PSI - Issue 5

G. Lesiuk et al. / Procedia Structural Integrity 5 (2017) 912–919 Lesiuk et al./ Structural Integrity Procedia 00 (2017) 000 – 000

915

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Lesiuk et al. (2015), De Jesus et al. (2015), Correia et al. (2015) etc. The low carbon content (and low Si level) indicate that the tested steel from 1902 year can show the tendency to microstructural degradation processes described in papers of Pękalski (1999), Lesiuk and Szata (2011). According to LM images, the presence of microstructural degradation processes is confirmed (see. Fig. 2c and 2d) in numerous of brittle separations inside ferrite grains. It can generally support the global brittleness of metallic members of the girder. Therefore, this type of material is representative for the efficiency of the strengthening strategy in terms of fatigue crack propagation in similar long term operated structures. As bonding, the Sikadur®-31 CF Normal glue was used. It is 2-component thixotropic epoxy adhesive. As product data sheet announces it is moisture tolerant, based on a combination of epoxy resins and extra filler. The bonding can be utilized in temperatures between +10˚C and +30˚C. It can be applied to join: concrete elements, natural stone, ceramics, bricks, mortar, masonry, steel, iron, aluminium, wood, epoxy and glass. The adhesive has the following advantages: easy to apply, good adhesion, High strength, hardens without shrinkage, no primer needed and good mechanical resistance. The adhesive can be applied easily with spatula and trowel. To reinforce steel specimens, the Sika® CarboDur® S1014/180 bands were applied which are pultruded carbon fibre plates on epoxy matrix for structural strengthening of structures. It presents very high strength, excellent durability, lightweight, outstanding fatigue resistance, service up to +150˚C and corrosion resistant (Sika, 2017). The basic mechanical properties of the used CFRP patches are collected in Table 3.

Table 3. Mechanical properties of Sika® CarboDur® S1014/180 (Sika, 2017) Property A (%)  u (MPa) E (GPa)  [g/cm 3 ]

t[mm]

Value

>1.7

>2800

>165

1.6

1.4

The specimen for FCGR (Fatigue Crack Growth Rate) investigation were extracted in longitudinal direction (LT). According to ASTM E647 (2015), the CT specimens ( W =50mm, t =8mm) were prepared. Before the strengthening, specimens underwent process of pre-cracking as it is described in ASTM E647 (2015), which basic aim is to obtain maximally sharp tip of crack enabling proper fatigue crack growth propagation, because after machining of the mechanical notch using EDM (Electro Discharging Machine) process. When specimen is fully prepared – standard specimen without patches (Fig. 3a) - CFRP patches can be glued to the specimen surfaces using two-component epoxy adhesive (Fig. 3b) which is applied with spatula. The edge of a CFRP patch is coincident with mechanical notch tip and perpendicular to crack line – fibres of a patch are laid parallel to force axis. (a) (b)

Fig. 3. Compact tension specimens ready for test: (a) without reinforcement; (b) with full CFRP patches