PSI - Issue 25

Tiago Bento et al. / Procedia Structural Integrity 25 (2020) 234–245 Tiago Bento / Structural Integrity Procedia 00 (2019) 000 – 000

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1. Introduction

In the aerospace industry, greenhouse gases emissions limiting measures along with several economic benefits, push the designs into a higher energy efficiency level. Therefore, weight reduction in the designs is one of the top priorities. The aerospace sector is highly regulated, constant certifications and validation processes of new manufacturing methods for structural components are required. Friction stir weld-bonding (FSWB) is an innovative alternative process that aims to unite the advantages of both friction stir welding and adhesive bonding. This hybrid method should be able to withstand continuous applied loads, providing the structures with high fatigue life along with static strength and ductility in several environmental conditions. The reliability of this technology would be significantly improved through monitoring of the stress/strain history during the manufacturing and service. This monitoring can be achieved by integrating sensors within the welds which up until now were only surface mounted (Richter-Trummer et al. 2010).

Nomenclature CWL Central wavelength 0 Effective refraction index FBG Fiber Bragg grating Friction stir welding FSWB Friction stir weld-bonding WL Wavelength Wavelength shift ∆ Strain range Normal strain Microstrain Brag g’s wavelength D iffraction’s net period Tensile strength Yield stress 0 Yield strength 2. Background FSW

Fiber photoelastic coefficien t

Although the number of filled patents has begun to stabilize, there has been an exponential growth in FSW applications up until 2016 (Magalhães, Leitão, and Rodrigues 2018). The process has applications in multiple domains such as civil, naval, railway, automotive and aerospace industries but its usage may spread to other domains due to its ability to change, repair or improve material properties. Although FSW is mainly employed in aluminum structures it can be utilized with magnesium and even some composites. Examples of applications of FSW are the Delta II rocket, produced by Boeing in 1999, the Eclipse 500 aircraft, in 2002, and the Legacy 450 and 500 aircraft in 2012 as stated in (Introduction to Friction Stir Welding (FSW) n.d.; Richter-Trummer et al. 2010). The stabilization in number of published patents since 2009 indicates that the technology reached its maturity maybe due to saturation of new solutions based on the friction stir welding principles. The introduction of smart joints in the market, like the ones studied in this paper, may help overcome some barriers that have been imposed to this technology so far. The use of fiber Bragg (FBG) grating based sensors may be justified because due to the frequency coding of the measurements, the output does not depend on the signal intensity which allows very long distances for data transmission, while still retaining adequate signal strength (Richter-Trummer et al. 2010). This work will explore the embedding of FBG sensors to monitor structural integrity in FSW joints is explored and a preliminary study of how these technologies can be joined is presented.