Abstract
This paper reports an experimental study on the mode I interlaminar fracture of carbon/epoxy multidirectional specimens with 0°/ θ° delaminating interfaces. The suitability of the double cantilever beam (DCB) specimens was first evaluated in three-dimensional (3D) finite element (FE) analyses, which addressed possible elastic couplings associated problems. In the DCB tests, extensive fibre bridging occurred as a result of transverse cracking of θ-oriented plies. This complex damage morphology caused pronounced R-curves with artificially high final values of the critical strain energy release rate, G Ic. In these circumstances, G Ic values of initiation from the film generated starter crack were considered the only true interlaminar properties. Moreover, they were found to be fairly independent of the delaminating interface, a result that could be interpreted in additional FE analyses concerning the interlaminar stress field ahead of the crack tip.
Highlights
Delamination is a common damage mode of high performance laminated composites which can be detrimental for structural behaviour
The characterisation of delamination resistance has been the subject of many studies, especially under mode I loading. Those efforts lead to the standardisation of the Double Cantilever Beam (DCB) test for measuring the critical strain energy release rate, GIc, of unidirectional (UD) [0]n laminates [1]
The majority of applications involve multidirectional (MD) laminates and it is observed that delaminations usually occur between layers of different orientations
Summary
Delamination is a common damage mode of high performance laminated composites which can be detrimental for structural behaviour. The characterisation of delamination resistance has been the subject of many studies, especially under mode I loading Those efforts lead to the standardisation of the Double Cantilever Beam (DCB) test (figure 1) for measuring the critical strain energy release rate, GIc, of unidirectional (UD) [0]n laminates [1]. The incorporation of thick symmetric [(02/90)6/02] blocks of 20 plies minimised the elastic couplings related problems mentioned above and effectively avoided large displacements, plastic deformations and intraply damage These features are favourable for DCB tests, as the following analysis confirms. The effects of residual stresses on GIc measurements were assessed with the above FE models by performing a preliminary 100 oC temperature decrease loading step The results showed they were already lower than 1.2 % for the small width-wise average GI ≈ 100 J/m2 assumed at this stage [14]. We could conclude that the proposed specimens were adequate for DCB tests
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