Flexural loading is among the types of loading most frequently occurring in practice and is therefore an important consideration when it comes to testing a wide range of materials. Flexure tests are accordingly used in determining the mechanical properties of steel, plastics, wood, paper, ceramics and other materials.
Characteristic values are generally obtained via 3-point or 4-point flexure tests.
When flexural loading is applied to a symmetrical cross-section, tensile stresses arise in the extreme (outer) fibers along one edge. Compressive stresses arise in the extreme fibers on the opposite side. The stresses on both sides increase with the distance from the neutral axis, so that the highest values in both cases occur in the edge zones. If the tensile or compressive yield point of the material is attained, plastic flow will occur.
During the flexure test shear stresses also arise in the specimen. To keep the shear stress percentage as low as possible during a test, the ratio of the span to the thickness of the specimen must be as high as possible. In most cases the span is specified in the relevant test standard, however.
Specimens with a rectangular cross-section are generally preferred for use in flexure tests.
The loading device consists of two parallel-positioned anvils for the specimen and an upper anvil positioned centrally between the anvils. The upper anvil applies the load to the specimen.
Depending on requirements (standard), the anvils and upper anvils must have fixed, rotating or rocking mountings to enable testing in accordance with specifications.
The test is mainly used for tough and elastic materials.
To minimize frictional influences during testing the anvils can be mounted so as to rotate around their longitudinal axis. Rocking mountings can be used with the upper anvil and anvil supports to ensure that they are parallel to the specimen.
As with the 3-point flexure test, the 4-point flexure test kit consists of two parallel-positioned anvils which, depending on testing requirements, must have fixed, rotating or rocking mountings.
The difference from the 3-point flexure test lies in the way in which the load is applied to the specimen. This is done via 2 upper anvils located symmetrically to the anvils. The bending moment is then constant between the two force application areas.
This test is primarily used to determine the modulus of elasticity in bending for brittle materials.
If, for lack of space, only relatively small specimens can be taken from injection-molded parts, the two Dynstat test methods offer interesting options
for the comparative characterization of static and dynamic material properties in quality assurance and product development.
Range of application
The Dynstat flexure tool can be used to determine characteristics of plastics under 4-point flexural loading to DIN 53435 – DB – G. It is used for the characterization of material properties in components made of plastics.
The Dynstat flexure test is designed as a four-point bending test. With this test fixture, the test can also now be performed on one testing machine. The spans at the start of the test are set to 12 mm and 2 mm. This means a specimen only 15 mm in length can be tested with this method. Flexural deformation is produced by the test fixture's rotational movement. The test result is the maximum flexural stress measured during the test. It is calculated using the bending moment and the resisting moment of the specimen. If the specimen does not break by the time it reaches the Dynstat bending angle defined by the standard, the flexural stress at this point is measured.
The upper anvils of the dies have a fixed radius r. They are used for 3-point flexure tests and connected at the top. All dies are connected to the mounting stud via the 16H7adapters (not included). In addition, all dies are case-hardened (hardness = 700 HV) and DNC-treated (DNC 520/5 µm), that is the surface is nickel-coated to a depth of 5 µm. For installation of the dies see diagram Connection system, upper.
Upper anvil holder
Upper anvil holders are used to hold the upper anvils listed later in this section. They are used for 3-point flexure tests and are installed above. All upper anvil holders have the same connection diameter, 16H7 mm. For installation of dies see diagram 'Connection system, upper'.
Based on their load capacity, the three different flexure table versions Type A, B, and C can be divided into three force levels:
- The light Type A flexure tables can be used for forces up to a maximum of 20 kN.
- Type B flexure tables can be used for medium forces. The maximum forces that these tables in the medium series can used for are 10 kN to 250 kN, depending on the size of the table and the type of installation.
- The heavy series of flexure tables are Type C. These can be used for the highest forces. Up to a table length of approximately 600 mm, all tables in this range can be loaded with a maximum of 250 kN. Only tables longer than this have loading restrictions.
All three types of flexure tables can be used for 3-point flexure tests (installed below) and 4-point flexure tests (installed above and below).
The span is adjusted manually on the flexure tables using screws or clamping levers. The left and right-hand sides must be adjusted separately (except for flexure table Type B, Item No. 317121). The (lower) connection of the flexure table can be via an adapter, a flange or directly to the crosshead (Types A and B flexure tables to the crosshead of table-top testing machines only). For installation of flexure tables see diagrams Connection system, top and Connection system, bottom. The T-slot nuts of Types A, B and C flexure tables are not compatible with each other.
Specimen flexure is limited at maximum span (flexure supports rotated).
Rockers and extensions
Rockers and extensions can optionally be used with Type B and C flexure tables for the 3-point flexure test kit (installation below) and 4-point flexure test kit (installation above and below). They are installed between flexure support and anvil. The rockers/extensions are to be used optionally with an extension or rocker on the flexure table. In the interests of stability a maximum of 1 rocker or extension should be used per side.
Additional elements for flexure test kits
- Rockers and extensions (option)
- Upper anvils/anvils, rotating
- Upper anvils/anvils, rigid
- Anvil plates
- Anvils, rigid
- Safety screen (option)
- Specimen stop (optional)