotched), reported in kJ/m². Failure behavior is classified as complete break (C), hinge break (H), partial break (P), or non-break (N).

Izod Impact Testing (ISO 180 / ASTM D256 / GB/T 1843)

In the Izod configuration, the specimen is clamped as a vertical cantilever and struck by the pendulum on the notched face at a fixed distance above the clamp. Notched Izod results per ASTM D256 are commonly reported in J/m of notch width, whereas ISO 180 reports in kJ/m². A minimum of five specimens (ten recommended for referee testing) is tested for each direction and condition, and the mean, standard deviation, and failure mode are reported. Edgewise, flatwise, and reversed-notch orientations must be clearly documented for sheet specimens.

Falling Weight and Falling Dart Testing (ASTM D5628 / ISO 7765 / GB/T 11548)

Falling-weight methods are particularly relevant to plastic sheets because they apply impact directly to the sheet face in a geometry resembling real service conditions. A hemispherical or tapered dart of defined geometry is dropped from increasing heights or with variable mass onto a clamped circular specimen. Using the staircase or statistical bracketing procedure, the F50 value, defined as the impact energy at which 50 % of specimens fail, is determined. This method is well suited to thin sheets, coated panels, and materials that do not break completely in pendulum tests.

Instrumented Puncture Impact (ISO 6603-2 / ASTM D3763)

Instrumented impact testing uses a instrumented striker, typically 20 mm in diameter for ISO 6603-2, driven through a clamped sheet specimen at a controlled high speed. The force-deflection curve is recorded continuously, yielding peak force, deflection at peak force, energy to peak force, and total puncture energy. This method distinguishes between ductile and brittle failure modes and is highly valuable for comparing sheet materials, evaluating low-temperature ductile-to-brittle transitions, and supporting finite element simulation with material data.

Low-Temperature Impact Testing

For outdoor and cold-climate applications, specimens are conditioned at the specified low temperature in an environmental chamber and tested within seconds of removal while maintaining the target temperature. Comparing room-temperature and low-temperature results quantifies the material's cold-weather embrittlement, a critical acceptance criterion for outdoor glazing, vehicle panels, and cold-chain packaging.

Applications

Impact strength testing of plastic sheets and panels supports a broad range of industrial uses:

• Building and construction: verifying roofing sheets, wall cladding panels, glazing, skylights, and greenhouse panels for hail, impact, and wind-debris resistance
• Transportation and automotive: qualifying interior trim panels, door modules, luggage compartment liners, and exterior trim made from PP, ABS, and composite sheets
• Safety and security: certifying machine guards, protective glazing, riot shields, and safety windows where breakage prevention is safety-critical
• Signage and display: ensuring acrylic and PC sign faces withstand installation handling, cleaning, and public contact
• Packaging and material handling: validating thermoformed trays, dunnage sheets, and rigid packaging against drop and transit shocks
• Material development and quality control: incoming resin verification, batch release testing, process change validation, and comparison of recycled versus virgin sheet materials in research and development programs
• Regulatory compliance and certification: providing test data for product certification files against ISO, ASTM, EN, GB, JIS, and UL-based requirements, supported by accredited third-party laboratories

FAQ

What is the difference between Charpy and Izod impact tests?

The Charpy test strikes a horizontally supported specimen at mid-span, while the Izod test strikes a vertically clamped cantilever specimen above the clamp. Charpy testing per ISO 179-1 is more widely used for sheet materials in international trade, whereas notched Izod per ASTM D256 remains the most common reference value in North American material datasheets. The two methods produce different numeric values and are not directly interchangeable.

Why test both notched and unnotched specimens?

Notched testing measures resistance to crack propagation from a sharp stress concentration, while unnotched testing measures the energy absorbed during gross deformation. Comparing the two values quantifies notch sensitivity, which is a key indicator of how a sheet will perform after scratching, drilling, or edge damage in service.

What thickness of sheet can be tested?

The preferred pendulum specimen is 4 mm thick. Thicker sheets can be machined down on one face to a standard thickness, provided the skin direction is documented, while thin sheets and multiwall panels are generally better evaluated using falling-dart or instrumented puncture methods, which accept original sheet geometry directly.

How many specimens are required for a valid result?

A minimum of five specimens per condition is required by ISO methods, while ASTM D256 recommends ten specimens for notched Izod testing. Falling-weight F50 determination typically requires 20 or more drops using the staircase procedure. Testing both machine and transverse directions doubles the specimen count.

How does temperature affect impact strength results?

Impact strength of most plastics decreases significantly at low temperatures due to the ductile-to-brittle transition. For example, a polycarbonate sheet may exhibit ductile, non-breaking behavior at 23 °C but shift toward brittle fracture below -30 °C. For this reason, materials intended for outdoor or cold environments should always be verified at the lowest expected service temperature.

What are typical impact strength values for common plastic sheets?

Indicative notched Charpy values at room temperature include approximately 2 to 8 kJ/m² for PMMA and polystyrene, 5 to 15 kJ/m² for rigid PVC, 10 to 25 kJ/m² for ABS and PP copolymers, and 60 kJ/m² or higher for polycarbonate. Actual values depend on grade, thickness, processing history, and notch geometry, so project-specific testing by accredited third-party laboratories is always recommended for certification purposes.