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Modulus, Poisson’s Ratio and Elongation
Modulus and elongation are two important properties of a material, which are determined by tensile tests. A stress-strain curve plays an important role in determining these material properties. The slope of the stress-strain curve, where the stress is proportional to strain, is called the Young’s Modulus or Modulus of Elasticity. It is a measure of the stiffness of the material. Since strain is unitless, modulus is measured by the units as that of stress. Generally, the epoxy modulus is expressed in psi or MPa units. Epoxies that are very stiff and rigid have a higher modulus as compared to flexible ones. The modulus value also varies depending on whether the system is filled or unfilled.
Poisson’s ratio is another important physical property of a material. It is defined as the negative of the ratio of lateral to axial strain. When a material is deformed in one way, in one direction, deformation occurs in the other two directions as well. Poisson’s ratio helps us to understand this theory. Poisson’s ratio is unitless, since it is a ratio of strains. The Poisson’s ratio also varies depending on whether the system is filled or unfilled.
Elongation is the measure of ductility of a material. It is expressed as a percentage and it is the ratio of change in axial length to the original length of the specimen. The elongation percentage of a flexible epoxy or a silicone is much higher than that of a rigid epoxy.
Master Bond offers products with varying modulus, Poisson’s ratio and elongation. The modulus varies from very low values for flexible epoxies to very high (around 500,000 psi) for rigid and filled epoxies. Elongation varies from as low as a few percent for rigid epoxies to more than 150% for flexible products. Poisson’s ratio for Master Bond’s epoxies typically varies from 0.29 to 0.34, depending on whether the system is filled or unfilled.