  ##### Common Adhesive Strength Tests Terms, Defined

When it comes to adhesive strength, there’s three main tests to determine the strength – tensile, shear and peel. That’s why we talked about adhesive strength tests. These tests are all used to determine how effective an adhesive will be when bonding certain types of materials. You’ve probably heard a lot of terms floating around when discussing adhesive strength, but what do they all mean?

Tensile Strength

Tensile strength is the resistance of a material to breaking under tension, used to determine the behavior of a material while an axial stretching load is applied. It refers to the ability of a material to resist loads under stress or deformation, without failure. The ability for an adhesive to resist failure under tensile stress is one of the most important and widely used measurements for adhesive strength.

When testing adhesives, the adhesive being tested is used to bond two sample substrates together. A test machine is then used to measure the tensile strength, which it does by applying tension to the sample which is held between two vises, and stretching the sample by pulling it straight apart. The maximum stress the sample withstands before fracturing is its ultimate tensile strength, and is calculated by dividing the peak tension force the sample withstands by its cross sectional area. This is expressed in pounds per square inch, or psi.

Shear Strength

Shear strength is the load that a material is able to withstand in a direction parallel to the face of the material. It’s the maximum shear stress that the material can withstand without failure, material’s ability to resist forces that can cause the internal structure of the material to slide against itself. The shear strength of an adhesive is the maximum shear stress in the adhesive prior to failure under torsional loading.

When doing adhesive strength tests, the adhesive being tested is used to bond two sample substrates together. A test machine is then used to measure the shear strength, which it does by applying force to the sample which is held between two vises, and pulling it in a sliding motion. The forces applied are parallel to the upper and lower faces of the sample, so that the substrates are pulled in opposing directions, as opposed to perpendicular to the surface. The peak shear strength is the highest amount of force sustained prior failure, and is calculated by dividing the peak shear force the sample withstands by its cross sectional area. This is expressed in pounds per square inch, or psi.

Peel Strength

Peel strength is the average force required to part two bonded materials, so is used mainly for adhesive testing. It is the average load per unit width of bond line required to part bonded materials, and is designed to measure adhesive resistance to highly localized stresses.

The adhesive being tested is used to bond two sample substrates together. A test machine is then used to measure the peak peel strength, which it does by pulling back the substrates in a peeling motion, at a set angle and at a constant rate. Depending on the sample, the test can be done at different angles, with 90° and 180° being the most commonly used. The peak peel strength is the highest amount of force sustained before pulling the substrates apart, and is calculated by dividing the average force required to separate the sample, divided by its cross sectional area. This is expressed in pounds per linear inch.

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