Surface Preparation

The selection and implementation of proper surface preparation procedures is equally as important as using proper gages and adhesives. High-elongation measurements demand closer attention to recommended procedures than do normal elastic strain measurements.

Micro-Measurements Instruction Bulletin B-129, Surface Preparation for Strain Gage Bonding , outlines steps for surface preparation on a variety of materials. These procedures produce a smooth, clean surface, usually having been abraded in a single direction during preparation. Because of higher bondline forces involved in high-elongation measurements, the surface should be altered further for greater bond strength, as indicated in the following procedures:

1. Completely prepare the surface of the test specimen as described in Bulletin-129.
    
2. Abrade the specimen surface in a direction 45 degrees to the intended axis of strain measurement. On soft materials, such as aluminum, use 320-grit silicon-carbide paper ; on harder materials, such as steel, use 60-grit.
    
3. Lightly abrade, with the appropriate grit paper as indicated in Step 2, in a direction 90 degrees to the first abrasion. This will produce a cross-hatched abraded surface. Typical surface roughness desired approximates 250 microinch (6.4 micrometer) rms.
    
4. Repeat the degreasing steps outlined in Bulletin B-129 .
    
5. Condition the surface with Micro-Measurements M-LINE M-Prep Conditioner A (if appropriate for the specimen materials as detailed in B-129), scrubbing with cotton-tipped applicators. Wipe dry with a gauze sponge.
    
6. Neutralize the surface with Micro-Measurements M-LINE M-Prep Neutralizer 5A (if appropriate for the specimen material), scrubbing with cotton-tipped applicators. Wipe dry with a gauze sponge.

Many materials oxidize readily in air. If allowed to form on the surface, oxidation will greatly reduce bond strength and elongation capabilities. It is strongly recommended that the gage bonding operation be completed as soon as possible after the surface is prepared.

Page 7 of 17