Issue 38

D. Carrella-Payan et alii, Frattura ed Integrità Strutturale, 38 (2016) 184-190; DOI: 10.3221/IGF-ESIS.38.25 185 I NTRODUCTION he increase of lightweight material in transportation industries is today facing more than ever questions on fatigue life prediction of composite structures. The critical step towards accurate prediction is to reproduce the loading conditions undergone by the composite component. In automotive application, the challenge is related to the variability of those conditions: multi-axial and variable amplitude on long duration fatigue loading. This is why Siemens PLM software has developed an innovative composite fatigue CAE methodology (patent pending) keeping track of the material degradation under such conditions. Sevenois [1] reviewed and compared the state of the art for fatigue model techniques of woven and UD composite. The study concluded that out of the four modelling methodologies (fatigue life, residual strength, residual stiffness and mechanistic model) the residual stiffness models are suitable for mechanical performance using experimental data and can also be combined with residual strength approach. The presented methodology is based on residual stiffness fatigue law combined with an efficient damage operator approach to calculate the residual stiffness. This approach will be able to perform fatigue simulations for variable amplitude loads and will allow ply-stacking optimization without additional testing or material characterizations. I NTRA -L AMINAR F ATIGUE S OLUTION W ITH D AMAGE J UMP he intralaminar fatigue model strategy is herein presented in the following order: definition of the stiffness degradation law, then calculation optimization algorithm (N-Jump and damage jump combined to the damage operator) and finally, description of parameter identification procedure used in Siemens PLM commercial software. Fatigue and Stiffness Degradation – Theory Fatigue damage laws The damage evolution law is based on the work of Van Paepegem for woven glass fibers [2]. Three intra-laminar damage variables D11, D22 and D12 are defined at ply level and linked to the stress tensor by the following behavior law, Eq.(1)   p HCH      (1) where C is the Stiffness tensor, εp is a permanent strain tensor and H is defined as D D D 11 22 12 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 0 0 H 0 0 0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1                         (2) In [2] the damage variables D ij were split into a positive and a negative part ( ij d  and ij d  ), where the positive part increases when the stress is positive, and the negative one when it is negative. At the end the two parts were added, including a crack closure coefficient for combination of tension of compression. In this work, only positive stress ratios are used, which means that there is no switch between tension and compression over a cycle and simplifies the problem. Either the stress is always positive and the damage D ij is equal to ij d  ; or it is always negative and it is equal to ij d  , Eq.(3) . Besides, the formulations of Van Paepegem [2] must be adapted to unidirectional plies. First of all, to account for the high in-plane orthotropy of unidirectional plies, independent c i parameters are defined for the three components of the damage. Therefore, fifteen parameters are used ( c i,jk ) instead of five. For the same reason, the coupling between D 11 and D 12 which T T