Hardness and Impact Testing of Materials (BHN)
Definition
Hardness is the property of a material that enables it to resist plastic deformation, indentation, scratching, and wear when an external force is applied.
Brinell Hardness Number (BHN) is the numerical value obtained from the Brinell hardness test, which expresses the hardness of a material based on the load applied and the surface area of the indentation made by a hard spherical indenter.
Impact strength is the ability of a material to absorb sudden or shock loading without fracturing.
Impact testing is a mechanical test used to determine the toughness of a material by applying a rapidly increasing load and measuring the energy absorbed during fracture.
Main Content
1. Hardness and Brinell Hardness Number (BHN)
Meaning of hardness
- Hardness indicates resistance to indentation or surface deformation. A hard material is difficult to scratch, dent, or cut. In engineering, hardness is often related to wear resistance and strength. For example, hardened steel is much harder than mild steel, and a ceramic material is harder than most metals.
Brinell hardness test principle
- In the Brinell test, a hardened steel ball or tungsten carbide ball of standard diameter is pressed into the surface of the test specimen using a specified load for a fixed time. After removing the load, the diameter of the indentation is measured, and the BHN is calculated. A smaller indentation means higher hardness.
Formula of BHN
where:
- = applied load
- = diameter of the ball indenter
-
= diameter of indentation
This formula expresses hardness as the load divided by the curved surface area of the indentation.
Standard test conditions
- Commonly used ball diameter is 10 mm, and the load is selected based on material type. For steel, a load of 3000 kgf is often used; for softer metals, lower loads are used. The test is performed for a specified dwell time, usually about 10–15 seconds for metals.
Advantages of BHN
- It is simple, gives reliable results for bulk materials, and is suitable for coarse-grained metals where other hardness tests may show variation. It is especially useful for castings and forgings.
Limitations of BHN
- It is not suitable for very thin materials, very hard materials, or small components where the indentation is too large. It also leaves a visible mark on the surface.
2. Impact Testing and Toughness
Meaning of impact testing
- Impact testing determines the amount of energy a material can absorb under sudden loading before fracture occurs. This is important because materials often fail not under slow loading but due to shocks, blows, or repeated sudden stresses.
Toughness vs hardness
- Hardness measures resistance to surface indentation, while toughness measures resistance to fracture under impact. A material may be hard but brittle. For example, glass is hard but has low impact strength, whereas mild steel has moderate hardness but high toughness.
Need for impact testing
- Many components such as automobile parts, railway wheels, machine frames, crane hooks, and structural members experience sudden loads during service. Impact tests help ensure that such parts will not fail catastrophically.
Energy absorption concept
- In impact testing, the specimen absorbs energy by plastic deformation, crack initiation, crack propagation, and fracture. Higher absorbed energy means greater toughness and better resistance to shock.
Effect of temperature
- Some materials become brittle at low temperatures and show a sharp drop in impact strength. This is especially important for steels used in cold climates, pressure vessels, and pipelines.
Applications in material selection
- Engineers use impact test results to choose materials for components that must withstand sudden forces, accidental overloads, or dynamic service conditions.
3. Types of Impact Tests and Their Significance
Charpy impact test
- In this test, a notched specimen is supported as a simple beam and struck by a pendulum hammer at the center opposite the notch. The energy absorbed in breaking the specimen is measured from the swing of the hammer. It is widely used because it is quick and economical.
Izod impact test
- In the Izod test, the specimen is clamped vertically like a cantilever beam, and the pendulum strikes it above the notch. This test is also used to assess toughness, especially for metals and some plastics.
Notch importance
- A notch acts as a stress concentrator and simulates real-life defects such as cracks, grooves, or abrupt changes in section. It helps determine how a material behaves under worst-case conditions.
Brittle and ductile behavior
- Ductile materials absorb more energy before fracture, showing plastic deformation. Brittle materials absorb very little energy and fracture suddenly. Impact testing helps distinguish these behaviors clearly.
Engineering significance
- The results of impact tests are used to check heat treatment quality, welding quality, notch sensitivity, and suitability of materials for dynamic applications.
Relationship with service failure
- Many sudden fractures in service are due to low impact toughness rather than low static strength. Therefore, impact testing is essential in design safety.
Working / Process
1. Hardness test by Brinell method
- Clean the surface of the specimen so that it is smooth and free from scale, rust, or dirt.
- Place the specimen on the test table and bring the Brinell indenter ball into contact with the surface.
- Apply the standard load gradually and keep it for the specified dwell time.
- Remove the load and measure the diameter of the impression in two directions using a microscope or measuring device.
- Calculate the BHN using the standard formula or a hardness chart.
2. Impact test by Charpy or Izod method
- Prepare a standard notched specimen of specified dimensions.
- Place the specimen in the correct position according to the test method: horizontally for Charpy or vertically for Izod.
- Raise the pendulum to a fixed height and release it so it strikes the specimen at the notch.
- Record the energy absorbed by the specimen from the difference in pendulum height before and after fracture.
- Analyze the result to determine the toughness and fracture behavior of the material.
3. Interpretation of results
- A smaller Brinell indentation indicates a harder material, while a larger indentation indicates a softer material.
- A higher impact energy value indicates a tougher material with better resistance to sudden loading.
- Compare the results with standard values to decide whether the material is suitable for the intended application.
- Use the test results to guide heat treatment, material selection, quality control, or failure investigation.
Advantages / Applications
Useful for material selection
- These tests help engineers choose suitable materials for specific applications based on hardness and toughness requirements.
Quality control in manufacturing
- BHN and impact tests are commonly used to verify heat treatment, check uniformity of material properties, and detect defects in manufactured parts.
Widely used in industries
- They are applied in automotive parts, structural steel, machine components, railway equipment, pressure vessels, tools, and castings.
Helps predict wear and service behavior
- Hardness gives an indication of wear resistance, while impact toughness shows how the material will behave under shock or sudden loads.
Supports failure prevention
- By identifying brittle materials or poor heat-treatment conditions, these tests help prevent sudden failure in service.
Simple and economical
- Brinell hardness testing and standard impact tests are relatively simple, fast, and cost-effective compared to many other mechanical tests.
Summary
- Hardness testing measures resistance to indentation, while impact testing measures resistance to sudden fracture.
- BHN is a standard way of expressing hardness using the Brinell ball indentation method.
- Impact tests such as Charpy and Izod help determine the toughness of materials under shock loading.