Brittle failure static load

Creep leads ultimately to rupture, referred to as creep-rupture, stress-rupture or static fatigue. Creep-rupture of thermoplastics can take three different forms brittle failure at low temperatures and high strain rates ductile failure at intermediate loads and temperatures and slow, low energy brittle failure at long lifetimes. It is this transition back to brittle failure that is critical in the prediction of lifetime, and it is always prudent to assume that such a transition will occur [1], A notch or stress concentration will help to initiate failure. 

Although some polymers may be satisfactory when used under the stress of static loads, they may fail when subjected to impact. The impact resistance, or resistance to brittle fracture, is a function of the molecular weight of a polymer. Thus uhmwpe is much more resistant to impact failure than general purpose high-density polyethylene (hdpe). The impact resistance of brittle polymers is also increased by the addition of plasticizers. Thus polyvinyl chloride (PVC), plasticized by relatively large amounts of dioctyl phthalate, is much less brittle than unplasticized rigid PVC. 

It has been proposed that surface degradation after UV exposure promotes brittleness and may therefore influence the failure mechanism. This is of particular importance for ABS pressure pipes that are continuously under static loading. A methodology for estimating the lifetime of UV exposed ABS pipes has been proposed (62). 

It may seem surprising that the notch support factor of brittle materials, like cast iron, is rather large (see figure 10.36). This is due to the fact that cracks in cast iron start at the graphite particles which act as inner defects and are statistically distributed. It is rather improbable that the crack that determines failure behaviour (the largest crack) is situated exactly at the notch root where the stress concentration becomes important. This is analogous to the dependence of the failure probability on the material volume according to the Weibull statistics (see section 7.3). This notch support in brittle materials also occurs under static loads. 

Impact failures of engineering components are not uncommon, but they are probably oumumbered by those from other brittle modes fatigue and slow crack growth. The fracture surfaces which witness all three modes are superficially similar, but profoundly different rate and temperature dependencies emphasise that different mechanisms are at work. Slow crack grow failures are favoured by long periods under static load at higher temperatures, whilst impact failures are favoured by rapid loading at lower temperatures. 

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. 

Equation 2 implies that the maximum acceleration or its equivalent inertial force controls the seismic failure of stmctures. This type of failure is the case of a brittle stmcture which collapses if the external load exceeds the resistance see Fig. 4. Because traditional masonry buildings and wooden houses were of brittle nature, seismic design by means of this static seismic design... 

In the case of silicate glasses, which break in a brittle manner, the fracture is time dependent. That is, for a given load, the fracture takes place after a period of time. This phenomenon is called static fatigue. The hme to failure is related to the fracture stress by the following equation ... 

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