Mechanisms of failure

Strength cannot be explained by relating it empirically to porosity or pore size distribution it is necessary to know what holds the material together and what happens when it fails. Many types of interatomic force exist in cement paste, and any distinction between chemical and physical forces is without obvious meaning. It has sometimes been assumed that van der Waals forces are important, but if this term is used in the restricted sense of London forces, this is unlikely. Although such forces exist between any atom and its near neighbours, in so strongly polar a material as hep they are 

Cohesion has often been attributed to the interlocking of fibrous or acicular particles. This could be important in the more porous parts of the material, but in the material as a whole, attractive forces between those parts of adjacent layers of C-S-H or other phases that are in contact are probably more important, both within particles and, in so far as the material is particulate, between them. The attractive forces could be direct, of the types mentioned above, or indirect, through interposed HjO molecules forming ion-dipole attractions and hydrogen bonds. Even for D-dried material, analogies with crystalline tobermorite and jennite indicate that much interlayer water is still present (Section 5.4). 

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The most promising approach to laboratory techniques for predicting performance is to understand the mechanism of failure and then use iastmmental methods to study the susceptibiUty of a coating to failure. The most powerful tool available now is the use of esr spectrometry to monitor the rate of free-radical appearance and disappearance (117—119) (see Magnetic spin resonance). 

Mechanisms of Failure. The causes of connector contact failute can be of a thermal, chemical, or mechanical nature, in addition to misapphcation and physical abuse. 

The path of failure of an adhesive joint can give information about the mechanism of failure if analysis of the elemental and chemical composition can be conducted along the path. Several authors have performed such analyses by loading the adhesive joint until it fractures and then using XPS to analyze each side of the fracture. 

Effects of Corrosive Environments on the Locus and Mechanism of Failure of Adhesive Joints... 

Ananth, C.R. and Chandra, N. (1995). Numerical modelling of fiber push-out test in metallic and intermetallic matrix composites mechanics of failure process. J. Composite Mater. 29, 1488-1514. 

Wilkins, D.J. (1983). Failure analysis and mechanisms of failure of fibrous composite structures. NASA CP-2278, 67-93. 

When Kevlar fibers were stressed in tension, a stranded form of fracture was observed, which resulted in multiple EE/PIE peaks spread over several hundred microseconds. The total emission from the entire fracture event correlated with the extent of "damage" to the fiber in fracture. Examination of the shape and intensity of the EE/PIE bursts could provide indications of the mechanism of failure, by differentiating between fibril formation and pull-out. 

The two predominant mechanisms of failure in adhesively bonded joints are adhesive failure or cohesive failure. Adhesive failure is the interfacial failure between the adhesive and one of the adherends. It indicates a weak boundary layer, often caused by improper surface preparation or adhesive choice. Cohesive failure is the internal failure of either the adhesive or, rarely, one of the adherends. 

A study was undertaken with a view to determine the basic mechanism of failure in series firing ability. The excitation time, transfer time and total rime of three different types of squibs (i) with LMNR as base (ii) with Lead Dimtro-ortho-cresol (LDNOC) as base and (in.) with charcoal plus potassium chlorate as base were determined... 

The problem of durability in cycles of freezing and thawing has received much attention. In cold climates, cyclic freezing and thawing is certainly one of the more common causes of durability failure of exposed concrete and other building materials. The currently accepted mechanisms of failure in portland cement and sulphur concretes are different. Both mechanisms are discussed for subsequently obvious reasons. 

With portland cement concretes, deterioration takes the form of horizontal cracks, pop-outs, D-cracking, spalling and scaling. Salts used as deicing agents compound the problem. Early theories attributed the mechanism of failure to the 9 per cent volume increase when water converts to ice. "Critical saturation" -moisture filling more than 91 per cent of the voids was considered important. 

Weeton, J. W., Mechanisms of Failure of High Nickel-Alloy Turbojet Combustion... 

A standard test report usually documents the resulting measurements, such as tensile shear strength and peel strength. It should also indicate all the pertinent conditions that are required to ensure reproducibility in subsequent testing. It is often very useful to describe the failure mode of the tested specimens. An analysis of the type (or mode) of failure is an extremely valuable tool to determine the cause of adhesive failure. The failed joint should be visually examined to determine where and to what extent failure occurred. The percent of the failure that is in the adhesion mode and that in the cohesion mode should be provided. A description of the failure mode itself (location, percent coverage, uniformity, etc.) is often quite useful. The purpose of this exercise is to establish the weak link in the joint to better understand the mechanism of failure. 

In Chaps. 13 and 15 we have already discussed several mechanisms of failure, so that we may confine ourselves to a short summary. This will be based on a very clear survey by Bin Ahmad and Ashby (1988). 

The overload mechanism of failure was confirmed by field measurement of stress on cargo ships, which showed cracking. A typical residual stress pattern obtained on the tie-down sockets installed on the ships is shown in Figure 2.28. 

An alternate method of classification of damage mechanisms in terms of environmental conditions such as stress, temperature, corrosion, wear, radiation is known as the failure wheel. In this representation secondary mechanisms are underlined as opposed to primary damage mechanisms. The boiler tube failure discussed can be represented in terms of failure wheel, as shown in Figure 2.31. The aim in any case is to identify definitively the underlying mechanism of failure to enable one to undertake remedial action such that future failures do not occur. 

The stress-corrosion cracking (SCC) mode of failure was later observed even in the case of ferritic stainless steels. The only clear message from this is that the exact mechanism of failure by this mode is not well established. Alloys containing >34% Ni were found to prolong the time of SCC failure. Ferritic type alloys 430 and 434 are resistant to SCC both in MgCl2 and NaCl environments in the mill-annealed condition, but not in welded conditions. Also, welding impairs the ductility and their resistance to SCC. 

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