Bending tests are commonly used materials mechanics testing methods to evaluate the strength, toughness, and resistance to deformation of materials. By observing and analyzing the deformation and failure behavior of materials under bending loads, we can gain a deeper understanding of the material's performance characteristics, providing important reference for engineering design and material selection.

1，What is the principle of a bending test?

The principle of a bending test is to use the action of force and bending moment to cause the material to bend. The mechanical properties of the material are evaluated by measuring the relationship between the deformation and the load. During the test, the material sample is fixed at two support points, and a force is applied to a fixed point above the sample, causing the sample to bend. By measuring the strain and the applied torque, the stress-strain relationship, bending strength, elastic modulus, and other mechanical property parameters of the sample can be calculated.

2，What are the testing items in a bending test?

Bending tests are conducted on a universal joint testing machine, with two loading methods: three-point bending and four-point bending. The bending test of ductile materials is called the cold bending test.

1. Three-point bending test: This is an experimental method to test the bending mechanical properties of metals. The specimen is placed on two support points at a certain distance. A downward load is applied to the specimen at the midpoint between the two support points. Three-point bending occurs when two equal moments are formed at the three contact points of the specimen, and the specimen will break at the midpoint.

2. Four-point bending test: This is a test method to measure the bending properties of materials. A strip specimen is placed flat in a bending test fixture, forming a simply supported beam. The distance between the two lower support points supporting the specimen is adjustable according to the specimen length. There are two symmetrical loading points on the top of the specimen.

3. Cold bending test: During the test, the specimen is loaded and bent to a certain degree. The surface of the specimen is observed for cracks.

3，What are the testing scopes of bending tests?

1. They can determine the bending strength of gray cast iron. Gray cast iron has better bending properties than tensile properties, and its bending strength is an important mechanical property indicator.

2. They can determine the bending strength of cemented carbide. These materials are difficult to process and easy to prepare tensile specimens. Bending specimens, on the other hand, have a simple shape, so bending tests are used to evaluate their performance and quality.

3. They can measure the bending strength of ceramic materials and tool steel. Determining the tensile strength of these brittle materials is difficult, and specimen processing is also challenging; therefore, bending tests are used.

4. They can be used to inspect and compare the quality and performance of surface heat-treated layers. Bending tests are sensitive to surface defects in materials.

5. They can be used to test the performance of materials under bending loads. Many mechanical parts (such as cutting tools made of brittle materials) operate under bending conditions, requiring bending tests for these parts.

4，What are the standards for bending tests?

Plastic Bending Test: GB/T 9341-2008 "Plastics - Determination of Bending Properties"

Metal Bending Mechanical Property Test: YB/T 5349-2006 "Metal Bending Mechanical Property Test Method"

Metallic Material Bending Test: GB/T 232-2010 "Metallic Materials - Bending Test Method"

Metal Wire Repeated Bending Test: GB/T 238-2002 "Metallic Materials - Wire - Repeated Bending Test Method"

Reinforced Concrete Steel Reinforcing Bar Bending and Reverse Bending Test: YB/T 5126-2003 "Reinforced Concrete Steel Reinforcing Bar Bending and Reverse Bending Test Method"

Thermal Bimetallic Hot Bending Test: GB/T 8364-2008 "Thermal Bimetallic Hot Bending Test Method"

Thermal Bimetallic Transverse Bending Test: GB/T 24298-2009 "Thermal Bimetallic Transverse Bending Test Method"

Welded Joint Bending Test: GB/T GB/T 1449-2005 "Test Method for Bending Joints of Welded Joints"

GB/T 1449-2005 "Test Method for Bending Properties of Fiber Reinforced Plastics"

GB/T 4283-2011 "Test Method for Bending of Welded Plastic Specimens"

GB/T 8812.2-2007 "Determination of Bending Strength and Apparent Flexural Modulus of Rigid Foamed Plastics - Part 2: Determination of Bending Strength and Apparent Flexural Modulus of Rigid Foamed Plastics"

GB/T 3844-2008 "Determination of Bending Strength of Rigid Rubber"

GB/T 13465.2-2002 "Test Method for Bending Strength of Impermeable Graphite Materials"

5，What is the test operation procedure?

1. Install the specimen

Place the specimen stably on the support rollers, ensuring the specimen axis is perpendicular to the support roller axis. For steel pipes or plates, adjust the support position to avoid eccentric loading.

Hydraulic testing machines lower the mandrel manually or automatically until it contacts the upper surface of the specimen; electronic machines use software to control the electric loading device for positioning.

2. Start the test

Manual/Automatic mode: For hydraulic testing machines, slowly operate the hydraulic valve handle to apply bending force evenly; for microcomputer-controlled machines, click the "Start Test" button in the software, and the system will automatically load according to the set parameters.

Real-time monitoring: Observe the changes in force, displacement, and bending angle data on the display screen. Pay attention to whether the specimen shows cracks, fractures, or other abnormalities. If any abnormality occurs (such as a sudden change in load), stop the test immediately.

3. Reach the target conditions

Stop loading when the specimen bends to the set angle (e.g., 180°) or fractures. For materials with good plasticity, obvious plastic deformation without fracture can be observed at the bending point of the specimen.

6，What are the safety precautions?

Protective measures: Wear protective goggles during testing to avoid injury from flying debris when the sample breaks; for hydraulic testing machines, prevent high-pressure oil pipes from bursting.

Load limits: Never exceed the maximum test force of the testing machine to avoid damaging the sensor or mechanical structure.

Sample fixation: Ensure the sample is securely installed to prevent slippage or ejection during bending.

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