Impact Testing: Izod vs Charpy Pendulum Dynamics & Fracture Mechanics

Polymer components subjected to sudden mechanical impacts (drop impact, collision, shock loading) must resist brittle fracture. Pendulum Impact Testing—via Izod and Charpy methods—measures the energy absorbed during rapid crack propagation across a standardized notched specimen. Understanding notch sensitivity, ductile-to-brittle transition temperature ($DBTT$), and the non-interchangeable differences between ASTM and ISO test standards is critical for automotive bumper and protective helmet material selection.

By completing this lesson, you will be able to:

• Differentiate Izod (cantilever beam) and Charpy (three-point supported beam) testing geometries.
• Calculate absorbed impact energy ($E_{absorbed}$) and impact strength.
• Compare ASTM D256 ($ext{J/m}$) and ISO 180 / ISO 179 ($ext{kJ/m}^2$) units and notch radii.
• Diagnose notch sensitivity and brittle failure modes.

graph TD
    A["Pendulum Released from Initial Height h1"] --> B["Impact Striker Strikes Notched Specimen"]
    B --> C{"Testing Configuration"}
    C -->|"Izod Method (ASTM D256 / ISO 180)"| D["Vertical Cantilever Beam (Struck on Notched Face Side)"]
    C -->|"Charpy Method (ISO 179)"| E["Horizontal Simply Supported Beam (Struck Opposite Notch)"]
    D --> F["Pendulum Swings to Reduced Height h2"]
    E --> F
    F --> G["Absorbed Energy E = m*g*(h1 - h2) - Friction Losses"]

Pendulum Energy Equation

Potential energy absorbed by specimen fracture:

$$E_{absorbed} = m \cdot g \cdot (h_1 - h_2) - E_{windage}$$

Unit Formats: ASTM vs ISO Standards

$$\text{Impact Strength}_{ASTM} = \frac{E_{absorbed}}{b} \quad (\text{J/m})$$ $$\text{Impact Strength}_{ISO} = \frac{E_{absorbed}}{b \cdot (h - a)} \times 10^3 \quad (\text{kJ/m}^2)$$

Where $b$ = specimen width ($0.0032 ext{ m}$), $h$ = total thickness ($0.0127 ext{ m}$), $a$ = notch depth ($0.00254 ext{ m}$).

Worked Numerical Example:

<div className="problem-statement">

Problem: A Polycarbonate notched Izod specimen ($b = 3.20\text{ mm} = 0.0032\text{ m}$, depth under notch $h - a = 10.16\text{ mm} = 0.01016\text{ m}$) absorbs $E_{absorbed} = 2.10\text{ Joules}$ during fracture. Calculate impact strength under both ASTM D256 and ISO 180 formats.

</div> <div className="solution-step">

Solution:

1. Calculate ASTM D256 Impact Strength ($ext{J/m}$):

$$\text{Impact Strength}_{ASTM} = \frac{2.10\text{ J}}{0.00320\text{ m}} = 656.25\text{ J/m}$$

1. Calculate ISO 180 Impact Strength ($ext{kJ/m}^2$):

$$\text{Cross-Section Area} = 0.00320 \times 0.01016 = 3.2512 \times 10^{-5}\text{ m}^2$$ $$\text{Impact Strength}_{ISO} = \frac{2.10\text{ J}}{3.2512 \times 10^{-5}\text{ m}^2} = 64,591.5\text{ J/m}^2 = 64.59\text{ kJ/m}^2$$

• Automotive Bumper Compound Validation: Polypropylene-EPDM tough impact testing at $-30^circ ext{C}$ in Chennai auto testing lab. (Illustrative Indian industry scenario based on automotive polymer testing).

• Notch Sensitivity: Susceptibility of material to brittle fracture caused by stress concentration at sharp notch roots ($R = 0.25 ext{ mm}$).
• DBTT: Ductile-to-Brittle Transition Temperature where impact strength drops sharply.

1. ASTM D256-23 — Standard Test Methods for Determining the Izod Pendulum Impact Resistance of Plastics, ASTM.
2. ISO 179-1:2023 — Plastics — Determination of Charpy impact properties, ISO.