18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS TRANSLAMINAR PLY FRACTURE TOUGHNESS OF ADVANCED COMPOSITES R. F. Teixeira 1 *, S. T. Pinho 1 , P. Robinson 1 , 1 Department of Aeronautics , Imperial College London , London, UK * Corresponding author (rita.teixeira07@imperial.ac.uk) Keywords : Translaminar Fracture Toughness, Ply failure, Compact tension, R-Curve, Fractography. 1 Summary the properties of the fibre, matrix and their interface The translaminar fracture toughness of fibre [5]. reinforced polymers (FRP) is important to Cruse and co-workers investigated several angle-ply characterise the failure resistance and response of laminates to investigate [6] and predict [2] the notched composite structures. fracture strength of notched carbon/epoxy laminates. Compact Tension (CT) tests of UD , [ They concluded that fibre diameter and ply thickness ] , [ ] and [ did not affect considerably the toughness and that ] laminates were conducted in order to LEFM could be applied, as long as the damage zone size is small when compared to the initial crack investigate the crack development and measure the length (and other specimen dimensions) and crack translaminar fracture toughnesses values for three plies of interest: , and . A quasi-isotropic propagation is co-planar with the initial crack (mode layup with , and plies was also I failure). Crack growth in isotropic materials, such as metals under Mode I loading, is usually co-planar investigated. with the initial notch. However, in composite Testing was monitored with digital image laminates, fracture mechanisms such as fibre failure, correlation (DIC). The test method and data matrix cracking, delamination and bridging, very reduction scheme are the result of extensive often lead to non co-planar propagation. background work [1]. Failure sequence was analysed Cruse [2] proposed that all plies should fail using optical micrography, SEM, C-Scan and X-ray. simultaneously and that each ply can be considered to separately while the crack propagates. 2 Introduction Waddoups et al. [7] used a stress intensity factor The fracture toughnesses in fibre reinforced (SIF) approach that relied on a modified effective polymers (FRP) play a key role in determining the crack length to predict the fracture toughness of a damage tolerance of composite structures and their notched specimen in laminated composites. response during damage propagation . To Pinho et al. [3] measured the fracture toughness characterise damage tolerance in composites, there is associated with mode I fibre tensile of a a need for reliable experimental procedures [1]. carbon/epoxy crossed ply laminate using compact Numerous studies have measured the translaminar tension specimens. The authors found that fracture fracture toughness of composite laminates [2], as toughness associated with mode I longitudinal reviewed in [4]. However, there have been few matrix failure was similar to the mode I interlaminar studies that have attempted to measure the fracture toughness of the carbon/epoxy system used. translaminar fracture toughness based on its This paper aims to relate the translaminar toughness individual plies and most studies only include of individual plies to the translaminar toughness of initiation values [2]. the laminate, by obtaining the resistance curves (R- The energy-absorbing mechanisms involved in the curves) both for the plies and laminate from the process of translaminar failure of a composite ply experimental data. The fracture surfaces were are matrix cracking, matrix-fibre debond and pull- examined using SEM to find relationships between out of individual fibres and bundles of different the features of these surfaces and the experimental lengths, and these depend on the constituents data. properties. These processes are responsible for the toughness of a composite and they can be related to
3 Experimental method and data reduction energy release rate was calculated using the change in compliance, , with crack length : Compact Tension (CT) specimens [3] with (1) dimensions and orientation shown in Fig. 1 were used. The compliance calibration curve was obtained by The material system used in this work was the FE, in approximately 1 mm increments, to capture unidirectional carbon-epoxy prepeg T800s/M21 the full compliance versus crack length response, for each layup of interest. The calibrated vs. data manufactured by Hexcel. The elastic properties were measured using standard tests (Table 1). was plotted and fitted according to: All the laminate configurations were manufactured (2) using hand layup and cured in an autoclave where , , and were calculated to best fit the according to the manufacture recommendations. A experimental data. These constants were used with wet saw was used to cut the rectangular plates into the experimental compliance to calculate the the specimen ’ s geometry (Fig. 1). The 8 mm holes corresponding crack length. were drilled using a carbide tipped drill. The notch For the fracture toughness tests, an effective crack was machined using a diamond coated disk-saw to length, , could be determined using the elastic guarantee an accurate and sharp crack tip. compliance measured from the load displacement Micrographs of the crack tip are presented in Fig. 2. curve of the test specimens: Six CT specimens of each layup were tested using an Instron machine with a 10 kN load cell, except (3) for the [ ] layup where a 1 kN load cell was used The critical strain energy release rate was then since much lower loads were expected for these determined using: specimens. Each specimen was loaded to a constant displacement rate of 0.5 mm/min with being in (4) the direction of the applied load. One of the main advantages of this data reduction All the experiments have been monitored with method is that it does not rely on the observation of digital image correlation (DIC). The ARAMIS DIC the surface of the specimen. The external plies of the camera was mounted on a tripod, which was specimen do not reflect the actual crack tip growth positioned facing the testing machine, perpendicular since the crack front is not constant. to the surface of the specimen. Two spotted stickers Once the critical energy release rate for the laminate were attached to the pin loads in order to better is obtained, the critical energy release rate for the identify the specimen displacement during testing. ply of interest is calculated by accounting for the Testing was conducted to investigate the toughness corresponding to matrix cracking in the layers. This procedure neglects other damage translaminar fracture toughnesses for each ply of interest, , and as well as quasi-isotropic modes such as delamination, as well as any laminates: , [ ] and [ interaction between matrix cracking and the fibre ] , respectively. A fifth layup dominated failure modes, and assumes that a single [ ] was analysed to better matrix crack parallel to the pre-crack occurs in the understand the failure interaction between and layers. This approximation appears to be plies. acceptable as the toughness of mode I fibre tensile failure contribution is considerably more important A compliance calibration method [1] was used for than that of the matrix. It has been proposed [2] that data reduction. This method used Finite Element a through thickness mode I crack for a general analysis to infer the crack length corresponding to orthotropic laminate can be related to the each stage of crack propagation from the corresponding ply simply by a law of mixtures experimental elastic compliance. A half CT expressed as: specimen was modelled using the FE ABAQUS ∑ package. Models were 1 mm thick and a 1 N load (5) was applied at the position of the loading pin. The represents the critical strain energy where elastic properties for each layup were calculated release rate for the laminate, represents the according to lamination theory. The critical strain
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