Creep of structures under non-isothermal conditions.
thesisposted on 19.11.2015, 08:58 by M. H. Walter
Although there has been extensive investigation of the creep behaviour of structures subjected to steady loads and isothermal conditions, the behaviour when temperatures vary both spatially and with time has received relatively little attention. Numerical solutions are extremely difficult to produce for time varying stress and appropriate constitutive relationships have yet to be evolved. The thesis is divided into two sections both of which are concerned with structural creep behaviour under time-constant applied loads: In the first section the behaviour of a few simple structures are investigated for spatially varying temperature fields which remain constant in time. Adopting an appropriate form of Norton's constitutive relationship it is shown that the stationary deformation of the structure may be related to a single reference material test conducted at a reference stress and a reference temperature, which is independent of material constants, thereby providing a generalisation of the reference stress technique used for isothermal conditions. Experiments on a simple beam structure are described which confirm that a good correlation between the structural behaviour and uniaxial reference test behaviour exists. In all cases considered the reference temperature remains close to the lowest temperature in the structure indicating that locally high temperatures may sometimes be tolerated without excessive structural deformation. In the second section the creep behaviour of a parallel two-bar structure and a uniform plate subjected to cyclic histories of temperature is analysed by means of a method of structural analysis which arises from certain bounding theorems. It is shown that these bounding theorems can describe thermal-creep interaction extremely well and general modes of creep behaviour are discernable when the non-linear viscous, strain-hardening or Bailey-Orowan constitutive relationships are adopted. Deformation maps that relate structural behaviour to a material parameter B are described and in certain circumstances a reference stress may be defined which is independent of other material parameters. This result indicates that a reference stress approach is applicable to variable temperature problems, but that the reference value depends upon the range of values of this quantity B. In order to substantiate the theoretical assumptions a preliminary experimental investigation of the two-bar structure subjected to cyclic histories of temperature is described. Tests using aluminium specimens indicate that a residual stress field is set up that varies quite slowly in time and remains effectively constant after a few cycles. It is found that the strain-hardening constitutive relationship provides a best fit to the structural behaviour.