Impact resistance is one of the most critical mechanical properties determining whether a plastic material can survive sudden loads, drops, and collisions in real-world service. This article provides a comprehensive comparative analysis of the impact performance of common plastics, covering Izod, Charpy, and falling dart test methods under ISO, ASTM, GB/T, and JIS standards. Engineers, quality managers, and material selection specialists will find practical guidance on specimen preparation, data interpretation, and grade-to-grade comparison for applications ranging from automotive components to electronic housings.
Technical Overview
Impact resistance describes the ability of a plastic material to absorb energy during a rapidly applied mechanical shock without fracturing. Unlike slowly applied loads measured in tensile or flexural testing, impact events occur within milliseconds, and the material response depends heavily on molecular structure, morphology, temperature, geometry, and stress concentration. A comparative analysis of impact performance is therefore essential whenever multiple candidate materials compete for the same application.
Plastics fail under impact in two characteristic modes. Ductile materials such as polycarbonate (PC) and some polyethylenes undergo extensive plastic deformation, whitening, and drawing before failure, absorbing large amounts of energy. Brittle materials such as general purpose polystyrene (GPPS) and PMMA fracture with minimal deformation and low absorbed energy. Many engineering plastics, such as polyamide and polypropylene, shift from ductile to brittle behavior as temperature drops, notch sharpness increases, or wall thickness increases — a phenomenon known as the ductile-brittle transition.
A rigorous comparison program must control all of these variables: identical specimen geometry, identical notch geometry, identical test temperature and humidity, identical conditioning history, and identical strain rate. Accredited third-party laboratories routinely perform such comparative studies so that the resulting data can support material substitution, incoming material qualification, and failure analysis with defensible, standards-based results.
Testing Samples
Amorphous Thermoplastics
Amorphous resins are frequently compared for housings and transparent parts. Typical candidates include polycarbonate (PC), acrylonitrile butadiene styrene (ABS), PC/ABS blends, high impact polystyrene (HIPS), general purpose polystyrene (GPPS), polymethyl methacrylate (PMMA), unplasticized polyvinyl chloride (PVC-U), and polysulfone. Within this group, PC typically delivers the highest notched impact values, followed by PC/ABS and ABS, while PMMA and GPPS occupy the brittle end of the scale.
Semi-Crystalline Thermoplastics
Semi-crystalline resins include polypropylene homopolymer and copolymer, high density polyethylene (HDPE), low density polyethylene (LDPE), polyamide 6 (PA6), polyamide 66 (PA66), acetal (POM), polybutylene terephthalate (PBT), and polyethylene terephthalate (PET). Polyamides deserve special attention in any comparison because their impact response changes dramatically with moisture content; dry-as-molded and 50% relative humidity conditioned specimens must be tested separately.
Modified and Reinforced Compounds
Comparative studies often include impact-modified and filled grades, such as PP toughened with ethylene-propylene rubber or POE elastomer (TPO), talc-filled PP, glass-fiber reinforced PA6 and PA66, flame-retardant ABS and PC/ABS, and mineral-filled PBT. Short glass fibers typically raise stiffness and heat deflection temperature but often reduce notched impact strength of the base resin, while elastomer modification substantially raises impact energy at the cost of rigidity.
Specimen Preparation and Conditioning
To ensure valid comparison, all specimens should be injection molded under controlled parameters using ISO 294-1 or ASTM D638 Type geometry guidance, with the gate location and flow direction documented because molecular orientation can bias results. Notches are machined after molding in accordance with ISO 2818 using a fresh cutting tool profile. Standard conditioning is 23 °C and 50% relative humidity for at least 40 hours per ISO 291 or ASTM D618. Polyamides require additional drying or controlled humidity conditioning so that the moisture state is known and reported.
