A Guide to Bend Testing for Plastic and Other Materials

Whilst it is clear to see why bend testing has to be conducted on inflexible materials like plastics (ASTM D790) and concrete (ASTM C1609), as a matter of fact, bend test applications are often seen in various industries.

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Universal testing machines outfitted with bend fixtures are used in calculating flexural strength, flexural modulus, yield point, and more. This article will explore the requisite equipment for typical testing standards as well as available equipment for use in less typical testing applications.

The two frequent kinds of bend fixtures are three-point and four-point bend fixtures.

Three-point bend fixtures: A flexural strength testing configuration where a sample is loaded at a location at the midpoint between two support bearings

Four-point bend fixtures:  A flexural strength testing configuration where a sample is loaded in a symmetrical way at two areas that are situated one quarter of the overall span, at a distance from the outer two support bearings

Note: Dependent upon the industry, terminology may change when using “three-point loading” and “four-point loading.”

ASTM D790 Bend Testing – Reinforced and Unreinforced Plastics and Electrical Insulating Materials

ASTM D790 is utilized to measure the flexural strength and flexural modulus of reinforced and unreinforced plastics, including materials that are high-modulus composites and electrically insulated. The standard includes comprehensive specifications on the endorsed bend fixture for this kind of testing, as the following lists:

• The rectangular specimen is loaded by a loading nose at the midpoint between the supports (three-point-loading).
• The recommended support span-to-depth ratio per ASTM D790 is 16 to 1. A ratio bigger than 16:1 may be required for laminated thermosetting materials and materials used for electrical insulation.
• Cylindrical rollers are the recommended loading nose and supports.
• The loading nose and supports’ default radii are 5.0 ± 0.1 mm (0.197 ± 0.004 in).
• If default radii are not used, the minimal radius recommendation radius is 3.2 mm (1⁄8 in). For specimens 3.2 mm or higher in depth, the radius of the supports may be up to 1.6 times the specimen depth.
• The loading nose’s maximum radius shall be no greater than four times the specimen depth.
• The sample shall be long enough to account for overhanging on each end of a minimum of 10% of the support span (at least 6.4 mm (1⁄4 in) on each end).

Cylindrical rollers for ASTM D790 bend testing. Image Credit: Admet, Inc.

ASTM C1609 Bend Testing – Fiber Reinforced Concrete Beams

ASTM C1609 is used to measure the flexural performance of fiber reinforced concrete beams. The objective of ASTM C1609 is to not just find the maximum strength of the concrete but also to determine the remaining strength of the fibers after the initial crack is apparent.

For this reason, the test continues to be conducted beyond the first peak. The standard also lays out that the test must be conducted in servo control at extremely slow net deflection rates. Net deflection is the average of two displacement sensors placed on either side of the specimen.

• Specimen size recommendations per ASTM C1609 are:
  • 150 by 150 by 500 mm (6 by 6 by 20 in)
  • 100 by 100 by 350 mm (4 by 4 by 14 in)
• The load is applied by two loading noses on the top of the beam.
• For a 150 by 150 by 500 mm (6 by 6 by 20 in) beam, the spacing between the loading noses is 150 mm (6 in).
• For a 100 by 100 by 350 mm (4 by 4 by 14 in) beam, spacing between the loading noses is 300 mm (12 in).
• The 3^rd point loading fixture features two supports underneath the beam.
• The span length, or the distance between the two support points, should be three times the specimen depth or 300 mm (12 in), whichever is higher.
• Loading nose and lower support rollers shall have the ability to rotate on their axes.
• A net deflection flex jig surrounds the beam and clamps to the beam at its neutral axis directly over the supports.

ASTM C1609 concrete bend testing fixture. Image Credit: Admet, Inc.

Two displacement transducers, one on each side of the beam, are mounted mid-span to take measurements of beam deflection. Each transducer’s output is averaged together to grant the measurement of net deflection, making sure of precise measurements of mid-span deflection and limiting errors caused by the concrete specimen twisting or seating in the supports.

