Exploiting thin-ply materials to establish controlled failure in carbon composites

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

The failure prediction of composite materials is hindered by the lack of accurate theories and difficulties in generating reliable experimental data. To complement the latter, this thesis aims to investigate the behaviour of carbon/epoxy materials under multi-axial loadings using thin-plies. By controlling the failure mechanism of composite laminates, the effect of different stress components can be looked at with regard to the failure strain/strength of the material. An expedient to that is utilizing a very thin-ply carbon/epoxy material (with a ply thickness of only 0.03 mm) that allows unwanted damage mechanisms to be suppressed. Multi-axial stress states are created in the unidirectional (UD) material under uniaxial tensile loading by specially designing the laminates. The thesis also incorporates the basic material characterisation of the thin-ply carbon/epoxy that provides the foundation for these novel tests.

A combined loading of longitudinal tension and transverse compression was applied to the UD thin-ply composite through the scissoring deformation of angle-ply blocks of the same material in which the UD plies were embedded. Three different angle-ply thin-ply laminates were designed using classical
laminate analysis (CLA) with varying amounts of transverse compression. It was found that the transverse stresses have very little effect on the failure strain of the thin-ply material. Additionally, one of the key limitations of tensile tests – stress concentrations arising due to gripping effects – was also addressed using
a recently developed novel hybrid composite test approach.

A biaxial stress state of longitudinal tension and transverse tension was induced in the UD ply block where transverse tension was due to the indirectly generated residual thermal stresses in the crossply laminates. The effect of ply thickness on the fibre failure strain was investigated by designing four
different cross-ply configurations with varying thickness 90° blocks adjacent to the central 0° plies. Advanced instrumentation including in-situ and quasi-in-situ X-Ray Computed Tomography (X-CT) and acoustic emission (AE) were utilized to detect and monitor damage accumulation in the specimens. It was found that thin-ply materials are indeed capable of suppressing transverse micro-cracking. The initiation of edge cracks and their progression into the bulk of the material were considered with respect to their effect on failure. A small degradation of the fibre failure strain was found as the 90°-layer thickness increased in
the laminates mainly due to the development of edge/transverse cracks. Overall, ply thickness was found to have a small effect on the longitudinal failure strain whereas there was no evidence to suggest an effect for the transverse tensile stresses.

Furthermore, a recently published novel tensile test method is investigated further and used to determine the longitudinal compressive strength under a combined longitudinal compression and transverse tension state of stress. An advanced failure criterion called ONERA progressive failure model (OPFM) and
the design of experiments (DOE) were used to design the laminates and optimize the stacking sequence. Material behaviour was investigated with respect to non-linearity of the carbon fibre as well as the effect of ply thickness. The compressive loading was indirectly applied to the 90° layers through the scissoring deformation from the high Poisson ratio of angle plies in which they were embedded. Measurements were carried out using AE, digital image correlation (DIC), video and virtual strain gauge measurements. Post
failure, optical microscopy and Scanning Electron Microscopy (SEM) imaging were carried out to highlight the damage state of the coupons. Stress concentrations were addressed by manufacturing shoulder ended specimens. Gauge section compressive failure (fibre kinking) was found in the specimens. The effect of
ply thickness on the fibre failure strain was investigated and it was shown that this resulted in higher failure strain/strength values for thinner 90°-layer thickness laminates.

Additionally, the development and fundamental characterization of thin-ply glass/carbon hybrid composite overload sensors which show a striped pattern if triggered are presented along with the proof of concept on a demonstrator application. By doing so, a visual indication of damage can be observed that can
be further utilized as a novel structural health monitoring concept for composite and metallic components or structures.
Date of Award23 Jun 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorMichael R Wisnom (Supervisor), Gergely Czel (Supervisor) & Ian P Bond (Supervisor)

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