Adhesive Selection: Measurements on Plastics and Composites

Adhesive Selection
Strain gage manufacturers typically supply adhesives which have been developed and validated for use in gage bonding, and can recommend the most suitable adhesive for any particular circumstances. For relatively short-term applications in a benign environment, cyanoacrylate adhesive (where compatible with the plastic) is often a convenient choice because of the simple, quick curing procedure. Epoxy adhesives that cure at room temperature or somewhat above are eminently suitable for bonding strain gages to most types of plastics and composites. These adhesives are available in a wide range of formulations with different characteristics and properties -- time/temperature/clamping pressure for curing, elongation capability, expected glueline thickness, etc. -- and should be selected accordingly. When necessary for elevated-temperature testing, epoxies which cure or post-cure at higher temperatures can be employed; assuming, of course, that such temperatures are suitable for the plastic or composite.

An unfilled adhesive is the normal choice for gage bonding, in order to produce a thin, creep-free glueline. However, a filled epoxy is sometimes used to level an irregular or textured surface prior to gage installation. In a composite which is reinforced with continuous fibers, for example, the fiber pattern may be replicated in the surface texture. For such cases, a layer of partially filled adhesive (or matrix resin) can first be applied, smoothed, and cured to form a level surface. After curing, the leveled area is cleaned and abraded in the usual manner, preparatory to bonding the gage with an unfilled adhesive. Although this technique solves the problem of the rough surface, the resulting gage installation may exhibit greater than usual creep, particularly if the intermediate layer of resin is thicker than necessary.

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