Testing Items
A complete comparative analysis of impact resistance typically includes the following test items:
- Notched Izod impact strength at 23 °C (ISO 180, ASTM D256, GB/T 1843, JIS K 7110)
- Unnotched Izod impact strength, often reported as "no break" for tough materials
- Notched and unnotched Charpy impact strength (ISO 179-1, ASTM D6110, GB/T 1043)
- Charpy impact with instrumented analysis of force-deflection curves (ISO 179-2)
- Falling dart impact resistance for films (ASTM D1709, ISO 7765-1)
- Gardner or falling weight impact for rigid sheets and painted parts (ASTM D5420, ASTM D5628)
- Instrumented high-speed puncture impact providing peak force, deflection at peak, and total energy (ISO 6603-2, ASTM D3763)
- Tensile impact strength for very thin sections (ISO 8256, ASTM D1822)
- Low-temperature impact testing at -10 °C, -30 °C, or -40 °C to map the ductile-brittle transition
- Comparative testing after thermal aging, UV exposure, or chemical contact to simulate service degradation
Testing Methods
Izod Impact Testing (ISO 180 / ASTM D256)
In the Izod configuration, a cantilever specimen is clamped at one end and struck by a pendulum hammer near the free end, on the same side as the notch for the standard notched procedure. The energy absorbed during fracture is corrected for windage and friction and divided by the specimen width to yield impact strength. ISO 180 reports results in kJ/m², whereas ASTM D256 reports J/m or ft·lb/in. Because ISO and ASTM specimens differ in geometry, notch detail, and clamping, values from the two systems must never be compared directly; each material in a comparison campaign should be tested under one consistent system.
Charpy Impact Testing (ISO 179-1 / ASTM D6110)
Charpy testing supports the specimen horizontally on two anvils and strikes it at midspan with a pendulum, either with an edgewise (1eA, 1eU) or flatwise (1fA, 1fU) orientation. Charpy is the dominant method in European and Chinese practice and is particularly useful for brittle materials and for low-temperature studies where complete fracture is expected. Instrumented Charpy per ISO 179-2 records the force-time history, allowing separation of crack initiation energy from propagation energy — a valuable distinction when ranking materials with similar total energy but different failure mechanisms.
Falling Dart and Falling Weight Testing (ASTM D1709 / ASTM D5420 / ASTM D5628)
These methods drop a weighted, rounded dart onto a clamped disk or film. The staircase (Bruceton) approach in ASTM D1709 and ISO 7765-1 statistically determines the energy at which 50% of film specimens fail, making it the standard comparison tool for packaging films. Gardner impact per ASTM D5420 uses a similar statistical treatment for rigid sheet, painted panels, and finished flat parts, and correlates well with in-service dent and crack resistance.
Instrumented Puncture Impact (ISO 6603-2 / ASTM D3763)
Instrumented puncture testing drives a striker through a clamped disk at high speed while a load cell and displacement sensor capture the full force-deflection curve. Key outputs include peak force, deflection at peak force, energy to peak force, and total puncture energy, together with a ductility index. This is the most informative method for comparing engineering materials because two compounds with identical total energy can differ greatly in stiffness, yield behavior, and failure mode.
Comparative Data Interpretation
The following typical ranges for notched Izod impact strength at 23 °C, based on ISO 180 with standard notch, illustrate the relative ranking of common materials. Exact values depend on grade, thickness, and processing, so these figures are indicative rather than specification limits.
- PC: approximately 60 to 90 kJ/m² — the benchmark for notched toughness among unfilled thermoplastics
- PC/ABS blends: approximately 40 to 70 kJ/m², balancing cost, flow, and impact
- Impact-modified PP (TPO): approximately 15 to 45 kJ/m², sometimes "no break" at room temperature
- ABS: approximately 15 to 25 kJ/m² across standard grades
- PA66, moisture conditioned: approximately 12 to 25 kJ/m², versus 5 to 8 kJ/m² dry-as-molded
- PP copolymer: approximately 6 to 12 kJ/m², roughly double that of homopolymer
- HDPE: approximately 6 to 15 kJ/m² with strong low-temperature retention
- POM and PVC-U: approximately 5 to 12 kJ/m² depending on grade and formulation
- HIPS: approximately 8 to 12 kJ/m², while GPPS and PMMA fall near 2 to 5 kJ/m²
- Glass-fiber reinforced PA and PBT: commonly 6 to 12 kJ/m², trading impact for stiffness
Statistical treatment is mandatory for a defensible comparison. A minimum of five valid specimens per material per condition is standard, and ten or more are recommended where ductile-brittle scatter appears. Results should be reported as mean, standard deviation, and the observed failure mode classification: complete break, hinge break, partial break, or non-break. When a material shows non-break with the selected pendulum, a higher energy hammer must be used, and the energy scale should never exceed 85% utilization of hammer capacity.
