All about Impact Testing in the North American Market

Know the Toughness of Material through Impact Testing  

Think of a plastic substance. If it is subjected to pressure or impact, it will deform. The material’s ductility can be determined by the point at which it is about to deform due to stress or impact. When a material resists deformation and only begins to show wear and tear after a long amount of time, you can say that it is ductile.

Nevertheless, ductility does not automatically equate to toughness in a material. A suitable balance of ductility and strength is required to be tough. A material with greater strength and ductility is thought to be tougher than one with lesser ductility and strength.

Calculating the area below the stress-strain curve using a tensile test is a good approach to assessing a material’s true toughness. The value is known as the toughness of the material. The amount of energy units per volume is used to express this. The toughness of a material is equivalent to a substance’s slow energy absorption.

Consider a stress-strain diagram as an illustration of the elasticity limit of a material. Put the diagram into two halves. The elastic range is located on the left. On the right side of the elastic limit lies the plastic range. The energy/unit volume that the material can absorb without experiencing permanent deformation is measured in the region under the elastic range’s curve OP.

The resistance modulus is the name of the value. The level of toughness is the amount of energy per unit of volume that the material can withstand (or the region under the entire diagram till the fracture point). This characteristic primarily belongs to the plastic category. The amount of elastic energy that may be categorized as part of the total absorbed energy is quite little.

Different elements that affect the toughness of a material 

The following elements significantly affect a material’s level of toughness:

• Stress level (or the rate of loading)
• Notch impact
• Temperature

When subjected to impact or a dynamic force, a given metal may fail while having a respectable amount of toughness under a static load. When the rate of loading is increased, both ductility and toughness tend to decline.

The notch effect is a different factor. This results from the dispersion of stress. a certain material that, when subjected to uniaxial stress, may exhibit respectable toughness. Yet, the specimen might not be able to withstand simultaneous plastic and elastic deformation originating from separate directions if stress in a multiaxial state is generated as a result of the presence of a notch.

Temperature is the third factor that has a substantial impact on material toughness. You’ll observe that when the temperature decreases, the material loses toughness and ductility.

What Distinctions Exist Between Tough and Strong?

Strong materials are those that can withstand a large amount of stress without rupturing or deforming. Hence, a substance with a high amount of tensile strength can be described as strong.

Contrarily, toughness is the quality that enables a person to withstand impact harm. Via the appropriate testing process, it can be measured. It is visible at the bottom of a stress-strain diagram that leads up to the fracture point.

The substance with the greatest tensile strength is the strongest. On the other hand, the substance that takes up the most space underneath the diagram can be regarded as the strongest substance.

• Impact Examining
• North American Impact Testing Support Anvil
• Anvil for supporting impact tests

Either the Izod or Charpy test procedures are used to gauge a material’s relative toughness. Naturally, the two bear the names of the test method’s original creators.

A specimen with a notch-like shape is used for the impact test. Two different types of notches are typically employed. Keyhole notch and the more popular V notch are the two different types of notches. You can use either Charpy or Izod with either kind of specimen.

The specimen is positioned on an anvil during the Charpy impact test so that it resembles a straightforward beam with supports at each end. In contrast, a vice is used to secure the specimen during an Izod test. This creates the appearance of a cantilever beam since one of the ends is free.

A notched specimen is struck once by a freely swinging pendulum during an impact test. Upon release, the pendulum begins to swing from a fixed height. You can predict the energy expended from the strike because you know the mass of the pendulum and the height at which it is elevated before it swings.

The pendulum impacts the sample in the case of the Charpy test specimen from behind the V notch. In contrast, the specimen fronting the pendulum with the V-notch is struck above the notch in an Izod test.

Impact testing using pendulum impact hammers in North America

High-capacity impact hammers with a pendulum

In both tests, the sample material is ruptured using some of the pendulum’s energy. In this technique, the pendulum rises to a specific height on either side of the device that is lower than the original height.

There is a way to show how much energy was absorbed to damage the sample material. For example, a Charpy machine may produce energy between 220 and 330 foot-pounds (or 300 and 400 kg).

Sometimes a machine with a smaller capacity is employed. The machine must, however, always have a capacity that is far more than the energy consumed by the specimen. The testing technique frequently requires a temperature other than ambient or room temperature.

Why the Test Temperature Matters

Izod or Charpy tests are frequently repeated, testing the specimen at a different temperature each time because the temperature has a substantial impact on toughness. The data is then utilized to create a temperature-dependent impact toughness graph for a specific material. The material’s resistance to impacts is then contrasted with the temperature graph.

Low temperatures make the specimen more fragile. It has a low impact toughness level, as you can see. On the other hand, it is more ductile and more impact-resistant in warm or high temperatures. The boundary separating brittle and ductile behaviour is known as the transition temperature. When selecting the material to utilize for a specific application, this temperature is typically a crucial factor to take into account.

When subjected to impact testing at vastly different temperatures, ferritic or body-centred cubic materials exhibit a significant behaviour change. A large amount of energy is absorbed by the impact specimen fracture brought on by ductile mechanisms (usually microvoid convalescence) at temperatures exceeding transition.

Lower temperatures cause the specimen to shatter brittlely (typically in the cleavage form), which releases less energy. In general, the resulting fracture within a particular transition range frequently combines both brittle and ductile fracture zones.

