The new CBPM-FBG filament holder (its chassis) is made of Ti6Al4V [Young’s Modulus (E) of 114 GPa, Poisson’s ratio (ν) of 0.342] class of alloy due to its high resilience [3232N/mm2]. In practice, the only means to prepare the CBPM precisely is to use the Wire Electro Discharge Machining (WEDM) technique (here: by Mectalent Oy, Finland). The optical fibre [E=70GPa, ν=0.224] and the carved FBG (sensor) are bonded onto the remaining space between the outer strips of the CBPM and the filament mounting stage (see Fig. 1b). The silica FBG sensor itself was prepared with a device-specific (here: by Instituto de Telecomunicações, Aveiro, Portugal). A step index single-mode glass fibre (GF1, Nufern®) was selected to conduct the laser signal in/out via the core with a diameter of 10μm. The cladding in the selected fibre has the (outer) diameter of 125μm. By using the phase mask method, a 3.0 mm-long FBG grating is inscribed into the fibre25. A W3/1050 series FBG interrogator (Smart Fibres®) is used for flashing and reading the sensor. This interrogator has a wavelength range of 1510–1590 nm and an accuracy of ±0.0006nm. The interrogator was controlled using a remote interface W3 WDM (version 1.04) at a sampling rate of 50 Hz.

For the MB test machine used here, the force data is measured with a load cell of 1 N (Futek, US), which is connected to a linear slide. The slide in-turn is connected to an absolute linear encoder (Numerik Jena, Germany). The main advantage of the encoder connected to the slide is to measure elastic deformation of the load cell during loading. A data acquisition device NI-6003 (National Instruments, US) is used to record the force signal at a 1 KHz sampling rate. Microtome blades of a type R35 with a thickness of 0.254 mm (Feather, Japan) are used to excite loading to the droplet on the filament (test per individual droplet). The blade movement along the filament long axis is controlled with a M-111-1DG1 DC motor operated with a C-863 controller (Physik Instrumente, Germany). The FBG signal (strain indication) is zeroed before each measurement and so is the load cell of the testing device. Before any series of measurements, the load cell is initially calibrated (maximum error in force = signal voltage ×(±0.001182) N). The experiments are carried out in (constant) ambient laboratory conditions (20.4±0.04C, 27.3±0.04 %RH, statistics for a week).

In this study, for the particular characterization of the CBPM mechanism, the blades are momentarily replaced by a tensile mechanism in which the tensile stage is displaced until the applied load reaches 0.8 N. The force and strain data in general are recorded simultaneously using a defined time stamp on the systems (the Fibrobotics test machine and laser interrogator). Cyclic tensile tests are also performed with the tensile setup (see Fig. 10a) for a range of load levels (0.74 N, 0.38 N and 0.13 N) and 30 cycles per each level.

(a) Experimental tensile setup; (b) CBPM-FBG filament holder prepared for tests.

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