ADMET material testing systems accurately and reliably measure the flexural properties of metals, concrete, plastics, medical devices and other products and components. Our software can calculate properties such as flexural modulus, flexural strength, flexural strain, and yield strength. Examples of bend tests include applying three-point or four-point flexural forces to materials to determine the amount of flexural force and strength needed to break the material or the product. This blog posts covers flexural property definitions, why flexural testing is done, and finally the testing equipment recommended by ASTM/ISO industry standards.
Common measurements obtained from flexural testing include:
Flexural strength– The maximum bending stress that the test specimen can sustain before it yields. Flexural strength measurements are expressed in force per unit area.
Flexural offset yield strength– The stress at which the stress-strain curve deviates by a given strain (offset) from the tangent to the initial straight line portion of the curve. Note that flexural offset yield strength may be a different value than the flexural strength.
Flexural strain– The nominal fractional change in the length of an element of the outer surface of the test specimen at midspan, where the maximum strain occurs. Flexural strain is a ratio, thus is unit-less.
Flexural modulus– The ratio of stress to strain in a flexural deformation of a material.
See graph below for a test results report from ADMET MTESTQuattro software, which automatically calculates these properties.
MTESTQuattro Test Report for ASTM D790
Why is flexural testing recommended?
Bend, or flexion, occurs with both tension and compression force on a material. The material is compressed at the bottom end while the top is elongated towards the bottom dead center from the sides to stabilize the shape. The bend may be invisible to the eye for certain materials such as metals and concrete, while for other materials such as plastics the deformation will be large enough to provide visual cues on how bend forces modify the material.
Flexural properties obtained from flexural testing provide information for use in real-world scenarios and are especially useful for quality control and specification purposes. Standardized test methods from organizations such as ASTM and ISO outline bend test methods that can be used to determine flexural stress and strain of materials undergoing tension, compression, and shear loading.
Wood specimen flexural testing per ISO 13061-3
Universal testing machines equipped with bend fixtures and closed loop controllers are recommended to run flexural testing. The testing machine should be able to move at a controlled, uniform rate of loading such that the load may be applied with a continuous motion. Force and displacement transducers should provide an accuracy within +/- 1% of the indicated value. Depending on where the load is applied to the specimen, bend fixtures may be configured for three-point or four-point bend testing. On a four-point loading configuration, the bending moment is held constant between the central force application members. Consequently, the maximum flexural stress is uniform between the central force application members. On a three-point loading configuration, the maximum flexural stress is located directly under the center force application member.
Below is a list of ADMET testing systems commonly used to run flexural testing.
eXpert 1600 Servohydraulic Testing Machines
eXpert 1600 universal testing machines are hydraulic testing systems that are used in high-capacity test applications. ADMET hydraulic testing systems are equipped with load cells that provide +/- 1% of reading down to 0.5% of capacity once calibrated per ASTM E4. eXpert 1600 servo-hydraulic testing machines are often used for flexural testing of concrete and metal specimens.
eXpert 2600 Dual Column Testing Machines
eXpert 2600 dual column testing machines are electromechanical testing systems that can be used to test a variety of materials in different loading scenarios, including tension, compression, flexion, and shear. These systems are available in table top or floor standing configurations from 2kN to 600kN.
The most common mechanical tests performed on silicon wafers, microchips, and semiconductor devices are three and four point bend tests. These tests are designed to measure both the flexural strength (of the edge and surface) and the capacitance of the sample under flexural strain. Picture below shows a four point bend configuration on an eXpert 2600 testing system.
Four point bend testing of electronics
eXpert 4000 MicroTester
eXpert 4000 MicroTesters are ideal for flexural testing of miniature samples. With capacities from milligrams to 5kN and speeds up to 500 mm/min, these systems can be used on microscope stages.
eXpert 5000 Modular Testing Machines
eXpert 5000 testing machines can be configured as single-column, dual-column, wide width, and more for a wide range of applications requiring flexibility and precise control. These configurations are especially useful for wider or longer specimens that are tested in flexion.
eXpert 7600 Single Column Testing Machines
eXpert 7600 single column testing machines are electromechanical testing systems with capacities up to 5kN. These systems can be used to test a variety of test types and methods including ASTM D790.
Fatigue Testing Machines
ADMET offers electromechanical fatigue testing systems that can be configured for running cyclic bend testing at small or large amplitudes over millions of cycles. Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) tests can be conduced up to 10Hz.