Issue 62

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

However, the strain rate is considered as a control factor in the PA6 response and creep behavior. It can be graphically determined for each stress level from the slope of the secondary creep portion of the strain-time curve of the experiment result, Eqn. 7.

      t

(7)

Contrary to the method used in previous research, where the strain rate is calculated in a global way and taking into account only two points of the curve, i.e. the total slope of the curve, in this work, an instantaneous strain rate will be considered in computation of the shift factor. For this purpose, the same numerical extrapolation proposed in rescaling process (Eqn.1) will be derived and adopted to calculate the strain creep variation, Eqn. 8.            1 1 1 2 2 2 ( ) exp exp t A t t t A t t t (8) After that, an average value is then computed over the dwell time for each stress level and substituted in Eqn. 6. Both the rescaling and the logarithmic shifting can be done graphically but it is better to follow a numerical procedure at each stress step. Estimation of shift factor   and t-axis shifted values at each stress level can be obtained following numerical method depicted in Fig. 2.

Start





     , j= , NS , NPS i t(i,j), j ,j= , NS t NPS(j), j= , NS NS rescale 1 1 , 1 , 1 , Read    

 

  1 1 1  

Δ t α σ



  1 1 11  t ,   

, t NPS



j

2

Yes

No

j NS 

    / t ,j

 j

 



 

j   1 1 Δ t



j α σ

t

rescale

Write   , t

1   j j

1  i

End

Nomenclature: NS : Number of load steps NPS : Number of time measurements in each load step (1D table). t : Response time (2D table) i : Time counter j : Load step counter t rescaling : Rescaling time of each step (1D table).   : Shift factor (1D table)

No

Yes

  j i NPS 

      j t i , j t i , j  

1   i i

 



j Δ t

    t , j j , j j t NPS )+ Δ t(j- 1 1       

 

Figure 2: Numerical flowchart of the horizontal shifting.

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