Strain true plastic

Plastic deformation is the permanent change in shape of a specimen due to applied stress. The onset of plastic deformation is seen as curvature in the stress—strain curve. Plastic deformation is important because it allows pharmaceutical excipients and drugs to establish large true areas of contact during compaction that can remain on decompression. In this way, good, intact, tablets can be prepared. 

For many metals, the flow curve, i.e. the true stress versus true plastic strain curve, can be expressed as a power law. 

Another common theory was proposed by Manson (27) and is referred to as the universal slopes equation. In this model, the plastic strain or permanent deformation, is considered as a measure of the damage imposed in the material. On this basis, the true plastic strain amplitude can be used as a measure of the fatigue behavior. Moreover, the fatigue curve can be predicted in terms of the monotonic stress-strain curve. This empirical approach was initially statistically correlated with many metals and takes the following form ... 

This classification is based on differences in mechanical performance specifically, differences in the strain response to applied stress. Elastic behaviour is defined by the deformation being proportional to the stress, and fully recoverable when the stress is removed. True plastic behaviour implies continuing flow under load, without recovery afterwards. In practice, polymers are viscoelastic , their behaviour being partly plastic and partly elastic, with the balance between the two extremes varying with the temperature and time under load. 

A tensile test is performed on a metal specimen, O and it is found that a true plastic strain of 0.16 is... 

The following true stresses produce the corre-O spending true plastic strains for a brass alloy ... 

For the simulation of isotropic thermoplastics elasto-viscoplastic material models are used. They are composed of an elastic part consisting of a constant Young s modulus and Poisson s ration and a plastic part being described by true stress/strain-curves depending on the true plastic strain-rate. As a failure criterion a maximal endurable hydrostatic stress, a critical equivalent plastic strain or a combination of these can be used. The strain criterion can also be set as a function of the strain-rate. 

True stress-strain curves for plastic flow... 

Conversely, at strains above the first damage limit recovery will be incomplete and permanent deformation should be expected and accounted for in the evaluation. This is true not only for plastics in general but also of reinforced plastics. When RPs are stressed... 

For the designer it is not important whether cracking develops upon exposure to a benign or an aggressive medium. The important considerations are the embrittlement itself and the fact that apparently benign environments can cause serious brittle fractures when imposed on a product that is under sustained stress and strain, which is true of certain plastics. 

The test can provide compressive stress, compressive yield, and modulus. Many plastics do not show a true compressive modulus of elasticity. When loaded in compression, they display a deformation, but show almost no elastic portion on a stress-strain curve those types of materials should be compressed with light loads. The data are derived in the same manner as in the tensile test. Compression test specimen usually requires careful edge loading of the test specimens otherwise the edges tend to flour/spread out resulting in inacturate test result readings (2-19). 

It follows that s = In Aq/A for large plastic strains. The quantity A0/A is a measure of the reduction in area due to deformation. One can define a quantity the true reduction in area (f), in a manner analogous to the definition of true strain ... 

It could be seen that for large plastic strains the true strain and the true reduction in area are identical. 

This relationship indicates that for plastic instability to occur the modulus of strain hardening, i.e., the slope of the true stress-true strain plot, should be equal to the true stress. This result is independent of any assumed functional relationship between o and s. If the relationship o = kt s " assumed to hold then one obtains... 

This relationship implies that the value of the true strain at which plastic instability sets in, i.e., necking starts to occur, is equal to the strain hardening exponent. 

At a certain level, denoted as true stress stabilises and a plateau is obtained. It corresponds to the plastic flow behaviour, associated with an increase of strain under a constant stress, the plastic flow stress, apf. 

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