Strength modelling of composite filament fabricated materials using classical laminate theory

Abstract

Additive manufacturing (AM) describes a manufacturing technique that builds 3-dimensional objects by adding layer-upon-layer of material in order to create a product. One subset of AM is 3D printing or rapid prototyping (RP), which has seen extraordinary popularity among engineers, designers and hobbyists. The most significant benefit additive manufacturing offers is the elimination of waste material. This aspect will have significant value for poorer countries as it reduces the cost. While designers have used 3D printing for some time now, it is not clear exactly how strong the printed materials are. The manufacture of composites using 3D printing is progressing and, therefore, a strength prediction model for the composite filament fabricated materials is needed to understand how the material reacts under loads. With this knowledge, designers can analyse a component to ensure that it will withstand the specified load parameters before going into production. It will also give engineers geometrical freedom when designing. By gaining a better understanding of the fabrication of composites and by which laminate failure theory the composite will abide, the strength of the components can be predicted. Some of the advantages of using composites are that they are lightweight and extremely high strength when fibres are oriented correctly due to the anisotropic nature of fibre reinforced composites. They also do not require subtractive manufacturing processes to produce. This dissertation focuses on predicting the strength behaviour of 3D printed composites. The material properties used in the composite are determined by a series of testing and used to develop a model to predict the individual mechanical properties of the materials in the composite and how they will react when subjected to a load. An investigation into laminate- and ply failure theories has been conducted. Research into laminate theories is relevant since, until recently, composites have been constructed by laying fibre on top of each other or weaving them into one another and using a resin as the matrix that holds them together. With the 3D printed process, the type of composite must first be determined before the laminate theory for the composite can be identified Experimental testing of the composite by ASTM standards has been done and the results processed with a method called micromechanics. This analyses the materials that contribute to the composite on the level of the individual elements that constitute them. The elastic modulus of the fibre used was determined to be 20.153 GPa, which is 4.20 per cent in range with the 21.00 GPa that Markforged® claimed. The data has then been used with the developed strength prediction model, based on the classical laminate theory to accurately predict the strength of the composite, given the fibre orientations. The software LAP® was then used to verify the results from the prediction model and compare them to the experimental values. With a 0° fibre orientation, composite with ten fibre layers and two nylon layers over 12.7 mm width, LAP® predicted a 16.477 GPa modulus of elasticity compared to the 16.870 GPa with experimental testing. Further investigation into the micromechanics input parameters is required, including the influence of gaps between the fibres and/or the fibre and nylon. This will alter the volume fraction of the materials in the composite and ultimately lead to a more accurate prediction model. Further research is needed in the flexural behaviour of the composite and the influence of temperature on the testing and printing of the composite. Measured against the outcomes, the project has been successful.