Tensile-impact property evaluation measures the energy required to fracture plastic specimens under high-speed uniaxial tensile loading, providing decisive insight into material toughness where bending-type impact tests fall short. Performed to internationally recognized standards such as ISO 8256, ASTM D1822, and GB/T 13525, this test supports quality control, material selection, and product qualification across automotive, electronics, and packaging applications. This article outlines the specimen requirements, testing items, test methods, calculations, and practical applications of plastic tensile-impact evaluation.
Technical Overview
Tensile-impact property evaluation is a mechanical testing technique that measures the energy required to fracture a plastic specimen under rapid uniaxial tensile loading. Unlike conventional tensile tests performed at low crosshead speeds, or bending-type impact tests such as Izod and Charpy, tensile-impact testing applies a high-speed tensile load to a dumbbell-shaped specimen using a swinging pendulum, causing fracture within milliseconds.
The method is particularly valuable for ductile plastics that may bend without breaking in flexural impact tests, as well as for thin-walled components, films, and parts whose in-service failure mode is dominated by tensile stress at high strain rates. The principal standards governing this evaluation include ISO 8256 (Plastics — Determination of tensile-impact strength), ASTM D1822 (Standard Test Method for Tensile-Impact Energy to Break Plastics and Electrical Insulating Materials), and GB/T 13525 (the Chinese national standard for the determination of tensile-impact properties of plastics).
- Core measurand: energy absorbed by the specimen at break, expressed as tensile-impact strength in kJ/m²
- Loading condition: high-speed uniaxial tension at pendulum impact velocities of approximately 3 m/s
- Failure assessment: classification of brittle, ductile, or mixed fracture behavior
- Scope: rigid and semi-rigid thermoplastics, thermosets, composites, and electrical insulating materials
Testing Samples
ISO 8256 Specimens
ISO 8256 specifies dumbbell-shaped specimens in several standardized types (Types 1 to 4) that differ in overall length, width, and thickness, allowing selection according to material form and available quantity. Specimens are injection molded or machined with a narrow parallel gauge section in which fracture is forced to occur. Specimens may be tested edgewise or flatwise, and the testing direction must be reported together with the results.
ASTM D1822 Specimens
ASTM D1822 defines Type L (long) and Type S (short) rectangular specimens with a reduced central cross-section. The Type L specimen is the preferred configuration, while the Type S specimen is intended for cases where material availability is limited, such as compound development or screening studies. Because absorbed energy depends strongly on specimen geometry, results obtained with Type L and Type S specimens are not directly comparable.
Specimen Preparation and Conditioning
Specimens should be prepared in accordance with the relevant material specification, with smooth edges free of notches, scratches, and machining marks that could act as stress concentrators. Prior to testing, specimens are conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291 or ASTM D618, unless otherwise specified. A minimum of ten specimens is recommended for each test condition, with additional specimens tested when data scatter is high.
Testing Items
A typical tensile-impact evaluation program covers the following measurable items:
- Total energy to break: the energy absorbed by the specimen during fracture, expressed in joules (J)
- Tensile-impact strength: the corrected absorbed energy divided by the original cross-sectional area of the narrow section, expressed in kJ/m²
- Corrected energy: absorbed energy adjusted for the kinetic energy imparted to flying fragments (toss correction), particularly significant for brittle materials
- Failure mode classification: brittle, ductile, or mixed fracture, assessed visually after the test
- Temperature-dependent behavior: optional testing at elevated or sub-ambient temperatures using an environmental chamber
Testing Methods
Apparatus and Test Principle
Testing is performed on a pendulum-type impact machine fitted with a dedicated tensile-impact fixture. One end of the specimen is clamped to a crosshead carried by the pendulum, while the other end is held by a stationary yoke. When the pendulum is released, the specimen is loaded in uniaxial tension at an impact velocity of approximately 3 m/s and fractures in the narrow gauge section. The absorbed energy is determined from the difference in pendulum height before and after impact, or measured directly with instrumented systems.
Test Procedure
- Select a pendulum capacity such that the absorbed energy falls within the reliable working range of the machine, commonly between 10 % and 85 % of the nominal capacity
- Measure the width and thickness of the narrow section of each specimen to determine the original cross-sectional area
- Mount the specimen in the fixture with careful axial alignment to avoid bending stresses superimposed on the tensile load
- Release the pendulum from the defined height and record the absorbed energy indicated by the machine
- Inspect the fracture surfaces and classify the failure mode as brittle, ductile, or mixed
- Repeat the test on at least ten valid specimens, replacing any specimen that breaks outside the gauge section
Calculation and Expression of Results
The tensile-impact strength is calculated by dividing the corrected energy absorbed at break by the original cross-sectional area of the specimen's narrow section, and is reported in kilojoules per square meter (kJ/m²). Under ASTM D1822, results may additionally be expressed as tensile-impact energy to break in joules. The test report should state the standard used, specimen type, testing direction (edgewise or flatwise), conditioning details, individual values, mean value, standard deviation, and the observed failure modes. Comparisons between data sets are valid only when the same standard, specimen geometry, and test conditions are applied.
Applications
Tensile-impact evaluation supports material development, quality control, and design verification across many sectors:
- Automotive: assessment of interior trim, clips, fasteners, and thin-walled structural parts subjected to dynamic loading and crash conditions
- Electrical and electronics: qualification of insulating materials, connectors, and housings that must withstand rapid deformation and impact
- Packaging: evaluation of films, thin rigid containers, and closures under high-speed handling and transport loads
- Appliances and consumer goods: comparison of resin grades, filler loadings, and recycled content with respect to brittle-ductile transitions
- Polymer research: study of strain-rate sensitivity, temperature effects, and the influence of additives, UV exposure, or aging on impact toughness
Manufacturers frequently submit materials to accredited third-party laboratories for tensile-impact testing as part of incoming inspection, supplier qualification, or compliance demonstration against product specifications referencing ISO 8256, ASTM D1822, or GB/T 13525.
FAQ
What is the difference between tensile-impact and Izod or Charpy impact testing?
Izod and Charpy tests load the specimen in bending and can yield a "no break" result for very ductile plastics, whereas tensile-impact testing loads the specimen in uniaxial tension at high speed and forces a definitive fracture. The two approaches probe different failure mechanisms, and their results are not interchangeable.
Which standard should be selected, ISO 8256 or ASTM D1822?
The choice depends on the target market and the product specification. ISO 8256 and GB/T 13525 are widely used in international and Chinese supply chains, while ASTM D1822 is commonly referenced in North American specifications. Because specimen geometries and energy scales differ, the specific standard must always be stated when reporting or comparing data.
How many specimens are required for a valid test?
Both ISO 8256 and ASTM D1822 recommend testing a series of specimens, typically at least ten per condition, to obtain a representative mean and standard deviation. Specimens that fracture outside the gauge section should be excluded and replaced.
Why is the toss correction necessary?
Part of the pendulum energy is consumed in accelerating the broken fragments rather than fracturing the material itself. For brittle plastics that shatter into flying pieces, this toss energy can represent a significant fraction of the indicated reading, so ISO 8256 applies a correction to obtain the true fracture energy.
What does a brittle versus ductile failure indicate?
A clean brittle fracture with low absorbed energy signals limited toughness, while a ductile failure involving yielding and high energy absorption indicates superior resistance to rapid tensile loading. Mixed failures suggest that the material is operating near its brittle-ductile transition at the tested temperature and strain rate.