Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics - PDF

​

ASTM D256 measures the Izod impact resistance of plastics. A notched bar is clamped upright as a cantilever and struck by a swinging pendulum; the energy it takes to break the bar, divided by the bar thickness, is the Izod impact strength in J/m (ft·lbf/in). It is the most quoted toughness number on plastic data sheets.

ASTM D256 Test Methods

• Method A: standard notched Izod, the method on most data sheets

• Method C: like Method A, but corrects for the energy used to toss the broken piece; used for materials below about 27 J/m

• Method D: measures notch sensitivity by testing specimens with different notch radii

• Method E: reversed notch, with the notch facing away from the striker, approximating unnotched toughness

The former Method B (Charpy) now lives in ASTM D6110.

Specimen and Notch

• Bar 63.5 × 12.7 mm (2.5 × 0.5 in), 3.0–12.7 mm thick; 3.2 mm (1/8 in) is the most common

• 45° notch with a 0.25 mm (0.010 in) tip radius

• Depth of material under the notch: 10.16 mm (0.400 in), so the notch is 2.54 mm deep

• Machine the notch with a single-tooth cutter and check it with an optical comparator

• Condition at 23 °C and 50% RH for at least 40 h (D618); test at least 5 specimens (10 if the results are scattered)

Test Procedure

• Choose a pendulum whose capacity gives readings within its accurate range; common sizes are 2.7 J and 5.4 J (2 and 4 ft·lbf)

• Striking velocity is about 3.46 m/s (11.4 ft/s)

• Clamp the bar vertically in the vise with the notch facing the striker and its center level with the vise top

• Release the pendulum, record the energy absorbed and correct for windage and friction

• Record the break type for every specimen

Break Types

• C – complete break: the specimen separates into two or more pieces

• H – hinge break: an incomplete break where one part cannot support itself above horizontal

• P – partial break: at least 90% of the depth under the notch is broken

• NB – non-break: less than 90% broken

• Report averages only for specimens with the same break type

Calculation

Izod impact strength = corrected energy (J) ÷ specimen thickness (m) = J/m

Example: a 3.2 mm thick polycarbonate bar absorbs 0.35 J after the windage correction. Impact strength = 0.35 ÷ 0.0032 = 109 J/m (2.04 ft·lbf/in).

Some data sheets also give kJ/m² (energy ÷ area under the notch), which is the ISO 180 unit. The two are not directly interchangeable.

Related Standards

• ASTM D6110 – Charpy impact of plastics

• ISO 180 – Izod impact of plastics (kJ/m²)

• ASTM E23 – Charpy and Izod impact of metals

• ASTM D618 – conditioning of plastics

• ASTM D4812 – unnotched cantilever beam impact

ASTM D256 FAQ

What units are used for ASTM D256 Izod impact strength?

Joules per meter (J/m) of specimen thickness, or ft·lbf/in. ISO 180 uses kJ/m² instead, so the numbers are not directly comparable.

What size is the D256 specimen?

63.5 × 12.7 mm, 3.0 to 12.7 mm thick (3.2 mm is most common), with a 45° notch, 0.25 mm tip radius and 10.16 mm of material under the notch.

What are the D256 methods?

Method A (standard notched Izod), Method C (toss-energy correction for low-energy materials), Method D (notch sensitivity) and Method E (reversed notch).

What is the difference between Izod and Charpy?

Izod clamps the bar upright as a cantilever and strikes it above the notch; Charpy supports the bar at both ends and strikes it behind the notch. D256 is Izod; D6110 is Charpy for plastics.