Napkin ring test
The napkin ring test produces a relatively uniform shear stress within an adhesive layer (i.e. 3% variation across the adhesive ring), and hence the method has been used for producing shear modulus and shear strength data for adhesives [56, 57]. The test consists of applying equal and opposite torque to two tubular adherends butt joined by an adhesive ring. This configuration is used to generate shear modulus and shear strength of adhesive films restrained by relatively higher modulus adherends [56]. Shear stress is a maximum at the outer radius. Significant stress concentrations are present at the inner and outer bondline perimeters. Bevelling the inner and outer adherend edges reduces these stress concentrations. A number of researchers have preferred to use a solid butt joint in preference to the napkin ring simply because it was easier to fabricate, and only the outer spew fillet needed to be removed [6].
The shear stress tzq at a radial distance R from the central axis is [6]:
where T is the applied torque, RO is the outer radial boundary and RI is the inner radial boundary of the cylinder.
As with all torsional loading configurations, there is a need for specialised test equipment and loading fixtures. Contact extensometers can be used to measure the angular rotation across the adhesive, although it may be difficult in obtaining high precision strain measurements, due to the very low angular displacements (1–2 degrees) associated with this test configuration. Little evidence exists to support the use of the napkin ring specimen for determining fatigue resistance under shear loading conditions.
| Advantages | Disadvantages |
|
Yields shear strength and shear modulus Compatible with all metals and PMCs Stress state relatively uniform over bondline Straightforward/economic Suitable for environmental testing |
Extensometers required Special bonding fixture required Accurate specimen machining required Small strains difficult to measure Stress concentrations present at bondline ends Bondline thickness difficult to control Stress analysis difficult (non-linear stress/strain) No existing national or international standards Limited fatigue capability |
Next: Skin-Doubler Test
