Flexure/Bend: How Is It Tested, What are the Standards and Why Is It Necessary?

The flexure/bend test plays a crucial role in assessing the strength and structural integrity of various materials. It is conducted using various test setups, including two-point, three-point, and four-point bending configurations. Flexure testing allows for the determination of various material properties depending on the specific material being tested.

This article aims to delve into the fundamental aspects of flexure/bend testing, including the testing methods employed, the relevant standards that govern the process, and the underlying reasons why this type of testing is essential.

What Is Flexure/Bend Testing?

Flexure/bend testing is a mechanical testing method used to evaluate the behavior of a material when subjected to bending forces. It involves applying a load to a specimen in such a way that it induces bending or flexural deformation. The test measures the response of the material to bending, including its ability to withstand the applied forces and its resistance to fracture or permanent deformation.

This testing method is widely used across different industries, including materials science, engineering, construction, and manufacturing. It helps determine the mechanical properties and performance characteristics of materials, such as: their flexural modulus, fracture toughness, flexural strength, and resistance to bending or deformation. 

How Does the Flexure/Bend Test Work?

During flexure/bend testing, a sample or specimen is placed on supports. A force is applied to the center or at specific points on the specimen. This force causes the specimen to bend or flex, and the resulting deflection or strain is measured. 

A flexure/bend test is performed using a universal testing machine. These machines are designed to assess the ductility, bend strength, fracture strength, and resistance to fracture of materials. A bend test is specifically designed to evaluate the crack resistance of ductile materials. During a bend test, forces are applied to the sample in a manner that induces deformation, emphasizing any flaws or cracks present in the material. This allows for the assessment of the material's ductility. These tests are typically conducted on ductile metals and metal products.  

What Flexure/Bend Test Standards Apply?

There are several well-established standards for flexure/bend testing developed by renowned organizations including: ASTM, BS, DIN, ISO, and MIL. Some of the most common standards that apply to flexure tests are listed below:

1. ASTM C674-88(2006): Outlines the standard test methods for evaluating the flexural properties of ceramic whiteware materials. 
2. ASTM D790-03: This comprises standard test methods for evaluating the flexural properties of both unreinforced and reinforced plastics and electrical insulating materials.
3. ASTM E855-08: Describes standard test methods for bend testing of metallic flat materials for spring applications involving static loading.
4. BS EN 2746:1998: This pertains to the standard for conducting flexural tests using the three-point bend method on glass-fiber-reinforced plastics. 
5. BS EN ISO 8491:2004: Focuses on the bend test for metallic materials in the form of tubes with full sections.
6. ISO 9538:2017: Addresses the planar flexure test for hydraulic tubing joints and fittings used in aerospace applications.
7. ISO 178:2019: This standard outlines the determination of the flexural properties of plastics. 
8. JEDEC JESD 22B113: Evaluates cyclic bend testing of mounted electronic products at board level.
9. IPC JEDEC 9702: Specifies monotonic bend testing of interconnects at (printed circuit) board level.

For more information, see our guide on What is ASTM International.

Why Is Flexure/Bend Testing Necessary?

Flexure tests offer a dependable and straightforward testing approach. They provide valuable insights into the stress-strain characteristics of material within low specimen strain. Flexure/bend testing helps to assess mechanical properties and behavior under bending loads. Flexure testing is crucial for quality assurance purposes to help ensure that only high-quality and reliable components are used in various applications. Additionally, flexure/bend testing is often required for regulatory compliance. Many industries have specific standards and regulations that mandate such testing to ensure that materials and products meet safety, quality, and performance requirements. 

What Do You Need for Flexure/Bend Testing?

There are several essential components and items of equipment needed to perform flexure/bend testing. The key requirements are:

1. Test Specimen: This is the material sample that will undergo bending or flexural loading during the test. The specimen is typically in the form of a rectangular or cylindrical shape, depending on the testing standard and the material being tested.
2. Testing Machine: A universal testing machine or a specialized flexure/bend testing machine is required. This machine applies the bending load to the specimen and measures the corresponding response. It consists of load cells, grips, or supports for specimen fixation and control software to regulate the test parameters.
3. Loading Configuration: Depending on the testing standard and the desired test type (e.g., three-point bend or four-point bend), the testing machine should be equipped with an appropriate loading configuration. This configuration includes supports or anvils for specimen placement and loading fixtures for applying the bending force.
4. Measurement Devices: Strain gauges, extensometers, or displacement transducers are used to measure the deflection or displacement of the specimen during the bending test. 

