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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3. Results and Discussions

Fig. 3 shows differences in load response of the ma teria l under uniaxia l and biaxia l loading. Load-time graphs in Fig. 3a were converted to load-stra in graphs in Fig. 3b by finding the stra in va lues from DIC ana lysis a t a given time. It was not possible to convert the load va lues to stresses as it was hard to measure reduction in the cross section area . The load increase behavior is similar for both conditions (Fig. 3a ). Ma teria ls deform with a lmost same hardening ra te, yet the sample under biaxia l tension fa ils a t 0.34 equiva lent stra in (Fig. 3b). After this point, the ma teria l under uniaxia l stretching continues to deform up to approximately 700 s, corresponding to an equiva lent stra in of 0.70 until fracture (Fig. 3b). Ma teria l shows no loca lized neck region and there is no remarkable load drop a t fracture under both conditions due to nature of the biaxia l testing appara tus and specimen design as expla ined in elsewhere (Güler and Efe (2018)). Wh ile ma teria l undergoes additiona l deforma tion under uniaxia l loading, serra tions of the curve due to the PLC effect seem to be more distinct in uniaxia l case. Observed serra tion numbers under uniaxia l loading are five times higher than tha t of under biaxia l loading. Average load jump and drop amount is approxima tely 15 N for biaxia l stretching, whereas it is 23 N for uniaxia l stretching. Un iaxia l condition lea ds to higher load jumps and drops during deforma tion. The first observed PLC band (serra tion) is seen a t earlie r stages (a t time nearly 180 s) of the testing under uniaxia l loading. Fu rthermore, there are no strict sequence of load jumps and drops with respect to time, indica ting tha t the characteristics of the PLC bands are near to the Type C.

a)

b)

Fig. 3. (a) Load versus timegraph (b) Load versus strain graph of thematerial during uniaxial and equibiaxial loading

2D DIC ana lysis gives the shape and stra in amount in the PLC band by ca lcula ting stra in distribution of the pit region by taking two consecutive images a t each load jump and drop during the test. The DIC results ( ε xx direction) in Fig. 4a . are taken from different load jumps and drops during uniaxia l tension. Red and yellow regions in the figure show stra in loca lized regions. Shape of the PLC bands is sharp and distinct under uniaxia l stretching, it a lso has an angle of 50 º to the loading direction. The b lue regions represent nearly stra in-free regions and approxima tely ha lf of the tota l area is stra in-free; therefore, the deforma tion of the ma teria l is heterogeneous, where the stra in is loca lized only in the PLC bands. Stra in ca lcula ted in the PLC band from beginning to end of the test differs by 0.015 and the maximum stra in is 0.026. Load increase during the test is responsible from the stra in increase in the formed PLC bands. Also, multiple PLC bands can form a t the same time, and these bands can either cross or be para llel to each other. The width of the bands differs from 150 μm to 400 μm . The stra in distribution of individua l PLC band has higher va lues through the centerline of the band and the stra in usua lly peaks at one end of the centerline. The PLC bands are a lso repetitive; they can from a t the same region a t any time. However, nuclea tion and propagation of the bands do not have a sequence and they appear to be completely random.