Impact toughness analysis is a critical evaluation process for determining how plastic materials behave under sudden, high-speed loading conditions, providing essential data for material selection, product design, and quality control. This comprehensive guide covers the principle testing methods—including Charpy, Izod, tensile impact, and instrumented falling dart tests—along with relevant ISO, ASTM, and GB standards, sample requirements, and typical industrial applications. Whether you are a materials engineer, quality manager, or product designer, understanding impact toughness analysis helps ensure the reliability and safety of plastic components in real-world service environments.
Technical Overview
Impact toughness refers to the ability of a plastic material to absorb energy during rapid deformation or fracture under sudden loading. Unlike metals, plastics exhibit a strong dependence of toughness on temperature, strain rate, notch geometry, and molecular structure. Many thermoplastics that appear ductile under slow tensile loading can become brittle when subjected to impact conditions, particularly at low temperatures or in the presence of stress-concentrating notches.
Impact toughness analysis quantifies the energy absorbed by a standardized specimen during fracture, typically expressed in units of kJ/m². The measured value is influenced by multiple factors, including polymer type, crystallinity, molecular weight, filler content, processing conditions, and internal defects. Common material classifications based on impact behavior include brittle failure, partial fracture, ductile failure, and non-break (hinge-type break) modes.
The two most widely recognized test geometries are the Charpy configuration (horizontal beam, struck at the center) and the Izod configuration (vertical cantilever beam, struck near the free end). Both notched and unnotched specimens are used, with notched testing evaluating notch sensitivity and unnotched testing reflecting overall material toughness. Additional methods such as tensile impact and instrumented falling dart impact provide complementary data for specific failure modes.
Testing Samples
Molded Specimens
Injection-molded or compression-molded bars are the most common sample form for impact testing. Typical dimensions for Charpy testing per ISO 179 are 80 mm × 10 mm × 4 mm, while Izod testing per ISO 180 and ASTM D256 generally uses bars of approximately 63.5 mm × 12.7 mm × 3.2–12.7 mm. Specimens must be free from sink marks, weld lines, flash, and internal voids that could distort results.
Machined Specimens
Samples cut from finished parts, sheets, extruded profiles, or thick stock may be machined to the required geometry. Machining parameters such as cutter sharpness and feed rate must be controlled to avoid introducing residual stresses or heat-affected zones that artificially reduce measured toughness.
Notched Specimens
Notches are machined into specimens using a standardized cutting tool with defined tip radius. Common notch types include Type A (0.25 mm radius), Type B (1.00 mm radius), and Type C (0.10 mm radius) per ISO standards. The notch acts as a stress concentrator, and notch quality—particularly tip radius and surface finish—strongly affects the measured impact strength.
Plate and Film Specimens
For falling dart and puncture impact tests, circular or square plaques cut from sheets or films are used, typically 60–100 mm in diameter. Film specimens for free-falling dart methods such as ASTM D1709 are prepared in accordance with the applicable film thickness requirements.
Testing Items
Impact toughness analysis typically includes the following measurable parameters:
- Charpy impact strength (notched and unnotched), expressed in kJ/m²
- Izod impact strength (notched and unnotched), expressed in kJ/m² or J/m (ft-lb/in)
- Tensile impact strength, expressed in kJ/m²
- Puncture impact energy and peak force from instrumented falling dart testing
- Ductile-to-brittle transition temperature
- Fracture mode classification: complete break, hinge break, partial break, or non-break
- Notch sensitivity comparison across notch radii
- Temperature-dependent impact behavior across a defined service range
Testing Methods
Charpy Impact Testing (ISO 179-1, ASTM D6110, GB/T 1043)
The specimen is supported horizontally on two anvils and struck at the midpoint by a swinging pendulum hammer. The energy absorbed during fracture is calculated from the difference between the initial and residual pendulum heights, corrected for windage and friction losses. Charpy testing is suitable for rigid and semi-rigid plastics and can be performed with notched or unnotched specimens. ISO 179-1 designates test geometries such as 1eU (unnotched, edgewise) and 1eA (notched, edgewise).
Izod Impact Testing (ISO 180, ASTM D256, GB/T 1843)
The specimen is clamped as a vertical cantilever beam and struck on the notched side by a pendulum at a fixed distance from the clamp. ASTM D256 reports results in J/m or ft-lb/in of notch depth, whereas ISO 180 reports kJ/m² based on the cross-sectional area beneath the notch. Izod testing is widely used for quality control and material datasheet comparisons, particularly in North American specifications.
