PSI - Issue 65

Maslov S.V. et al. / Procedia Structural Integrity 65 (2024) 139–146 Author name / Structural Integrity Procedia 00 (2024) 000–000

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Fig. 2. Determination of the strain gauges creep limit during cyclic tests: (a) – relative decrease in the output signal: (b) – building functions for determining the creep limits.

4. Accounting and minimization of strain gauge creep when processing the findings of SSS field studies

The above analysis shows that it is impossible to directly use formula (4) to estimate creep under variable deformations occurring in a full-scale structure. At the same time, expression (2) cannot be applied either due to no data on defining the function in case of an arbitrary nature of the stress change function in a full-scale structure. In this regard, a combined method for accounting for the creep of strain gauges is proposed, based on several estimation options for constant and cyclically varying deformation. Let us consider the application of this method using the example of estimating the strain gauge creep for a deformation change function having a time derivative variable in sign (Fig. 3).

Fig. 3. Assessment of strain gauges creep for the analyzed loading process.

The function of changing the output signal of the strain gauge obtained from a field experiment is shown in Fig. 3 as a solid line. The function has 1 maximum and 1 minimum, which are marked with round marks on the graph. At the end of the analyzed process marked on the graph with a triangle, the rate of deformation change decreases. To estimate the creep of the strain gauge at the point of a positive maximum, an envelope function is used, which has the shape of a sine wave (curve 1), and reaches a maximum at time τ1. Curve 1 intersects with the time axis at points τ0 and τ2. The creep value at time τ1 is determined using the graphs shown in Fig. 2b for the maximum temperature acting on the strain gauge in the range from 0 to τ2. In a similar way, the envelope function (curve 2) is constructed for the second extremum (local minimum) of deformation. At the end of the analyzed process, in addition to constructing the envelope function (curve 3), it is also advisable to perform an estimate in the form of the creep sum at the point of the local maximum and the nominal creep value of the strain gauge calculated by formula (4) for the highest temperature in the area from τ 2 to τ 3.