Napkin ring test

In conventional Shear tests, there is considerable non-uniformity of stress throughout the joint. The napkin ring test was introduced to provide a test where the variation of shear stress was minimal. It consists of two thin-walled tubes joined end to end by a thin layer of adhesive. The torque required to break the joint is recorded. 

Specific tests of adhesion are described in more detail under the following articles Blister test, Climbing drum peel test, Fracture mechanics. Napkin ring test. Peel tests. Rubber to metal bonding - testing. Shear tests. Tensile tests. Wedge test and in Refs. [1-5] see also Standards for adhesives and adhesion and Appendix. 

Napkin ring test D E PACKHAM Calculation of shear stress refinements... 

In this case, an independent check on the magnitude of t was found by deteimining the stress-strain curve of the adhesive layer when constrained between two aluminum plates in a napkin-ring test [60]. 

Ref, [13] contains further information about this instrument and test coupon. It should be noted that the amplification of the relative displacement signal is extremely great and that it has been found necessary to correct the initial modulus of the curve to compensate for the small distortions of the aluminum adherends themselves. This is accomplished by subjecting an equivalent one-piece notched coupon, with no adhesive layer, to the same loads as the bonded coupons, with the instruments mounted in the same locations. Although there were earlier such curves measured on napkin-ring test coupons, the coupons for that set-up were difficult to fabricate because the radial width of the bond layer was so narrow and the adhesive tended run out. In addition, it was diflicult to ensure uniformity of bond thickness around the perimeter. It is for these reasons that the Krieger... 

Uses a napkin ring test for determining shear modulus and shear strength of structural adhesives... 

The napkin-ring test (see Table 6.1 and Section 6.4.8) more nearly... 

Although the thick adherend test piece may be a cheaper and more simple joint to manufacture than the napkin ring test piece, it has a number of limitations to its applicability that appear to be not generally recognized. In particular, the measurement of the small glue-line deformations requires considerable experimental care and accuracy, if reliable results are to be obtained. 

But favoured for many years is the napkin ring test shown in Fig. 82. By applying equal and opposite torques T, the adhesive is stressed purely in shear and the maximum stress, t, will be that at the outside radius. To, and is given by ... 

In a modification of the napkin ring shear test, the adherends are solid bars and the adhesive forms a penny-shaped slab similar to the butt joint tensile test. Such a test will give the relationship between torque and twist, but whereas in the napkin ring test it may be assumed that all the adhesive is at the same stress and strain, with a solid butt joint there is a radial variation of strain, but a non-linear variation of stress after yield. It is then necessary to use the Nadai correction (see Adams and Wake") to determine the true stress-strain curve of the adhesive. 

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