Shrinkage differential stresses

Differential stress from mean at hot and cold faces of lining due to shrinkage, ots-... 

Local variations in process parameters result in local shrinkage differentials. The different types of internal stress may certainly occur at the same time, for example due to cooling, dwell pressure, and crystallization during injection molding of semicrystalline thermoplastics. The different types of internal stress are also not sharply differentiated. 

Local differences in degree of crosslinking, for example in thick-walled moldings between the inside of the wall and the outer skin, result in shrinkage differentials and thus to internal stresses. This is observed when the temperature falls below the glass transition temperature - i.e. in duroplastics usually on or above 100°C. 

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. 

Abrupt changes of section cause poor flow and differential shrinkage, giving sink marks (Fig. 28.11 - you can find them on the surface of many small polymer parts), distortion, and internal stress which can lead to cracks or voids. The way out is to design in the way illustrated in Fig. 28.12. Ribs, which are often needed to stiffen polymer parts, should have a thickness of no more than two-thirds of the wall thickness, and a height no more than three times the wall thickness. Corners are profiled to give a uniform section round the corner. 

The radial (compressive) stress, qo, is caused by the matrix shrinkage and differential thermal contraction of the constituents upon cooling from the processing temperature. It should be noted that q a, z) is compressive (i.e. negative) when the fiber has a lower Poisson ratio than the matrix (vf < Vm) as is the normal case for most fiber composites. It follows that q (a,z) acts in synergy with the compressive radial stress, 0, as opposed to the case of the fiber pull-out test where the two radial stresses counterbalance, to be demonstrated in Section 4.3. Combining Eqs. (4.11), (4.12), (4,18) and (4.29), and for the boundary conditions at the debonded region... 

Stresses in solvent based coatings arise from the differential shrinkage between the thin film coatings and the corresponding substrates. These stresses are due to volume changes associated with solvent evaporation, chemical reaction (i.e. cyclization in polyimide formation) and differences in thermal expansion coefficients of the coating and substrate (4>5). The level of residual stress depends on the material properties such as modulus, residual solvent content and crosslinking (5) and its thermal-mechanical history. 

At the end of the molding cycle, the mold is opened and the part is ejected from the core. The part is allowed to cure under ambient temperature, during which shrinkage may occur because of the residual stresses developed owing to differential cooling at variable sections of the part. 

Thermosets are often used in intimate contact with materials of much lower coefficient of linear thermal expansion p. A thermoset film may be coated on a surface, or sandwiched between two surfaces as an adhesive. A thermoset matrix may be filled with high-modulus fibers in aerospace structural materials and in other composite materials. A residual stress other material(s) to changes in temperature. The value of or typically reflects the balance between the driving force to produce residual stresses due to differential shrinkage upon cooling and the temperature-dependent capacity to relax these stresses. The worst such effects... 

The variation in coefficients of thermal expansion between polymers gives difficulties where materials are composites or where several different material types are in close contact On cooling, the differential shrinkages may generate stress between the materials, causing them to fail or to come apart from each other. 

The load or force that causes stress around a discontinuity may be applied externally to a material or structure. More often, in bonded joints the force arises from differential shrinkage and swelling of the bonded members or particles. In adhesive joints and most wood products, forces tending to cleave the joint (mode I loading) are of primary importance. Sliding shear (mode II) and torsional shearing (mode III) forces are less important. However, most wood joints experience a combination of mode I with either mode II or mode III shear. 

---