What Are the Benefits of Flexure/Bend Testing?

Flexure/bend testing offers several benefits in material testing and analysis, including:

1. Allows for the evaluation of various material properties such as flexural strength, flexural modulus, bending stiffness, and flexural resistance.
2. Offers a closer examination of the material's surface behavior, providing insights into how it responds to bending stresses.
3. Aids in material selection by comparing the performance of different materials under bending loads.
4. Provides essential data for research and development purposes to study and improve materials, develop new products, optimize designs, and enhance structural integrity.

What Are the Challenges of Flexure/Bend Testing?

Flexure/bend testing, while valuable, also presents some challenges that need to be considered. These are listed below:

1. The size and thickness of the samples used in flexure/bend testing can influence the results. For thin samples, ensuring uniform stress distribution across the specimen width can be challenging. Additionally, the sample size should be representative of the intended application to provide meaningful data.
2. There are various standards available for flexure/bend testing, and choosing the appropriate standard for a specific material or application can be challenging. 

What Are the Types of Flexure/Bend Testing?

Flexure tests can be classified into three main types based on the number of support points involved:

• Two-Point Flexure Test: In a two-point flexure test, the sample is clamped/fixed at one end, and load is applied at a single point at a specified distance from the clamping point. This test configuration creates a bending moment that causes the sample to deflect or bend. The measurement of the resulting deformation or stress provides information about the material's flexural properties. Figure 1 is an example of a two-point bending test:

The 2-point flexure test method is specifically designed for testing the bending properties of materials such as paper, cardboard, and films. It is commonly used to determine the bending stiffness and bending resistance of these materials using the beam method.

• Three-Point Flexure Test: The three-point flexure test is the most commonly used configuration. It involves supporting the sample at two outer points and applying the load at the center point as shown in Figure 2 below:

This arrangement creates a bending moment that causes the sample to bend. Based on specific requirements and standards, the anvils and upper anvils used in the test can be equipped with rotating, fixed, or rocking mountings to facilitate testing according to the specified specifications. The anvils can be designed to rotate around their longitudinal axis to mitigate any potential impact from friction during the test. Additionally, rocking mountings can be utilized for the upper anvil and anvil supports to ensure parallel alignment with the specimen. These measures aim to enhance the accuracy and reliability of the flexure test results.

This type of test is primarily employed for assessing the properties of tough and elastic materials.

• Four-Point Flexure Test: The four-point flexure test involves supporting the sample with two outer points and applying the load at two inner points as shown in Figure 4: 

This configuration creates a more complex bending moment and allows for a more uniform stress distribution along the length of the sample. The primary application of this test is for brittle materials to determine their modulus of elasticity in bending.

What Materials Can Be Subjected to Flexure/Bend Test?

A wide range of materials can be subjected to flexure/bend testing. Some of the common materials include:

1. Metals: Various types of metals, including steel, aluminum, copper, and alloys, can be tested for their flexural properties.
2. Plastics: Different types of plastics, such as thermoplastics and thermosetting plastics, can undergo flexure testing to evaluate their bending behavior and strength.
3. Composites: Composite materials, which consist of a combination of different materials like fibers and resins, are often tested using flexure/bend tests to assess their mechanical properties.
4. Ceramics: Brittle materials like ceramics and ceramic composites can be subjected to flexure testing to determine their resistance to bending forces and evaluate their structural performance.
5. Wood: Wood and wood-based materials, including plywood and particleboard, can be tested for their flexural strength and stiffness.
6. Paper and Cardboard: Flexure tests are also performed on paper, cardboard, and other similar materials to assess their bending properties, stiffness, and resistance to deformation.

Note that the suitability of a specific material for flexure/bend testing depends on its mechanical characteristics and the specific testing standards or requirements applicable to that material.

How Is Flexure/Bend Measured?

A flexure test is primarily conducted to determine the flexural modulus and flexural strength of a material. The flexural modulus is derived from the slope of the stress versus strain deflection curve. Flexural strength, on the other hand, refers to the maximum stress experienced by the outermost fiber on the tension or compression side of the specimen. Both parameters provide valuable insights into the material's ability to withstand bending or flexural forces. For more information, see our guide on What is Flexural Modulus.

How Important Is the Flexure/Bend Test for Engineering?

Engineers rely on tests such as bend tests to give them a better understanding of material behavior under loads. It helps engineers assess the structural integrity of materials and components. Engineers can determine their ability to withstand applied loads and deformations by subjecting them to bending forces. This information is crucial in designing and analyzing structures such as: beams, columns, bridges, and other load-bearing elements.

How Important Is the Flexure/Bend Test for Manufacturing?

Manufacturers rely on the results of flexure testing to ensure the quality of their products and meet industry standards. They can evaluate their strength, flexibility, and resistance to deformation or failure by subjecting materials or components to bending forces. Flexure tests are a reliable method to verify if the materials or products can perform as intended.

How Important Is the Flexure/Bend Test for Aerospace Industry?

The flexure/bend test is essential for ensuring the reliability, safety, and performance of materials and structures used in aircraft and spacecraft. Aerospace structures, such as wings, fuselages, and landing gear, are subjected to a variety of bending loads during flight and landing. The flexure/bend test allows engineers to evaluate the structural integrity of materials and components under these bending forces. It helps identify potential failure modes, deformations, and the ability of the structure to withstand expected loads, ensuring the overall safety and performance of aerospace systems.

Is the Flexure/Bend Test Required for Engineering?

Yes, in many engineering fields, flexure/bend testing is necessary and highly recommended. The requirement for flexure/bend testing in engineering depends on the specific application, industry, and regulatory standards involved. For example, industries such as aerospace, automotive, construction, and consumer goods, have established standards and regulations that specify the testing methods and requirements for materials and components. In many cases, these standards include flexure/bend testing as a mandatory or recommended test procedure. 

Flexure/bend testing provides valuable insights into the mechanical behavior and performance of materials and structures under bending loads. It helps engineers understand the flexural strength, stiffness, ductility, fatigue resistance, and other properties that are critical for assessing the suitability of materials and designing structures to withstand bending forces.

Is Footing a Flexural Member?

No, a footing (or foundation) is not considered a flexural member. Beams are structural members that primarily support transverse loads and are subjected to flexure or bending. While some degree of axial load may be present in any structural member, beams are typically treated as flexural members when the axial load is negligible compared to the bending forces. The design of beams focuses on their resistance to bending moments.

On the other hand, footings are designed to support and distribute the loads from columns or walls to the underlying soil. Their main purpose is to provide a stable foundation and prevent excessive settlement or movement of the structure. Footings are primarily subjected to vertical loads and are designed to resist failure modes such as punching shear, two-way shear, and flexure. However, the primary design consideration for footings is often punching shear, which governs the depth of the footing.

While footings may experience some degree of bending due to the applied loads, they are not typically treated as flexural members in the same sense as beams. The design and analysis of footings focus on different failure modes and considerations, such as the prevention of punching shear failure and the distribution of loads to the underlying soil.

What Is the Difference Between the Flexure/Bend Test and the Ductility Test?

The flexure/bend test and ductility test are two mechanical tests used to evaluate the deformation behavior of materials, but they differ in their specific objectives and testing procedures. The flexure/bend test is primarily performed to assess the strength and stiffness of a material when subjected to bending loads. This test involves applying a three-point or four-point load to a test specimen, measuring its deflection and the load required to induce bending. Parameters such as modulus of elasticity, flexural strength, and flexural modulus are evaluated to determine the material's resistance to deformation and fracture under bending forces.

On the other hand, the ductility test focuses on a material's ability to undergo plastic deformation without fracturing under tensile stress. A cylindrical or rectangular test specimen is subjected to a uniaxial tensile load until it fractures. Throughout the test, elongation, and reduction in the cross-sectional area are measured. The parameters evaluated in a ductility test include elongation, reduction in area, percentage elongation, and percentage reduction in area. These measurements provide insights into the material's capacity to stretch and deform under tension without breaking. For more information, see our guide on What is a Ductility Test?

Summary

This article presented flexure/bend testing, explained it, and discussed its standards and why it's necessary. To learn more about flexure/bend, contact a Xometry representative.

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