Pull-Off Adhesion Testing: Why Disc Diameter Changes Your Results

A pull tester reading of 100 pounds-force does not mean 100 psi. To interpret the result, you also need the area being tested.

Disc diameter affects the force required to reach a specified stress, the capacity your tester needs, and the calculation in your report.

Start with the test requirements

Confirm the applicable procedure, substrate, fixture size, and acceptance criteria before selecting your equipment. Different procedures apply to different materials and assemblies. Follow the project specification and applicable manufacturer instructions.

Convert force to stress

For a circular test area, calculate the area in square inches as π × diameter² ÷ 4. Then divide the force in pounds-force (lbf) by the area to obtain the pull-off stress in pounds per square inch (psi).

A 2.25-inch-diameter circular area is approximately 3.976 square inches. A 4-inch-diameter area is approximately 12.566 square inches.

At the same recorded force of 100 lbf:

• A 2.25-inch diameter gives approximately 25.2 psi.
• A 4-inch diameter gives approximately 8.0 psi.

These are calculation examples, not acceptance limits. They assume a fully bonded circular test area matching the stated diameter. Use the area required by your test procedure.

Check tester capacity before starting

A 4-inch area is approximately 3.16 times the area of a 2.25-inch area. Reaching the same stress therefore requires approximately 3.16 times the force.

For example, reaching a hypothetical 40 psi requires about 159 lbf with a 2.25-inch diameter, versus 503 lbf with a 4-inch diameter. A tester limited to 400 lbf cannot reach the second target.

Check the tester’s rated capacity, useful measurement range, accuracy, and required fittings. Do not change a specified disc size simply to fit the tester available.

Record where the test failed

The numerical result needs context. Separation may occur at an interface, within a material, or in the adhesive used to attach the fixture. A glue failure does not automatically identify the strength of the bond being assessed.

Record the fracture location alongside the force and calculated stress. Keep the preparation, adhesive, fixture size, and loading conditions consistent when assessing a series of tests.

Choose discs for your testing workflow

Once the required fixture size is established, consider disposable or reusable adhesion discs. Before ordering, confirm the disc diameter, tester connection, and any accessories required for your setup.

Explore disposable discs | Explore reusable discs

This guide explains calculations and equipment selection. It does not replace the applicable test method, project specification, or equipment instructions.