Impact toughness is an important mechanical property index of materials' ability to absorb energy and resist fracture under impact load, which is widely used in the selection and evaluation of engineering materials. It not only reflects the ductile-brittle transition characteristics of materials under low temperature or high speed impact, but also has great significance for preventing structural failure and improving safety.
　　Impact toughness is usually measured by impact test, and the most commonly used methods include Charpy V-notch test and Izod impact test. Among them, Charpy impact test has become one of the most commonly used impact toughness testing methods in the world because of its high standardization and simple operation.

First, Charpy impact test principle
　　The basic principle of Charpy impact test is to use a pendulum impact tester to apply an impact load to a standard sample with a V-notch or a U-notch, measure the energy absorbed by the sample during fracture, and observe its fracture behavior. The specimen is usually a rectangular metal bar of 10mm×10mm×55mm, and a notch with a specific shape is machined in the middle to simulate the stress concentration in the actual structure.

During the test, the pendulum is released from a certain height and breaks the sample at the moment of impact. By measuring the energy change corresponding to the height difference of the pendulum before and after impact, the impact work (in joules) absorbed by the sample can be calculated. The greater the absorbed energy, the better the impact toughness of the material.

Second, the factors affecting the impact toughness test results

1. Temperature: Temperature has a significant effect on the impact toughness of materials. In particular, metallic materials will undergo ductile-brittle transition at low temperature, and the impact toughness will decrease significantly. Therefore, impact test is often used to determine the ductile-brittle transition temperature curve (CVN curve) of materials.
　　2. Notch shape and size: Different notch forms (such as V-shaped or U-shaped) will affect the stress concentration, and then affect the impact value. The international standard ISO 148-1 has strict regulations on the processing of notches.

3. Material structure and composition: grain size, alloy composition and heat treatment process will significantly affect the impact toughness performance of materials.

4. Accuracy of test equipment: such as pendulum calibration, energy reading accuracy, etc., is also a key factor to ensure the accuracy of test results.

Third, the application of impact toughness test

Impact toughness testing is widely used in the evaluation of structural materials such as ships, bridges, pressure vessels, piping systems, etc. which bear low temperature or impact load. For example, the pressure vessel steel of nuclear power station must pass strict impact toughness test to ensure that brittle fracture accidents do not occur under operating conditions.

In addition, the impact toughness test results are also used in material quality control, process optimization and failure analysis. In the research and development of materials, researchers optimize the properties of materials by comparing the impact toughness of materials under different technological treatments.

Fourth, the development trend of impact toughness testing

With the development of materials science, the traditional impact test is gradually combined with digital and automation technology. Modern impact testing machine has the functions of automatically recording energy and analyzing fracture morphology, and some systems can also be combined with low-temperature or high-temperature environment boxes to realize the study of material properties under extreme conditions.
　　In a word, impact toughness is a key index to evaluate the toughness of materials under dynamic load, and its measurement method has become mature after long-term development. The standardized impact test method can provide scientific basis for engineering design and material selection, thus ensuring the safety and reliability of the structure.