What is Toughness?
Before you learn what is Impact Testing, it is important to understand the Toughness as a material properties because the impact testing or also called CVN testing is the only test used for the measured for the impact toughness for the materials (metals as well as non-metals) including welds. When we say toughness, its means the total amount of energy that is needed to fracture a material- or a component such as pressure vessels or ship or bridge, It can be anything. For example, toughness of a steel rod will the amount of energy required to make it to fracture. When we say impact toughness, it refer to the amount of energy required to sudden break or fracture a material and it is measured using the Charpy impact machine. Generally, as the amount of energy required to fail a material increase so the toughness of the material also increases.
What is Charpy V Notch bar or impact testing?
Charpy V-Notch test (Named after Prof. Georges Charpy) performed with set of 3 standard machined (Notched) samples & breaking them with impact load (high strain rate) at minimum operating temp. as per material specification. CVN testing measures the amount of energy absorbed during fracture; which is a measure of the material’s notch toughness. This test is particularly useful for ferritic steels that show a ductile to brittle transition (DBTT) behavior with decreasing temp. Therefore, it is important to know the Notch toughness at mini. specified temp. to ensure that selected material is suitable for particular application during mini. design service temp.
The zone under a stress/strain curve (darker zone in below picture) resulted from a tensile test is also a toughness test but with slow loading conditions and not represents a impact toughness. When we talk about Impact test, the toughness is represented by notch toughness. This notch toughness is the measure of the material resistance to brittle or sudden fracture when a notch is present and loading is fast, a sudden impact in case of CVN Impact Test.
Historically, the attention on impact toughness was not a consideration for this material properties. However, due to the brittle fractures of the welds in the Liberty ships faced by the USA in II world war, the emphasis were put on fracture toughness. This work became the foundation of toughness and fracture tests. With new research and development, Impact toughness was developed.
Importance of Charpy V Notch or Toughness Test
Why Charpy V notch or Toughness Test is so much important?
Because when a material is utilized below Ductile Brittle Transition Temperature or also called DBTT temp., it is prone to brittle failure. A brittle failure means there will be little or no warning before the material catastrophically fails. This type of failure mechanism must be avoided at all costs.
Brittle & Ductile Fracture in Charpy Impact test
If CVN fractured specimen show flat, shiny fracture with no shear lip then it is termed as brittle fracture, otherwise good ductile materials show dull-rough grey appearance with shear lips. As shown in below figure, Specimen A have ductile fracture, specimen B is mix of ductile& brittle and specimen C is having brittle fracture.
Factors affecting toughness of a Material
Several variables have an essential influence on the toughness of the material, and they are the following:
- Strain rate or rate of loading
- Notch effect
You should know that metal can possess a satisfactory level of toughness under static loads, but it can fail under some dynamic loads or even impact. If we can speak about a general rule, both ductility and toughness can decrease while the rate of loading increases. Temperature is another variable that has an essential influence on material toughness. When the temperature lowers, both the ductility and toughness decrease. The last variable is termed notch effect, and that’s due to the distribution of stress. Sometimes, a material that can display a high toughness when the stress we apply is uniaxial. Still, when we use multiaxial pressure in the presence of a notch, the material cannot resist the two-type of deformation: both plastic and elastic, in various directions.
Impact Test Procedure
To carry out the test the standard specimen is supported at its two ends on an anvil and struck on the opposite face to the notch by a pendulum as shown in the right side animation. The specimen is fractured and the pendulum swings through, the height of the swing being a measure of the amount of energy absorbed in fracturing the specimen.
Conventionally three specimens are tested at any one temperature, and the results averaged.
The Impact toughness of a material defines its ability to absorb energy before it breaks. The impact toughness can be determined with a Charpy test. The test consists of a specimen being broken by a single overload by a pendulum. A pointer is used to measure how far the pendulum swings after it has hit the specimen. The energy absorbed by the specimen is calculated from this value, which is the impact toughness.
Test Specimen for Charpy Impact Test
The Charpy Test Specimens can have one of three different types of the notches used as a ‘keyhole’, a ‘U’, and a ‘V’. For brittle materials (e.g. cast iron) keyhole and U-notch are suitable.
For welding tests and ductile materials, the Charpy V-notch or also called CVN specimens are used worldwide.
The dimensions of a standard Charpy-V specimen, shown in the below figure are 55 X 10 X 10mm with a 2mm deep notch with a tip radius of 0.25mm machined on one face.
Charpy Impact Testing Standards
- ASTM E23: Notched bar impact testing of metallic materials
- DIN EN ISO 148-1 2011: Charpy pendulum impact test part 1 test method
- EN 10045-1: Charpy Impact test on metallic materials – Test method
- EN-875: Charpy impact test on welds
- ISO 9016: Destructive tests on welds in metallic materials- impact testing- test specimen location, notch orientation, and examination.
- ASTM E2248 – 18: Standard Test Method for Impact Testing of Miniaturized Charpy V-notch Specimens.
Ductile to brittle transition in impact testing
Carbon steel and low alloy steel are characterized by the fact that the fracture behavior changes as the temperature drops, and the fracture type changes from ductility to brittleness. When the impact test is run at a range of temperatures, the absorbed energy and temperature results can be plotted to obtain the “S” curve shown in the following figure.
This indicates that as the temperature drops, the fracture of this type of steel changes from ductile to brittle changing the transition area as shown in above figure. Many specifications describe the transition temperature as the temperature at which fracture behavior changes from ductile to brittle. This temperature is often determined by completely arbitrary choice of the temperature at which the metal reaches a notched impact strength value of 27 joules. For example, referring to EN10028 Part 2 Steel Notched Impact Strength Requirements for pressure purposes.
This curve shows that ductile fracture of the same material absorbs more energy than brittle fracture. Therefore, if the structure is operated at a higher shelf temperature, it is very important to know the temperature at which the fracture behavior changes, ideally when considering the operating temperature of the structure in use.
The shape of the S-curve and the position of the upper and lower region are all influenced by the chemical composition, type of heat treatment, whether the steel is welded, weld heat input, weld deposit chemistry, and a number of others. cause. If you need good notch toughness, you need to control all the elements. This means that tight control of welding parameters is essential if impact testing is a specification requirement.
Quantitative results & Qualitative results
- The Quantitative result of the impact tests the energy needed to fracture material and can be used to measure the toughness of the material. There is a connection to the yield strength but it cannot be expressed by a standard formula. Also, the strain rate may be studied and analyzed for its effect on fracture.
- The Qualitative results of the impact test can be used to determine the ductility of a material. If the material breaks on a flat plane, the fracture was brittle, and if the material breaks with jagged edges or shear lips, then the fracture was ductile. Usually, a material does not break in just one way or the other, and thus comparing the jagged to flat surface areas of the fracture will give an estimate of the percentage of ductile and brittle fracture
Percentage Crystallinity and Lateral Expansion in Charpy Impact test
Brittle fractures are shiny and crystalline, and ductile fractures are dull and fibrous. Therefore, percentage crystallinity is a measure of the degree of brittle fracture, as determined by assessing the degree of crystal or brittle fracture on the surface of the broken sample.
Lateral expansion is simply a measure of sample ductility. When the ductile metal breaks, the specimen deforms before it breaks, causing a pair of “ears” on the sides of the pressure plane of the specimen to pop out, as shown in the figure above. The amount of deformation of the sample is measured and expressed as millimeters of lateral expansion. For example, in ASME B31.3, instead of specifying an impact value, the lateral extension of threaded materials and steels with a UTS greater than 656 N / mm2 should be 0.38 mm.