Can strain gages be bonded to a device or component made from stamped, galvanized steel? Yes, in a manner of speaking. Galvanizing (zinc plating) is a common means of protecting steel from corrosion. This plating is rather soft and will “self-heal” since it tends to migrate a bit, a characteristic that makes it very desirable for protecting parts from outdoor conditions. Being rather thick and soft, the coating must be removed prior to starting a standard strain gage bonding surface preparation operation. If the requirements for the strain gage installation are known in advance, it is preferred to mask the gage location to prevent plating at the gage site.
Removal of the plating can be quite challenging. The zinc tends to clog most abrasive sanding mediums. Oftentimes, a torch is used to melt the zinc and it either flows off the steel or a technician will use a wire brush to “sweep away” the molten material. It is imperative to use respiratory, eye, and hand protection during this operation.
Once the plating is removed, conventional surface preparation techniques for steel are required to prepare the exposed steel. For complete instructions, refer to Micro-Measurements bulletin B-129-8: Surface Preparation for Strain Gage Bonding. http://www.vishaypg.com/docs/11129/11129B129.pdf.
Although quite commonly used for simple, low-cost transducers, stamped parts present their own set of challenges. The stamping process creates residual stresses, which can affect accuracy and repeatability in transducers and may also degrade overall fatigue life. The design engineer must take this into consideration when making the decision to use stamping to form a transducer element.
One method employed to avoid such issues is photo-chemical machining (etching) to produce the metal elements. The etching process is quick, economical, and doesn’t introduce unwanted characteristics to the metal, such as,residual stresses.