Impact testing is applicable to castings of various materials, such as stainless steel castings, alloy steel castings, and carbon steel castings. Impact testing is a dynamic mechanical test in which a notched specimen of a specific shape is pulled, twisted, or bent to rapidly fracture, and the work required to achieve this fracture is measured. This effectively measures the material's sensitivity to notches. In engineering, the Charpy pendulum bend test, using a single impact, is often used to determine a metal's ability to withstand impact loads. It is also an effective method for examining a material's resistance to brittle fracture (at room temperature, low temperature, or high temperature).
Impact testing can be categorized into tensile impact, bending impact, cantilever beam impact, simply supported beam impact, and torsion impact. Bending impact testing is the most commonly used.
Bending impact testing can be categorized into cantilever beam (where one end of the specimen is clamped and the other end is struck, also known as the Izod impact test) and simply supported beam (where the specimen is not clamped and the center is struck). Depending on the notch shape of the specimen, there are various other methods, including the Mayer impact test, the Charpy impact test with different notch shapes, and the German DVM impact test. The cantilever beam (Izod) test is widely used in the UK, US, and other countries, but it can only be conducted at room temperature, limiting its application. The simply supported beam impact test can be conducted at various temperatures and with specimens having different notches of varying geometry and depths, making it a more widely used method. The test is conducted in accordance with GB/T 229-2007, "Metallic Materials - Charpy Pendulum Impact Method."
1. Overview
A specimen of a specified shape and size, with a notch of a specified geometry and depth, is placed between two supports. A pendulum with a specified kinetic energy and velocity is applied from the back of the notch to the notch, breaking the specimen in one stroke. The pendulum blade radius is either 2mm or 8mm, selected based on the relevant standards. A 2mm pendulum radius is generally used for domestic testing machines and material acceptance.
2. Specimen Preparation
Charpy notch specimens used for impact testing are available in U- and V-shaped notches. U-notch specimens are available in 2mm and 5mm notch depths. When stock is insufficient for small parts, small-section specimens can be used. For brittle materials, the specimen form shall be as specified in the technical agreement.
3. Impact Absorbed Energy
The energy absorbed by a specimen of specified shape and size when it breaks under a single impact test force is called the impact absorbed energy, expressed in kilojoules (J). For different notch shapes and pendulum radii of 2mm or 8mm, the absorbed energy can be read directly on the impact machine's dial in joules (J).
If the specimen does not completely break due to insufficient impact energy from the testing machine, the absorbed energy cannot be determined. The test report should indicate that the test was conducted using a ×J testing machine and that the specimen did not break.
The relationship between impact test results and test temperature should be noted. Within a narrow temperature range, impact absorbed energy varies dramatically. If a test temperature is specified for the impact test, it should be conducted within 2°C of the specified temperature. Room temperature tests should be controlled within 23°C ± 5°C.
In addition, whether a material is ductile or brittle at the test temperature can be determined by characteristics such as the shear area ratio and lateral expansion value of the specimen at the fracture surface. The higher the percentage of fibrous fractures on the fracture surface, the better the material's toughness. Conversely, the greater the proportion of crystalline regions on the fracture surface, the coarser the grains, and the greater the brittleness. Lateral expansion (LE) is another important indicator of material toughness. In fact, the total energy K absorbed during material fracture consists of two components: crack formation energy and crack propagation energy. The material's toughness depends not only on the total energy K absorbed but also on the ratio of these two energy components. The lateral expansion reflects the crack propagation rate. Low crack propagation energy and a small lateral expansion value indicate that the crack will fracture immediately upon formation, resulting in brittle fracture. High crack propagation energy and a large lateral expansion value indicate that the crack will propagate slowly after initiation, resulting in a ductile fracture characterized by a predominantly fibrous fracture surface.
4. Low-Temperature Impact Test
The impact specimen should be cooled using appropriate methods, with the medium temperature within 1°C of the specified temperature, for at least 5 minutes (for liquids) and 20 minutes (for gases). If the test temperature is not lower than -70°C, the specimen can be placed in alcohol and the temperature adjusted with dry ice. If a lower temperature is required, liquid ammonia or a special cryogenic device is required.
Currently, commercially available cryogenic tanks are available that can reach test temperatures of -100°C.
5. Determination of the Ductile-Brittle Transition Temperature
As the test temperature decreases, the impact absorption capacity (K) decreases. When the temperature drops to a certain value, the K value drops rapidly, and the material changes from ductile fracture to brittle fracture. This transition is called the ductile-brittle transition, and the transition temperature is called the ductile-brittle transition temperature. GB/T 229-2007, "Metallic Materials - Charpy Pendulum Impact Test Method," recommends using Charpy V-notch specimens for testing. Based on agreement between the supplier and the purchaser, the ductile-brittle transition temperature can be determined using one of the following methods:
1) The temperature corresponding to a specific impact energy absorption value (e.g., 27 J).
2) The temperature corresponding to a certain percentage of the upper plateau (e.g., 50%) of the impact energy absorption.
3) The temperature corresponding to a certain percentage of the shear fracture area (e.g., 50%). 4) The temperature at which the lateral expansion value reaches a certain value (e.g., 0.9 mm).
Shear area ratio and lateral expansion value are determined according to GB/T 229-2007, "Metallic Materials—Charpy Pendulum Impact Test Method."
Post time: Sep-25-2025