Constant Mechanical Load Research Articles

An Extended Shakedown Theory for Structures That Suffer Cyclic Thermal Loading, Part 1: Theory

The paper describes an extension of classical shakedown theory for structural problems involving constant mechanical loads and cyclic variations in temperature. The objective of the theory is to provide a simple means of distinguishing between circumstances where thermal cycling can cause structural ratchetting for small or zero mechanical loads from those where very substantial thermal stresses can occur with no danger of ratchetting. This distinction is of particular importance in the design of Liquid Metal Fast Breeder Reactors and current design codes do not address this problem directly. An extended definition of material shakedown is described that provides a conservative theory, taking into account cyclic strain hardening. Some simple experiments on a two-bar structure demonstrates the relevance of the theory to observed structural behavior.

An Extended Shakedown Theory for Structures That Suffer Cyclic Thermal Loading, Part 2: Applications

In an accompanying paper an extended shakedown theory was described for structures subjected to constant mechanical loads and cyclically varying thermal loads, a circumstance of interest in Fast Reactor design. In this paper the upper bound theorem is used to construct interactive diagrams for a sequence of sample problems. These examples imply that for many shell problems, where the temperature variations occur along the surface, ratchetting occurs in two distinct ways. For low thermal loading the structure deforms in the same mechanism that occurs at plastic collapse when only the mechanical loads are applied. For high levels of thermal loading the mechanism changes to a local concentration of strain. This behavior differs significantly from that of the Bree problem. In one example this mechanism involves through-thickness shear deformation of a plate; as a consequence the use of thin shell theory which ignores shear deformation in numerical solutions of problems of this type is likely to result in nonconservative estimates of inelastic deformations.

An experimental and theoretical investigation into the creep properties of a simple structure of 316 stainless steel

The paper describes a set of creep experiments conducted on 316 austenitic stainless steel in a coupled pair of uniaxial creep testing machines which simulate the performance of a two bar structure subjected to constant mechanical and cyclic thermal loading in the temperature range of 550–600°C. The experimental results are compared with theoretical predictions using a non-interactive viscous creep/plasticity model and a model which includes recovery effects. In addition, an upper bound solution based upon the “rapid cycle” creep solution is calculated. The comparison shows that the recovery model is in close agreement with experiment and that the rapid cycle solution can provide a simple conservative estimate of the average creep rate. Several of the tests specimens exhibited strain instabilities during the initial cycles of temperature when sudden increases in inelastic strain of up to 0.2% occurred. As the material had not shown such effects in standard isothermal constant stress tests, the effects of simultaneous variation of stress and temperature which occurred in the two bar tests seems to have contributed to the occurrence of these strain instabilities.

Experimental theramal ratchetting data for a component with a stres concentration

The result from eleven tests of lead alloy model shouldered tubes, subjected to constant axial mechanical loads and cyclic, axisymmetric thermal shocks are described. Dwell periods between 0.5 h and 24 h allowed creep to occur between the thermal shocks. Electrical resistance strain gauges were used to measure ratchet and creep strains. At low mean axial stresses (< 0.94σy), it was found that the ratchet strains increased with load and dwell period. It was also found that the ratchet strains in the fillet region were significantly larger than those in the shank and are induced at lower mechanical loads than those in the shank. For higher stresses, the ratchet strains were found to be relatively insensitive to both load and dwell period. Also, the ratchet strains in the fillet and shank were found to be of the same magnitude.

An experimental study of a three-bar structure subjected to variable temperature

Experiments on a three-bar structure subjected to constant mechanical load and to variable temperature are reported. After about thirty temperature cycles, the structure settles down to exhibit a steady increment of displacement per cycle. Reference parameters presented in an earlier paper provide acceptable predictions of the displacement rates occurring in the cyclic stationary state, the predictions erring on the safe side.

Application of bounds for creeping structures subjected to load variations above the shakedown limit

Previous work established upper bounds on work and displacement for creeping structures subjected to load variations above the shakedown limit. The present paper applies these bounds to three structures subjected to constant mechanical loading and to variable imposed strains. The results are in good agreement with those of more detailed calculations.

Connectors for Aluminum Cables: A Study of the Degradation Mechanisms and Design Criteria for Reliable Connectors

The mechanisms of degradation which occur in mechanical connectors on aluminum power conductors have been examined. The factors investigated include constant and cycling mechanical load, stress relaxation, plastic deformation, surface finish, temperature, temperature cycling, and supertemperature. The aluminum-aluminum interface is shown to be stable, and the design requirements for stable connections to aluminum conductors are given.