Napkin ring shear test

Test Method for Shear Strength and Shear Modulus of Structural Adhesives (Napkin Ring Style Test Piece)... 

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... 

E-0229. Test Method for Shear Strength and Shear Modulus of Structural Adhesives. This method employs torsional shear loading on a napkin ring specimen to determine the shear strength and shear modulus of structural adhesives in thin gluelines on high modulus adherends. 

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

As stated in Section 6.4.2, an annular butt joint, or napkin-ring, specimen tested in shear minimizes the variation of shear stress in the adhesive and has been used by many workers to assess the shear strength and shear stress versus strain behaviour of adhesives [14,17,111-115] and is also listed for this purpose as ASTM E 229 (see Table 6.1). The independence of the measured strength upon specimen geometry has been substantiated by Bryant and Dukes [112] and Foulkes et al. [110] and the shear stress distribution calculated using linear elasticity theory is ... 

In practice, most structural adhesives exhibit considerable plastic deformation when subjected to a shear stress, and it is quite probable that the small volume of adhesive near the adherend corner will yield without causing premature failure of the joint when loaded in torsion. However, the spew fillet also makes the joint stiffer in torsion and, since this would lead to an overestimation of the shear modulus of the adhesive, it is probably advisable to remove any spew fillet present on test specimens, although this may be difficult on the inside surface of napkin ring specimens. 

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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