PSI - Issue 25

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

237

4

Table 2. AA6082-T6 mechanical properties (Braga and December 2017) Density [ kg/m 3 ] Young’s modulus [GPa] Rupture Tensile Strength [MPa] Yield Tensile Strength [MPa]

Elongation at Break [%]

2700

67.1

322.9

276.2

17.5

Table 3 presents the mechanical properties of the Araldite 420 A/B adhesive. This adhesive is a room temperature curing paste adhesive of high strength (peal and shear) and toughness, suitable for a wide variety of metals and resistant to high temperatures (Hunstsman Advanced Materials 2004). The two components should be mixed with a nozzle mixer and the mixture should have a dark blue/green colour. Only after 4 or 5 days at ambient temperature the adhesive will achieve 90% full strength and 100% full strength will be achieved in 1 to 2 weeks’ time (Hunstsman Advanced Materials 2004).

σ u [MPa] τ u [MPa] G I c [N/mm] G I c I [N/mm] 30 22.5 3 6.5

Table 3. Adhesive basic properties, adapted from (Braga and December 2017) Cure temperature E [GPa] G [GPa]

Room temperature

1.57

600

As for the surface treatment, besides the degreasing and sanding treatments, the plates were also subjected to a chemical treatment by exposure to 3M™AC -130. 3.3. Bragg fibers These sensors act like fi lters re fl ecting back a speci fi c wavelength of light when exposed to a broad specter light source. The re fl ected light consists of a very narrow light peak. When external loading is applied on the Bragg grating sensor, the wavelength of the reflected light beam is changed . The fiber sensitivity to strain is the combination of physical deformation and refraction index matrices which are altered via photoelastic effect. This relation between strain, ∆ε, and wavelength, ∆ , is expressed as: ∆ = ( 1 + 1 0 0 ) ∆ = (1 − )∆ (1) Where, is the the period of the diffraction’s net, 0 the effective refraction index of the fiber’s nucleus, a nd fiber photoelastic coefficient. To measure the strain, the fibers were connected to a machine that acted both as a large wavelength light source a nd as an interrogating unit. This machine, BraggMETER™ has high dynamic range and output power and al lows high resolution to be attained even for long fibers and lossy connections (Fiber Optics Sensors & Systems Monthly Newsletter July 2010). In Fig. 1 (b) the effect of tension and compression on a Bragg grating is represented.

Interrogator unit & wide specter light source - BraggMETER ™

Stretched grating

Undeformed grating

Compressed grating

Specimen with Bragg fiber

PC

(a)

(b)

Fig. 1. (a) Spectral displacement of a deformed Bragg grating; (b) Spectral displacement of a deformed Bragg grating.