International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ Fatigue analysis of CFRP composite laminates with circular cutouts 1 Abhilash S Mirji, 2P.K. Sahoo, 3B. Raghuvir Pai M. Tech Graduate, MIT Manipal, Senior Scientist, STTD, CSIR-NAL, Professor, MIT, Manipal Email: 1abhilashmirji002@gmail.com, 2pks@nal.res.in, 3raghuvir.pai@manipal.edu. Abstract— The aerospace structural components, during the service of aircraft, are subjected to fatigue loadings. Accurate fatigue evaluations of those components are very essential for assessing the structural integrity of the aerospace structures. In the present investigation, fatigue analyses of composite laminates with circular cutouts are performed at stress ratio of 0.1 and frequency of 10Hz. The composite laminates are made up of carbon-epoxy AS4/3501-6 and of different stacking sequences such as [0/902]s, [0/904]s, [02/90]2s and [45/-45]2s First, two dimensional finite element analyses on such laminates have been conducted. Then these stress analysis results are post processed to predict fatigue life of laminates with cutouts using ABAQUS and FE Safe finite element tool. The analysis results were compared with results available in the literature for all the laminate configurations. Both the results are found to be in good agreement. Index Terms— Composite materials; 2D finite element; Stress analysis; Fatigue analysis. I. INTRODUCTION Composite materials have recently replaced conventional metals in application to related industries. This class of materials has offered substantial improvement over conventional metal applications in advanced engineering and high performance aerospace structures. Even though being used extensively, they are not immune from degradation and damage. Infact fatigue failure is one of the main failure mechanism associated with composites. Varvani-Farahani et al. distinguished damage regions as the number of cycle propagates to failure i.e. matrix, fiber- matrix interface and fiber regions. [1] Lian and Yao performed 2D finite element analysis for plain E-glass/ epoxy and simulated fatigue failure. The 'ABAQUS' FEA package was employed for this purpose and individual plies were analysed. [4] The above review of literature reveals the scarcity of 2D finite element simulation of fatigue failure. In the present work, fatigue life of carbon-epoxy laminates are predicted taking into account stress analysis. The damage modes are also obtained from FE Tool. II. MULTICONTINUUM THEORY The fundamental premise of continuum mechanics is that any physical quantity of interest is evaluated at a material point by averaging the quantity over a representative volume element that surrounds the point of interest known as representative volume element (RVE). In the case of a typical unidirectional FRC, the fibers tend to have a somewhat random spacing within the matrix material; therefore, the RVE used to characterize a material point must be large enough to contain numerous fibers in order to provide an accurate statistical representation of any quantity that is averaged over the RVE. The concept of a multicontinuum simply extends the notion of a continuum to reflect distinctly different materials that coexist within the representative volume element used to characterize a material point. Unidirectional fiber-reinforced composite is viewed as two interacting continua (Fiber and matrix continuum). In such a representation, three different volume averages are relevant to the mechanics of the composite material as mentioned below P. Papanikos et al [2] developed a fatigue damage model for predicting the fatigue failure and life of carbon-epoxy laminates with different stacking sequences. Analysis was performed by developing 3D element in ANSYS FE code Naderi and Maligno developed 2D finite element model to simulate fatigue failure of different unidirectional and multidirectional plies. [3] Multicontinuum Theory traditional continuum mechanics by expanding the focus to include two additional issues: Composite average quantity or homogenized average quantity indicated by superscript c. Fiber average quantity indicated by superscript f. Matrix average quantity indicated by superscript m. _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-5,6, 2014 83 International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ The development of relationships between the various constituent average quantities of interest Developing relationship that links the composite average quantities to the constituent average quantities. 