Study on fibre optic sensors embedded into metallic structures by selective laser melting

Abstract

Additive Manufacturing, which builds components layer by layer, opens up exciting possibilities to integrate sophisticated internal features and functionalities such as fibre optic sensors directly into the heart of a metal component. This can create truly smart structures for deployment in harsh environments. The innovative and multidisciplinary study conducted in this thesis demonstrates the feasibility to integrate fibre optics sensors with thin, protective nickel coatings (outer diameter ~350 μm) into stainless steel (SS 316) coupons by Selective Laser Melting technology (SLM). Different concepts for fibre embedment by SLM are investigated. The concepts differ in which way the fibre is positioned and how the powder is deposited and solidified by the laser in respect to the optical fibre. Only one approach is found suitable to reliably and repeatable encapsulate fibres whilst preserving their structural integrity and optical properties. In that approach SS 316 components are manufactured using SLM, incorporating U-shaped grooves with dimensions suitable to hold nickel coated optical fibres. Coated optical fibres containing Fibre Bragg Gratings (FBG) for strain and temperature sensing are placed in the groove. Melting subsequent powder layers on top of the FBGs fuses the fibre’s metallic jacket to the steel and completes the integration of the fibre sensor into the steel structure. Cross-sectional microscopy analysis of the fabricated components, together with analysis of fibre optic sensors’ behaviour during fabrication, indicates proper stress and strain transfer between coated fibre and added SS 316 material. During the SLM process embedded Fibre Bragg grating (FBG) sensors provide in-situ temperature measurements and potentially allow measuring the build-up of residual stresses. Subsequently, FBG sensors embedded into SS 316 structures using our approach follow elastic and plastic deformations of the SS 316 component, with a resolution of better than 3 pm*μɛ-1. Temperature measurements are also conducted with a precision of 3 pm*K-1. Such embedded fibre sensors can also be used to high temperatures of up to ~400 °C. However, at elevated temperatures issues arise from the significantly larger thermal expansion coefficient of SS 316, leading to delamination of the more rapidly expanding metal from the glass. Rapid thermal expansion of the metal also leads to high axial stresses within the glass exceeding the fibres tensile strength and ultimately leading to structural damage of the optical fibre.