Impact Strength Test Procedures: A Complete Step-by-Step Experimental Guide

Impact strength testing measures a material's ability to resist fracture under sudden, high-speed loading and is one of the most widely performed mechanical tests in materials laboratories. This guide explains the complete impact strength test procedures, covering specimen preparation, notching, equipment setup, test execution, and result calculation in accordance with ISO and ASTM standards. Whether you work with rigid plastics, composites, or metals, the following experimental steps will help you obtain accurate and repeatable impact data.

Technical Overview

Impact strength, also known as impact toughness, quantifies the energy a material absorbs before fracturing when subjected to a rapidly applied load. Unlike tensile or flexural tests performed at slow strain rates, impact testing simulates real-world events such as drops, collisions, and sudden loads, where many otherwise ductile materials behave in a brittle manner. The test is essential for evaluating product durability, comparing material formulations, and verifying compliance with safety requirements.

Pendulum impact testing is the most common approach, in which a swinging hammer of a known energy capacity strikes a supported specimen. The energy absorbed by breaking the specimen is read directly from the scale or digital display of the instrument. The principal international standards governing these procedures include:

• ISO 179-1: Determination of Charpy impact properties of plastics
• ISO 180: Determination of Izod impact strength of plastics
• ASTM D256: Standard test methods for determining the Izod pendulum impact resistance of plastics
• ASTM D6110: Standard test method for determining the Charpy impact resistance of notched specimens of plastics
• ASTM D4812: Unnotched cantilever beam impact resistance of plastics
• ASTM E23 / ISO 148: Notched-bar impact testing of metallic materials
• GB/T 1043, GB/T 1843, and JIS K 7110/JIS K 7111: Chinese and Japanese national equivalents

Results are expressed either as energy per unit notch length (J/m or ft-lbf/in, typical of ASTM Izod) or as energy per unit cross-sectional area (kJ/m², typical of ISO Charpy and Izod). Because the two systems use different specimen geometries and calculations, ASTM and ISO values are not directly comparable.

Testing Samples

Rigid Plastic Specimens

For pendulum impact tests, specimens are produced by injection molding, compression molding, or machining from sheet or finished parts. Standard dimensions differ between the two major standard systems:

• ASTM specimens: 63.5 mm long, 12.7 mm wide, and typically 3.2 mm thick (other thicknesses such as 6.4 mm are permitted when noted)
• ISO specimens: 80 mm long, 10 mm wide, and 4 mm thick (type 1A multipurpose bar)

All specimens must be free of visible defects, sink marks, flash, voids, and machining marks that could act as unintended stress concentrators.

Notched Specimens

Notching creates a controlled stress concentration so that fracture occurs at a reproducible location. Key notch parameters include:

• Notch type A: radius of 0.25 mm, the standard notch for most notch-sensitivity evaluations
• Notch type B: radius of 1.0 mm, used for more notch-sensitive comparisons
• Notch type C: radius of 0.10 mm, for very sharp stress concentration
• Residual notch depth: 10.2 mm for ASTM specimens (2.54 mm of the 12.7 mm width removed at the notch center) and 8 mm width remaining for ISO specimens

Notches are machined with a dedicated single-point cutter, and cutting conditions must be controlled because a dull tool, excessive feed rate, or overheating can introduce residual stresses that distort results. After notching, specimens should be conditioned at 23 °C and 50% relative humidity for a minimum of 40 hours, in line with ASTM D618 or ISO 291, unless the material specification dictates otherwise.

Other Material Forms

Beyond rigid plastics, impact procedures extend to thin films and sheets via free-falling dart methods (ASTM D1709, ISO 7765), to metals via notched-bar Charpy testing at ambient or subambient temperatures, and to fiber-reinforced composites per ISO 179-2 instrumented methods.

Testing Items

Typical impact strength test items requested by manufacturers and accredited third-party laboratories include:

• Notched Izod impact strength, the most frequently specified plastic toughness metric
• Unnotched Izod impact strength, which reflects total energy absorption without a machined stress raiser
• Notched and unnotched Charpy impact strength
• Tensile impact strength (ISO 8256, ASTM D1822) for materials too flexible for bending fracture
• Falling dart or drop weight impact (ASTM D1709, ASTM D5420, ISO 7765) for films, sheets, and painted panels
• Instrumented impact analysis providing force-displacement curves, peak load, and total energy
• Temperature-dependent impact testing to determine the brittle-to-ductile transition temperature
• Validation of annealing, moisture conditioning, or low-temperature embrittlement effects

Testing Methods

The following steps describe the standard pendulum impact test procedure. Always consult the governing standard for the specific material and product before testing.

Step 1: Verify Specimen Dimensions and Conditioning

Measure the width and thickness of each specimen near the notch with a calibrated micrometer to 0.01 mm precision. Confirm that all specimens have completed the required conditioning period at 23 °C and 50% relative humidity. Record the dimensions because they enter directly into the final calculation.

