PSI - Issue 75

Benjamin Causse et al. / Procedia Structural Integrity 75 (2025) 205–218 Author name / Structural Integrity Procedia (2025)

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3.2. Influence of biaxiality (k=  II /  I ) and average (mean) stress (  m ) Figure 5 presents the results of service life as function of factor k=  II /  I and average stress effect  m /  R (directly linked to ratio R = (  m -  R )/(  m +  R ), and remember  =2  R ). The results are shown for  =2  R =200 MPa, but the charts can be used safely for lower values.

Fig. 5. Bi-axiality and average stress effects on service life according to Dang Van compared with uniaxial Eurocode – for a proportional signal ; and application example for k=  II /  I =0.72 and (  m /  R )=1.6, then N DV  /N E =64%.

• Biaxiality effect : k=  II /  I First of all, for k=  II /  I = 0, it can be verified graphically that the calibration in R 0 of the Dang Van criterion is indeed carried out on the basis of the Eurocode S-N curves considered obtained in alternating pure tension (Causse et al. 2024). In fact, for a pure alternating tension test ((  m /  R )=1, i.e. R = (  m -  R )/(  m +  R )=0), our results give the same service life with Dang Van as with the Euocode : N DV /N E = 1. By varying k,  II changes, and so k=  II /  I changes to. There is a difference in results between N DV and N E . N E does not change when  II changes because the Eurocode is uniaxial, whereas N DV changes when  II changes : N DV service life therefore decreases. Even more precisely : for a state of tension in the first main direction (  I >0), if compression is applied in the second main direction (  II <0), the cracks will tend to open more quickly than if tension is added (  II >0): this phenomenon is clearly shown in the graphs: when k path is 0 to -1, N DV /N E decreases more rapidly than when k path is 0 to 1. Finally, the Dang Van criterion takes good account of the variation in principal stress in the second direction. The effect of the average stress is measured by the change in the ratio  m /  R (directly linked to the ratio R = (  m -  R )/(  m +  R )). The Dang Van criterion shows a reduction in service life (N DV /N E < 1) when the average stress is a tensile stress, and conversely, an increase in service life (N DV /N E > 1) when the average stress is a compressive stress. • Average stress effect :  m /  R (directly linked to ratio R = (  m -  R )/(  m +  R )).