EN 14651 Bend Testing – Metallic Fibered Concrete

European Standard EN 14651 is the test method for taking measurements of metallic fiber concrete’s flexural tensile strength. It is quite alike to ASTM C1609. The key variations are:

• The dimensions of the test specimen are 150 by 150 by 550 mm (6 by 6 by 22 in).
• The load is applied by one loading nose on the top of the beam.
• The span length, the distance between the two lower support points, equals 500 mm (20 in).
• The loading nose and support rollers should be built from steel and the cross-section should be circular with a 30 mm (1.2 in) diameter. Roller length should be no less than 10 mm (0.4 in) longer than the width of the sample.
• The loading nose and one of the support rollers should be articulating rollers, that rotate freely around their axis.
• EN 14651 needs one transducer

ASTM D2344 Shear Testing – Polymer Matrix Composite Materials

ASTM D2344 is of use in determining high-modulus fiber-reinforced composite materials’ short-beam strength. Short-beam strength is in reference to the shear stress developed at the specimen mid-plane at failure. The beam specimen is loaded in three-point bending with the use of the specific bend fixture as the standard describes.

• The load is applied by one loading nose on the top of the beam. The specified diameter is 6 mm (0.25 in).
• The special support platen system with two moving slides offers two supports below the beam. Support diameters are 3 mm (0.125 in).
• The span length, the distance between the two lower support points, equals 12 mm (0.5 in).

The cross-sections of the loading nose and support rollers should be circular.

ASTM D2344 bend test fixture. Image Credit: Admet, Inc.

ASTM C1161 Bend Testing – Ceramics

ASTM C1161 is used in determining the flexural strength of advanced ceramic materials through the use of either a three-point or a four-point bend fixture. Consider that the three-point measurements of flexural strength will be greater than four-point flexural strengths. It is preferential to make use of the four-point flexure  and endorsed for the majority of  characterization purposes.

• The standard lists three different configuration options shown in the following table :

Cross-sections of the loading nose and support rollers should be cylindrical with independent articulation (dependent upon the material of the specimen, rollers may be fully articulated or semi-articulating).

ASTM C1161 bend testing fixture. Image Credit: Admet, Inc.

ASTM C158 Bend Testing – Glass

ASTM C158 is of use in finding the modulus of rupture in bending, defined as the maximum tensile or compressive stress’ value (whichever causes failure) of glass and glass-ceramic samples. There are two ways of testing in ASTM C158:

Test Method A – Test For Modulus of Rupture of Flat Glass

Test Method B – Comparative Test for Modulus of Rupture of Glass and Glass Ceramics

The following description is for a four-point bend fixture that is a requisite for Test Method A that is possible with big sizes of specimen laid out in the standard.

• The dimensions of the test specimen are 250 mm (10 in) in length and 38.1 mm (1/8 in) in width.
• The cross-sections of loading noses and support rollers should be cylindrical and 3-mm (1⁄8-in.) radii.
• The distance between the two upper loading noses equals 100 mm (4 in).
• The distance between the two lower support points, the span length, equals 200 mm (8 in).

Customizing a Bend Fixture with Different Loading Noses and Supports

Not every test follows a testing standard. Products and materials are also tested following custom testing procedures that may require non-standard equipment. It is possible to order bend fixtures with various span widths and lengths. What ensures a bend fixture is unique to the specimen and application it is used for is how the loading noses and supports are customized.

Hardened supports on a standard bend fixture. Image Credit: Admet, Inc.

The most typically used, hardened supports come with radius options as follows: 0.5, 1, 1.5, 2, 3.2, 5, and 10 mm. Lower supports come with two of the radius options listed previously to grant the user some flexibility with regards to the chosen radius.

Adjustable rollers. Image Credit: Admet, Inc.

Interchangeable rollers are cylindrical rollers that are fixed into place through the use of O-rings. This attachment enables the length of the cylinders to be larger than the span width and grants the user the ability to choose and interchange two differing diameters of roller.

Interchangeable rollers. Image Credit: Admet, Inc.

Glass specimen can be tested using the special flat supports as depicted below.

Also demonstrated are movable lower supports with needle-roller bearings.

Longer supports with 1 mm radius as well as supports with large widths are attainable for running different testing procedures with various sample materials.

Is should be noted that it is possible to order bars that do not have loading noses and supports as well as fixture configurations that may be of use in the running of both 3-point and 4-point tests.

Bend fixture support for glass samples. Image Credit: Admet, Inc.

Fixture with needle-roller bearings. Image Credit: Admet, Inc.

Wide sample support. Image Credit: Admet, Inc.

Longer specimen support. Image Credit: Admet, Inc.

This information has been sourced, reviewed and adapted from materials provided by Admet, Inc. - Materials Testing Equipment.

For more information on this source, please visit Admet, Inc. - Materials Testing Equipment.