Material failures

Fractures and cracks can be classified in three groups, depending on whether their main cause is mechanical, thermal, or corrosive. 

Mechanical fracture can be due to monotonic increase of the load overload fracture or forced fracture) or due to cyclic loads fatigue fracture). Overload 

An overload fracture is characterised by a mainly monotonously increasing load that is applied moderately fast or abruptly [90]. These conditions discriminate overload fracture from fatigue fracture (non-monotonous, cyclic load) and creep fracture (long loading times at high temperature). 

The fracture can occur as shear fracture, cleavage fracture, or a mixture of both. The two characteristic forms will be discussed in the following. 

Shear fracture (microscopically ductile fracture) occurs by plastic deformation with slip in the direction of planes of maximum shear stress (see sections 3.3.2 and 6.2.5). Therefore, it occurs only in ductile materials. In most cases, shear fracture is associated with large macroscopic deformations, as, for example, in a tensile test. However, if this is prevented by the component geometry, the component may fail macroscopically brittle, but still with a shear fracture. This may happen if there are notches or cracks in the material (see chapters 4 and 5). 

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In addition to the reduction in performance, flow maldistribution may result in increased corrosion, erosion, wear, fouling, fatigue, and material failure, particularly for Hquid flows. This problem is even more pronounced for multiphase or phase change flows as compared to single-phase flows. Flow distribution problems exist for almost all types of exchangers and can have a significant impact on energy, environment, material, and cost in most industries. 

Water chemistry is important to the safe and reflable operation of a nuclear power plant. Improper conditions can lead to equipment and material failures which ia turn can lead to lengthy unscheduled shutdown periods for maintenance (qv) and repair operations. Water chemistry can also have an impact on the radiation levels duriag both power operations and shutdown periods. These affect the abiUty of personnel to perform plant functions. 

With the advances in computing hardware that have occurred over the last decade, three-dimensional computational analyses of shock and impact problems have become relatively common. In Lagrangian calculations, element erosion schemes have provided a means for handling the large deformations and material failure that is often involved, and Fig. 9.28 shows results of a penetration calculation which makes use of this methodology [68]. 

Dasgupta, A. and Pecht, M. 1991 Material Failure Mechanisms and Damage Models. IEEE Transactions on Reliability, 40(5), 531-536. 

Materials selection is as much an art as a rigorous science, and another computational approach to it, based on ideas of artificial intelligence, has been proposed by Arunachalam and Bhaskar (1999). They call their approach bounded rationality and exploit it to analyse the background to some notorious disasters based on material failure. We can always learn from failure as well as from success. 

Most faults are attributed to poorly machined surfaces causing out-of-specification tolerances, although defective material failures also occur. Inadequate material hardness and poor strength factors contribute to many premature failures. Other common causes are improper coupling selection, improper installation, and/or excessive misalignment. 

Obviously, the designer must take thermal expansion and contraction into account if critical dimensions and clearances are to be maintained during use where material is in a restricted design. Less obvious is the fact that products may develop high stresses when they are constrained from freely expanding or contracting in response to temperature changes. These temperature-induced stresses can cause material failure. 

The first step in the data analysis process is to choose the level of decomposition. A selection level early in the decomposition is desired since the mechanism is more likely to be related to the process of the actual failure onset point of the material (i.e., thermal decomposition). The analyst must be cautious to use former experience with the construction of the model construction of the method so as not to select a level too early and cross material failure with the measurement of some volatilization that is not involved in the failure mechanism. A value of 5% decomposition level (sometimes called conversion ) is a commonly chosen value. This is the case in the example in Fig. 4.25, and all other calculations from the following plots were based on this level. 

A-2.11.1 Storage Vessel Failure. The release of GH2 or LH2 may result in ignition and combustion, causing fires and explosions. Damage may extend over considerably wider areas than the storage locations because of hydrogen cloud movement. Vessel failure may be started by material failure, excessive pressure caused by heat leak, or failure of the pressure-relief system. 

The epoxy matrix, containing multiple hydroxyl and amino groups, has a high tendency to adsorb water, a major cause of material failure in many applications. Finally, when one uses the epoxy as a fiber matrix, there is a distinct probability that voids will appear due to reaction by-products which do not escape during cure. Once the material vitrifies, the mobility of the molecules is reduced, making it difficult for the small molecular weight products to be eliminated. 

Unlike ductile metals, composite laminates containing fiber-reinforced thermosetting polymers do not exhibit gross ductile yielding. However, they do not behave as classic brittle materials, either. Under a static tensile load, many of these laminates show nonlinear characteristics attributed to sequential ply failures. One of the difficulties, then, in designing with laminar composites is to determine whether the failure of the first ply constitutes material failure, termed first-ply failure (FPF), or if ultimate failure of the composite constitutes failure. In many laminar composites, ultimate failure occurs soon after first ply failure, so that an FPF design approach is justified, as illustrated for two common laminar composites in Table 8.9 (see Section 5.4.3 for information on the notations used for laminar composites). In fact, the FPF approach is used for many aerospace and aircraft applications. 

Figure 3.3 Stress-strain plots representative of brittle, ductile, and elastomeric polymeric materials. Failure is denoted by . (After J. Fried, Plastics Engineering, July 1982, with permission.)...

Explosive decompression due to pressure or temperature shifts can cause catastrophic material failure. 

Surface fatigue is usually the result of high periodic loads in the contact zone that lead to changes of the material structure and finally to material failure at the surface. 

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