applied to directionally solidified columnar‐grained pure S‐2 ice (Sinha 1978b) using a conventional dead‐load lever system. However, state‐of‐the‐art, computer‐controlled, servo‐hydraulic technology has provided us with the opportunity to load fully and unload completely in fractions of second for a wide range of stress. Moreover, improvements in measuring specimen strain at high temperatures and controlling it by closed‐loop systems (such as truly constant rate) provide a measure of deformation that was not possible in the past (details are provided in Chapter 4). This is the main reason why the above‐described creep and recovery methodology required a new name – SRRT.
To better understand the need for a new name, let us divert our attention a bit to stress relaxation. Engineering components of nuts and bolts face serious problems at high temperatures, because the bolts lose their grip with time. To understand this issue, Stress Relaxation Tests (SRTs) are performed. An SRT is performed by suddenly applying a strain (constraint) and monitoring the decrease in stress with time. SRT is a universally accepted name. To be consistent, why not use “strain relaxation test” (also SRT) for the test in which a stress is suddenly applied and the increase in strain is monitored thereafter? But that term may create confusion. A clear distinction can be made, however, by adding the “hindsight” or the “recovery” aspect of the new test method to the name. Hence, the name SRRT was chosen for the test approach described above.
As mentioned, the SRRT approach was first applied to soda–lime–silica glass in late 1960. It was extended to natural water ice during the late 1970 and finally to a wide‐ranging nickel‐, titanium‐, and iron‐base complex superalloys in late 1990 and early 2000. The authors have not performed SRRTs on geologic materials and are not aware of any SRRT type of test methodology applied to rocks. However, Chopra (1997) reported two CL creep and recovery tests (on full unloading), essentially SRRTs, on an olivine basalt. Chopra focused on the delayed elastic recovery in order to model transient creep, but inadvertently missed the fact that the permanent strains, he reported, could provide a measure of the average viscous strain rate during loading time, equivalent to the reported steady‐state strain rate. This is presented in Figure 5.5. and discussed further in Section 5.6 in Chapter 5.
Significant progress has been made in physically based holistic modeling of microstructure‐sensitive reversible and irreversible deformation and failure processes based on SRRT. The basic principles of the model can be applied to performance problems of wide‐ranging materials at high temperatures. It includes some very new ideas in the field of gas turbine materials engineering, which could have important practical implications if it stands up to the close scrutiny by others working in the field.
SRRT is a novel approach to the way in which creep and creep‐rupture properties are measured and have been measured for a great many years. This book deals with work that has been done at various homologous temperatures and works well for a number of wide‐ranging materials. What is needed now is to understand if there are any limitations to the technique in terms of the temperatures and stresses that can be used. The SRRT method has a major advantage in that it may now be possible to develop cost‐effective stress rupture properties of alloys at relatively lower homologous temperatures; tests that otherwise take a very long time to complete at great expense.
SRRT‐based test technique for measuring elastic strain, ε e, delayed elastic strain, ε d, and viscous strain, ε v, was developed, as mentioned earlier, first for glass, an amorphous medium without any recognized “grain boundaries.” It was extended to polycrystalline ice, and grain‐size effect was introduced. The transparency of pure polycrystalline ice helped in identifying and quantifying stress and temperature dependence of the initiation and the multiplication of intergranular cavities and cracks, and the role of dislocations in the high‐temperature embrittlement processes. During primary creep period, the pile‐up of dislocations against grain boundaries may not be the major source for the initiation and multiplications of intergranular cracks. At a constant temperature, the first intergranular crack forms during the early primary creep when a critical des, ε d c, corresponding to a critical grain‐boundary shearing (gbs) displacement, x c, is reached irrespective of stress. The kinetics of crack multiplication depends exponentially on (ε d –ε d c ) corresponding to (x–x c ). This makes ε d the Achilles heel for creep activities. Yet, historically, materials scientists in general (metals, ceramics, and rocks) have not paid much attention to the primary creep, and certainly not to delayed elasticity. Traditionally the primary focus has been on minimum creep rate. Even though the rate of ε d decreases with time leading to a so‐called steady state, cracks are initiated during the primary creep and void density increases rapidly during the later stage of the primary creep, leading to minimum creep rate and ultimately to failure.
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