How do they determine if a component is tough enough to withstand heavy impact? Materials undergo a drastic change in varying temperatures. If the temperature goes below -40°C to -198°C, the physical properties of the material change. Some cracks and flaws form at a microscopic level. Some defects, even at a microscopic level, could make the material weak thereby causing failure under impact. Read our partners as well: Lebronze Alloys Bornel. How much impact energy a material can withstand is measured through impact testing. The most common impact testing method is swinging pendulum impact testing. The pendulum is used to strike a notched specimen and calculate the impact energy. Curious about how the test is done? Read the article to understand impact testing thoroughly.
What Is Impact Testing?
Impact testing signifies the toughness of a material, which is the ability to absorb energy during plastic deformation. Toughness takes into account both the strength and ductility of the material. There are two distinct types of toughness mechanisms. In this case, it is appropriate to consider a notch as a very high local stress concentration. The first type of mechanism occurs in a ductile material. This is because very high stresses at the end of the notch produce local yielding of the material and local plastic flow at the crack tip. This blunts the sharp tip of the notch, reducing the stress concentration effect. The second mechanism occurs in fibers, wood materials, etc., which have a weak interface. Local tensile stress developed at the front of a propagated crack opens up the interface and produces a crack sink, effectively increasing the radius of the crack tip. The stress concentration at the notch increases with decreasing notch radius. Toughness is the ability of the material to withstand cracks, preventing the transfer or propagation of cracks across its section, thus avoiding failure.
Impact testing machines consist of a pendulum suspended from a short shaft that rotates in ball bearings and swings midway between two rigid upright stands supported on a rigid base. According to Indian Standard, the speed of the pendulum at the instant of striking shall be 4.5 - 7 m/s, and the plane of swing of the striker shall be vertical and within 0.5 mm of the plane midway between the supports. The pendulum can be raised to any desired height and rested at that position. It is supported in the starting position by a catch and can be released by a trigger. The mechanism is designed so that the pendulum is not disturbed when the catch is released. The striking energy of the testing machine should be 300±10J for standard testing.
Charpy Impact Test
Charpy tests determine whether a metal can be classified as brittle or ductile, particularly useful for materials such as ferritic steels that show a ductile to brittle transition with decreasing temperature. A brittle material will absorb a small amount of energy when impact tested, while a tough ductile metal absorbs a large amount of energy. The appearance of a fractured surface also gives information about the type of fracture: a brittle fracture will be bright and crystalline, and a ductile fracture will be dull and fibrous. The percentage crystallinity is determined by judging the amount of crystalline or brittle fracture on the surface of the broken specimen, which measures the amount of brittle fracture.
Test Specimen
The standard test piece shall be machined all over and have a square cross-section of 10 mm x 10 mm sides, 55 mm long, with a U-notch or V-notch of specified depth with a 1 mm root radius in the center of one face. Where the standard test piece cannot be obtained from the material, one of the subsidiary test pieces with a rectangular cross-section shall be used, with the notch cut in one of the narrower faces.
Schematic
Procedure:
- A Charpy V-notch specimen is placed across parallel jaws in the impact-testing machine.
- The pointer is set to its maximum value (300 J).
- The hammer is released from the initial height downward towards the sample.
- Observations and the energy absorbed are recorded and tabulated.
- Steps 1-3 are repeated for another type of metal.
Izod Impact Test
In the Izod test, the specimen is supported at one end like a cantilever beam. From the amount of swing of the pendulum, the energy dissipated in breaking the specimen is obtained; this energy is the impact toughness of the material. Unlike hardness test conversions, no quantitative relationships have yet been established between Charpy and Izod tests.
- A test specimen with a V-shaped notch is fixed vertically.
- The specimen is broken by striking it from the same side as the notch with the hammer.
- The fracture energy is determined from the swing-up angle of the hammer and its swing-down angle.
- The Izod impact value (J/m, kJ/m²) is calculated by dividing the fracture energy by the width of the specimen.
Applications
The Charpy and Izod tests are commonly used to evaluate the relative toughness or impact toughness of materials. They are often used in quality control applications where a fast and economical test is needed. These tests are more comparative than definitive.
Conclusion
Changing temperatures affect material properties, so checking a material's toughness is crucial. Charpy and Izod impact testing procedures ensure accurate measurements of how much impact load a material can withstand. Machines and machine components generally undergo high energy impact loading. Therefore, to ensure these components and machines do not fail under impact stresses, the material must be tested for toughness before use in the machine. This is the significance of impact testing.
