Incorporation of fiber Bragg grating sensors in additive manufactured Acrylonitrile butadiene styrene for strain monitoring during fatigue loading
Introduction
Additive manufacturing (AM), also known as 3D printing, creates a structure by printing individual layers of material on top of each other until the desired shape forms. Compared to subtractive manufacturing, this technique offers unique benefits such as reduced tooling costs and the ability to produce complex component geometries [1]. These factors have made 3D printing popular in the aerospace and biomedical industries, specifically as a prototyping tool [2]. 3D printed materials have even advanced to the point where they are now beginning to be considered for selection in components that undergo service loads [3]. Although much research has been conducted on their static properties, much less has been done on these material's dynamic mechanical properties [1]. Some previous work includes the fatigue characterization of a 3D printed elastomer material [4], the fatigue behaviour of additively manufactured polylactide [5], and the effect of raster orientation on the static and fatigue properties of 3D printed ABS polymer [3].
Along with additive manufacturing, the past two decades have also seen a rise in what is known as “smart materials”. These materials possess inherent sensing capabilities, making them useful for structural health monitoring in civil and aerospace applications [6]. A technology that has aided in the creation of these materials is Fiber Bragg Gratings (FBGs). FBGs consist of a periodic grating structure at the core of a single-mode fiber. Fabrication occurs by exposing the fiber to a pattern of intense laser light, causing a permanent periodic change in the fiber’s refractive index [7]. This concept was first demonstrated by Hill et al. when they exposed the core of a silica fiber to an intense argon-ion laser for several minutes and afterward saw an increase in reflected light [8]. This behavior occurs because the fiber's internal periodic structure causes certain incident light to be reflected while letting the rest pass through. The wavelength at which the light reflects is known as the Bragg wavelength. This wavelength is determined by Equation 1 shown below, where λB is the Bragg wavelength, neff is the index of refraction, and ΛB is the period. Both the index of refraction and the period of the grating shift when the fiber experiences changes in strain or temperature, thus changing the Bragg wavelength as well. Therefore, by tracking the change of the Bragg wavelength one can actively measure changes in strain or temperature, making FBGs a useful sensing technology.
Equation 1: Bragg Wavelength Equation
These sensors can be used for structural, medical, and chemical applications since they can sense vibration, temperature, strain, and impact. They work well because they possess high sensitivity, good long-term stability, immunity to magnetic and electromagnetic interferences, a small formfactor, are lightweight and are resistant to corrosion [9].
For use in smart materials, FBGs have commonly been embedded in composite materials [9], [10], [11], [12] and have more recently begun to be embedded in additive manufactured plastics materials such as PLA and ABS [2], [13], [14], [15], [16]. Due to their small size and compatibility with common polymeric materials, this embedding can easily be done without inducing significant weakening of the material [17]. These benefits are helpful in many structural health monitoring (SHM) applications, including fatigue measurements [9], [12], [18], [19] and damage detection [20], [21]. Embedding FBGs into 3D printed materials may increase reliability since this would allow for real-time online monitoring structure stress, strain, damage cracks during manufacturing or operation [22]. Many studies have also been done with embedded [9], [12], [19], [20], [23], [24], [25] or surface mounted [17], [26], [27], [28] FBGs, or both [21]. Each method gives certain advantages, such as fiber protection with embedding and easy installation with surface mounting.
The application of FBG sensors in fatigue monitoring of additive manufactured materials is a particularly interesting area to examine since it brings together these two new technologies in a way that utilizes the unique advantages of both. The real-time strain monitoring provided by embedded FBGs can allow for the characterization of the dynamic properties of additively manufactured parts which is essential for their wider adoption. Also, due to the 3D printing’s low cost and high speed in terms of adjustability, it is perfectly suited to embedding FBGs which can be difficult in less adaptable manufacturing processes.
This work investigated the combination of additive manufacturing and FBGs by comparing fatigue test results between additive manufactured Acrylonitrile Butadiene Styrene (ABS) specimens with and without FBG sensors. This study is the first of its kind to directly view how the introduction of the FBGs effects the dynamic properties of a 3D printed material. Furthermore, the purpose of this work is to demonstrate the effectiveness of these sensors under fatigue loading when embedded in ABS through the utilization of FBGs’ real-time online strain monitoring. Strain measurements between different FBG incorporation techniques are also compared along with results from finite element analysis.
Section snippets
Experimental details
Test specimens were manufactured in a MakerBot Replicator 2X FDM 3D printer using MakerBot’s True White ABS filament. The printer bed was covered with blue painter’s tape and set to a temperature of 115 °C while the filament was extruded at a temperature of 240 °C. The filament, which had an original diameter of 1.77 mm, was extruded at a speed of 30 mm/s for the first layer, 40 mm/s for outlines, and 60 mm/s for the infill. The infill density was set to 80%, as this was the highest level at
Results & discussion
A summary of the static tensile test results for both the standard specimens and those with the through-hole for the FBG can be seen in Table 1. The average ultimate tensile strengths (UTS) were seen to be between 10 and 12 MPa for the 2 specimen types. This average is lower than what is found in the literature however, these findings range widely from 15 MPa to 45 MPa [1], [3], [31], [32]. This is thought to be caused by several factors such as infill density, infill pattern, and raster
Conclusion
The properties of additive manufactured ABS and FBG sensors were both characterized in this study. This was done by tensile fatigue testing under differing specimen and sensor configurations as identified in section 2: standard, containing an inserted FBG sensor and containing an embedded FBG sensor. Results showed that the incorporation of the sensor altered the specimens’ fatigue life differently for high and low load levels. The through-hole was seen to increase the variation in the number
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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