Mode-I interlaminar fracture of carbon/epoxy cross-ply composites (original) (raw)
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Mode I interlaminar fracture of carbon/epoxy multidirectional laminates
Composites Science and Technology, 2004
An experimental study was conducted on the mode I interlaminar fracture of carbon/epoxy multidirectional specimens with starter delaminations in 0/0 and 0/θ interfaces. The specimens selected for Double Cantilever Beam (DCB) tests had [(0 2 /90) 6 /0 2 //(0 2 /90) 6 /0 2 ] and [(0 2 /90) 6 /0 2 //θ/(0 2 /90) 6 /0 2 ] stacking sequences, where // denotes the position of the starter delamination and θ = 22.5 to 90 degrees. The incorporation of the thick symmetric [(0 2 /90) 6 /0 2 ] block of 20 plies aimed at minimising elastic couplings related problems, such as non-uniform width-wise distributions of the strain energy release rate G I , spurious modes and effects of residual stresses. This was verified in preliminary three-dimensional (3D) Finite Element (FE) analyses, which also showed the applicability of the Corrected Beam Theory data reduction scheme. The DCB specimens were tested according to the procedures defined in the ISO 15024:2001 standard for unidirectionally reinforced specimens. In the tests, intraply damage occurred in the θ-oriented ply soon after initiation from the starter crack, as it is commonly observed in multidirectional specimens. For θ = 22.5 and 45 degrees, a second crack propagated close to a neighbouring interface, while for θ = 67.5 and 90 degrees the starter crack ran inside the θ-oriented ply and proceeded in a wavy propagation trajectory. This resulted in extensive fibre bridging and pronounced R-curves with artificially high final values of the critical strain energy release rate, G Ic. Therefore, only G Ic values of initiation could be considered true interlaminar properties. Moreover, it was found that they were fairly independent of the ply angle θ. An additional FE analysis of the interlaminar stresses ahead of the crack tip was performed to interpret the delaminating interface effect on G Ic values. The results indicated the existence of an interface independent fracture toughness, K Ic , probably resin controlled, thus reinforcing the idea that the measured initiation G Ic were pure interlaminar properties.
Interlaminar Fracture Characterization of a Carbon-Epoxy Composite in Pure Mode II
Materials Science Forum, 2010
The interlaminar fracture toughness in pure mode II (G IIc ) of a Carbon-Fibre Reinforced Plastic (CFRP) composite is characterized experimentally and numerically in this work, using the End-Notched Flexure (ENF) fracture characterization test. The value of G IIc was extracted by a new data reduction scheme avoiding the crack length measurement, named Compliance-Based Beam Method (CBBM). This method eliminates the crack measurement errors, which can be nonnegligible, and reflect on the accuracy of the fracture energy calculations. Moreover, it accounts for the Fracture Process Zone (FPZ) effects. A numerical study using the Finite Element Method (FEM) and a triangular cohesive damage model, implemented within interface finite elements and based on the indirect use of Fracture Mechanics, was performed to evaluate the suitability of the CBBM to obtain G IIc . This was performed comparing the input values of G IIc in the numerical models with the ones resulting from the application of the CBBM to the numerical load-displacement (P-) curve. In this numerical study, the Compliance Calibration Method (CCM) was also used to extract G IIc , for comparison purposes.
Mode-II interlaminar fracture toughness of carbon/epoxy laminates
Iranian Polymer Journal, 2003
The interlaminar fracture behaviour of unidirectional carbon/epoxy composites has been studied under flexural loading by using end-notched flexure (ENF) speci-mens. GIIc values were calculated as total fracture toughness energy at the maxi-mum load sustained by the materials as ...
Engineering Fracture Mechanics, 2019
The dynamic propagation behavior of the mode I interlaminar crack in unidirectional carbon/ epoxy composites was investigated by using double cantilever beam (DCB) specimens. The dynamic interlaminar propagation toughness was obtained using a hybrid experimental-numerical method. Using a novel electromagnetic Hopkinson bar system, pure mode I fracture was guaranteed by symmetrical opening displacement rates in the range of 10-30 m/s. The crack velocities before crack arrest were between 100 and 250 m/s, which was monitored by crackpropagation gauges and high-speed photography. To model the interlaminar crack, a user-defined cohesive element was developed, which integrated the experimentally measured crack propagation history. The propagation toughness was calculated by the energy balance method and the dynamic J-integral technique. Results from extensive studies indicate that the dynamic propagation toughness is not a single-valued function of the crack velocity for mode I interlaminar crack. Both the external dynamic loads and the interaction with the bending waves emanating from the moving crack tip affect the behavior of the crack propagation.
