ISO 180 Izod Notched Impact Strength Testing: Principles, Specimens, Methods, and Applications

ISO 180 Izod notched impact strength testing is the internationally recognized method for quantifying the impact resistance and notch sensitivity of plastic materials. This technical guide explains the test principle, specimen and notch requirements, step-by-step testing procedure, calculation of results, and practical applications across the automotive, electronics, and polymer processing industries.

Technical Overview

ISO 180, titled "Plastics — Determination of Izod impact strength," specifies a standardized pendulum-impact method for measuring the energy required to fracture a plastic specimen. In this test, a rectangular bar containing a machined notch is clamped vertically as a cantilever beam and struck by a swinging pendulum on the notched face at a defined point above the clamping line. The energy absorbed during fracture, corrected for friction and windage losses, is divided by the cross-sectional area beneath the notch to yield the notched Izod impact strength, expressed in kJ/m².

The notch serves as an intentional stress concentrator. It forces a triaxial stress state at the notch root, promotes brittle rather than ductile fracture, and allows the test to differentiate materials by their sensitivity to sharp defects, scratches, and sharp corners in real parts. Results are reported with a method designation such as ISO 180/1A, where "1" identifies the specimen type and "A" identifies the notch radius.

• Test result: notched Izod impact strength a_iN, in kJ/m²
• Preferred specimen: Type 1 bar, 80 mm × 10 mm × 4 mm
• Standard notch: Type A, 45° included angle, 0.25 mm root radius
• Reference impact velocity: approximately 3.5 m/s
• Machine verification: performed according to ISO 13802
• Related methods: ISO 179 (Charpy) and ASTM D256 (Izod, inch-based)

Testing Samples

Injection-Molded Multipurpose Specimens

The preferred test piece is the Type 1 specimen measuring 80 mm × 10 mm × 4 mm, typically produced directly by injection molding using the multipurpose test specimen defined in ISO 3167. Direct molding eliminates machining variables and gives results representative of the material in its as-processed state.

Machined Specimens from Sheet, Plate, or Rod

When molding is impractical, specimens may be machined from compression-molded or extruded sheet, plate, rod, or other semi-finished stock in accordance with ISO 2818. Machining must be performed with sharp tools, controlled feeds, and adequate cooling so that residual heat or tool marks do not influence the measured impact strength.

Specimens Taken from Finished Parts

For quality audits, incoming inspection, or failure analysis, specimens can be cut from molded components, pipes, fittings, or housings, provided the required bar geometry and notch can be produced. Because part geometry, flow direction, orientation, and processing history affect results, such values are valid for comparison within the same part family but are not interchangeable with data from molded Type 1 bars.

Notch Geometry Requirements

The notch is machined into the 10 mm wide face to a depth of 2.0 mm for the standard bar, leaving a ligament of 8.0 mm beneath the notch root. ISO 180 defines three notch radii: Type A (0.25 mm), Type B (1.00 mm), and Type C (0.10 mm). The Type A notch is the default and the basis of most datasheet values. The notch root radius must be checked with a calibrated optical or profilometric device, since radius deviations as small as 0.05 mm can shift results significantly.

Testing Items

Laboratories performing ISO 180 testing typically offer the following test configurations, each of which answers a distinct materials question:

• Notched Izod impact strength, Type A notch (designations 1A, 2A, etc.) — the standard notch-sensitivity index
• Notched Izod impact strength with Type B or Type C notches — evaluation of radius-dependent brittleness
• Unnotched Izod impact strength (a_iU) — baseline toughness of a defect-free material
• Direction-dependent testing — impact bars cut parallel versus perpendicular to flow or grain direction
• Temperature-dependent testing — sub-ambient testing (for example at −40 °C or −30 °C) to locate the ductile-to-brittle transition
• Conditioned and aged states — dry-as-molded, moisture-conditioned, heat-aged, or UV-exposed specimens
• Quality-control lot release testing and datasheet verification for resins and compounds

Testing Methods

Specimen Preparation and Notching

Specimens are molded or machined to the required bar geometry, then notched with a single-point milling cutter or dedicated notching machine using a shaped cutter matched to the 45° notch angle and target root radius. Notching must be carried out with a sharp cutter, low feed rate, and generous coolant, followed by careful deburring only where permitted. The notch radius, depth, and angle are verified before testing, and any specimen with a torn, burned, or non-conforming notch is rejected.

Conditioning

Specimens are conditioned before testing in a standard atmosphere of 23 °C ± 2 °C and 50 % ± 10 % relative humidity in accordance with ISO 291, normally for a minimum of 16 hours. For moisture-sensitive polymers such as polyamide, special conditioning states (for example, the standardized humid state per ISO 1110) may be agreed between interested parties and must be documented with the results.

Equipment and Calibration

Testing employs a pendulum impact machine fitted with an Izod support and striker. The machine, its supports, and its energy scales are verified according to ISO 13802. Pendulums with nominal energies of 1 J, 2.75 J, 5.5 J, 11 J, and 22 J are commonly available; the capacity must be chosen so that the corrected absorbed fracture energy lies between 10 % and 80 % of the pendulum capacity. If the measured energy falls outside this window, a different pendulum is fitted and the test repeated on fresh specimens.

