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 by placing a specimen on two support anvils, which is bent through applied force on 1 or 2 loading anvils. The force is applied with either a single upper anvil at the midpoint, which is a 3-point bend test, or two upper anvils equidistant from the center, a 4-point bend test.
In a 3-point test the area of uniform stress is quite small and concentrated under the center loading point. In a 4-point test, the area of uniform stress exists between the inner span loading points (typically half the length of the outer span). Depending on the type of material being tested, there are many different flex fixtures that may be appropriate.
Why Perform a Bend Test?
Engineers often want to understand various aspects of material’s behavior, but a simple uniaxial tension 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, the material also needs to return to its original shape if any bending occurs.
Performing a Test and Calculating Results
Bend tests are generally performed on a universal testing machine using a 3 or 4 point bend fixture. Variables like test speed and specimen dimensions are determined by the ASTM or ISO standard being used. 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 bend 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.
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.
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.