The impact test is done to test
Toughness
Understanding the Impact Test and Material Properties
The impact test is a fundamental mechanical test used to evaluate a specific property of engineering materials, particularly metals and plastics. This test is designed to measure the ability of a material to withstand a sudden applied load or shock without fracturing.
Let's look at the material properties presented in the options:
- Strength: This refers to a material's ability to withstand a static load without deformation or fracture. It's often measured by tensile tests (yield strength, ultimate tensile strength) or compressive tests.
- Ductility: This is the ability of a material to undergo significant plastic deformation before fracturing. Materials like copper and aluminum are ductile, meaning they can be stretched or drawn into wires. This is typically measured in a tensile test by calculating the percentage elongation or reduction in area.
- Toughness: Toughness is the capacity of a material to absorb energy and deform plastically before fracturing when a load is applied. More specifically for impact tests, it measures the energy absorbed during fracture under a sudden or impact load. A tough material can withstand shocks and impacts without brittle failure.
- Hardness: Hardness is the resistance of a material to localized plastic deformation, usually by indentation or scratching. Tests like Brinell, Rockwell, and Vickers are used to measure hardness.
How the Impact Test Works
Common impact tests include the Charpy V-notch test and the Izod test. In these tests, a standard notched specimen of the material is struck by a heavy pendulum from a specific height. The pendulum swings through and breaks the specimen.
The energy absorbed by the material during the fracture process is calculated from the difference in the pendulum's height before and after breaking the specimen. A material that absorbs a large amount of energy before breaking is considered tough. A brittle material, which fractures easily under impact, absorbs very little energy.
Therefore, the impact test directly measures the energy absorbed by the material during a high-strain-rate fracture event, which is the definition of impact toughness.
Why Toughness is Measured by Impact Tests
Unlike static tests (like tensile tests for strength and ductility), the impact test applies a load very rapidly. Materials can behave differently under sudden loads compared to slow, steady loads. Some materials that are strong under static load might be brittle and fail catastrophically under impact if they lack toughness. The impact test helps predict how a material will behave in applications where sudden shocks or impacts are expected.
Based on the principles of the test and the properties it quantifies, the impact test is done to test the toughness of a material.
| Material Property | What it Measures | Common Test Method |
|---|---|---|
| Strength | Resistance to static deformation/fracture under load | Tensile Test, Compression Test |
| Ductility | Ability to deform plastically before fracture | Tensile Test (% Elongation, % Reduction in Area) |
| Toughness | Energy absorption capacity before fracture (especially under impact) | Charpy Test, Izod Test |
| Hardness | Resistance to surface indentation/scratching | Brinell Test, Rockwell Test, Vickers Test |
Revision Table: Mechanical Properties Testing
Understanding the different mechanical properties and how they are tested is crucial in material science and engineering. Each test provides insight into specific aspects of a material's behavior under load.
| Property | Definition | Typical Test Type | What the Test Measures |
|---|---|---|---|
| Strength | Ability to resist deformation/failure under static stress. | Tensile, Compression, Shear | Yield Strength, Ultimate Strength |
| Ductility | Ability to undergo plastic deformation before fracture. | Tensile | Percent Elongation, Percent Reduction in Area |
| Toughness | Ability to absorb energy up to fracture, especially under impact. | Impact (Charpy, Izod) | Energy absorbed during fracture (e.g., Joules) |
| Hardness | Resistance to localized surface deformation or indentation. | Indentation (Brinell, Rockwell, Vickers) | Hardness Number (e.g., HRB, HRC, HB, HV) |
Additional Information: Impact Testing Methods
The two primary types of impact tests, Charpy and Izod, differ mainly in the way the specimen is supported and struck.
- Charpy Impact Test: The specimen is supported horizontally at both ends and struck in the center by the pendulum on the side opposite the notch.
- Izod Impact Test: The specimen is clamped vertically at one end, and the pendulum strikes the un-clamped end on the notched side.
Both tests require careful preparation of notched specimens because the notch creates a stress concentration point, which is important for obtaining consistent and relevant impact toughness data. The results are often temperature-dependent, so tests are frequently conducted at various temperatures to determine the ductile-to-brittle transition temperature (DBTT) for materials like steel, which is a critical parameter in many applications.
Important Questions from Testing of Materials
The radius of curvature at the root of the V-notch in an Izod impact test specimen is
The ability of a material to be drawn into a thin wire without breaking is termed as:
___________ is the collective term for the physical manifestations of the defects like cracks, spalling, pop out, staining and corrosion.
In Rockwell hardness testing method, the hardness of a material is measured by-
The property of the material or a structure indicating the extent to which it can deform beyond the limit of yield deformation before failure or fracture is termed as:
