Bend testing, sometimes called flexure testing or transverse beam testing, measures the behavior of materials subjected to simple beam loading. It is commonly performed on relatively flexible materials such as polymers, wood, and composites. At its most basic level, a bend test is performed on a universal testing machine by placing a specimen on two support anvils and bending it through applied force on one or two loading anvils in order to measure its properties.

Why Perform a Bend/Flex Test?

Engineers often want to understand various aspects of a material’s behavior, but a simple uniaxial tensile or compression test may not provide all necessary information. As the specimen bends or flexes, it is subjected to a complex combination of forces including tension, compression, and shear. For this reason, bend testing is commonly used to evaluate the reaction of materials to realistic loading situations.

Flexural test data can be particularly useful when a material is to be used as a support structure. For example, a plastic chair needs to give support in many directions. While the legs are in compression when in use, the seat will need to withstand flexural forces applied from the person seated. Not only do manufacturers want to provide a product that can hold expected loads, but the material also needs to return to its original shape if any bending occurs.

3-Point Bend Test

A 3-point flex test balances a specimen between two lower anvils while applying force from a single upper anvil centered at the midpoint. The area of uniform stress is quite small and concentrated under the center loading point. Different testing standards may require the anvils to be fixed, rotated, or rocking.

4-Point Bend Test

A 4-point flex test differs from the 3-point test in that it has two upper anvils positioned equidistant from the center of the specimen. In this test, the area of uniform stress exists between the inner span loading points (typically half the length of the outer span). Again, depending on the requirements of the testing standard, the anvils may need to be fixed, rotated, or rocking. Typically, 4-point tests are used to measure modulus of elasticity in bending for brittle materials.

Bend tests are commonly performed on universal testing machines. These systems consist of a test frame that is equipped with a load cell, testing software, and application-specific grips and accessories, such as extensometers. The type of material being tested will determine the type of accessories needed, and a single machine can be adapted to test any material within its force range simply by changing the fixturing.

Flex Test Setup
Tensile Grips Universal testing machine load frames come in single or dual column configurations and are available in force capacities up to 2,000 kN.
Software Testing software allows users to configure test methods and output results.
Load Cell The load cell is a transducer that measures the force applied to the test specimen. Instron load cells are accurate down to 1/1000 of load cell capacity.
Test Fixture Flex testing requires upper and lower anvils to apply force to key points of the specimen. The number of anvils is determined by the test type being performed.

 

Universal testing machines are available in a variety of different sizes and force capacities ranging from 0.02 N to 2,000 kN. Most low force testing is performed on an electromechanical single-column or dual-column tabletop machine, while higher force applications require floor model frames. Instron's 6800 Series systems are available in capacity ranges up to 300 kN and can perform a wide range of different test types, including tensile, compression, bend, peel, tear, shear, friction, torsion, puncture, and more. Instron's Industrial Series servohydraulic systems are designed for even higher capacity testing of high strength metals, alloys, and advanced composites, and the ElectroPuls Series is designed for dynamic fatigue testing.

Universal Testing Systems
up to 300 kN

Single and dual column table model and floor model testing systems with a force capacity range of 0.02 N (2 gf) to 300 kN.

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Industrial Universal Testing
Systems up to 2000 kN

Instron’s Industrial Series includes frames with single or dual test spaces and range in force capacity from 300 kN to 2000 kN.

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ElectroPuls All-Electric Dynamic and Fatigue Testing Systems

Linear electric motor driven dynamic test machines for fatigue and fracture mechanics testing available in force capacities up to 20 kN.

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Flexural tests are typically performed to ISO, ASTM, or other recognized standards, which will prescribe variables such as the required test speed and specimen dimensions. Specimens are generally rigid and can be made of various materials such as plastic, metal, wood, and ceramics. The most common shapes are rectangular bars and cylindrical-shaped specimens.

A flex test produces tensile stress in the convex side of the specimen and compression stress in the concave side. This creates an area of shear stress along the midline. To ensure that primary failure comes from tensile or compression stress, the shear stress must be minimized by controlling the span to depth ratio: the length of the outer span divided by the height (depth) of the specimen. For most materials, S/d=16 is acceptable. Some materials require S/d=32 to 64 to keep the shear stress low enough.

Maximum fiber stress and maximum strain are calculated for increments of load. Results are plotted on a stress-strain diagram. Flexural strength is defined as the maximum stress in the outermost fiber. This is calculated at the surface of the specimen on the convex or tension side. Flexural modulus is calculated from the slope of the stress vs. deflection curve. If the curve has no linear region, a secant line is fitted to the curve to determine slope.

Calculated values such as maximum force and maximum extension can be recorded just like a normal tension or compression test based on load cell and extension readings. Stress and strain values are calculated differently, as they incorporate the flex fixture support span and loading span (for 4-point bend testing). It is just as important to record these measurements as it is to properly record the specimen’s dimensions. Once these values are entered into Bluehill Universal, calculations such as flexural modulus are automatically calculated when requested.

Polymers

Polymers are most commonly tested with a 3-point bend test. Specimen deflection is usually measured by the crosshead position, and test results include flexural strength and flexural modulus.

Wood and Composites

Wood and composites are most commonly tested with the 4-point bend test. The 4-point test requires a deflectometer to accurately measure specimen deflection at the center of the support span. Test results include flexural strength and flexural modulus.

Brittle Materials

When a 3-point bend test is done on a brittle material like ceramic or concrete, flexural strength is often called modulus of rupture (MOR). This test provides flex strength data only, not stiffness (modulus). The 4-point test can also be used on brittle materials, though alignment of the support and loading anvils is critical in these cases, and the test fixture for these materials usually has self-aligning anvils.