Thermal contraction, stress from

In polycrystalline ceramics, microcracks can develop at the grain boundaries. The reason for this is the difference in the orientation of two adjacent grains. When the material is heated to a higher temperature, the grains will expand differentially along their different axes. On cooling from this higher temperature, the contraction also will be differential. Because of this differential expansion and contraction, thermal stresses will be built up in the material. 

Stresses from welding result principally from the effects of differential thermal expansion and contraction arising from the large temperature difference between the weld bead and the relatively cold adjacent base metal. Shrinkage of the weld metal during solidification can also induce high residual stresses. Unless these residual stresses are removed, they remain an intrinsic condition of the weldment apart from any applied stresses imposed as a result of equipment operation. 

Quantification of residual stresses after manufacture. The build up of thermal stresses starts during fabrication of the laminate when it is cooled from the stress free temperature to room temperature. The stress free temperature in the case of an amorphous thermoplastic as used in this study is taken as the glass transition temperature [1] Tg of the Polyetherimide used is 215°C). On a fibre-matrix scale, the contraction of the matrix ( = 57 x 10 /°C) is constrained by the presence of the fibre (cif = -1 x 10 /°C for the carbon in the fibre direction). This results in residual stresses on a fibre-matrix scale (microscale). On a macroscopic scale, the properties of a unidirectional layer can be considered trans ersally isotropic. This means, in turn, that a multidirectional composite will not only contain stresses on a microscale, but also on a ply-to-ply (macroscopic) scale. 

Low temperatures can also affect materials by thermal contraction. The thermal expansion coefficient is a function of temperature. For many materials, which are cooled down from room to cryogenic temperature, more than 90 % of the total contraction experienced will have already taken place at 77 K. Rule-of-thumb figures of thermal contraction are 0.3 % in iron-based alloys, 0.4 % in aluminum, or over 1 % in many plastics [43]. Cryogenic vessels or piping systems must account for this contraction to avoid large thermal stresses. 

When a weld solidifies, it undergoes a volume contraction. Further contraction occurs while it cools out, except that the transformation from austenite to lower temperature transformation products gives an expansion, counterbalancing some of the previous contraction. Thermal contraction gives rise to distortion and residual stresses, the balance between the two depending on the level of restraint (q.v.). 

Stresses can, of course, result from an applied load on the component undergoing oxidation. However, additional stresses are generated by the oxidation process. These are growth stresses, which develop during the isothermal formation of the scale, and thermal stresses, which arise from differential thermal expansion or contraction between the alloy substrate and the scale. 

The authors have recently calculated the residual thermal stresses in HMS4/PEEK (19) and have shown that significant levels of axial compression would be expected in the fibres because of high levels of thermal contraction in PEEK on crystallisation and cooling from the processing temperature. 

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