Mode-I interlaminar fracture of carbon/epoxy cross-ply composites

Abstract

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.

Multiscale Polymer Composites: A Review of the Interlaminar Fracture Toughness Improvement

“…A lot of experimental and analytical research has gone into enhancing fracture toughness of laminated composite structures [29][30][31][32][33]. Many factors can affect the ILFT value; the factors influencing the ILFT as found in the literature are material system, lamina stacking and laid up configurations [34][35][36][37], fabrication quality and fabrication procedure [38][39][40], specimen thickness [41], and environmental effects [42,43]. Among all, lamina stacking, laid up configurations, and fabrication procedure are the main parts of composite manufacturing and hence are the most important parameters to understand properly and to be discussed in detail here (Refer Figure 2).…”

Results of Finite Element Analysis for Interlaminar Fracture Reinforced Thermoplastic Composites

“…The above expression will underestimate the compliance as the beam is not perfectly built-in [32]. Thus, a correcting factor ∆ is introduced assuming that the real crack is slightly longer (α+∆) than the one measured experimentally.…”

Karbon Lifi/Epoksi Tabakalı Kompozit Malzemelerin Farklı Ortam Şartlarındaki Mod I Kırılma Davranışı

“…Açılı katlı tabakalı kompozit malzemelerde malzeme salt Mod I veya Mod II yüklemesine maruz kalsa da eğilme-burulma ve burulma-burulma kuvvet çiftlerinin etkisi ile çatlak ucunda hem Mod I hem de Mod II kırılması eş zamanlı olarak ortaya çıkmaktadır [16]. Açılı katlı tabakalı kompozit malzemelerde lif yerleşim açılarının neden olduğu anticlastic eğilme veya asimetrik eğilme ile çatlak ucunun çarpılmasına neden olan karışık mod yüklemeleri nümerik metotlar ile de incelenmiştir [17][18][19][20][21][22][23].…”