Abstract

The scale effect in Composite Materials, consisting in the delay in the first damage appearance as ply thickness decreases, is, at present, a phenomenon under study due to the use of ultrathin plies (whose thickness may reach 20 microns) in the most leading industries. Based on this, the appearance of the first damage is experimentally analysed in this work for [04,90n,04] conventional laminates where n=1,2,4,8,16,32. The objectives are to study damage mechanisms occurrence as the thickness of the weakest layer varies when subjecting the laminate to uniaxial tension and to check if the scale effect is present. Acustical techniques have been employed for damage detection and optical and electronical microscopy have been used for its observation. The conclusions lead to well-diferenced mechanisms of damage for the different thicknesses analysed: high thicknesses show unstable damage resulting in an explosive failure of the whole laminate, whereas low thicknesses show a more stable damage, it being less significant for the laminate integrity. In addition, an scale effect is detected, since the load at which the failure is generated decreases as the thickness increases. In future works the same analysis will be presented for ultrathin plies.

Abstract

Numerous studies on the use of laminates manufactured with ultra-thin composite laminates have shown the existence of a significant delay in the appearance of damage, a phenomenon known as the Scale Effect. To study the effect that layer thickness may have on the damage mechanisms that appear at higher values of the load, this work analyses whether the use of ultra-thin laminates exhibits a better behaviour in the instants prior to failure and laminate rupture. For this purpose, different laminates have been manufactured with ultrathin thickness material formed by the same number of laminae oriented at 0 and 90 degrees, but with different stacking sequences, distributing the laminae such that the thicknesses of the resulting layers are different. The laminates have been subjected to uniaxial tension up to values close to the ultimate load in order to observe and compare the different damage morphologies that appear in the 90 and 0 degrees layers, using optical microscopy techniques for the observation of the damage. On the other hand, the loads for which the failure of the selected laminates happens have also been compared. The results obtained from this study show different behaviour of the laminates depending on the thickness of their 0 and 90 degrees layers in the moments prior to failure and at the instant when failure occurs.

Abstract

In this work, the authors propose an experimental study on the possible improvement of the mechanical behaviour produced by ultra-thin plies in laminates subjected to cyclic loading after impact. More specifically, this work introduces the use of these plies in carbon fibre quasi-isotropic laminates, with the aim of studying the evolution of impact damage under tensile cyclic loads. To this aim, the behaviour of quasi-isotropic laminates consisting of conventional thickness plies is compared with laminates with 90° ultra-thin plies, keeping the same stacking sequence. When considering the same loading level, the damage evolution shows a lower progress and a later failure for the laminates containing ultra-thin plies.

Abstract

A recent study about the ''edge effect'' phenomenon in cross-ply laminates made of ultra-thin plies composites has shown the presence of a relevant stress component through the laminate thickness. This fact implies that there is a biaxial stress state in the 90° ply block, which is present both before and after the cyclic loading tests. 4 cross-ply laminates were analysed, only varying the 90° ply block thickness. In each case, the biaxial stress state was obtained, selecting the most detrimental one to analyse the Energy Release Rate (G) with respect to the fibre/matrix interface crack growth. This analysis is performed using a BEM model from a previous study of the authors. An exhaustive microscopic revision is performed both before (only sanded and polished after curing process) and after cyclic testing, corroborating the occurrence of different crack growth of the present longitudinal fibre/matrix interface debonds. In conclusion, the single fibre numerical model using BEM and the experimental microscopic observations shed light on the micromechanical behaviour of the interface cracks which are subjected to a biaxial stress state.