Tensile Impact Testing (ISO 8256, ASTM D1822)
A dumbbell-shaped specimen is fractured in tension by a swinging pendulum crosshead. This method is appropriate for materials that are too flexible for flexural impact tests and evaluates energy absorption under high-speed tensile loading, which better reflects certain real-world failure modes.
Instrumented Falling Dart Impact (ISO 6603-2, ASTM D3763, GB/T 15248)
A dart with an instrumented tup strikes a clamped plate specimen at a controlled velocity, and a load cell records the full force–displacement curve. This method distinguishes between brittle failure (sharp force peak) and ductile failure (energy absorption through deformation), providing both peak force and total absorbed energy. It is especially valuable for engineering plastics, automotive components, and packaging materials.
Free-Falling Dart Methods (ASTM D1709, ASTM D5270)
Used primarily for plastic films, these methods determine the impact resistance by dropping a dart with a rounded head from a fixed height onto the film and statistically analyzing the failure mass using the staircase (Bruceton) approach.
Gardner / Falling Weight Impact (ASTM D5420, ASTM D5628)
A weight is dropped through a guided tube onto a supported specimen, with impact energy adjusted by varying drop height or mass. These methods are common for painted plastics, rigid sheets, and finished parts where failure is defined by visible cracking or deformation.
Test Condition Control
All impact tests require strict conditioning per ISO 291 or ASTM D618, typically 23 °C and 50% relative humidity for at least 40 hours. For low-temperature evaluation, specimens are conditioned in an environmental chamber and tested within a specified time window after removal. A minimum of five specimens per condition is standard, with ten or more recommended for materials exhibiting scatter.
Applications
Impact toughness analysis supports material qualification and product safety across a broad range of industries:
- Automotive: bumpers, interior trim, fuel system components, battery housings, and underbody shields requiring validated impact performance at low temperatures
- Consumer electronics and appliances: housings, connectors, and structural frames subjected to drop and handling impacts
- Packaging: films, containers, and closures evaluated for puncture and drop resistance during transport
- Pipes and fittings: PVC, PE, and PP systems tested for toughness to ensure long-term service reliability
- Construction: profiles, panels, and glazing materials requiring impact resistance for safety compliance
- Medical devices: components that must withstand sterilization and mechanical abuse without brittle fracture
- Material development: comparing base resins, impact modifiers, recycled content levels, and processing windows
- Quality control: incoming resin inspection, lot-to-lot consistency verification, and failure analysis of returned parts
FAQ
What is the difference between Charpy and Izod impact tests?
The primary differences are specimen orientation and support. In Charpy testing, the specimen lies horizontally on two supports and is struck at the center; in Izod testing, the specimen stands vertically clamped as a cantilever and is struck near the free end. The two methods produce different numerical results for the same material and are not directly interchangeable, so the test method must always be specified when reporting data.
Why are notched impact results lower than unnotched results?
The notch creates a localized stress concentration and a triaxial stress state that suppresses ductile deformation and promotes brittle crack initiation. Notched impact strength therefore measures the material's resistance to crack propagation from a sharp flaw, while unnotched values reflect the overall energy absorption capability of the material.
Why does impact toughness decrease at low temperatures?
As temperature drops, molecular segmental mobility decreases and many amorphous polymers approach or pass through their glass transition region. The material loses its ability to dissipate impact energy through plastic deformation and transitions from ductile to brittle failure. This ductile-to-brittle transition is a key design consideration for outdoor and cold-climate applications.
How many specimens should be tested?
ISO and ASTM standards generally require a minimum of five specimens per test condition for rigid plastics, and ten or more when results show significant scatter or mixed failure modes. Statistical reporting of mean, standard deviation, and failure type classification is recommended.
Can impact test results from different standards be compared directly?
No. Differences in specimen geometry, notch radius, support span, hammer energy, and result calculation (energy per unit area versus energy per unit length) mean that data from ISO 179 and ASTM D256, for example, cannot be compared numerically. Comparisons should only be made within a single test standard and specimen configuration.
Which factors during processing affect the measured impact strength?
Molding temperature, injection speed, holding pressure, cooling rate, and gate location influence molecular orientation, residual stress, crystallinity, and weld line formation—all of which can significantly reduce impact performance. Testing specimens cut from actual parts, rather than only from molded test bars, is recommended when process effects are a concern.
How should impact toughness testing be outsourced?
Testing should be performed by accredited third-party laboratories with calibrated pendulum and falling dart equipment, controlled environmental conditioning, and traceability to ISO, ASTM, or GB standards. A complete test report should identify the material, specimen preparation method, notch type, test geometry code, conditioning, test temperature, individual and mean results, and observed fracture modes.