Design and characterization of advanced pseudo-ductile unidirectional thin-ply carbon/epoxy–glass/epoxy hybrid composites

In a recent article written by Czél et al. and published in Composite Structures (2016), a set of immediately applicable pseudo-ductile composite materials was presented exploiting the great potential of the thin-ply hybrid composite concept recently developed with the assistance of analytical modelling.

High performance fibre reinforced composites exhibit outstanding strength, stiffness and very low density compared to other structural materials. However, their catastrophic failure and poor residual integrity have hindered their penetration into several high-volume applications including mass-produced automotive and construction. On the other hand, high performance ductile or pseudo-ductile composites have generated significant interest due to their safe, progressive failure process offering a wide margin between damage initiation and final failure.

As shown in the paper, interlayer or ply-by-ply hybrids have good potential for avoiding the brittle failure of ordinary non-hybrid composites. In the study, hybrid laminates were manufactured simply by stacking layers of different prepregs together. The presented work is the continuation of the initial research where Czél et al. (2013), demonstrated pseudo-ductility using standard thickness E-glass and emerging thin-ply carbon prepregs suitable for suppressing unstable delamination in UD interlayer hybrid composites during fragmentation of the low strain material. However, pseudo-ductile strains (widths of the stress plateau) were moderate because of limited E-glass/epoxy failure strain. In this study, higher strain S-glass plies were used and favourable stress-strain responses were obtained in tension with a second rising part after the plateau and high pseudo-ductile strains (please refer to the figure).

Thin carbon plies in the studied interlayer hybrids showed favourable progressive damage by suppressing unstable delamination after first carbon layer fracture in the glass-carbon hybrid laminates. The key damage mechanisms were dispersed fragmentation of the carbon layer, and stable, localized delamination (pull-out) of carbon layer fragments from the undamaged glass layers resulting in a pseudo-ductile failure process.

Standard thickness S-glass/epoxy prepregs, thin S3-glass/epoxy with high failure strains and various thin carbon/epoxy prepregs were used as design materials. Suitable design criteria adopted from the previous study for assuring stable pseudo-ductile failure of the UD glass/carbon interlayer hybrids included minimum strength and thickness for the outer higher strain (S-glass) layers, and an energy release rate (G_II) lower than the mode II fracture toughness of the glass-carbon interface at the expected failure strain of the low strain (carbon) layer. The stress-strain response results indicated that the thinnest specimen made with only one thin TR30 carbon prepreg ply (SG/TR30/SG) had a low relative carbon layer thickness resulting in relatively low initial modulus and moderate decrease in slope after carbon fragmentation. SG/TR30₂/SG with two carbon plies showed a favourable behaviour with pseudo-yielding and a wide stress plateau while SG/TR30₃/SG and SG₂/TR30₄/SG₂ showed load drops rendering them unsuitable for pseudo-ductility. It was noted, that the initial modulus of hybrid materials is controlled by the relative carbon layer thickness and the elastic modulus of the carbon fibres. With carbon prepregs of high modulus HS40 fibres, the SG/HS40/SG configuration was very attractive, with relatively high initial modulus, but the failure was not stable enough. The same combination with double glass plies each side, SG₂/HS40/SG₂ had a lower energy release rate and gave very stable response. These results show that a slightly thinner carbon ply with single S-glass plies could have been more beneficial for this material combination if it had been available.

The stress-strain responses of the intermediate modulus carbon configurations were found beneficial and significantly different from each other. S-glass with a single thin, high strain T1000 carbon ply resulted in a high pseudo-yield stress while S-glass with a bit thicker, lower strain MR40 ply provided a more balanced overall shape with a wide stress plateau. These results confirm experimentally the trade-off between pseudo-ductile strain and pseudo-yield stress of hybrid configurations as identified earlier by the authors (Jalalvand et al, 2015).

The pseudo-ductile stress-strain curves of the S3G/M55/S3G and SG/XN80₂/SG configurations made with high and ultra-high modulus carbon plies showed high initial modulus of 113 GPa and 124 GPa respectively with the latter being similar to that of non-hybrid high strength carbon/epoxy and a final failure strain similar to that of S-glass/epoxy with pseudo-ductile strain of up to 2.64%.

Good agreement was obtained between analytical predictions and experimental observations of failure modes and pseudo-ductile strains. The paper also provided an insight into the important factors for optimal hybrid configuration design.

CITATIONS 

Gergely Czél, Meisam Jalalvand, Michael R, Wisnom. Design and Characterisation of Advanced Pseudo-ductile Unidirectional Thin-Ply Carbon/Epoxy-Glass/Epoxy Hybrid Composites. Composite Structures, 2016, Volume 143, pp 362-370.

Czél G, Wisnom MR. Demonstration of pseudo-ductility in high performance glass–epoxy composites by hybridization with thin-ply carbon prepreg. Compos A Appl Sci Manuf 2013; 52:23–30

Jalalvand M, Czél G, Wisnom MR. Parametric study of failure mechanisms and optimal configurations of pseudo-ductile thin-ply UD hybrid composites. Compos A Appl Sci Manuf 2015; 74:123–31.