An investigation into the role of fibre bridging as a toughening mechanism in engineered composite materials
Huxford, Bobby
Huxford, Bobby
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Publication Date
2022-06-27
Type
Thesis
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Abstract
Fibre reinforced composites are becoming increasingly popular in engineering design due to their high strength to weight ratio. In one of the failure mechanisms, which can occur during impact, fibres can bridge between each of the newly created crack surfaces. This extrinsic toughening mechanism can significantly increase the material toughness. The objective of this thesis is to provide frameworks to characterise this behaviour and capture it correctly in computational models, i.e., using the finite element method. While previous research has represented this behaviour in models, the procedures to identify the input cohesive properties can be subjective. Using the most commonly used representation of fibre bridging (tri-linear traction separation law), a thorough examination of the possible parameter space is performed. The effect of each parameter is described and a robust method to identify the properties is outlined. This work is performed by virtually mimicking the typical experimental characterisation techniques, i.e., a Double Cantilever Beam (DCB) test. This work is also packaged in a format that allows use by a non-expert user, e.g., in an industry setting. The outputs can be used to compare different material systems or as inputs in larger scale simulations of applications of composite laminates. To further understand the mechanism of fibre bridging, it is necessary to consider the precise nature of the tractions exerted behind the crack tip. A recently developed experimental method seeks to do this by applying a fixed curvature to cantilever arms of a typical DCB specimen. A theoretical analysis is performed to show how the fracture energy can be related to the applied loads and how the crack length is directly controlled by the applied displacement. Further analysis establishes a method to calculate the tractions generated as a function of crack separation. Simulations using these findings show the conditions under which the approach can give accurate results. For both materials tested with this technique, the data suggests a second local maximum in the traction separation response; this contrasts with the majority of the literature which considers only a single peak associated with intrinsic toughness. To further explore the detailed relationship between traction and separation for fibre bridging, a simpler test method based on the standard DCB approach was developed. Using a series of captured images and rotation tracking features on the beams, the slope, curvature, and internal moments were determined along the beam and from this the tractions are derived. These analyses support the existence of the second peak in the bridging tractions. The proposed methods are easily incorporated into standard test methods and the data is directly comparable to standard test methods. This thesis presents work which can improve the design of composite material systems by quickly assessing the changes in fracture behaviour with increased detail in the extrinsic toughening mechanisms. The research provides experimental and computational methods to accurately characterise the behaviour and facilitate inclusion in simulations of composite applications.
Publisher
NUI Galway