Tertiary creep

Tertiary creep represents the final stage of creep deformation and involves an acceleration of the creep rate followed by failure of the component. This stage does not occur in all ceramics, and as previously noted certain ceramics exhibit only primary creep. Tertiary creep involves the formation of cavities that lead to crack formation, often along grain boundaries. The cracks can propagate rapidly, particularly under tensile (17.11) loading. 

Fig. 5. Tensile elongation vs time demonstrating creep behavior of ceramics. Section I is primary creep II, secondary or steady-state creep III, tertiary...

Here o is the stress, A and n are creep constants and Q is the activation energy for creep. Most engineering design against creep is based on this equation. Finally, the creep rate accelerates again into tertiary creep and fracture. 

The final stage of creep is known as tertiary creep at which time the creep rate increases rapidly culminating in failure. This acceleration in creep is due mainly to the formation of voids and microcracks in the material which form along the grain boundaries causing the fracture path to be predominantly intercrystalline. 

Tertiary amine polymethacrylates, 20 471 tertiary-butyldimethylsilyl (TBDMS), cleavage of, 16 559, 560 Tertiary creep, 13 472 Tertiary diperoxyketals, 18 457 Tertiary metal phosphates, 18 840 Tertiary nitriles, 12 180 Tertiary phosphine oxides, 19 66 Tertiary phosphines, 19 64 Tertiary recycling, 21 449 of PET, 21 450... 

Figure 5.43 Representative creep curve illustrating primary, secondary, and tertiary creep. Reprinted, by permission, from W. Callister, Materials Science and Engineering An Introduction, 5th ed., p. 226. Copyright 2000 by John Wiley Sons, Inc.

On the other hand the accelerating homogeneous deformation in the tertiary creep phase (Fig. 10a) indicates material weakening through the accumulated geometrical and structural changes (reduced cross-section, dilata-... 

In the initial stage, known as primary creep, the strain rate is relatively high, but slows with increasing strain. The strain rate eventually reaches a minimum and becomes near-constant. This is known as secondary or steady-state creep. This stage is the most understood. The characterized creep strain rate , typically refers to the rate in this secondary stage. The stress dependence of this rate depends on the creep mechanism. In tertiary creep, the strain-rate exponentially increases with strain [1-9]. 

A typical tensile creep curve for a particulate reinforced ceramic matrix composite, siliconized silicon carbide (Si/SiC),28 is shown in Fig. 4.1. In comparison to the behavior of metals and metallic alloys, tertiary creep is suppressed in this material. There is only a slight upward curvature of the creep curve prior to failure. In many other ceramic matrix composites, tertiary... 

Fig. 4.1 Tensile creep curves for siliconized silicon carbide (Carborundum KX01). Over most of the data range, these data can be represented by a constant creep rate there is a short primary creep stage, and almost no tertiary creep. The rupture strain decreases with increasing creep rate. The strain to failure, =1.5%, indicates brittle behavior even at low rates of creep detormation. Figure from Ref. 28.

The tertiary stage, or tertiary creep, is characterised by a rapid increase in the strain, leading to failure or creep rupture (Figure 10.28). At this stage, voids form in the region of the fracture and there is considerable grain boundary movement. As would be anticipated, this part of the curve is difficidt to analyse theoretically. 

Initially, primary or transient creep occurs in Stage I. The creep rate, (the slope of the curve) is high at first, but it soon decreases. This is followed by secondary (or steady-state) creep in Stage II, when the creep rate is small and the strain increases very slowly with time. Eventually, in Stage III (tertiary or accelerating creep), the creep rate increases more rapidly and the strain may become so large that it results in failure. 

The primary creep has a rapidly decreasing strain rate. It is essentially similar in mechanism to retarded elasticity and, as such, is recoverable if the stress is removed. The secondary or steady-state creep is essentially viscous in character and is therefore nonrecoverable. The strain rate during this state is commonly referred to as the creep rate. It determines the useful life of the material. Tertiary... 

What processes are usually occuring in tertiary creep ... 

Tertiary creep a rapid increase in creep rate just before failure. This stage is often missing for ceramics. 

Viscoelastic creep manifests itself in the time-dependent deformation of a material. Experimental data obtained from a laboratory creep test under constant applied stress for a viscoelastic solid is shown in Fig. 12.1. Traditionally, a creep curve consists of three stages. In the first stage, also known as primary creep, the creep strain rate decreases with time until it reaches a constant value. The second stage, known as steady state creep, is defined as the region where the slope of the creep strain is a constant with respect to time. In the third and final stage, termed tertiary creep, the creep strain rate increases with time through progressive failure and terminates with the rupture of the specimen. 

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