Bend test of metallic materials

Bend test is one of the basic methods for testing the mechanical properties of materials, mainly used to determine the mechanical properties of materials subjected to bending loads. In the bend test, one side of the specimen is unidirectional tensile, the other side is unidirectional compression, the maximum positive stress appears on the surface of the specimen, so it is very sensitive to surface defects. This feature makes the bend test is often used to check the surface defects of the material, such as carburisation or surface quenching layer quality. bend test is mainly used to determine the brittle and low plasticity materials (such as cast iron, high carbon steel, tool steel, etc.) bend strength, and reflect the plasticity index of the deflection. For brittle materials, because of the eccentricity sensitivity, the use of tensile testing is not easy to accurately determine its mechanical properties indicators, so the bend test is often used to determine its bend strength, and the use of comparative method to determine the deformation of the material's deformation capacity.

The types of specimens generally used for guided bend test are rectangular ones machined from plates and pipes.The weld-face surface of the fat specimen contains the greater width of the weld material, while the opposite side is called the weld-root surface.Weld bend test is generally divided into face bend，root bend, side bend;

Side bend: so that either one of the side surfaces becomes the convex surface of the bent specimen；

Face bend: so that the weld-face surface becomes the convex side of the bent specimen;

Root bend: so that the weld-root surface becomes a convex surface of the bent specimen;

Bend test in metal materials has important significance, mainly reflected in the following aspects:

1、Assess the bending performance of the material: bend test can assess the performance of metal materials when subjected to bending force, such as bending strength, bending modulus, etc., so as to understand the bending deformation capacity of the material and the ability to resist bending damage.

2、Evaluate the fatigue life of the material: through the bend test, it can simulate the force of the material in actual use, evaluate its fatigue life, and provide an important basis for the selection of materials.

3、To ensure the safety of engineering structures and stable operation: metal materials in engineering applications are often in the force state, the bend test can ensure the safety of engineering structures and stable operation, to avoid safety accidents due to inadequate material performance.

Regarding the testing steps of bend test, it usually includes the following links:

Specimen preparation: according to the test requirements, prepare specimens that meet the standards, including the shape, size, tolerance and surface requirements of the specimen.

Installation of specimen: Install the specimen on the testing machine to ensure that the specimen is installed correctly and the force is uniform.

Apply load: Apply the bending load slowly according to the test requirements and record the data and phenomena during the test.

Observe and analyse: Observe the deformation and damage form of the specimen, analyse the test data and derive the test results.

When conducting the bend test, the following matters need to be noted: strictly comply with the test standards and operating procedures to ensure the accuracy and reliability of the test.

Damage or deformation to the specimen should be avoided as much as possible during specimen preparation to maintain the original performance of the specimen.

When applying load, the rate and size of load increase should be controlled to avoid sudden fracture of the specimen caused by too fast load increase.

During the observation and analysis process, attention should be paid to recording the test phenomena and data, and the test results should be analysed and conclusions drawn in time.

Different metal materials and application scenarios may require the use of different bend test methods and standards, so the choice needs to be made on a case-by-case basis in practical applications.

The following are some commonly used bend test standards:

ISO 7438:2020 《Metallic materials - Bend test》

ASTM E290 《Standard Test Methods for Bend Testing of Material for Ductility》

ASTM E190 《Standard Test Method forGuided Bend Test for Ductility of Welds》

GB/T 232 《Metallic materials—Bend test method》

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