PSI - Issue 21

O. Berk Aytuna et al. / Procedia Structural Integrity 21 (2019) 120–129 O. Berk Aytuna et al. / Structural Integrity Procedia 00 (2019) 000 – 000

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magnifica tion with 2448x2050 pixels resolution. Acry lic pa int (diluted with acetone) was sprayed to the monitored region with a irbrush for these magnifica tions. Spraying achieved the proper speckle pa ttern and contrast for DIC with average 25 μm black speckles on electropolished shiny surface. The recording ra te varied from 1 fps (for spa tia l ana lysis) to 5 fps (for tempora l ana lysis). Matlab based 2D digita l image correla tion software Ncorr v1.2 was used to ana lyze stra in distribution of the monitored region. Stra in resolutions were 1.08 μm and 5.05 μm for 3X and 1.4X magnification, respectively. Error ca lculated by translation experiments was 0.14% for these magnifica tions. All images recorded with 5 fps ra te were used to obta in stra in-time graphs whereas consecutive images recorded with 1 fps ra te were used to obta in spa tia l ana lysis of PLC bands. Median stra in va lues were collected to plot stra in -time graphs. Software ca lcula ted stra ins as Green-Lagrangian stra ins in x and y direction and they were converted to the true stra in with the formula of = ln⁡(√2 + 1) where ε is true st ra in and E is Green-Lagrangian stra in. Moreover, equiva lent stra in ca lcula tion was done by: _ = √2/3( _ 2 + _ 2 + _ 2 ) . Stra in in thickness was obta ined by assuming tha t summa tion of stra ins in three axes equa ls to zero.

c)

F 4

F 1

b)

a)

F 3

F 2

20X Objective Lens

d)

LED Coaxial Light Source Zoom Lens System CCD Camera

8 mm

2 mm

2 mm

Fig 2. (a) Specimen geometry, (b) portable biaxial testing apparatus, (c) specimen attached to apparatus arms (d)DIC imaging unit.

2.3. Scanning electron microscopy and electron backscatter diffraction (EBSD)

FEI Nova Nano SEM 430 field emission scanning e lectron microscope a t 18 kV accelera ting voltage was used to monitor the surface irregularit ies, defects, slips and micro cracks after testing and to ana lyze fracture surfaces. EDAX EBSD camera equipped to SEM obta ined the diffraction da ta with the software of Orienta tion Imaging Microscopy (OIM) Da ta Collector. OIM Ana lysis software was used to plot o rienta tion maps and pole figures. Larger areas such as 450x450 μm 2 were studied for the representative pole figures. Standard cleanup procedures were applied in this software. Sample prepara tion for EBSD was done with the same electropolishing parameters and solution.