Mode II interlaminar fracture of carbon/epoxy multidirectional laminates
Composites Science and Technology, 2004
This paper reports an experimental study on the mode II interlaminar fracture of carbon/epoxy multidirectional laminates. A 3D finite element analysis was first performed to define appropriate stacking sequences for end-notched flexure (ENF) specimens with starter delaminations on θ/−θ and 0°/θ interfaces. The analysis concerned the shape of widthwise distributions of the mode II strain energy release rate, GII, the level of mode-mixity, the effect of residual stresses on GII and the applicability of data reduction schemes. The experimental GIIc values increased with the ply angle θ for both θ/−θ and 0°/θ specimens. These trends were found to be in agreement with an average interlaminar stress fracture criterion. However, this criterion proved to be of limited practical usefulness, as its predictions were dependent on a characteristic length. Therefore, GIIc measurements on multidirectional specimens remain essential for the application of accurate fracture mechanics based design cr...
Interlaminar and intralaminar fracture characterization of composites under mode I loading
Composite Structures, 2010
The interlaminar and intralaminar fracture of laminated composites under mode I loading was studied using the double cantilever beam test. The effect of bridging on the measured fracture energy was assessed by cutting fibres during crack propagation. The fracture energy was evaluated considering a previously developed data reduction scheme based on the beam theory and crack equivalent concept. The model only requires the applied load-displacement data and provides a complete R-curve allowing the definition of the critical energy from the plateau value. A cohesive damage model was used to validate the procedure. It was verified experimentally that bridging phenomenon is pronounced, being more important in intralaminar tests. However, a detailed observation of the intralaminar R-curves showed that the intrinsic toughness, without fibre bridging effects, is similar to the interlaminar one.
2006
In this work a parametric study of the Edge Crack Torsion (ECT) specimen was performed in order to maximize the mode III component (GIII) of the strain energy release rate for carbon-epoxy laminates. A three-dimensional finite element analysis of the ECT test was conducted considering a (90/0/(+45/-45)2/(-45/+45)2/0/90)S lay-up. The main objective was to define an adequate geometry to obtain an almost pure mode III at crack front. The geometrical parameters studied were specimen dimensions, distance between pins and size of the initial crack. The numerical results demonstrated that the ratio between the specimen length and the initial crack length had a significant effect on the strain energy release rate distributions. In almost all of the tested configurations, a mode II component occurred near the edges but it did not interfere significantly with the dominant mode III state.
Fatigue & Fracture of Engineering Materials & Structures, 2008
One of the major difficulties in interlaminar fracture tests of multidirectional laminates is the high tendency for intralaminar cracking and the resulting wavy crack propagation. Experimental work showed that this occurred in double cantilever beam (DCB) tests of cross-ply laminates having a starter crack on a 0 • /90 • interface. Moreover, under steadystate propagation conditions, the apparent values of the critical strain energy release rate G Ic were two times higher than those of 0 • /0 • specimens. In this paper, a finite-elementbased progressive damage model was used to simulate crack propagation in cross-ply specimens. The results showed that transverse cracking alone cannot be responsible for the above difference of G Ic values. Therefore, the higher propagation G Ic values for crossplies must be attributed to the more extensive fibre bridging observed and to plastic deformations of the 90 • interfacial ply.
Mode III interlaminar fracture of carbon/epoxy laminates using a four-point bending plate test
Composites Part A: Applied Science and Manufacturing, 2009
The mode III interlaminar fracture of carbon/epoxy laminates was evaluated with the edge crack torsion (ECT) test. Three-dimensional finite element analyses were performed in order to select two specimen geometries and an experimental data reduction scheme. Test results showed considerable non-linearity before the maximum load point and a significant R-curve effect. These features prevented an accurate definition of the initiation point. Nevertheless, analyses of non-linearity zones showed two likely initiation points corresponding to G IIIc values between 850 and 1100 J/m 2 for both specimen geometries. Although any of these values is realistic, the range is too broad, thus showing the limitations of the ECT test and the need for further research.