Shrinkage stresses

Polyester resin floor toppings, similar in performance to the epoxy toppings, have been used but, as indicated earlier, polyester systems tend to shrink and, without careful formulation and laying, shrinkage stresses with polyester resin systems can develop at the interface between the topping and the concrete substrate. Coupled with the additional stresses due to the differences in their coefficients to thermal expansion, this can cause failure at the surface of the concrete substrate." ... 

ISO 14236 2000 Plastics pipes and fittings - Mechanical-joint compression fittings for use with polyethylene pressure pipes in water supply systems ISO 14616 1997 Plastics - Heatshrinkable films of polyethylene, ethylene copolymers and their mixtures - Determination of shrinkage stress and contraction stress ISO 14632 1998 Extruded sheets of polyethylene (PE-HD) - Requirements and test methods... 

Enabled by the high resolution of spectra, which is enhanced by the use of spatial filter assembly having a small (200 pm) pin hole, the principle of the strain-induced band shift in Raman spectra has been further extended to the measurement of residual thermal shrinkage stresses in model composites (Young et al., 1989 Filiou et al., 1992). The strain mapping technique within the fibers is employed to study the... 

Fig. 7.18. Source of shrinkage stresses (a) rigid inclusion embedded in a matrix (b) resin pockets surrounded by fibers in hexagonal and square arrays. After Hull (1981).

Fig. 7.24. Predicted fracture toughness of carbon and glass fiber-polymer matrix composites (CFRP and GFRP) with varying matrix shrinkage stress, n. After Piggott (1981).

Daniel, I.M. and Durelli (1962). Shrinkage stresses around rigid inclusions. Exper. Mech. 2, 240-255. 

Harris, B. (1978). Shrinkage stresses in glass/resin composites. J. Mater. Sci. 13, 173-177. 

Koufopoulos T. and Theocaris P.S. (1969). Shrinkage stresses in two-phase materials, J. Composite Muter. 3, 308-320,... 

The nature and type of initiation scheme plays an important role in the performance of the adhesive [194,202-204]. Stresses due to polymerization shrinkage lead to the creation of a gap between the adhesive and tooth material. In the case of bulk chemical initiation, shrinkage stresses tend to create gaps at all interfaces, drawing material inward isotropically. With a photoinitiation scheme, polymerization begins at the free surface and pulls the material away from the dentin towards the free surface [194]. Thus the gap is created at the... 

Thermal shrinkage stresses must be superposed on the stress of Equation 3. Assuming an elastic sphere embedded in an infinite elastic matrix, the complete analytical solution for thermal stress is obtained ... 

If we assume that the above processes limit the thermal shrinkage stress o>r to 1000 psig and if we superpose thermal stresses on the mechanical stresses obtained by inserting appropriate boundary conditions in Equation 3, we obtain at r = a ... 

Since T is of the order of 6000 psig during the yielding of an ABS resin, the 500-psig thermal shrinkage stress is negligible compared with the maxima in aee. 

Finally, it should be realized that, even with a well-defined material in a standard test method, the properties may be dependent on the way in which the test sample has been made the preparation conditions may be different, and, moreover, may strongly deviate from those present in a technical processing operation. This applies, e.g. to shrinkage stresses, nature of the surface, crystalline structure, and, in particular, chain orientations. 

Assuming planar symmetry, for a two dimensionally constrained film the shrinkage stress would approach 16 MPa in equal biaxial tension. 

To determine if this balance between stress level and swelling is a non-equilibrium effect, long-term experiments were performed. Figure 4 shows the determination of steady state shrinkage stress. 

These results indicate that the solvent removal process in poly(amic acid) film formation is dependent on the residual stress level. Solvent evaporation creates volume changes which in turn generate shrinkage stresses ( 8 MPa at room temperature). This will continue until there is a balance between the internal stress and the driving forces for solvent removal. Further solvent may be removed if this balance is altered by a change in stress or temperature. 

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