Contents Plastic Deformation

The local dissolution rate, passivation rate, film thickness and mechanical properties of the oxide are obviously important factors when crack initiation is generated by localised plastic deformation. Film-induced cleavage may or may not be an important contributor to the growth of the crack but the nature of the passive film is certain to be of some importance. The increased corrosion resistance of the passive films formed on ferritic stainless steels caused by increasing the chromium content in the alloy arises because there is an increased enhancement of chromium in the film and the... 

Brittle Failure (8). Brittleness is a principal consideration in selecting construction materials for liquid hydrogen service. Brittle fracture can result in the essentially instantaneous release of a vessel s contents, the hazard being a combined one of PV energy release and the possibility of fire and/or explosion. Three conditions must exist for a brittle fracture to occur 1) a stress riser, a crack, notch, or other discontinuity, 2) a section where the actual stress exceeds the yield stress of the material, and 3) a temperature below which failure occurs without appreciable plastic deformation. Metals that are satisfactory for liquid hydrogen service include aluminum, stainless steels, brass, and copper. Carbon steel is not suitable. 

These ionic crosslinks decrease chain mobility, less energy is dissipated and less plastic deformation takes place. Also, as we will describe, the ionomers with low ion content show poorer fatigue performance with increasing ion content. Evidently, the loss of chain mobility causes embrittlement of glassy polymers, and this is probably responsible for the observed effects seen here in ionomer samples with low ion content. 

Hydrophillic colloids are dried to within a narrow range of moisture content. This level leaves the polymer sufficiently wet to plastically deform, yet not too wet allowing a strong bond to form. 

Spin-lattice relaxation has been studied by Cu NMR for dilute Cu-Mn and Cu-Cr alloys at temperatures of 0.32-20K while the NMR lines in deformed Cu alloys with Si, A1 and Ga were investigated as a function of plastic deformation. Variations in Cu Knight shifts in Cu-Zn alloys have been reported. Spectra from NMR experiments performed on binary Cu-Au ordered alloys and martensitic ternary Au-Cu-Zn alloys in order to study crystallographic Cu sites microscopically have revealed different distributions of Cu atoms that change appreciably with Cu content. 

Figure 18. Effect of interparticle distance, A, on plastic deformation of matrix strands between particles (a) definitions of the size parameters D = particle diameter, vP = particle volume content, aQ = applied stress, and aK = stress concentration (b) with a small interparticle distance, a uniaxial stress state is dominant between the particles and microvoids after cracking of the particles, and plastic yielding can be obtained and (c) with a large interparticle distance, thick matrix strands favor a triaxial stress state between the particles and microvoids, and plastic yielding is hindered.

Particle Diameter Because plastic deformation depends on stress concentration in the whole volume between particles and not directly on the stress at the particles, D is not of primary importance. The only precondition is that for a given particle volume content, the particles must be small enough to ensure that A is less than the critical value according to equation 1. Therefore, the most important function of particles is to produce a dense pattern of microvoids. Recently, an increase of the toughness of modified PA with increasing tendency to form microvoids inside the particles (with decreasing stress to crack the particles or with decreasing cavitation strain) was found (6). The cavitation stress of an elastomer is dependent on its modulus (27). 

Macroscopic Appearance of Fracture Surfaces. The most striking observations (Table III) of fracture surface topography are the occurrences of stress-whitening and extensive plastic deformation at the higher water contents, and the occurrence of arrest lines at high values of AK. 

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