Critical Variables in Any Comparison
- Temperature: repeat testing at 23 °C and at the lowest expected service temperature to expose the ductile-brittle transition
- Notch radius: sharper notches (0.1 mm versus 1.0 mm) systematically depress impact values and amplify notch sensitivity differences
- Wall thickness: thicker sections promote triaxial stress states and plane-strain brittleness in many resins
- Processing heritage: weld lines, flow orientation, sink marks, and residual stress can override intrinsic resin differences
- Moisture and aging: polyamide conditioning state and any thermal or UV exposure history must be documented and matched
Applications
Comparative impact data directly drives material selection across every major polymer end market:
- Automotive: bumper fascias and trim formulated from impact-modified PP and TPO, interior panels in PC/ABS, air intake and underhood components in glass-filled PA, all validated through low-temperature impact programs
- Consumer electronics and electrical housings: PC and PC/ABS chosen for drop resistance of laptop enclosures, chargers, and switchgear covers, with flame-retardant grades balanced against UL and IEC impact and glow-wire requirements
- Pipes and fittings: PE and PVC-U pressure systems qualified by falling weight and Charpy testing, particularly for cold-climate installation where brittle failure must be excluded
- Packaging: blown and cast films ranked by falling dart (ASTM D1709) and tear data to balance downgauging against puncture resistance
- Appliances and power tools: housings in ABS or PC/ABS compared on drop impact and Gardner impact after assembly-level simulation
- Safety equipment: helmet shells, protector plates, and sports gear relying on PC, ABS, and PA grades selected through instrumented impact mapping
- Medical devices: housings and IV components where crack initiation resistance and sterilization history affect comparative rankings
FAQ
Question: Which test should I choose for comparing plastics — Izod or Charpy?
Answer: Both rank materials consistently when specimens, notching, and temperature are identical. Izod under ASTM D256 remains dominant in North American specifications, while Charpy under ISO 179-1 is preferred in Europe and Asia and is better suited to low-temperature studies. The critical rule is to use one method and one standard system throughout the entire comparison.
Question: Why do tough materials often report "no break"?
Answer: When a specimen bends or hinges without fracturing, the pendulum cannot lift through the full arc and the absorbed energy cannot be calculated meaningfully. Standard practice records the result as non-break, and a higher-energy hammer is used. Comparisons should note how many specimens in each batch failed to break, because a "no break" result at a given hammer energy is still comparative information.
Question: Can I convert ISO kJ/m² values to ASTM J/m values?
Answer: No. The two methods use different specimen dimensions, notch details, and support conditions, so there is no valid universal conversion factor. Comparing an ISO-tested material against an ASTM-tested material requires retesting both under the same standard.
Question: Why does the same resin show different impact values from different batches?
Answer: Impact strength is highly sensitive to molecular weight, melt flow index, reground content, moisture, molding conditions, and notch machining quality. A controlled comparative study fixes molding parameters, tool condition, and conditioning history so that observed differences reflect the material rather than the process.
Question: Does higher notched impact strength always mean better real-world durability?
Answer: Not necessarily. Notched pendulum tests measure resistance to crack propagation from a sharp defect, while service failures often involve multiaxial impact, drop orientation, rib geometry, and weld lines. The best practice combines notched and unnotched pendulum data with instrumented puncture data (ISO 6603-2 or ASTM D3763) and, where possible, component-level drop testing.
Question: How does glass fiber reinforcement affect the comparison?
Answer: Short glass fibers typically raise stiffness, strength, and heat deflection temperature but reduce notched impact strength relative to the unfilled resin in many systems. However, fiber-reinforced compounds often show flatter impact retention at low temperature and after aging, so the ranking can invert depending on the test temperature selected for the study.
Question: How many specimens are required for a valid comparison?
Answer: Standards require at least five specimens per condition as a minimum, and most comparison protocols use ten to capture scatter, especially near the ductile-brittle transition. Outlier rejection rules in ISO 179-1 and ASTM D256 must be followed and documented.
Question: Which standards organizations govern these methods?
Answer: ISO publishes ISO 179-1/179-2 (Charpy), ISO 180 (Izod), ISO 6603-2 (instrumented puncture), ISO 7765-1 (film dart impact), and ISO 8256 (tensile impact). ASTM International publishes ASTM D256, D6110, D1709, D3763, D5420, D5628, and D1822. Chinese national standards GB/T 1043, GB/T 1843, and GB/T 11548 mirror the ISO approaches, while JIS K 7110 and JIS K 7111 cover the Japanese market. Accredited third-party laboratories can execute any of these systems and issue test reports acceptable for international supply chains.