Materials imperfections

On one hand, inherent flaws or perturbations in a fracturing body, which are the sites of internal fracture nucleation, have been recognized as important in determining characteristic fracture spacing and, consequently, the nominal fragment size in a fracture event. Theoretical work based on a physical description of these material imperfections has been actively pursued (Curran et al., 1977 Grady and Kipp, 1980). 

There is considerable literature on material imperfections and their relation to the failure process. Typically, these theories are material dependent flaws are idealized as penny-shaped cracks, spherical pores, or other regular geometries, and their distribution in size, orientation, and spatial extent is specified. The tensile stress at which fracture initiates at a flaw depends on material properties and geometry of the flaw, and scales with the size of the flaw (Carroll and Holt, 1972a, b Curran et al., 1977 Davison et al., 1977). In thermally activated fracture processes, one or more specific mechanisms are considered, and the fracture activation rate at a specified tensile-stress level follows from the stress dependence of the Boltzmann factor (Zlatin and Ioffe, 1973). 

Unbalance. This stimulus is caused by material imperfections, tolerances, etc. The mass center of gravity is different from the geometric case, leading to a centrifugal force acting on the system. 

Since fatigue always occurs at locations of stress concentrations or material imperfections, peak stresses and stresses at welding locations have to be calculated and evaluated. 

Recent studies have shown that this is not the case in polyacetylene improvements in synthesis and orientation have resulted in electrical conductivities as high as 10 S/cmA Furthermore, the absence of a metallic temperature dependence with resistivity decreasing as the temperature is lowered - implies that the measured conductivity is still limited by material imperfections. Consequendy, it is quite clear that the intrinsic electrical conductivity of polyacetylene and, by implication, of other conducting polymers, may be significantly greater than that of copper. 

The microstructure is defined as a network of one or more crystalline phases with various types of imperfection. The crystalline phase, which covers the greater part of the whole system, is the origin of the intrinsic properties of a specific material. Imperfections are defects to which most of the processing-dependent properties are attributed. Typical examples of imperfections include solid/vapor and solid/liquid interfaces, grain boundaries, phase boundaries, pores, secondary phases, and so on. Among those listed, the first three defects - that is, the solid/vapor and solid/liquid... 

Qualitatively, the temperature dependence of the conductivity is the reverse of that of metals since, at least up to about 200 K, the conductivity increases with increasing temperature. The ratio of room-temperature to zero-temperature conductivity is less than 10 down to 2-3. This temperature dependence indicates that the conductivity is still limited by various material imperfections. 

Penetration regards the existence of material imperfections or defects as pores, sealing joints default and providing opportunities for transition to chemical in contact with the glove. 

The problem of secondary flexure in axial tensile tests is considered in view of the tension-softening process. The procedures aimed at elimination of that parasite effect were studied by Akita et al. (2001) and Novak et al. (2006) who have shown possibilities of nonlinear fracture mechanics simulation in analysis of the experimental data, including post-peak descending branch of the load-displacement curves, taking into consideration material imperfections and heterogeneities. 

Measling/Crazing, Delamination, Blistering, and Weave Texture/Exposure. The IPC formed a special committee in 1971 to consider base material imperfections and to define them better with illustrations and photographs. The conditions are defined and discussed here ... 

Base Material Voids after Thermal Stress. Heat from soldering operations wiU transfer quickly through the copper holes and pads in a PCB to the base material in close proximity to the copper. This rapid heat movement can easily cause imperfections to occur in the base material and this area near the holes is exempted from evalnation for base material imperfections. This area is defined as the thermal zone and described in detail by IPC-A-600. Outside this thermal zone, base material voids should not exceed 0.08 mm for rigid PCBs and 0.5 mm for flexible PCBs or flexible portions of rigid-flexible PCBs. Base material voids found in the same plane of flexible or rigid-flexible PCBs should not have a combined dimension greater than 0.5 mm (see Rg. 51.20). 

This at best is a proposition confined to one description of causes and the inference is fair either that the Massachusetts convention considered that as the only class of federal causes, in which the trial by jury would be proper or that if desirous of a more extensive provision, they found it impracticable to devise one which would properly answer the end. If the first, the omission of a regulation respecting so partial an object, can never be considered as a material imperfection in the system. If the last, it affords a strong corroboration of the extreme difficulty of the thing. 

NMRI is a means of detecting and imaging previously invisible material imperfections in fabricated articles. Its potential applications in the field of polymeric materials are many and diverse [4]. They include the detection and imaging of subsurface defects, including interfacial flaws and microcracks, and the detection and characterization of areas modified through the introduction of foreign substances such as additives, degradation products, and contaminants. 

Crude oil is a strategically important raw material and therefore it is necessary to pay increased attention to its transportation and storage. The dominant load case of cylindrical tanks is loading of the inner shell by hydrostatic pressure of the oil. The article is aimed at the analysis of failure probability of the inner shell with regard to random geometric and material imperfections. The article... 

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