Failure creep

A single creep test in tension or flexure gives information on long-term dimensional stability of a load-bearing element. When combined with a variable temperature, the test provides a simple means of measuring the heat deflection temperature. 

Creep is the phenomenon of high-temperature deformation under a constant load. When the deformation results in the failure of the material, the failure is termed as the creep failure. In the case of polycrystalline materials, the deformation is mainly due to grain boundary sliding. Tensile stresses are built up at the boundaries. These stresses develop pores and cracks at the boundaries. As deformation increases, the pores grow. 

Development of cracks in alumina during creep deformation. 

Creep failure is represented by creep-rupture curves. In this plot, applied stress is plotted against time to failure in the logarithmic scale. 

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Here A, m and Q are the creep-failure constants, determined in the same way as those for creep (the exponents have the opposite sign because tf is a time whereas e, is a rate). 

Materials subjected to high temperatures during their service life are susceptible to another form of fracture which can occur at very low stress levels. This is known as creep failure and is a time dependent mode of fracture and can take many hours to become apparent (Fig. 8.88). 

Although the creep behavior of a material could be measured in any mode, such experiments are most often run in tension or flexure. In the first, a test specimen is subjected to a constant tensile load and its elongation is measured as a function of time. After a sufficiently long period of time, the specimen will fracture that is a phenomenon called tensile creep failure. In general, the higher the applied tensile stress, the shorter the time and the greater the total strain to specimen failure. Furthermore, as the stress level decreases, the fracture mode changes from ductile to brittle. With flexural, a test specimen... 

An understanding of the mechanism of creep failure of polymer fibres is required for the prediction of lifetimes in technical applications. Coleman has formulated a model yielding a relationship similar to Eq. 104. It is based on the theory of absolute reaction rates as developed by Eyring, which has been applied to a rupture process of intermolecular bonds [54]. Zhurkov has formulated a different version of this theory, which is based on chain fracture [55]. In the preceding sections it has been shown that chain fracture is an unlikely cause for breakage of polymer fibres. 

As shown in Sect. 2, the fracture envelope of polymer fibres can be explained not only by assuming a critical shear stress as a failure criterion, but also by a critical shear strain. In this section, a simple model for the creep failure is presented that is based on the logarithmic creep curve and on a critical shear strain as the failure criterion. In order to investigate the temperature dependence of the strength, a kinetic model for the formation and rupture of secondary bonds during the extension of the fibre is proposed. This so-called Eyring reduced time (ERT) model yields a relationship between the strength and the load rate as well as an improved lifetime equation. 

Fig. 62 The creep failure curves of a PpPTA fibre with an initial modulus of 91.8 GPa calculated with the same parameters as used in Fig. 61...

Fig. 64 Creep and creep failure can be modelled by the time-dependent shear deformation as described by the Eyring reduced time model...

Another common location for creep failures of encapsulated assemblies is at sharp corners or edges. Many encapsulants such as polyimides must be applied in thin coats, and coverage of points, edges, or corners is difficult or impossible. Sharp corners, characteristic of most thin-film devices provide ideal conditions for the initiation of creep failures due to the resulting irregularity of the encapsulant coverage. 

The ubiquity of this power-law behaviour in SCG tests on PE has been the subject of considerable discussion, usually based on the assumption of a fibril creep failure mechanism [43, 45, 46, 47, 76, 79]. At high and intermediate K, after a certain induction period, steady-state crack advance is generally observed to occur by a stick-slip mechanism all or part of the fibrillar zone breaks down rapidly after an incubation time during which fibril creep takes place. The crack-tip then advances rapidly over a short distance and a new fibrillar zone stabilises, as sketched in Fig. 12. 

Life prediction methodology embraces all aspects of the numerous processes that could affect the function of the element—in this case the bulk adhesive. The first step is to define the function of the adhesive clearly enough for a failure criterion to be derived. This failure criterion may be an unacceptable reduction in tensile strength, time to creep failure under a given stress, reduction in modulus due to moisture ingression, increase in modulus due to oxidation, unacceptable crack depth, or a variety of other possible criteria. It is also important that the criteria be related to practical adhesive joint performance. This is where it is difficult, and one must presume, at least for this limited analysis, that the adhesive will fail via a bulk (cohesive) property. 

Understanding of the mechanism of creep failure of polymeric fibres is required for the prediction of lifetimes in technical applications (Northolt et al., 2005). For describing the viscoelastic properties of a polymer fibre use is made of a rheological model as depicted in Fig. 13.103. It consists of a series arrangement of an "elastic" spring representing the chain modulus ech and a "shear" spring, yd with viscoelastic and plastic properties... 

In this respect also the time needed for creep failure, t / is also an important property of fibres. According to Northolt et al. this lifetime reads... 

Thus, it has been concluded that the creep process of elastomers is one of the suitable processes for separating the second factor from the canbined state and that the creep failure experiment gives us a good information for estimating the failure mechanism of polymers. 

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