3. Kinetic theory of fracture V. FINITE ELEMENT ANALYSIS The element type chosen is conventional shell element S48- A four node doubly curved thin or thick shell with six degrees of freedom at each node (three translations and three rotations). The number of nodes and elements for laminates with cutout it is 689 and 616 respectively. The finite element model of laminate with circular and with boundary conditions is shown in figure 1. The kinetic theory of fracture (KTF) describes the process of bond breaking via thermally activated processes. Helius: Fatigue uses KTF to predict fatigue failure in the matrix constituent, which translates to composite failure. The bond breakage rate due to thermal energy is given by Equation 1 [5] Kb kT U exp h kT (1) The effects of applied stress on bond breakage rate are given by equation 2 [5] Kb kT U exp h kT (2) (a) FE model (b) FE model with BC’s The base line equation for determining fatigue life of composites is shown in equation 3. [5] Figure 1: Finite element model with boundary conditions applied on it t eff kT U n(t ) n0 n01 (1 ) exp exp d (3) h kT 0 kT VI. RESULTS AND DISCUSSIONS PROBLEM DEFINITION Composite laminates made of CFRP AS4/3501-6 material of dimensions 50*10*0.127 (each lamina thickness) and following stacking sequences of [0/902]S, [0/904]S, [02/902]S, and [45/-45]2s is considered to perform fatigue analysis. The properties are tabulated in table 1. The multidirectional lamina properties shown in table indicates that lamina is transversely isotropic because elastic properties in second and third direction is same i.e. E2=E3 and also G12=G13. But laminate is orthotropic, because elastic properties in all three directions are different. Stress analysis for composite laminates was carried out in ABAQUS and the results are as shown in figure 2 (a)- 2(d) below. (a) [0/902]s (b) [0/904] Table 1: Material properties of AS4/ 3501-6 [3] E1 (MPa) E2 (MPa) E3 (MPa) G12 (MPa) G13 (MPa) G23 (MPa) ν12 ν 23 ν 13 138*103 9*103 9*103 5.7*103 5.7*103 3*103 0.3 0.45 0.3 (c) [02/902]s (d) [45/-45]2s _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-5,6, 2014 84 International Journal of Electrical, Electronics and Computer Systems (IJEECS) _______________________________________________________________________________________________ Figure 2: Stress analysis results for laminates with circular cutout The above stress results are post processed in FE safe to obtain fatigue life of composite laminates with cutout as shown in figure 3(a)- 3(d) respectively. (c) [02/902]s (d) [45/-45]2s Figure 4: S-N plots for different laminates with circular cutout CONCLUSIONS: (a) [0/902]s (b) [0/904]s (c) [02/902]s (d) [45/-45]2s Figure 3: Fatigue analysis results for laminates with circular cutout. Similarly fatigue life cycles to failure were obtained at different stress levels by changing the boundary conditions optimistically to obtain S-N plots for above laminates. These results were compared with results obtained from literature. Both are found to be in good agreement. Fatigue failure analysis is carried out for AS4/ 3501-6 carbon/ epoxy laminates subjected to tensile cyclic loading conditions with stress ratio as 0.1 and frequency as 10 Hz. Fatigue life for different stacking sequences is predicted and compared with available results from literature [3] as in figure 4(a)-4(d) . It was found that stress values were high at cutout sections were failure is possible very easily and as well as magnitude of fatigue cycles to failure was also high at cutout region. This observation concludes that fatigue crack is initiated at cutout region and this crack will propagate to failure as number of cycle increases. S-N curves were plotted for different stress levels and were compared with results available in literature [3]. Both were found to be in good agreement. REFERENCES [1] Varvani-Farahani, H. Haftchenari, M. Panbechi.” An energy-based fatigue damage parameter for off-axis unidirectional FRP composites”. Composite Structures 79 (2007) 381–389. [2] P. Papanikos et al. “Modelling fatigue damage progression and life of CFRP composites”. ISTARM, Institute of structure and advanced materials, Patron-Attinan. Greece [3] M. Naderi and A.R. Maligno. “Fatigue life prediction of carbon/epoxy laminates by stochastic numerical simulation”. Composite structures 94(2012)1052-1059. [4] Wei Lian, Weixing Yao.”Fatigue life prediction of composite laminates by FEA simulation method”. International Journal of Fatigue 32 (2010) 123–133. [5] FE Safe composite theory manual, edition 2010. (a) [0/902]s (b) [0/904]s _______________________________________________________________________________________________ ISSN (Online): 2347-2820, Volume -2, Issue-5,6, 2014 85