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 [1].
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 [1]:
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 [2].
When an impact load exceeds the impact strength, the material may exhibit any of the following types of failures [3]:
A material’s impact strength or toughness may be measured through any of the following tests:
The Charpy impact test makes use of a pendulum arm attached to a precalibrated energy gauge [4]. 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 [5].
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 [6].
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 [6].
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:
Because impact strength denotes a material’s brittle-ductile transition [1], the parameter is significant for the following industries and applications:
[1] Kluwer Academic Publishers. (1999) Factors Affecting Impact Strength. In G.M. Swallowe (Ed.), Mechanical Properties and Testing of Polymers. (p. 127-128). Dordrecht, The Netherlands: Kluwer Academic Publishers
[2] (n.d.) Impact Testing and Ductile-Brittle Transition. Polymer Properties Database. Retrieved from: http://polymerdatabase.com/polymer%20physics/ImpactTest.html
[3] John Wiley & Sons. (2005) Impact Properties. In M. Kutz (Ed.), Handbook of Materials Selection. (p. 555). New York, NY: John Wiley & Sons
[4] (n.d.) Impact Toughness. NDT Resource Center. Retrieved from: https://www.nde-ed.org/EducationResources/CommunityCollege/Materials/Mechanical/ImpactToughness.htm
[5] (n.d.) Charpy Impact Test. UNSW Sydney School of Materials Science and Engineering. Retrieved from: http://www.materials.unsw.edu.au/tutorials/online-tutorials/1-charpy-impact-test
[6] (n.d.) Difference Between Izod and Charpy Impact Test. Guide By Tips. Retrieved from: HYPERLINK "http://polymerdatabase.com/polymer%20physics/ImpactTest.html"https://guidebytips.com/difference-between-izod-and-charpy-impact-test/