Flexing Your Muscles - Defining Flexural Properties in Plastics

Flexing Your Muscles

In the stress analysis of plastics, the question is often asked “What is the strength of a given material?” The first step to determine the strength is to determine if your application is more dependent on tensile or flexural strength. Tensile strength measures a plastic material’s resistance to stretching or tension. Flexural strength measures resistance to bending or deflection.

In this article, we will discuss the flexural properties of plastics. The two main properties measuring flexural properties are Flexural Strength and Flexural Modulus of Elasticity. ASTM D790 measures the flexural properties of a material while under a bending strain or deflection while ISO 178 is used as a European standard to measure flexural properties. The two common Flexural properties to determine the strength of plastics for resistance of bending are Flexural Modulus of Elasticity and Flexural Strength.

Flexural Modulus of Elasticity

This is the slope of the initial linear portion of the load-deflection curve while the material is still in the elastic phase. It is essentially a measurement of the material’s stiffness while still recoverable. Very often these values, measured in psi or MPa, correlate with tensile modulus, as plastic materials are often isotropic in nature and thus resist stress equally in both the X and Y axis.

Flexural Strength at Yield

The maximum stress a plastic material can withstand to the point of yield, which is defined as the point where the elastic characteristics of the material no longer apply, and the material undergoes permanent deformation. Measured in psi or MPa, flex strength is useful in determining the highest resistance to stress a material can take just before yield. Often flexural strength values are higher than tensile strength as polymers are long-chain molecular structures and thus resist bending stress more along the Y axis.

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