Steps to Consider in Surface Preparation (Part 1 of 3)

The first strain gage used by Dr. Arthur Ruge at MIT was made of wire, cigarette paper, and bonded with Duco Cement. While the technology has advanced a long way, the base principles have not changed. An intimate bond with a substrate transfers in-plane strain to the sensing foil of the gage which causes the gage resistance to change. Without proper bonding, a gage may come entirely detached from the part, or worse, give unreliable data. The first step to any strain gage application will be preparing the surface appropriately before applying adhesives and protective coatings. Following best practices, the success of strain gage applications will be much higher, and you can have faith in the data which is generated.

Grease will typically be the first target for surface preparation. The organic molecules from grease, machining oil, or anything similar will prevent the proper wetting of an adhesive to the surface so they must be removed prior to abrasion. If grease is not removed prior to abrasion, those contaminants will be forced down into the substrate underneath the bond line. This will leave the adhesive susceptible to attack from those molecules after the gage has been installed. In cases where a chemical like a rust inhibitor has been applied to a large casting which has travelled overseas before gaging, additional steps must be taken to draw contaminants from the substrate. There are other cases where a substrate may be sensitive to the aggressive degreasing from something like CSM-3 Degreaser. Plastics are typically the most vulnerable, but it is also the case that some composites require the less aggressive counterpart found in a reagent grade Isopropyl Alcohol. Several passes of cleaning are required, but this is much less likely to chemically attack a plastic substrate. Understanding the steps your part has undergone before reaching stress analysis testing is critical to performing the proper preparation steps.

After degreasing, the next step is to determine the level of abrasion required for the given application. Surface coats of paint or any additional layer above the target substrate should be removed. Remembering that a strain gage reads in-plane strain, it will collect data from whatever is immediately beneath the gage. If you are seeking the strain in a coating, your approach to abrasion and removal of material will change.
The target strain range for your testing is critical in determining the RMS surface finish after abrasion. Larger RMS finishes leave a coarser surface which is ideal for high elongation testing to ensure the gage remains bonded to the desired point of measurement. For low and stable strain readings (like a loadcell) a smoother (smaller RMS) surface finish is desirable. The goal is to provide a surface for the selected adhesive to grip the substrate. The backing of each Micro-Measurements gage is prepared for bonding as part of the manufacturing process for the same reason.

Silicon Carbide Abrasive Paper is preferred in surface preparation because one can perform wet abrasions without a worry of mastic and grit being redeposited to the surface. In combination with mechanical abrasion, Micro-Measurements uses a mild-phosphoric acid etchant, M-Prep Conditioner A which, when used in a wet abrasion, functions to remove organic contaminants as well as promoting mechanical abrasion.

For personalized support, detailed advice, or to explore our range of surface preparation solutions, reach out to us at mm@vpgsensors.com or call 919.365.3800, option 1, option 2.

Let's ensure your strain measurement setup is built on a solid foundation.