**1. Introduction**

In classical terms, creep is defined as a process that takes place under constant load, which ensures that stresses are usually within the Hooke's law range, and at elevated temperature. The loads applied to products working under creep conditions cause stresses and strains whose values may change over time, not only as a result of overloads occurring during service, but mainly at the time when loading remains unchanged and results in stresses lower than the yield strength [1].

A characteristic feature of creep is that after strain relief of the material in service, the strain decreases and the so-called strain recovery takes place [2].

In the creep of materials, both plastic and reversible strains occur. As a result of the process, their shapes and dimensions change constantly during service. After the elapse of sufficient time of working under creep conditions, the components are degraded and their further service is prevented [3–5]. The degradation processes start at the moment when the rate of strain changes from its minimum and proportional values to disproportional ones as it is increasing [6].

The service within the time as assumed by designers as well as the extension of the lifetime of the materials and products made from them which have worked the design service life are most often determined by the calculation method based on creep strength data and positive results of comprehensive mechanical tests [7–11]. Among materials, the products operating under creep conditions are of particular importance. In their evaluation, the condition assessment of their materials is important and necessary [12–16]. It is carried out using destructive and nondestructive materials testing the result of which is referred to the available characteristics of materials both in the as-received condition and after service [17–22]. The results of these tests, supported by the extensive material characteristics database, allow for good estimation of the material condition and degree of exhaustion and determination of the time of further trouble-free service until the next scheduled inspection.

However, in a number of cases, to reach the design service life for products working under creep conditions, not only good estimation of their service life but also its determination based on destructive tests in a selected representative test area is required.

Yet, it is not always possible in practice. Therefore, the aspect of knowledge (experience), frequently acquired during long-term scientific and industrial practice, and also supported by thorough cost-effectiveness calculation made for such a procedure, is also important.

The problem in evaluation of creep resistance of materials operated under creep conditions for a long time is the time necessary to perform creep tests for the assessment of performance within their design service life and frequently also of the residual life, i.e., the remaining time of service beyond the design service life.
