shear test

Is shear testing a good alternative for adhesion testing?

There are multiple approaches for adhesion testing, but the shear test is an easy and inexpensive way to help determine if the coating system is adhering properly. Especially if there are multiple coatings involved. To follow the method in ASTM D3359, testers will need pressure-sensitive tape and a few of the best practices to ensure accuracy. This method is generally applicable to most substrates. The standard says that it applies to soft substrates such as wood and plastic but is often used on metallic substrates. While the standard does state that the test has limited accuracy, make sure you follow the test method to get the best results.

How to perform shear testing

You need to be careful when handling testing tapes to make sure the reading is as accurate as possible. Before the test begins and while you’re conducting it, you need to pay attention to several things:

  1. the condition of the tape,
  2. the cleanliness of the surface being tested
  3. the environment at the site.

While the test method does not specify a specific tape. There are several tapes commonly used including the D3359-TAPE that is available at MTest. Be aware that tape ships with a certificate of conformance.

The blade needs to be in good condition and have the proper squareness to the surface. Using a dull blade will affect the results of the test. Finally, the humidity and temperature of the area should be considered. The coating should be dry and around room temperature.

Individually these factors may not render the test inaccurate, but they can have a cumulative effect when taken altogether. Users can grade the test based on a scale of 0 to 5, with 5 meaning that none of the area was removed, and 0 indicating 65% or more was removed.

It is important to note that the ISO grading system (ISO 2409) is the exact opposite of the ASTM grading system so be careful when recording results or reading other people’s reports.

Is tape needed at all?

There is one adhesion test that uses a knife to test adhesion without needing tape. It is qualitative, not quantitate, but can come in handy of you do not have the tape available, ASTM D6677 “Standard Test Method for Evaluating Adhesion by Knife”

The MTest handbook states, intercoat cleanliness, overcoating and coating failures can all call for an adhesion test. As coating systems dry, they can put shear forces on the existing coatings. This makes D3359 an ideal test to see if a coating is in good enough condition to overcoat.

“This is a quick and easy test to see if an existing coating is in good enough condition to Overcoat.”

Tom Swan

Perhaps most importantly, inspectors should limit use of this test to situations when there are clear coating problems because this and all adhesion tests are destructive.

Getting the most out of the test

If doing a crosshatch test, consult the standard to check on the required spacing for the cuts. ASTM D 3359 recommends Method A, the X Cut, for coatings 5 mils thick and above. For this you can use a sharp utility knife with a straight edge, the TQC SP3000 or MTest’s new X-Cut Template that includes everything you need for an X Cut test.

For thinner coatings, up to 5 mils, the best choice is ASTM D3359, Method B. To run the cross-hatch test, you can use the TQC-SP3000 Crosshatch Template or one of the TQC adhesion testers that make all the cuts at once, such as the TQC CC1000 or CC2000.

While D3359 is similar to ISO 2409, keep in mind that the two are not the same. There are some spacing differences as well as a reversed grading scale.

Above all, clean the surface before you place the tape. The method says to use a pencil eraser to make sure the tape firmly adheres to the surface. When you do the pull, make sure you pull at a 180-degree angle to the surface. Use a quick steady pull. Do not let it sit for longer than two minutes and report important information such as how many tests you performed in a particular area.

15 tips for proper adhesion testing

  1. Tape should be pulled at an 180 degrees Angle.
  2. Make sure blades are sharp (preferably new).
  3. Keep blades square to the surface.
  4. Cut in as even a motion as possible.
  5. Make sure the tape is new or has been stored properly.
  6. Make sure the surface is clean and dry before applying tape.
  7. Rub tape onto surface using pencil eraser.
  8. Pull tape within 1 to 2 minutes; the bond strength generally increases with time.
  9. Run two more confirming tests in the same area.
  10. Keep the tape as a record of the test
  11. It is often easier to read the tape than the panel.
  12. For X-cut tests use a ruler to measure failure
  13. For X-Cut use a straight edge.
  14. Report Number of Tests, Mean, Range, where failure occurred, type of test used, environmental conditions, location of tests
  15. Keep a copy of ASTM D 3359 (or ISO 2409) available

This article was written by Tom Swan from MTEST

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