Creep, definition conditioning

The effect of temperature on the mechanical properties has definite place in specification of a material for any particular application. Some materials become dangerously brittle under arctic conditions, and all metals exhibit at elevated temperatures a phenomenon called creep. 

A sketch of the physical problem and a definition of the cylindrical polar coordinates (r, creeping-flow limit, the governing equations and boundary conditions are... 

For the pendant drop in figure I, the equations above are subjected to the no-slip boundary conditions at solid surfaces and the kinematic condition on the free surfaces. The kinematic condition implies that there is no liquid crossing the boundary into the gas phase, or in other words forms a definite boundary between the phases. For creeping flows into an atmosphere of gas with minimal velocities there will be no interfacial shear stress tangential to the surface and the normal stress inside the fluid is balanced by the surface tension as described by the famous Young-Laplace equation. 

Mechanical testing procedures in common use involve other patterns of stress history than the simple creep and relaxation experiments on which the definitions of the transient viscoelastic functions are based, and the sinusoidally varying stress which is inherent in the definitions of the so-called dynamic properties. Certain relations between the behavior under coniplicated conditions and the basic viscoelastic functions are presented here together with some related problems. They are limited to linear viscoelastic systems and hence small strains, but in some cases could be extended to describe larger deformations, especially for simple extension. 

All of the above conclusions have been drawn for isothermal conditions without considering a thermal fluctuation under creep conditions but which in practice represent the usual case during the service life of structural adhesive joints. Considering temperature effects in creep-dependent lifetime, prediction can either follow a worst case scenario in which the definition of the load-dependent fracture envelope or the test for compKance with predefined strain limits is carried out at the maximum temperature to he expected during service life. This conservative approach is likely to lead to excessive contingency reserves. 

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