Carbon fibre reinforced thermoset polymer (CFRP) components are becoming increasingly prevalent in aerospace and automotive industries where reduced weight and increased fuel efficiency is required. The manufacturing process typically requires the net shape to be edge trimmed, using a milling process, to achieve final part shape. The cutting process can cause defects on the trimmed edge which, due to the anisotropic nature of the CFRP material, may not be adequately captured by traditional, metallic material based surface quality metrics. More fundamentally, the effect on mechanical performance, in particular flexural strength, is not well understood.
The aim of this project is to investigate links between machined edge surfaces and static flexural properties. The effects of machine stiffness and cutting tool design, the effects of tool coating and tool wear, and finally, the effect of machining temperature on the surface quality and subsequent flexural strength are assessed. This is completed through the use of a robust framework to assess materials, machines and tools used in experimentation. Dynamometer data is captured and assessed through an original metric and current state-of-the-art 3D areal metrics are used to assess the machined surface topography. Additionally, scanning electron microscopy (SEM) is used to provide further qualitative data. Chips are collected and analysed, in a first for composite materials, to determine average geometry and changes due to machining variables. Finally, to address the shortcomings of current available metrics, a novel metric to observe sub-surface defects is proposed, validated and used to assess effects of machining variables on edge quality.
It has been found that edge quality does alter the mechanical strength of edge trimmed CFRP through static four-point bend analysis. Flexural strength of coupons machined by the 6-axis robotic system is 25.9% greater than the 5-axis gantry. Tool wear and machining at elevated temperatures can reduce flexural strength by 7.1 and 8.7%, respectively. Design of experiment (DoE) and analysis of variance (ANOVA) methods employed to show statistical correlations with machining variables and surface metrics. The edge quality of CFRP, machined using prescribed variables, has been successfully linked to amplitude and volumetric 3D areal metrics (p < 0.05). Cutting mechanisms of different fibre orientations have been successfully characterised through SEM and areal analysis. Analysis of machining chips has confirmed cutting mechanism changes when the CFRP material is pre-heated up to glass transition onset. A novel, validated strategy for measuring sub-surface defects, was able to observe defects in edge trimmed samples, particularly in the 90° fibre region where matrix smearing previously prevented observation of damage.