Step 2: Inspect Notch Quality

Examine each notch under magnification to confirm the correct radius, depth, and surface finish. Discard any specimen showing chatter marks, melted or smeared polymer at the notch root, or an incorrect notch geometry. Notch dimensions may be verified optically or with a notch depth gauge.

Step 3: Prepare and Calibrate the Pendulum Impact Tester

Confirm that the machine is level and securely anchored. Verify the energy scale zero by releasing the pendulum with no specimen installed; the indicated energy after a free swing represents friction and windage loss, which must fall within the tolerance stated in the standard. Select a hammer whose energy capacity allows the specimen to absorb between roughly 10% and 80-90% of the available energy, ensuring adequate resolution.

Step 4: Mount the Specimen

For an Izod test, clamp the specimen vertically as a cantilever with the notch facing the striker, positioning the notch line level with the top of the clamp within the specified tolerance. For a Charpy test, place the specimen horizontally on two anvils with the notch centered in the gap and facing away from the striker, so the pendulum strikes the face opposite the notch. Tighten supports without deforming the specimen.

Step 5: Conduct the Test

Raise and latch the pendulum at the defined release angle, verify that the energy indicator is zeroed, and confirm bystanders are clear. Release the pendulum smoothly so it strikes the specimen at the specified velocity (typically about 3.5 m/s for plastics). Allow the pendulum to complete its swing, then read and record the absorbed energy indicated after friction correction. Examine the broken halves to classify the failure mode:

• Complete break: specimen separates into two or more pieces
• Hinge break: an apparently broken specimen held together by a thin layer
• Partial break: incomplete cross-section fracture
• Non-break: specimen bends past a defined angle without cracking

Only complete breaks, or the break types permitted by the standard, are valid for calculating a standard impact strength value.

Step 6: Repeat Testing

Test at least five specimens and preferably ten, because impact data are inherently scattered, particularly for notch-sensitive materials. If any specimen gives an anomalous result traceable to a processing or notching flaw, the standard allows replacement. Test in sequence rather than batching specimens in and out of conditioning.

Step 7: Calculate and Report Results

Calculate the impact strength for each valid specimen and then the arithmetic mean and standard deviation. For ASTM Izod, divide the absorbed energy by the specimen thickness to obtain J/m (or ft-lbf/in). For ISO Charpy and Izod, divide the absorbed energy by the cross-sectional area beneath the notch to obtain kJ/m². The report should include:

• Material identification, source, and processing history
• Specimen dimensions, notch type, and notching method
• Test standard, hammer capacity, and absorbed-energy range
• Conditioning atmosphere, test temperature, and humidity
• Individual values, mean, standard deviation, and break classifications

Applications

Impact strength test data directly support design decisions and quality assurance across many industries:

• Automotive: interior trim, bumper systems, battery housings, and under-hood components that must survive crash loads and low-temperature exposure
• Packaging: blow-molded bottles, caps, and transport containers verified by drop and dart impact methods
• Construction: PVC window profiles, pipe fittings, and roofing sheets requiring certified cold-weather toughness
• Consumer electronics and appliances: housings and enclosures evaluated for drop durability and assembly-line handling
• Medical devices: sterilization trays and device components that must resist accidental impacts
• Material development: ranking toughness of new resin grades, filler loadings, recycled content, and impact modifiers
• Incoming inspection and lot release: comparing production batches against specification limits in accredited third-party laboratories

FAQ

What is the main difference between the Izod and Charpy impact tests? The Izod test strikes a vertically clamped cantilever specimen on the notched side, while the Charpy test strikes a horizontally supported beam on the face opposite the notch. Both use a pendulum, but specimen geometry, support conditions, and result units differ.

Why are specimens notched before testing? The notch concentrates stress at a known point so fracture initiates reproducibly and at a measurable energy level. Without a notch, many tough plastics simply bend without breaking, making comparisons impossible.

How many specimens should be tested? A minimum of five valid breaks is common for screening, while ten specimens give better statistical confidence. Standards specify the exact requirements for reporting averages and break classifications.

Why do test results scatter so widely? Impact fracture is highly sensitive to internal defects, notch machining quality, molecular orientation, and thermal history. Careful specimen preparation and conditioning, plus adequate sample numbers, reduce variability.

Does temperature affect impact strength? Yes, strongly. Many plastics transition from ductile to brittle behavior as temperature drops, so tests intended for cold-climate service are performed in a temperature-controlled chamber or after conditioning at the specified low temperature.

Can ASTM and ISO impact values be converted into each other? No direct conversion exists because the two systems use different specimen sizes, notch geometries, support spans, and calculation bases. Data must be generated under the specific standard required by the specification.