What is Flexural Modulus?

Flexural modulus is a measure of how a certain material will strain and potentially even deform when weight or force is applied to it. This is an important calculation in engineering and architectural fields, since it tells builders and designers the maximum weight that different materials can bear. Calculations often shape choices made in the building and design of everything from skyscrapers and homes to industrial machinery, cars, and basic consumer appliances. As a result, it plays an important role in civil, mechanical, and aerospace engineering and design, and is frequently used to select the correct materials for parts that will support loads without flexing or warping.

In basic terms, the calculation describes the ability of a material with a specific cross-section to resist bending when placed under stress. The number necessarily varies depending on what sort of material is at issue, though. Even surfaces and structures that look comparable can have different capacities when it comes to weight-bearing ability.

Ductile materials like steel and brass, which can be deformed a great deal before failure, have a well-defined modulus ranking, for example. Brittle materials like glass and concrete typically have little or no plastic deformation before failure. For this reason, many brittle materials do not have a clearly defined measurement, and they are often described by their flexural strength — which is the maximum amount of bending stress that can be applied before rupture or failure of the material occurs.

The term is used to define the relationship between a bending stress and the resulting strain. Strain is a measure of the amount that a material will deform when a stress is applied. Elastic strain is reversible and will disappear after the stress is removed, meaning that the material will return to its original state. At high levels of stress, however, a material will permanently deform and will usually not return to its original dimensions. This is referred to as “plastic strain” or “yielding.”

Engineers and architects typically plot out the modulus on a graph that can serve as a visual representation of the interplay between stress and strain. The result is typically known as a “stress-strain curve,” and it shows how strain changes with applied bending stress. The slope of this curve in the region where elastic strain occurs defines the flexural modulus of the material. The units of measure are, in most cases, pounds per square inch (psi) or Newtons per square meter, perhaps more commonly known as pascals (Pa).

Actually getting a precise measurement of flexural modulus can be somewhat complicated, but is almost always determined through a series of intensive laboratory tests. In most cases engineers will start with a sample of the target material with a specific shape and known dimensions. The “flexural test” is essentially a measurement of the force necessary to bend a sample, often known as a “beam,” that has defined dimensions. Technicians typically apply force at three points: the beam is usually supported on the bottom side near both ends and a force is applied to the top at the center point, between the bottom supports. This is known as three-point loading conditions. Once the force has been entered, any deflection or movement of the beam is measured and recorded, and then later analyzed.

Flexural modulus has been determined for a wide variety of structural materials, including metals, wood, glass, concrete, and plastics. It is usually measured at ambient temperature conditions. The properties of some materials, like many plastics, will change with temperature. The flexural test is sometimes conducted at lower or higher temperatures to simulate the intended end-use environment; this sort of knowledge is really important for people building at high altitudes or in extreme climate conditions, particularly in arctic zones or near the equator.