Plastics yield behavior

Plastic Forming. A plastic ceramic body deforms iaelastically without mpture under a compressive load that produces a shear stress ia excess of the shear strength of the body. Plastic forming processes (38,40—42,54—57) iavolve elastic—plastic behavior, whereby measurable elastic respoase occurs before and after plastic yielding. At pressures above the shear strength, the body deforms plastically by shear flow. 

The variety of epoxy resins offers a wide range of molecular structures that exhibit different yield behavior at the macroscopic level. The study of plastic deformation in different epoxy resins can help understand the structure/property relationship of plasticity in thermoset resins. 

This paper rerports an investigation of the yield behavior of several amine and anhydride cured DGEBA resin systems. The Argon theory is used to assess the controlling molecular parameters from the experimental results. Such parameters are then compared with the known chemical structures of the resins. The mechanisms of plastic flow in thermoset polymers such as epoxies is demonstrated. 

A new ASME code for calculating high pressure vessels (Sect VIII Div. 3) is based on the formulae to determine the internal pressure pcompi-pi for complete plastic yielding through the full wall with some assumptions, e.g. perfectly elastic-plastic material behavior and the GE-hypothesis [2]. 

Figures 8.51 and 8.52 show differences between the behavior of polypropylene filled with glass beads at different temperatures. In both cases, the debonding between filler and the matrix requires the lowest level of energy and confirms that this is the most likely mode of failure. The volume fraction of filler has little effect on debonding, cavitation, and yielding at 0°C. At -6()°C, yielding is improved by increasing concentration of filler. Debonding is initiated at the poles and begins plastic yielding in the matrix which ultimately leads to failure. Strain required to initiate failure is reduced when the filler concentration is increased. ...

Forming techniques used for clay-based ceramics require control of water content in the batch. Water content, in turn, affects the response of the clay during forming [27], As the water content of the batch increases, the yield point of the clay-water mixture, and thus the force required to form the desired shape, generally decreases [26], However, the relationship is complex and depends on the composition of the clay, its structure, additives to the batch, and other factors [14], One method for quantifying the behavior of clay-water pastes is to measure the plastic yield point as a function of water content [14], The water contents and maximum yield points in torsion are compared for several clays in Table 9. Kaolins and plastic fire clays require the least amount of water to develop their maximum plasticity, ball clays require an intermediate amount, and bentonite requires the most. 

The contour integral [17] was applied to fracture analysis in 1968 by Rice [18] to characterize elastic-plastic material behavior ahead of a crack. This nonlinear energy release rate was expressed in the form of a line integral, which was described as the J-integral. evaluated along an arbitrary contour around the crack. The analyses [ 19.20] showed that J can yield a nonlinear stress intensity parameter as well as energy release rate. 

In metal forming the deformation, by definition, is plastic and so the yielding behavior of materials is the most important region to consider. 

At temperatures sufficiently below the glass transition and under stresses well below the plastic yield stress to be defined later, all polymers exhibit reversible elastic behavior, which is quite often anisotropic, particularly when it relates to a polymer product that has undergone substantial prior deformation processing. 

The formulations described in the previous section show yield values coupled with viscous and viscoelastic flow, but not with thixotropy. It is possible to introduce thixotropy into plastic fluid behavior by allowing Y to depend on the history of tr dl This approach was applied to fluids with viscoelastic response above the yield value by Suetsugu and White [S26] and later by Montes and White [M35]. 

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