Integration and performance of distributed optical fibre sensors in thermoplastics and thermoplastic composites

Abstract

Today, thermoplastics and fibre reinforced thermoplastic composites, are increasingly being used in all fields of commerce and industry, for their improved performance with respect to impact resistance, recyclability, and the manufacturing cost. However, the damage and performance of thermoplastics and their composite structures is difficult to predict due to viscoelastic mechanical behavior and complicated failure mechanisms. Structural health monitoring (SHM) systems provide effective tools to detect and characterize how material degradation occurs and develops with time, assuring the safety of engineering structures. Optical fibre (OF) sensors are often considered for future SHM systems, because they enable both massive data acquisition and miniaturization of sensors in size and in power consumption. The performance of optical fibres depends to a large extent on how they are integrated into the host structure. For thermoplastics and their composite, OF sensor attachments typically use similar methods than have been used for traditional structural materials. The integration technology is still mostly manual work thereby producing inconsistent sensor measurements. The OF integration has three major issues; 1) lack of well-controlled OF integration methods; 2) lack of assessment methods for the OF attachment quality; 3) coupled sensitivity for thermal and mechanical effects on the OF. This research aims to improve upon these three aspects. The lack of automated, well-controlled OF integration methods induces inconsistent bonding which causes a non-uniform strain transfer from the host structure to the OF sensors. In this work, a novel method has been developed to integrate OF sensors into thermoplastics and thermoplastic composite materials by utilizing the material extrusion 3-D printing process. A procedure for OF insitu integration during the printing process with a fused deposition modeling (FDM) 3-D printer was proposed. The material behavior during and after the integration process have been investigated. Tensile testing and creep experiments were carried out to investigate the OF/thermoplastic interface bonding and the mechanical response of the substrate-OF system. Residual strains were measured on the integrated OF after 3-D printing and the residual strain creation mechanisms have been discussed. The measurement accuracy of the novel OF based method is compared with traditional methods. Quality and consistency of the OF bonding layer affects the measurement accuracy of the OF sensor directly. A good practical non-destructive method to test and evaluate the OF integration quality is currently missing. In this work, residual strains created by the integration process were used to assess the integration quality of both the novel OF integration by 3-D printing and a selection of traditional methods. Residual strains and their fluctuations along the OF are shown to be correlated to the specific OF integration method and it appears to be a useful quantitative metric for evaluating the attachment quality of the integrated OF. The coupled sensitivity of temperature and strain is one of the most significant limitations for using Fibre Bragg Gratings (FBGs) as well as Rayleigh backscattering OFDR (adopted in this work) based optical fibre measurements. This coupled sensitivity can introduce large errors for signal interpretation. The relationship between strain and the temperature effects under coupled mechanical-thermal loadings is studied in detail and a mechanical-thermal loading superposition model is confirmed for decoupling strain and temperature effects. A polynomial formula is deduced from physics-based temperature effect models to overcome the poor accuracy of widely used linear calculation principle. This more accurate calculation model is realized and tested in practice to distinguish pure mechanical strains from pure thermal loading on the OF.