Issue 62

M. Tedjini et alii, Frattura ed Integrità Strutturale, 62 (2022) 336-348; DOI: 10.3221/IGF-ESIS.62.24

14.27 days approximately (t=1.233  10 6 s). However, a significant improvement can be shown by the power function, where t=2.315  10 7 s, (8.92 months) is obtained. In contrast, a long creep time is estimated at about 2.45 years (t=7.624  10 7 s) using the exponential function.

1,6

4,0

SSM Experimental result

5 MPa 10 MPa 15 MPA 20 MPa 25 MPa 30 MPa

3,5

1,4

3,0

1,2

2,5

Creep strain (%)

Creep strain (%)

1,0

2,0

0,8

1,5

1,0

0,6

0,5

0,4

0,0

0

10000

20000

30000

40000

50000

0

10000

20000

30000

40000

50000

Time (s)

Time (s)

(a)SSM test

(b) Elimination elastic zone and vertical shifting

1,6

1,6

Polynomial Power Exponential

Polynomial Power Exponential

1,4

1,4

30 MPa

30 MPa

1,2

1,2

25 MPa

1,0

Creep strain (%)

25 MPa

20 MPa

1,0

0,8

15 MPa

20 MPa

0,8

0,6

10 MPa

5 MPa

Creepstrain(%)

15 MPa 10 MPa

0,4

 0 =0.37 (%)

0,6

t resc (polynomial) t resc (power) t resc (exponential)

0,2

5 MPa

0,4

0,0

0

10000

20000

30000

40000

50000

0

2000 4000 6000 8000 10000 12000

Time (s)

Time (s)

(c) Extrapolation and rescaling curves

(d) Rescaling curves

Figure 4: Required handling of the raw data of SSM

Rescaling time (h) polynomial

Rescaling time (h) Power law

Rescaling time (h) Exponential

Load (MPa)

5

-0.2917 1.3750 3.1306 4.8667 6.7222 8.5389

0.0016 1.7914 3.6851 5.5500 7.4322 9.2080

-0.0056 1.8194 3.7889 5.6722 7.6139 9.5083

10 15 20 25 30

Table 1: Rescaling time (t rescaling )

This large discrepancy between the third-order polynomial, power law and exponential models can be related to the accuracy of the rescaling time assessment in the beginning of each extrapolation functions. An inadequate value has a significant influence on the horizontal rescaling and shifting processes and affects, therefore the final master curve obtained by each model. However, a numerical criterion that could be helpful to judge the quality of the predictions, assumes that the rheological model of the creep behavior should be operative at all stress level. It found that the third order polynomial is less able to imitate this behavior in the reference stress (Tab. 1). In contrast, the exponential (Prony series approximation) and the power law models that fulfill this assumption, and satisfy the zero time value for zero creep strain, can be considered the better prediction.

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