Flexural Testing Essentials

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.

Definitions

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

Testing Equipment

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.

Picture below shows the eXpert 5952 electrodynamic testing systems to test the fatigue properties of solar panels. The system uses a modified four point bend arrangement and includes a camera to monitor and record crack propagation on the solar cell. Deflection is recorded using both crosshead travel and a linear displacement probe.

Four-point bend fixture on the eXpert 5952 Solar Panel Test System

Bend Fixtures

ADMET offers a variety of bend fixtures in order to match the requirements of all our customers. Fixturing may also be designed per ASTM/ISO specifications. Follow the steps below to order the bend fixture to meet your testing application.

1.Choose configuration:

Bend fixtures are most often offered as three-point or four-point bend testing configurations (five-point bend also available).

Three-point bend fixtures: Configuration of flexural strength testing where a specimen is loaded at a location midway between two support bearings

Four-point bend fixtures: Configuration of flexural strength testing where a specimen is symmetrically loaded at two locations that are situated one quarter of the overall span, away from the outer two support bearings

Four-point bend fixture

Note: Depending on the industry, terminology may vary with the use of “three-point loading” and “four-point loading.”

2. Select maximum force capacity:

ADMET bend fixtures are offered from 2.5kN capacity up to 100kN and more. Maximum force capacity would depend on the flexural strength of the material tested.

3. Select span length & span width:

Bend fixtures may be ordered with different span lengths and span widths. ADMET bend fixtures come with supports that can be moved to allow customers to modify span lengths.

4. Select supports & diameter:

It is recommended that the loading noses uniformly contact the specimen across its width as lack of uniform contact can affect flexural properties by initiating damage by crushing and by non-uniformly loading the beam.

Bend fixtures may be supplied with hardened supports or cylindrical rollers. Support radius options include 0.5, 1, 1.5, 2, 3.2, 5, and 10 mm. ADMET offers interchangeable roller/support design allowing users to select two radii with one bend fixture.

Deflectometers

Deflection of the specimen in a flexural test application may be calculated based on the crosshead displacement values or based on measurements taken by a deflectometer. The deflectometer is often mounted so that it is in contact with the specimen at its center of the support span. A load-deflection curve is generated from deflection measurements and since the results obtained from crosshead displacement and the deflectometer differ, the deflection measurement method should be noted on the test results report.

Flexural properties such as the flexural strength, chord or secant modulus or the tangent modulus of elasticity, are obtained from load-deflection curves.

Environmental Chambers

Many materials are used in extreme heat or cold applications in real-world applications, thus, a flexural test setup may require testing specimens in an environmental chamber. ADMET offers environmental chambers set to low-temperature testing (LN2), high temperature testing, and both. Environmental chambers come with their own temperature controller and the temperature settings and recordings may also be controlled with MTESTQuattro software.

eXpert 9613 with environmental chamber

Test Standards & Tips Based On Different Materials

Flexural testing and the flexural test setup may vary depending on the material being tested. Some materials such as electronic chips break at low forces while other materials such as concrete have high flexural strength that require high capacity, often hydraulic, testing machines. In addition, the deflection of certain materials may be minimal compared to others. Organizations such as ISO and ASTM have standardized flexural test methods outlining the test procedures and the recommended equipment. See below for a list of test standards based on the material being tested as well as testing tips:

• Ceramics Flexural Testing

  • Common Test Standard:
    • ASTM C1161 – Standard Test Method for Flexural Strength of Advanced Ceramics at Ambient Temperature
  • Tip: Three-point and four-point test configurations can be used to test ceramics. Keep in mind that the three-point flexural strength measurements will be higher than four-point flexural strengths. Using the four-point flexure is preferred and recommended for most characterization purposes.

• Composites Flexural Testing

  • Common Test Standard: ASTM D7264 – Standard Test Method for Flexural Properties of Polymer Matrix Composite Materials
  • Flexural properties may vary depending on which surface of the specimen is in compression, as no laminate is perfectly symmetric (even when full symmetry is intended); such differences will shift the neutral axis and will be further affected by even modest asymmetry in the laminate.

• Concrete Flexural Testing

  • Common Test Standards:
    • ASTM C1609 – Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam With Third-Point Loading)
    • ASTM C78 – Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading)
  • Tip: Prepare flexural test setup with a compressometer assembly consisting of displacement transducer, bend fixture, and a compressometer surrounding the concrete specimen.

ASTM C1609 Setup on an eXpert 2600

• Glass Flexural Testing

  • Common Test Standard: ASTM C158 – Standard Test Methods for Strength of Glass by Flexure (Determination of Modulus of Rupture)
  • Tip: Glass surface flaws should be taken into account especially near the surfaces in regard to the number and severity of stress-concentrating discontinuities and the degree of prestress existing in glass specimens.

• Plastics Flexural Testing

  • Common Test Standards:
    • ASTM D790 – Standard Test Methods for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials
    • ASTM D6272 – Standard Test Method for Flexural Properties of Unreinforced and Reinforced Plastics and Electrical Insulating Materials by Four-Point Bending
    • ISO 178 – Plastics – Determination of flexural properties
  • Tip: The most common flexural testing method, ASTM D790, is applicable for materials that yield at or before 5% strain.

• Wood Flexural Testing

  • Common Test Standard:
    • ISO 13061-3 – Small clear wood specimens — Part 3: Determination of ultimate strength in static bending
  • Tip: When testing long wood specimen using dual column universal testing machines, rotate the bend fixture so that it is perpendicular to the crosshead. This will provide the required horizontal testing space.

Conclusion

Flexural testing measures flexural properties such as flexural strength, flexural strain, and flexural modulus. Testing standards, which are often categorized based on the material being tested, outline test procedures and provide information on the recommended test equipment. There is a variety of test equipment options for flexural testing. Discuss your flexural test application with an ADMET Sales Engineer to ensure your product meets the required specifications.