Charpy Pendulum Impact Test, commonly referred to as impact test for short, is a mechanical property test used to determine a material’s ability to resist fracture under high-speed impact loading. If a tensile test measures how strong a material is when pulled slowly, an impact test evaluates how durable a material is when struck suddenly. For critical structural components such as pressure vessels, bridges and ships, the impact test is the most critical method to determine whether a material will undergo brittle fracture.
1. Test Principle: Similar to Chopping Wood
Imagine holding an axe (pendulum), lifting it high, then striking forcefully at a wooden stick (specimen) with a notch in the middle.
- Specimen: Typically a rectangular metal bar with a V-notch (Charpy V-notch, CVN) or U-notch (Charpy U-notch, CVU) machined in the center.
- Pendulum: The pendulum of the testing machine is raised to a specified height to gain potential energy.
- Impact: The pendulum is released and falls freely, striking violently against the back of the specimen’s notch.
- Fracture: The specimen breaks instantaneously.
- Energy Absorption: Energy is consumed when the pendulum breaks the specimen, causing the pendulum’s swing height to decrease.
- Reading: The machine automatically calculates the energy difference of the pendulum before and after impact; this difference is the impact absorbed energy (K), usually measured in Joules (J).
2. Core Index: Impact Energy (Ak or K)
Test results are generally expressed by a single value, namely impact absorbed energy.
Symbols
- Ak (commonly used in GB standards): Represents impact absorbed energy.
- K (commonly used in ISO/ASTM standards): Represents impact toughness value.
- The suffix V or U indicates the notch shape (e.g., AkV stands for V-notch impact energy).
Meaning
- Higher value: Indicates better toughness of the material, higher resistance to fracture, and greater energy absorption capacity.
- Lower value: Indicates the material is more brittle and will shatter like glass under slight impact.
3. Purpose of the Test (Preventing Cold Brittleness)
This is the most important significance of the impact test. Some steels appear tough at room temperature (passing tensile tests), but suddenly lose toughness and undergo brittle fracture when exposed to low temperatures, notches or impacts. This phenomenon is known as cold brittleness.
Case Study
During World War II, many large oil tankers sailing in the cold North Atlantic suddenly broke in two simply due to wave impact. Subsequent investigations revealed that the steel had extremely poor impact toughness at low temperatures.
Application in Pressure Vessels
Explosions of pressure vessels are rarely caused by overpressure stretching and rupturing the vessel (tensile fracture), but rather by brittle fracture triggered by minor vibrations when the material becomes brittle at low temperatures. Therefore, steels used in low-temperature pressure vessels must undergo low-temperature impact tests.
4. Key Concept: Ductile-Brittle Transition Temperature (FATT)
A material’s toughness varies with temperature. Through impact tests, an impact energy-temperature curve can be plotted.
- Ductile-Brittle Transition Temperature (FATT): The critical temperature at which a material changes from a ductile state (fibrous, dull fracture surface) to a brittle state (crystalline, bright silvery fracture surface).
- Engineering Significance: During design, the operating temperature of equipment must be higher than the material’s ductile-brittle transition temperature to ensure safety.
5. Relevant Standards
- Chinese Standard: GB/T 229-2020 Metallic materials — Charpy pendulum impact test method
- International Standards: ISO 148, ASTM E23
