Impact strength – also called impact toughness – is the amount of energy that a material can withstand when the said load is suddenly applied to it. It may also be defined as the threshold of force per unit area before the material undergoes fracture .
The strength of materials may be defined in many ways, and some of the most common parameters include tensile strength, yield strength, and creep limit. These material strength properties involve the gradual application of force until the material breaks or shows deformity.
In contrast, impact strength involves the application of force in mere milliseconds or less. The near-instantaneous implementation of load causes the material to absorb the energy. When the amount of energy exceeds that which it can accommodate, the material will experience fracture, tear, or damage. In this case, it can be said that the impact strength of the material has been surpassed.
A general misconception is that materials with high impact strength also have a high degree of hardness; however, this isn’t always the case. In fact, an impact modifier or certain fibrous fillers may be added to a particular material enabling the latter to have an increased capability to absorb the excess energy and prevent rupture.
Some of the factors that affect impact strength include :
- temperature: impact strength increases with increasing temperature.
- material thickness: increasing the thickness reduces impact strength.
- notch radius: a smaller notch tip radius lowers the impact strength.
Meanwhile, other intrinsic factors that dictate a material’s impact resistance include morphology, in which impact resistance is inversely proportional to the material’s crystallinity and the amount of voids, and molecular weight, where a higher molecular weight enhances the material’s impact resistance .
When an impact load exceeds the impact strength, the material may exhibit any of the following types of failures :
- brittle fracture
- slight cracking
- ductile fracture
How is impact strength measured?
A material’s impact strength or toughness may be measured through any of the following tests:
Charpy impact test
The Charpy impact test makes use of a pendulum arm attached to a precalibrated energy gauge . The material specimen is customised to take the shape of a bar with a small V- or U-shaped notch in the middle.
To conduct the experiment, the pendulum arm is set at a particular position correspondent to an energy setting. The arm is released and its hammer end is allowed to hit the centre of the specimen. The impact strength of the material is determined by the amount of energy needed to break or fracture the specimen .
This kind of impact test is similar to the Charpy test in the sense that it also uses a hammer attached to a pendulum arm to hit a custom-made specimen bar and measure the energy needed to fracture it.
The main difference between the Izod test and the Charpy test is the orientation of the specimen in the measuring equipment. While the specimen is set horizontally in the Charpy impact test, the Izod test examines a vertically positioned sample with a V-Notch. Here, the pendulum hammer is made to strike the upper tip of the notched specimen .
Other differences include the specimen size, notch face direction, type of hammer, and type of tested material. The Charpy test examines metal specimens with the notch facing away from a striking ball peen hammer. The Izod test, on the other hand, is used to test relatively longer metal or plastic specimens with the notch facing towards a farming hammer .
Impact strength of materials
As a general observation, materials with high yield strength and low modulus of elasticity tend to exhibit high impact strength.
Here are some materials and their respective impact strengths:
- Polyethersulphone (PES) Granules (SU306311) = 8 kJ/m² (Charpy and Izod notched)
- General Polyethylene, high density (PE-HD) = 6-50 kJ/m² (Charpy notched)
- extrudr PLA NX2 Filament (renewable thermoplastic) = 7.5 kJ/m² (Charpy notched), 64 kJ/m² (Izod notched)
- extrudr ABS Filament (styrene) = 250 kJ/m² (Izod notched)
- NITRONIC® 60 Bar (austenitic stainless steel) = 310 kJ/m² (Charpy notched)
Applications where impact strength is important
Because impact strength denotes a material’s brittle-ductile transition , the parameter is significant for the following industries and applications:
- Industrial plastics: Polymers and plastics generally exhibit higher impact strength than other hard materials. Knowing the impact strength of plastics allows manufacturers to classify the materials for specific purposes, such as polyamides and polyethylene.
- Weather resistance of building structures: Materials used in bridges and buildings should have high impact resistance against weather and the elements. A sudden ocean wave or a strong seismic activity may hit the structure and potentially damage it if the impact strength isn’t determined before construction.