Charpy Test

Summary

The Charpie impact test measures the toughness of materials by assessing the energy absorbed by a specimen under impact loading. The test involves releasing a pendulum to strike a specimen and measuring the energy difference between the starting and ending positions. Results show that strain-aged plain carbon steel absorbs less energy, exhibiting brittle behavior, while normalized steel absorbs significantly more energy and demonstrates toughness with plastic deformation. The test also reveals how materials behave at different temperatures, with body-centered cubic steels showing a ductile-to-brittle transition, while face-centered cubic materials maintain ductility even at low temperatures, ideal for low-temperature applications.

Takeaways

• 😀 The Charpie impact test is designed to measure the toughness of materials under impact loading and multiaxial stress.
• 😀 A pendulum impact testing machine is used to carry out the test, featuring a heavy weight that swings to hit the specimen.
• 😀 The machine's drag indicator is checked to ensure it is set correctly before the test begins, proving proper setup and friction compensation.
• 😀 The test specimen is often made of standardized materials like plain carbon steel (S235), machined to a V-shaped notch for consistency.
• 😀 The pendulum swings down, hits the specimen, and absorbs part of its energy, with the energy loss being recorded on the drag indicator.
• 😀 The energy absorbed by the specimen is calculated by the difference in potential energy before and after the impact.
• 😀 The first specimen, made from strain-aged plain carbon steel, absorbs only a small amount of energy, indicating brittle behavior.
• 😀 The second specimen, made from normalized steel (S235), absorbs a significantly higher amount of energy, showing more toughness and plastic deformation.
• 😀 The test demonstrates the difference in toughness between materials, with normalized steel displaying superior energy absorption and toughness.
• 😀 Materials like plain carbon steels with a body-centered cubic structure show a characteristic S-shaped curve of impact energy versus temperature, indicating a ductile-to-brittle transition.
• 😀 Materials with a face-centered cubic crystal structure retain ductility even at low temperatures, making them ideal for low-temperature applications.

Q & A

What is the purpose of the Charpie impact test?

The Charpie impact test is designed to measure the toughness of materials under impact loading and multiaxial stress.

How does the Charpie impact test machine work?

The machine uses a pendulum with a heavy weight at the end. The pendulum is lifted to a starting position, then released to hit the test specimen. The energy absorbed by the specimen is measured.

What is the role of the drag indicator in the Charpie impact test?

The drag indicator is used to ensure that the machine is correctly adjusted. It is checked by turning it downwards and releasing the pendulum without a specimen to ensure it reaches the correct starting position and compensates for friction.

What is the significance of the test specimen's V-shaped notch?

The V-shaped notch is a characteristic feature of the test specimen, designed to create a specific point of stress concentration, which helps in measuring the material's toughness and energy absorption during impact.

What does the amount of energy absorbed by the specimen indicate?

The amount of energy absorbed by the specimen, measured using the drag indicator, indicates the material's toughness. The more energy absorbed, the tougher the material.

How is the energy absorbed by the specimen calculated?

The energy absorbed is calculated as the difference between the potential energies before and after the impact. The initial potential energy is based on the height and mass of the pendulum, and the energy absorbed is the difference in height after the pendulum strikes the specimen.

What does the fracture surface of the first specimen indicate?

The first specimen, made from strain-aged plain carbon steel, shows a mostly flat and slightly glittering fracture surface. This suggests that the material absorbed very little energy and exhibits brittle behavior.

How did the normalized steel behave in the impact test compared to the strain-aged steel?

The normalized steel absorbed significantly more energy (182 joules) and showed much greater plastic deformation, indicating a tougher behavior. In contrast, the strain-aged steel absorbed only 13 joules and showed brittle fracture.

How do the temperature and crystal structure of materials affect their impact behavior?

Materials with a body-centered cubic (BCC) crystal structure, like plain carbon steels, show a characteristic S-shaped curve of impact energy versus temperature. They behave toughly in the upper shelf region (high temperatures) and brittle at low temperatures. Materials with a face-centered cubic (FCC) crystal structure retain ductile behavior even at low temperatures, making them suitable for low-temperature applications.

Why are FCC materials suitable for low-temperature applications?

FCC materials do not experience a ductile-to-brittle transition, which means they retain their ductility even at low temperatures, making them ideal for applications in cold environments.