Four dog-bone-type specimens and four rectangular specimens were printed for each of the four purchased filaments, with the commercial materials described below. The printing machine used was a 3D printing: A2V4 printing machine (3nt r) and Ultimaker 2+.
Epocyl is the trade name of an epoxy resin with a indetermined load of CNTs that was purchased from Nanocyl. To cure mixtures a mixture of accelerator and hardener were used: Aradur 1571 (50%)/ Accelerator 1573 (50%). The 24 parts of the mixture by 76 of the mixture of resins and we cure at 120º C for 2h. The catalyst mixture is heated slightly before hand-mix it well and is mixed with the resins just before curing.
- Prepreg layup manufacturing process
The layers of Carbon Fiber prepreg, purchased from Toray, were cut manually with (0º,90º, + 45º,-45º). First, a release agent was applied on the tool surface. The stack of plies was piled up manually. After the first ply was laid, debulking was made to ensure a good adhesion of the pre-preg to the tool surface. The rest of the plies were positioned following the stacking sequence. Debulking was performed for each four plies or when wrinkles appeared during stacking. This step was accomplished by placing a non-perforated bleeder film and breather layer under a flexible vacuum bag, and by applying 0,9 bar of vacuum. After placing all the prepreg layers, the coupon was covered by a peel ply, a layer of perforated bleeder film and a non-woven breather cloth. Before introducing the tool on the oven, a vacuum test was performed to check the bag’s integrity. Afterwards, the coupon was left under vacuum following the programmed curing cycle. After curing a demoulding phase was carried out.
- Measurements of electrical properties.
Once manufactured, two silver contacts 1 cm thick and 5 cm apart are painted and the ohmic resistance is measured with crocodiles in the acquirer. Data acquisitor system Keysight DAQ970A with an integrated DAQ901A-20 channels armature multiplexer module. This system acquires simultaneously measurements of electrical resistance and mechanical strain to analyse the SHM behaviour.
- Estimation of Conductivity and resistivity
Shape Factor (F) = S/L = 2x0,5/5 (mm) F = 0,002 m
Conductivity σ = 1/ρ = 1/R*0,02 (S/m)
They were carried out following the ASTM D3039/D3039M standard, the speed was 1mm/min, 10 cycles of ascent and descent were carried out in displacement control, until reaching the target tensions of 50, 100, 150, 200, 250, 300, 350, 400, 450 and 500 MPa. The speed of the section from 0 to 50 MPa is carried out at 0.5 mm/min. The speed of the rest of the sections is 1 mm/min. Due to the placement of the sensors on the specimen, the transverse extensometer cannot be installed.