Tese de mestrado integrado. Engenharia Mecânica. Faculdade de Engenharia. Universidade do Porto. 2012

Mechanical performance of composite structures is influenced by the accumulation of damage from the manufacturing process and throughout the whole service life. For instance, an aircraft is subjected to a combination of mechanical loading and the thermo-oxidative environment from the take-off to the landing. Therefore, this degree project consists of studying the damage initiation and evolution in carbon fibre reinforced polyimide composites and assesses the thickness effect of the laminated composites. After manufacturing, the level of residual thermal stresses occurring at room temperature lead to the occurrence of microcracks in bundles of the quasi-isotropic composites. Further cooling to cryogenic temperature creates new cracks were appearing. This reinforces the conclusion that cracks are created due to thermal stresses. Comparison between a baseline composite made of carbon fibre T650 8-harness satin weave with thermosetting polyimide resin (ply thickness= 190µm) and thin-ply textile laminate made of Textreme carbon fibre IMS65 (ply thickness=83µm) with the same resin shows that the ply thickness has a significant effect on suppressing or delaying the occurrence and the propagation of microcracks after mechanical loading. It is assumed that there are some edge effects leading to different damage state in 90° and ±45° layers.

Composite layups have been continuously used over many years in various applications. It is necessary to optimize its composition by studying various parameters influencing the mechanical properties and studying the failure behavior. In this master thesis, the objective was to test five different plies manufactured using thick and thin plies and various combinations of thick and thin laminates called hybrid laminates. Bearing tests are performed for five layups with each layer has its thickness varying from 40μm to 130μm, and a combination called hybrid laminate, and the results from the tests are investigated. The resulting system has a good performance with onset damage above 700 MPa and an ultimate failure above 1130 MPa, using fibers' full potential. Also, the different failure modes like fiber kinking, matrix crack, delamination, and their effect on the layup's strength are investigated using fractography. This paper also investigates the influences of the thickness of the laminate on the strength of hybrid composites jointed using different mechanisms, and its failure modes are checked. Results from this experiment are used to validate in the form of FEM model, which is a part of an internal project at RISE SICOMP AB. This thesis is suitable for an engineering student in mechanical engineering, material science interested in composite materials and fractography.

Thin-ply composite materials display remarkable versatility and hold great promise for applications in the space industry. They are characterized by exceptional attributes such as a high strength-to-weight ratio, fatigue resistance, and the ability to conform to high curvatures without failure. This study investigates the behavior of thin-ply composite materials and structures, with a particular emphasis on their relevance to deployable space applications. Deployable structures such as solar sails, are large structures that are designed to be compactly folded into small volumes to fit inside the spacecraft for the purpose of carrying them to space. These structures utilize the strain energy during folding, to facilitate the deployment sequence and attain the intended original configuration of the structure. However, the viscoelastic nature of the composite material leads to a reduction of strain energy over the storage period, leading to shape inaccuracies after deployment. Our research includes an in-depth analysis of the viscoelastic properties of the composite material and the behavior of structures following folding and subsequent deployment. The viscoelastic mechanical properties of the materials were assessed through a numerical multi-scale homogenization approach. We examined thin-ply laminates with varying orientations and ply arrangements and conducted experimental studies to validate the numerical models. We subsequently incorporated the viscoelastic properties of the laminates into the simulation of deployable structures. The laminate properties were evaluated both at the ply level and at the laminate level. Numerical simulations were conducted to study the behavior of a composite boom during folding, stowage, deployment, and subsequent shape recovery. Our research extended to characterizing the composite material based on available test data, as well as examining the stowage and recovery behavior of a structure constructed from unidirectional composites.