Creep deformation tertiary stage

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. 

Tertiary Creep or Third Stage The creep rate rapidly increases. There is a drastically increased strain rate associated with a rapid deformation, terminating in stress rupture. As flie cross sectional area of the material reduces (necking) the stress level is increased. 

Secondary creep or stage 11 creep is often referred to as steady state or linear creep . During the tertiary creep or stage 111 creep , the creep rate begins to accelerate as the cross-sectional area of the specimen decreases due to necking, which decreases the effective area of the specimen. If stage III is allowed to proceed, fracture will occur. The instantaneous strain, Sq, is obtained immediately upon loading this is not a creep deformation, since it is not dependent on time and is, by its nature, elastic. However, plastic strain also contributes in this case. 

The acceleration of the deformation speed dining the tertiary stage of creep results most often from the formation and coalescence of cavities at grain boimdaries. This form of generalized damage is accompanied by an appreciable decrease in the density of stmctures dining creep and leads to an intergranular mpture. 

A typical creep curve has three stages primary, secondary, and tertiary. The test specimen has an instant extension as soon as the load is applied. It is marked as the initial strain, in Figure 4.14. The deformation rate will gradually slow down in the primary creep stage, and reaches a constant creep rate in the secondary creep stage. This constant creep rate is also the minimum creep rate and is usually referred to as the steady-state creep rate, or simply the creep rate. The slope of the curve can be calculated using Equation 4.19, where e is the creep rate and e and t are creep deformation and time, respectively. 

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. 

Since the tertiary creep stage, accelerating creep, embraces a short time region, compared with the previous stages, it is difficult to investigate the kinetics of deformation on this stage. The next Section will be devoted to the final phenomenon of creep evolution, the fracture. 

FIGURE 24.11. A schematic of a creep curve. A = instantaneous initial deformation which may contain a piastic component B = primary, C = secondary and D = tertiary creep stage. 

Creep failure results whenever the deformation in a part accrues over a period of time under the influence of stress and temperature until the accumulated dimensional changes interfere with the ability of the part to perform satisfactorily its intended function. Three stages of creep are often observed (1) transient or primary creep during which time the rate of strain decreases, (2) steady-state or secondary creep during which time the rate of strain is virtually constant, and (3) tertiary creep... 

The presence or even predominance of one of the three creep stages depends upon the following factors (a) the material properties and microstructure (b) the temperature and (c) the applied stress. In the case of Sn-Ag-Cu lead-free solders exposed to the accelerated aging conditions of - 55 and 125 °C (- 67 and 257 °F) and hold times of 15 min., time-dependent deformation will be largely primary creep. On the other hand, service conditions that expose the Sn-Ag-Cu solder to several hours at temperatures exceeding 125 °C, or 257 °F (e.g., automotive underhood applications) will potentially place (depending on the applied stress) the material into the steady-state regime. At this time, there is no data that correlate the final failure of a solder interconnection to laboratory tertiary creep data. This lack of correlation is likely due... 

As of this writing, several finite-element based life prediction models have been proposed (Ref 1, 41) for SAC type alloys without Pb contamination. Some models feature primary and secondary creep of SAC solders, others ignore primary creep. According to recent, unpublished studies by this author, tertiary creep appears to be a significant deformation stage, but has not been considered in life prediction models. Moreover, existing models have not ex-... 

---