PSI - Issue 75

Laurent Gornet et al. / Procedia Structural Integrity 75 (2025) 129–139 Author name / Structural Integrity Procedia (2025)

131

3

axis. If the laminate exhibits two main damage mechanisms (for example, matrix cracking and matrix plasticity), the method consists in plotting the asymptote at the origin and the asymptote corresponding to the first slope change in the curve. The endurance limit is then determined at the intersection of these two asymptotes. Matrix cracking and matrix plasticity can lead to the appearance of a hysteresis loop in the stress-strain response, modeling is presented in Krasnobrizha et al 2018. The average loading conditions of self-heating tests for each laminate configuration are listed in Table 2. This table presents the average stress applied at the laminate level in the tensile direction ( ) and the average stress at the ply level in the tensile shear direction ( 12 ) for the ±45° plies and ( 22 ) for the 90° plies. The quasi-isotropic laminate [+45/-45/90/0] S was tested at a load level that results in stress levels in the ±45° and 90° plies corresponding to those tested in the [+45/-45] 2S and [0/90/] 2S laminates, respectively. The self-heating curve and the fatigue limit are shown Fig. 1a. The apparent average stress is given in Table 1 under the assumption that linear elasticity is applicable at the limit of fatigue. For the quasi-isotropic laminate, the endurance limit defined at 5 million cycles is 400 MPa for an apparent axial stress of 250 MPa. The fatigue limit obtained for the [+45/ – 45/90/0] ₛ laminate was consistent with those measured on elementary configurations, namely [+45/ – 45] ₂ ₛ and [0/90] ₂ ₛ coupons. Fig. 1 presents the main self-heating curve for the quasi- isotropic laminate [+45/−45/90/0] S showing the variation of temperature as a function of the maximum stress amplitude. Fig. 1b shows the relationship between the applied maximum stress and the number of cycles to failure. These plots correspond to what is commonly known as Wöhler curves (or S-N curves).

Table 2. Fatigue loading conditions studied as a function of laminate configurations. Fatigue limits obtained by the [+45/-45/90/0] S laminate is in agreement with those obtained on elementary coupon [+45/-45] 2S and [0/90] 2S

Fatigue tests on laminate specimens

Imposed loading stress =50 =335 =250

Stress at the ply scale ply 45°: 12 =25 12 =34.5

Experimental Limit of fatigue Self heating ply 90°: 22 =71 ply 45°: 12 =32 ply 90°: 22 =69

[+45/-45] 2S

ply 90°: 22 =41 ply 45°: 12 =25 ply 90°: 22 =42

[0/90] 2S

[+45/-45/90/0] S

Fig.1. (a) Self-heating curves of the [+45/-45/90/0] S laminate under an average stress = 250 . The self-heating fatigue limit is 371 MPa. (b) Fatigue tests on the [+45/-45/90/0] S laminate; The fatigue limit is 371 MPa. The fatigue limit derived from the self-heating method matches that obtained through classical fatigue tests (Westphal 2014).