NON LOCAL APPROACH FOR PREDICTION OF DELAMINATION ONSET P. Nimdum 1* - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS NON LOCAL APPROACH FOR PREDICTION OF DELAMINATION ONSET P. Nimdum 1* , J. Renard 1* 1 Mines-Paris Tech, CNRS UMR 7633, BP 87, F-91003 Evry, Cedex * Corresponding authors ( pongsak.nimdum@ensmp.fr, jacques.renard@ensmp.fr ) Keywords : Thick composite, Criterion, Delamination, Interlaminar stresses Abstract delamination, fiber breaking, fiber/matrix interface debonding …) are acting alone or coupled. This study is mainly focused on inter-ply delamination The objective in this study is to predict initiation of which could be more critical for structural parts and delamination in epoxy reinforced carbon fiber of 2/2 then has definitely to be avoided. twill weave fabric composites laminates during However, the application of unidirectional static testing. First, Micromechanical three- composites has several drawbacks such as impact dimensional finite element models of the twill weave resistance and tolerance in presence of a woven fabric are proposed to calculate ply delamination. Therefore, the trend for composites equivalent behaviour using homogenous process. applications is undergoing a transition towards the Then, we shall propose a criterion for onset of use of textile composite, also known as “woven delamination under static loading. This criterion is fabric composites”. These materials present various based on average values of the components of the attractive [1-3] since it provides improved impact stress field and the law of Coulomb describing the resistance, better in out-of-plane mechanical friction between two bodies following assumptions: properties and improved damage tolerant in the (i) a normal negative strength (compression) delays presence of the delamination due to the non-planar the delamination onset in shear (modes II and III); interply structure of woven fabric composites. (ii) on the contrary, a positive normal stress Nevertheless, the stiffness and strength behaviour of accelerate the delamination onset and (iii) a normal woven fabric composites are dependent on many negative strength is shear equal zero, cannot provoke parameters such as the characteristics of fibers and the delamination onset. Identification of the different matrix and weave architecture [4] (weave type, parameters of this criterion has been made with packing density of yarns, undulation angle etc.). experimental Edge Delamination Tests (EDT). Interlaminar normal and shear stresses can make Validation was made with tensile tests performed on edge delamination to appear, even up to laminate’s angle-ply textile laminates with drilled circular hole. failure. An efficient method for predicting Further numerical predictions are in good agreement delamination onset is thus needed. Two basic with experimental results. methode may be used for the delamination onset 1. Introduction prediction: the stress method or the energy release rate one. Stress method requires interlaminar stresss Thick composite laminated structures able to support to be determined at each interface and then to be significant efforts, are more and more used for compared with the material’s strength engineering structural parts. Then it is necessary to characteristics. However, this methode usually consider the ability of such laminates to resist from requires the help of the finite element method with damage development, the consequence of which is fine meshing and supposes some specific ability. mechanical degradation of properties (stiffness Most of stress criteria are expressed with average decrease). Thus, it requires appropriate design tools interlaminar stress [5-7] and the influence of a to prevent from damage evolution and predict the compressive stress stress is assumed to be negligible influence of damage on mechanical properties . [6-7]. Lagunegrand et al. [8] shown that the Damage mechanisms up to failure are rather compression should delay the delamination onset. complex in composites laminates. One reason is that The objective in this study is to predict initiation of several damage phenomena (matrix cracking, delamination in epoxy reinforced carbon fiber of 2/2

twill weave fabric composites laminates during become unstable and lead to the final failure of static testing. specimen. 2. Experimental procedure 2.1 Material The composite material of this study was carried out on carbon (T800)/epoxy composite material. The carbon fiber density was 12.81 g/cm 3 . This study focuses on woven fabrics composites which the interlacing of the fill and warp yarns was formed (a) according to the 2/2 twill weave pattern. The presence of weave structure induces very specific physical phenomena. We study on woven angle-ply ° ± ° ± laminates ( 0 , 20 n ) , ( 0 , 30 n ) and s s − ° − ° − ° ( 30 , 10 , 50 ) with n = 1 and 2. One ply n n s thickness is 0.65 mm and the fiber volume fraction = is V 52 . 9 % . f 2.2 Experimental procedure (b) All specimens were cut from plates using the Fig.1. (a) Mixed-mode and (b) open-mode diamond wheel saw and were bonded with delamination during static loading. glass/epoxy or aluminium tabs onto each specimen end. During tensile tests, the specimen surface Table 1 (length 65 mm) is recorded at different loading with Experiment result on angle-ply laminates a digital CCD camera under white light illumination. σ Interface Experimen Laminate 3. Experimental result onset of delamination (MPa) ° ± ° + ° − ° 3.1 Damage mechanism on angle-ply ( 0 , 20 ) 20 / 20 425-450 s ° ± ° + ° − ° ( 0 , 20 ) 20 / 20 312.5-337.5 2 2 s 2 2 In generally, due to edge effect lead to free-edge ° ± ° + ° − ° ( 0 , 30 ) 30 / 30 405-430 s stress singularity at interface of adjacent layers and ° ± ° + ° − ° ( 0 , 30 ) 30 / 30 287.5-321.5 result in the onset of delamination, also called 2 2 s 2 2 − ° − ° − ° − ° − ° ( 30 , 10 , 50 ) 10 / 50 270-295 interlaminar delaminattion. Fig. 1 illustrate the onset s − ° − ° − ° − ° − ° + ° − ° ( 30 , 10 , 50 ) 10 / 50 265-290 delamination at interface 20 / 20 and 2 2 s 2 2 n n − ° − ° 10 / 50 . These delaminations are not straight n n 4. FEM analysis (plan) but bended. They propagate to follow the interface of adjacent yarns and the crimp yarns. 4.1 Homogeneous procedure These delaminations are considered as shear mode In order to take place heterogeneous woven ply by (mode II and III) for the first and the second one but equivalent homogenous ply, a homogenization as a mixed-mode for the last one, respectively. Table procedure in the twill-weave unit cell (9.2x9.2mm) 1 shown the experimental result of onset of (Fig. 2) is used in this study. First, 3D finite element delaminates at free edge in angle-ply. taken into account the effects of yarns interlacing After the interlaminar delamination appeared, we and orientation of adjacent layers. The mechanical investigate on stiffness degradation and find that the response of a yarn is determined and validated by modulus decrease can be divided into three stages: tensile tests. The yarn (straight region) was assumed (i) initial region with a slightly decrease stiffness transversely isotropic and linear elastic and is given reduction of about 2% - 4%, and then, (ii) a rapid in Table 2, while the property of the epoxy matrix is decrease of stiffness (about 10%), and finally