The next physical characteristic to fall under our gaze is flexural strength, which is simply resistance to bending along the sample’s axial plane — it is different from torsional, or twisting, strength. The standard flexural strength test for plastic materials, including coatings, paints and films, is ASTM D790. This test comes in three-point and four-point varieties, and simply applies pressure to the central region of a bendable sample. Flexural strength is of secondary importance for materials applied to surfaces that don’t bend, i.e. concrete floors. It is more pertinent to coatings applied to flexible substrates, like unsecured strips of thin metal or plastic, and as a general indication of the material’s quality.
A Little Physics
With the ends of the sample free to move and pressure applied to the center, the test machine forces a plastic sample to flex, or bend. Two different and opposing forces are at work, because the sample has thickness. As the weight pushes down from above, the sample’s top surface compresses under the load, while the bottom surface experiences tension as the intermolecular bonds begin to stretch. Normally, the lower surface fails first. If the sample is completely homogenous, the flexural and tensile strength should be the same. However, if the sample has imperfections, the bending stresses will be localized and the material will show higher flexural, rather than tensile strength.
The formula for flexural strength, σ, under a three-point load is equal to 3FL/2bd2, where:
- F = force at fracture point
- L = support span length
- b = width of sample
- d = thickness of sample
A related physical quantity is flexible modulus, E, the ratio of stress (force) to strain (deformation). The formula is E = L3F/4bd3D, where D is deflection.
Testing Flexural Strength
As we’ve mentioned in previous articles in this series, the first step is to mix up the test material and pour it into molds. For ASTM D790, a mold size of 5 inches long by 1/2 inch wide by 1/8 inch thick is suitable. The material is allowed to cure for the recommended time, usually no less than 24 hours, removed from the mold and placed on a testing machine. The tester has two end posts, often 2 or more inches apart, that support (but do not clamp) the sample from below, and a central crosshead that presses down on the sample from above. The head contacts the central point of the sample and presses down with slowly increasing pressure, at a rate dependent upon factors such as the support span length and depth of sample. The test ends when either event occurs:
- The sample breaks
- The sample deflects 5 percent
The test equipment measures the final force, expressed in psi.
In this series on physical characteristics, we’ve been using Dur-A-Flex Poly-Crete MD, a 100% solids, aromatic, cementitious urethane, as our reference material. It is used to protect concrete, polymer reinforced screeds or water-resistant plywood. Poly-Crete MD has a flexural strength of 5,076.3 psi, which is more than twice its tensile strength.