After adhesively bonded mechanical test specimens have been tested to failure, the failure mode must be interpreted and quantified. Areas of the adherent that are bare (no residual adhesive remains) have undergone adhesive failure. The remainder of the surface has undergone cohesive failure. The ability to distinguish and accurately quantify the relative amounts of cohesive and adhesive failure on a failed bonding surface is of tremendous importance in the field of mechanical testing, and for the development of bonded assemblies. Some adhesives (and adherents) are fluorescent, meaning they re-emit light at a different wavelength after being irradiated by some lighting source. This property allows for quantitative analysis of the adhesive failure mode (adhesive and cohesive). A digital image of the fluorescing adhesive or adherent can be analyzed and quantified using publicly available software to determine the relative areas of exposed and covered adherent surface.
Failure mode results are an important (and in some instances, the only) type of data extracted from a mechanical test. Failure mode analysis can help determine the cause of failure and is often used to screen surface treatment techniques in a research environment.
It is normally difficult, if not impossible, to quantify the relative areas of surface that are occupied by either nothing or by a layer of adhesive. The bare areas and adhesive residues on an adherent are usually very small, irregularly shaped, and poorly contrasted in visible light.
Normally, the relative amounts of adhesive and cohesive failure are determined by direct visual inspection. Operator approximation and visualization error are common; the operator approximation varies from person to person. Slight changes in color and lighting could have a profound effect on the collected data.
The technique requires a non-fluorescent adherent and a fluorescent adhesive (common for metal specimens that have been bonded with adhesive), a camera with high sensitivity (preferably a CCD sensor), and a light filter to remove reflected light but allow fluorescent light to reach the camera sensor. A UV light source that will cause the adhesive to fluoresce brightly enough to be captured digitally is required. Software is needed to analyze the images.
Use of this fluorescence technique allows a quantitative, repeatable, and objective measurement of the relative amounts of adhesive and cohesive failure in a specimen. The technique is fast, simple, and allows the collection of valuable data in a clear and unambiguous way.
This work was done by Christopher J. Wohl and Thomas W. Jones of Langley Research Center, and Frank L. Palmieri of the National Institute of Aerospace. LAR-18215-1