Test Procedure

• Measure the specimen thickness and the width beneath the notch to the nearest 0.02 mm at the notch plane.
• Clamp the specimen vertically in the vise with the notch plane aligned to the top edge of the clamp and the notched face facing the striker.
• Confirm the free length above the clamp and the position of the center of strike against the machine geometry.
• Set the total energy scale, release the pendulum freely, and allow the specimen to be struck and fractured.
• Read the absorbed energy and apply the prescribed corrections for friction and air resistance.
• Classify the fracture as complete break (C), hinge break (H), partial break (P), or non-break (NB).

Calculation and Expression of Results

The notched Izod impact strength is calculated as the corrected absorbed energy divided by the original cross-sectional area of the specimen at the notch, multiplied by 10³ to express the result in kJ/m². In practice, the ligament area equals the specimen thickness multiplied by the width beneath the notch. Individual values, the arithmetic mean, and the standard deviation are reported together with the full method designation, notch type, fracture designations, and any deviations. Only complete and hinge breaks are conventionally used for mean-value reporting; partial breaks and non-breaks are recorded and listed separately rather than averaged with brittle fracture data.

Reporting and Validity

A compliant test report identifies the material, its conditioning state, the specimen type and preparation route, the notch type and machining parameters, the test temperature and humidity, the pendulum capacity used, the individual and mean impact strengths with standard deviation, and the observed fracture behavior. Duplicate testing with fresh specimens is required whenever a specimen shows a defective notch, slips in the clamp, or yields an energy outside the valid measurement window.

Applications

ISO 180 notched Izod data is one of the most widely quoted mechanical properties in the plastics industry and supports decisions ranging from resin selection to part failure analysis:

• Automotive engineering: bumper systems, body panels, interior trim, clips, and brackets that must withstand low-temperature impacts
• Electrical and electronics: housings, connectors, enclosures, and appliance components requiring certified toughness
• Pipe and fitting manufacture: impact qualification of PVC, PE, and PP systems for pressure and drainage applications
• Compounding and material development: ranking of impact modifiers, fillers, and rubber-toughened grades
• Incoming material inspection and lot release against datasheet or purchase-specification limits
• Failure analysis: comparison of impact strength between failed parts and reference material to detect degradation or contamination
• Certification and datasheet generation for engineering plastics such as PC, ABS, PA, POM, PMMA, PP, PE, PVC, and PS

FAQ

What is the difference between ISO 180 and ASTM D256?

Both are pendulum Izod tests, but they differ in specimen dimensions, notch preparation details, clamping geometry, and units. ISO 180 uses a 4 mm thick Type 1 bar and reports kJ/m², while ASTM D256 typically uses 3.2 mm or 6.4 mm thick specimens and reports ft·lbf/in. Because thickness strongly affects notch constraint, the two values correlate only approximately and should never be converted one-to-one; comparisons must always reference the same standard.

How does Izod testing differ from Charpy testing (ISO 179)?

In the Izod configuration the specimen is clamped as a vertical cantilever and struck on the notched face, whereas in the Charpy configuration the specimen lies horizontally on two supports and is struck on the face opposite the notch. The stress states and energy losses differ, so Izod and Charpy results are not interchangeable, even on identical materials.

Why is the specimen notched before testing?

The notch concentrates stress and imposes a triaxial stress state at its root, which suppresses ductile yielding and promotes brittle fracture. This makes the test far more discriminating between materials, reveals notch sensitivity that smooth-bar tests hide, and mirrors the behavior of real parts containing sharp internal corners, scratches, weld lines, or sink marks.

How many specimens should be tested?

At least 10 notched specimens are normally tested for each condition because impact results are statistically scattered, particularly for materials near a ductile-brittle transition. Additional specimens should be tested whenever the scatter is high, when fracture modes are mixed, or when a lot-release decision depends on the value.

What does a "non-break" result mean?

A non-break (NB) designation means the specimen did not fracture completely; it may bend, dent, or tear only partially while absorbing the full pendulum energy. This indicates an extremely tough material for the chosen notch and temperature, but the recorded energy is not a true fracture energy, so NB values are reported separately and are not averaged with complete-break results.

What are typical notched Izod impact strength values for common plastics?

Indicative ranges at 23 °C with a Type A notch are as follows; actual values depend strongly on grade, moisture state, and processing:

• Polycarbonate (PC): approximately 60 to 90 kJ/m²
• ABS: approximately 10 to 30 kJ/m²
• Impact-modified PP copolymer: approximately 6 to 15 kJ/m²
• HDPE and rigid PVC: approximately 5 to 15 kJ/m²
• POM and PET: approximately 4 to 10 kJ/m²
• General-purpose PS and PMMA: approximately 2 to 4 kJ/m²

Can ISO 180 results be used to predict real-world impact performance?

Notched Izod strength is a comparative ranking tool under a specific, severe stress state, not a direct design property. It is highly effective for material screening, specification control, and trend analysis, but component-level performance should be validated with product-specific impact, drop, or crash testing that reproduces the actual geometry, speed, and temperature of service.