Residual stress, plastics mechanics

Nondestmctive testing (qv) can iaclude any test that does not damage the plastic piece beyond its iatended use, such as visual and, ia some cases, mechanical tests. However, the term is normally used to describe x-ray, auclear source, ultrasonics, atomic emission, as well as some optical and infrared techniques for polymers. Nondestmctive testing is used to determine cracks, voids, inclusions, delamination, contamination, lack of cure, anisotropy, residual stresses, and defective bonds or welds in materials. 

Residual stresses and molecular orientation play an important role in the toughness enhancement of plastics, because toughness is primarily based on the mechanics of... 

Extensive theoretical investigations devoted to calculation of residual stresses have been carried out for metals. The principal theme of this work is assumption that residual stresses and strains are the result of differences between pure elastic and elastic-plastic deformations under fixed loading.127 128 The same mechanism, i.e., the appearance of plastic deformed zones, is responsible for the residual stresses arising during crystallization of metals, which occurs on quenching from the melt or cooling after welding. 

The phase transition rate in the crystallization of polymeric materials is of the same order as the rates of the heat exchange processes accompanying crystallization. Consequently, the boundary between phases becomes spatially dispersed. This excludes the possibility of using methods based on the front transition model proposed for metals to calculate residual stresses in plastics.148 It is possible to split the general problem and to find the temperature-conversion field independently. Then, assuming that the evolution of temperature T(x,t) and degree of crystallinity a(x,t) in time t and in space (x is the radius vector of an arbitrary point in a body) is known, we can analyze the mechanical problem.143... 

The presence of residual elastic stresses is inevitable in a surface which contains a plastically deformed zone whose thickness is limited compared with that of the bulk s pecimen. Alternatively, the relief of these stresses may cause distortion of the specimens in cases where the thickness of the two is comparable. Very little work has been done on this important subject and, so far as can be ascertained, none that can be related to the complexities of die plastically-deformed layer. Considerable complications are introduced because the residual stresses may be of thermal as well as mechanical origin and because those of mechanical origin may be altered by the thermal effects. 

The competition between the interfacial reactivities, the residual stresses and the elasto-plastic behavior of the components will be strongly dependent on the mechanical stability of the coating-substrate combination. Mechanical stability control has been assessed when making ceramic/metal junctions at high temperature (700°C - 1000°C) during which thick reaction zones tend to form by reactive diffusion in volume intermediate layers. ... 

An alternative strengthening mechanism is prestressing, called autofrettage, which ensures a more uniform stress distribution under load [16-20]. This intentional over-pressurizing of the vessel leads to a plastic-elastic interface that moves outward as the pressure increases. When the pressure is released, the residual stresses left as a result of radial expansion allow the vessel to be used up to the pressure at which it was subjected to autofrettage, without exceeding the yield point of the material. 

The mechanisms of fatigue crack closure have been identified for metals. These are illustrated by Anderson [16], who has also expanded upon the effects of crack closure on the fatigue properties of metals. The phenomenon can also manifest itself in polymer-based materials because of the plastic deformation, viscoelasticity, residual stresses, and environmental conditions. 

Material removal processes, such as abrasive processes, take effect on the residual stresses in two ways Generally material volume exposed to residual stresses is removed so that a new mechanical equilibrium establishes. This leads to form changes and a new residual stress state. The second way is the generation of residual stresses by local plastic deformation (creation of stress sources) which disturb the initial equilibrium and consequently also lead to form changes and a new residual stress distribution. 

A mechanical interaction of the abrasive grains with the workpiece usually leads to compressive residual stresses by localized elastic deformation and plastic flow. The predominance of mechanical process effects can be achieved by chip formation with increased ratio of micro-plowing. This usually occurs in grinding with small chip thickness and low cutting speeds (Fig. 2). 

During machining, the material subsurface is heated temporarily to high temperatures. By this the surface-near layers expand as a result of thermal compressive stress. The surface-near layers deform plastically in a state of reduced yield strength. After cooling down to room temperature, tensile residual stresses result. As in machining both mechanical and thermal influences act concurrently, they interfere with each other strongly nonlinear. A simple superposition is not... 

Flat tape, whether forced flat by mechanical constraints or held flat by gravity or good chemistry, experiences Level 2 stress and may or may not undergo Level 3 plastic deformation. We have never seen a tape that did not store residual tensile stress in the polymer matrix. This stored stress will be discussed in detail in later sections, along with methods for alleviating the stress. Plastic deformation cannot really be measured, but can be assumed when the same slip without a Type II plasticizer shows multiple Level 4 behaviors, but with the Type II plasticizer lays flat. 

With a manufacturing method (or MEH method), deformation mechanisms in the presence of compressive residual stress around particles in relation with the toughening conditions, relative stress components, and fracture morphology have been analyzed using Mohr circles. A major difference in deformation between MEH and non-MEH methods was found to be in the location of plastic deformation under plane-strain. The plastic deformation of non-MEH was dominantly in matrix and appeared in the form of matrix cavitation. In the case of MEH, it was dominantly in microspheres. It was suggested that compressive residual stress promotes plastic deformation of microspheres caused by extrusion effect. 

Residual stresses and molecular orientation play an important role in the toughness enhancement of cold-worked plastics, because toughness is primarily based on the mechanics of craze formation and shear band (crazes and flaws) formation. The shear bands determine the fracture mode and toughness of a polymer when subjected to impact loads. The amount of energy dissipated will depend on whether the material surrounding the... 

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