Skin-doubler specimen

Krieger has argued that the fatigue life of structural adhesives cannot be usefully predicted by thick adherent lap shear specimens.He has described a test sample configuration more appropriate for determining the durability of skin-doubler assemblies. The skin-doubler specimens proposed were found to survive peak cyclic strains that produced failures with conventional thick adherent specimens. 

The specimen, or model, which is selected is shown in Figure 1. It is named the skin-doubler specimen, because it comprises an infinitely long skin with a very long doubler bonded to it. When tension stress (strain) is introduced in the skin, the adhesive forces the doubler to strain until its strain and skin strain are equal. At this point, there is no longer any need for loading the doubler and so the adhesive shear force becomes zero. It will also be seen that the adhesive shear is a maximum at the doubler tip. 

FIGURE 1. Adhesive shear stress distribution for a skin-doubler specimen. 

It is proposed that the skin-doubler specimen does represent a basic model for stress analysis concepts and has great value in guiding the design of test specimens. 

The foregoing analysis of the skin-doubler specimen shows that it is essential to know the stiffness characteristics of the adhesive. Since good design practice places bond lines in shear, it was decided that the shear modulus is the primary stiffness parameter. Furthermore, it is recognized that more than the initial portion of the shear stress-strain curve was required. It was clear that the total curve was not linear. It was anticipated that the nonlinearity portion would bear heavily on creep and fatigue performance. Accordingly, the primary requirements for the strain measuring device were set as follows ... 

With the advent of accurate adhesive strain data via the KGR-1 extensometer, it became possible to verify the skin-doubler equations in Figure 1. To do this another extensometer was needed to measure the adhesive shear strain at the doubler tip. This measured value could then be compared to the calculated value. The second extensometer is identified as the KGR-2 and is shown in Figure 5. As before, the adhesive shear movement, Aa, at the doubler tip is made to move the core of an LVDT. The voltage change is fed to a recorder, which plots a curve of load vs. adhesive strain at the doubler tip. Tests were run on a skin-doubler specimen and the... 

FIGURE 7. Shear stress-strain data at room temperature for three adhesives on thick-adherend and skin-doubler specimens. 

The need is to develop a test to produce adhesive fatigue properties which are useful to the designer. We consider that the skin-doubler specimen has this potential. It combines the capability of stress analysis with a failure mode faithful to that of the actual structure (see Ref. 4). The results will reflect the strength (durability) of the attachment in terms of stress (load) in the skin. 

To fully appreciate this, we must explore the skin-doubler specimen beyond its fundamental value, i.e., that of providing the decisive shear at the doubler tip. This foundation allows us to predict the adhesive shear displacement Aa (Figure 5) at the doubler tip. We speak of this as a shear stress, because we can relate displacement to stress with thick adherend data. This has value as a practical simplification, but in reality, the adhesive at the doubler tip sees secondary and tertiary deformations. These are implicit in Aa but not precisely defined or measured. They cannot be ignored, because they must be decisive in initiating the fatigue failure. To calculate them is formidable at best, and fruitless at worst, if they are reproduced faithfully in a test specimen. We suggest that the skin-doubler specimen may faithfully reproduce these deformations, and do so in terms of correct proportions for actual structure. 

---