Local plasticity

The application of load in materials produces internal modifications such as crack growth, local plastic deformation, corrosion and phase changes, which are accompanied by the emission of acoustic waves in materials. These waves therefore contain information on the internal behaviour of the material and can be analysed to obtain this information. The waves are detected by the use of suitable sensors, that converts the surface movements of the material into electric signal. These signals are processed, analysed and recorded by an appropriate instrumentation. 

Measurements of stress relaxation on tempering indicate that, in a plain carbon steel, residual stresses are significantly lowered by heating to temperatures as low as 150°C, but that temperatures of 480°C and above are required to reduce these stresses to adequately low values. The times and temperatures required for stress reUef depend on the high temperature yield strength of the steel, because stress reUef results from the localized plastic flow that occurs when the steel is heated to a temperature where its yield strength is less than the internal stress. This phenomenon may be affected markedly by composition, and particularly by alloy additions. 

We first consider strain localization as discussed in Section 6.1. The material deformation action is assumed to be confined to planes that are thin in comparison to their spacing d. Let the thickness of the deformation region be given by h then the amount of local plastic shear strain in the deformation is approximately Ji djh)y, where y is the macroscale plastic shear strain in the shock process. In a planar shock wave in materials of low strength y e, where e = 1 — Po/P is the volumetric strain. On the micromechanical scale y, is accommodated by the motion of dislocations, or y, bN v(z). The average separation of mobile dislocations is simply L = Every time a disloca-... 

Fig. 4. Model of local plastic deformation of lamellae beneath the stress field of the indenter. The mosaic block structure introduces a weakness element allowing faster slip at block boundaries leading to fracture (right)...

Polyethylene pellets were Very abundant" on the Mediterranean coast of Spain, particularly near plastics fabrication factories (39, 40). Wastes from these factories and spillage during cargo loading and transport of raw materials were considered to be the major sources. Similarly, wastes from local plastic fabrication factories were thought to be the source of pellets on the beaches of Lebanon (40). 

In the unstrained material far from the center of an indentation, dislocations can move freely at much lower stresses than in the material near the center where the stress (and the deformation) is much larger. Thus, local plastic shear bands can form at the edges of the indenter, and do (Chaudhri, 2004). The lengths of these shear bands are often several times the size of an indentation. The leading dislocations in these bands move in virgin (undeformed) material, so they can move at lower stresses than the dislocations in the strain-hardened material near the center of an indentation.. The patterns they form are called rosettes. ... 

The changes in fibre flexibility may affect not only the rigidity of the fibre but also the local plasticity of the cell wall, and this may be important in determining the ease with which the inter-fibre hydrogen bonds are formed during drying. Pulp types also differ in... 

Dislocations are commonly present in two regions. A layer with high mismatch may relax so that interface dislocations are created to accommodate the strain. A network at the interface is thns observed. Shp dislocations may be generated by local plastic deformation due to thermal or mechanical strain and propagate elsewhere in the layer. Dislocations in the layer itself may also be generated during the growth process, dne, for example, to the presence of inclusions. 

Also, mechanical data on the influence of low volume fractions (0.03-0.05) of rigid filler particles provide evidence of a localized plastic deformation which would not seem understandable by reference to a uniformly crosslinked network. A non-uniformly crosslinked matrix might also be invoked to account for insensitivity of the rate of diffusion of water on the apparent degree of crosslinking. However, an observed increase in the uptake of water with apparent degree of crosslinking remains unexplained. 

Therefore If the softening Is, Indeed, due to localized plastic deformation then the difference between unfilled specimens and ones with a low volume fraction of filler should be reduced by radiation crosslinking. This was found to be the case (Fig. 6). 

In Sect. 5, the macroscopic extension at break of the glassy networks considered was shown to be s, 4-6% and only weakly dependent on the network s chemical composition. However, local plasticity markedly depends on the chemical composition of the network. L( in the samples with an excexs of amine (P = 1.3) is about 10 times larger than in the samples with an excess of epoxy groups (P = 0.8). This is an additional argument in favour of the assumption of the keyrole of 3-linked chemical crosslinks in network plasticity (see Sect. 5 and Fig. 20). It is evident that the excess of amine in the initial mixture leads to a relatively high concentration of 3-type crosslinks in cured resins. 

Hydrogen-induced plasticity. Another approach is related to the fact that hydrogen increases local plasticity. Two models have been proposed based on the adsorption and absorption of hydrogen. 

The second is the absorbed hydrogen-enhanced local plasticity mechanism (HELP). This is based on the fact that the local decrease of the flow stress by hydrogen leads to highly localized failure by ductile processes, while the local macroscopic deformation remains small. Shear localization results from local hydrogen absorption, giving a macroscopically brittle fracture related to microscopic localized deformation.95... 

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