Depending on the test subject, a transition from one type of behaviour to another has a different temperature range. As already discussed, the strength of a certain material is affected differently by different temperatures. It is important to note that the specimen exhibits properties of a ductile material at room temperature. Under -200o C, however, it tends to behave just like a brittle material would.

You can infer from the aforementioned paragraphs that material specifications frequently call for testing at a certain test temperature for the amount of energy absorbed, lateral expansion, the appearance of fracture, or any combination of these.

Here is a concrete example to help:

Although the steel used to construct the hull of the famous ship Titanic had adequate strength, it had poor toughness in freezing temperatures. Instead of creating bent steel and little fractures (which indicate ductile failure) when the enormous ship hit an iceberg, the crack expanded quickly and became very huge in a short amount of time (because of brittle failure).

Further details on impact testing can be found in the following ASTM specifications:

• E23 – These are the accepted test procedures for metallic materials that use the notched bar impact test.
• These are the accepted test procedures and definitions for mechanical testing of steel goods (A370).
• The Charpy impact testing protocol’s requirements are also included in ASTM A370. But all of its attention is on steel items.

All of the testing specifications provide information on how to accurately measure the energy absorbed by the material of the notched specimen when it comes into contact with a swinging pendulum.

A Charpy test typically calls for the notched metal sample to be horizontally supported by the supports at the ends and in contact with the nearby vertical anvils. After that, a striker mounted on a hammer or pendulum that is swinging strikes the sample material at the middle line opposite the anvils.

You need samples of the relevant product or material to measure the metal toughness using the Charpy impact test. Normally, samples are 55 mm long with a 10 mm by 10 mm section and a centre V notch that is 2 or 3.3 mm wide, though they can alternatively be 5 mm wide. The testing protocol also calls for reduced section specimens with a 10 mm or 10 mm width.

• 55x10x10mm Standard Impact Testing Specimens
• 55x10x10mm Standard Impact Testing Specimens

In unique circumstances, the thickness can be placed in between the specimen and the supports to ensure the required flawless strike axiality. The device that caused the impact is exceptionally safe, and its solutions go above and beyond the norm.

For instance, the metal bar that physically separates the armed pendulum bar from the location where the operator manually loads the specimen.

Also available is the fully interlocking protection guard. It includes two access doors to make it simple to retrieve the specimen’s post-test components. The operator can watch the impact test thanks to the clear Lexan panels. A motorized hammer return to the release point is included with the gadget. This avoids injuries by doing away with raising the hammer by the handle.

The specifications for the Charpy impact testing specimens are laid out in the ASTM E23 standard. Additionally, some applications demand certified outcomes from the National Institute of Standards & Technology or NIST.

The NIST stipulates that the samples must first be cooled to -40° C before being examined at that same temperature. As a result, it’s crucial to have a high-series energy break cooling bath. Upon the completion of the test, it is critical to transmit the documentation to the NIST for certification evaluation. This documentation must include the impact energy absorbed by each specimen as well as the specimens’ broken pieces.

You can anticipate receiving either a certification (passed) or a failure report with suggestions for improvements to pass subsequent NIST certification efforts. The NextGen Service Center will handle the full procedure if you submit it to them.

The accuracy of the measurement device can also be verified using samples approved by the BCR. There are different sizes of Charpy pendulums. 150, 300, 450, and 750 Joules are these values. To avoid further foundation work, customers can purchase the units with plinths in their pre-compressed condition.

• Impact Testing in the North American Aerospace Industry
• Impact Testing for the Aerospace Sector

For Charpy testing, a typical V-notch specimen is 55 mm long, 10 mm square, and has a 2 mm-deep notch with a 0.25 mm radius tip drilled into one face.

In some circumstances, tests are conducted on subsidised Charpy samples having a thickness of less than 10 mm.

The specimens are supported at the anvil’s two ends before being struck by a pendulum on the face opposite the notch. The sample is subsequently fractured, and the energy absorbed is then measured. This reveals the notch toughness of the test material. The pendulum swings during the experiment. The swing height serves as a gauge for the energy absorbed during specimen fracture.

Three samples are typically examined independently at a specific temperature. The outcomes are next averaged. On the other hand, tests are run at various temperatures to create a curve showing when a material becomes brittle and ductile.

A Charpy test can assist in identifying if a metal is brittle or ductile, which is useful for ferritic steels that exhibit a ductile to brittle transition at lower temperatures.

When put through impact testing, a metal that is deemed fragile will only absorb a little amount of energy. On the other hand, a strong and ductile metal can hold a lot of energy. A fracture surface’s appearance also reveals the type of fracture that took place; a brittle fracture is crystalline and brilliant. Fibrous and ductile fractures.

By assessing the degree of brittle or crystalline fracture on the surface of the fractured specimen, the crystallinity percentage can be calculated. It serves as a gauge for brittle fracture volume.

The specimen’s ductility is measured by lateral expansion. The test piece exhibits some deformation before breaking in tests when a ductile metal is bent or broken, and the material squeezes out on the sides of the compression face. In terms of mm of lateral expansion, the quantity of energy that causes the specimen to deform is expressed.

The percentage of crystallinity and lateral expansion are optional in test reports for Charpy tests, but the absorbed energy (expressed in J) is always reported.

The qualitative nature of Charpy impact tests must be emphasized. The findings can only be compared to one another or to those that meet certain criteria. They cannot be used to calculate a metal’s parent or weld metal’s fracture toughness.