PSI - Issue 13

Anke Schmiedt et al. / Procedia Structural Integrity 13 (2018) 22–27

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A. Schmiedt et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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Fig. 2. a), b) Quasi-static stress-strain curves (Schmiedt et al., 2018a) and c) tensile strengths of brazed joints prior to and after pre-corrosions.

The tensile behaviour of brazed AISI 304L/BAu-4 joints in dependence of the VDA pre-corrosion condition is compared, based on the ultimate tensile strength  UTS and the 0.2 % yield strength  YS , determined for 10 mm line elements (LE10) and a gauge length of 10 mm (GL10), respectively . The applied nominal tensile stress  N is calculated based on the gross cross-sections. Compared to the as-received condition with  UTS = 617 ± 1 MPa and  YS = 226 ± 2 MPa , a significant decrease of  UTS down to 82% (  UTS = 507 ± 10 MPa) is detected within the first two weeks of pre-corrosion. After the maximum investigated time span of 6 weeks, a reduction down to 69% (  UTS = 428 ± 28 MPa) has to be considered, Fig. 2c. Values of  YS remain at a constant level, because local strain concentrations at the brazing seam are expected to be barely taken into account when using a gauge length of 10 mm (GL10, LE10). In further studies, the effect of the gauge length on the yield strength will be investigated . 3.2. Fatigue behaviour The material reactions within LIT of the brazed specimens prior to and after 3 and 6 weeks (3w., 6w.) of pre-corrosion were recorded by various measuring techniques to evaluate the fatigue damage behaviour. Hence, the progression of the controlled maximum stress  max as well as the total strain amplitude  a,t , the total maximum strain  max,t , the loss energy density w, the change in AC voltage  U and the change in temperature  T are plotted as functions of the load cycles N for the as-received brazed joint, Fig. 3a. The parameters  a,t ,  max,t and w were determined based on strain measurements using an extensometer with a gauge length of 12.5 mm (GL12.5). For the as-received specimen, values of  a,t and  max,t remain constant within the stress levels and increase linearly with an increasing stress up to approx.  max = 290 MPa. Subsequently, both curves show a pronounced progressive increase in combination with a degressive trend within the respective stress levels. A considerable total maximum strain  max,t of approx. 30% is reached at the failure maximum stress  max,f (LIT) = 520 MPa. The determined directional accumulation of plastic strain is well known as ratcheting fatigue, which is more pronounced for elevated mean stresses and stress amplitudes. The degressive trend of the cyclic deformation curves within the constant stress levels is consistent with the literature on AISI 304 austenites and is expected to be a result of the rearrangement of the dislocation structure (Hahnenberger et al., 2014). As the curve of  U corresponds well with  max,t , electrical measurements are well suitable to characterise the cyclic deformation behaviour and ratcheting fatigue effects. During the cyclic loading, the energy dissipation leads to a self-heating of the specimen with a maximum change in temperature  T of 7.2 K at failure. The temperatures for each stress level increase linearly up to 20∙10 4 cycles, followed by a progressively increasing trend. For  max ≥ 380 MPa, a regressive decrease has to be considered within the stress levels. The effect is more pronounced with increasing stresses with e.g. a decrease of  T from 7.0 K down to 6.1 K for  max = 510 MPa. For a correlation with the cyclic deformation behaviour, the loss energy density w is calculated based on the areas of the hysteresis loops with w = ∮  d  (Smaga et al., 2008). For  max ≥ 350 MPa, w decreases regressively within the stress levels, indicating cyclic hardening effects. The decrease is more pronounced with increasing stresses with a maximum change of 1.5∙10 4 J/m 3 for  max = 510 MPa. The direct correlation of the temperature and the cyclic deformation behaviour was explained by Smaga et al. (2008) with the fact that i.e. 95% of the loss energy density dissipates in heat. The reason for a cyclic hardening is determined to be a deformation-induced formation of martensite, detected with a feritscope sensor (Schmiedt et al., 2018a) . Thus, fatigue damage processes, which are well known for metastable austenites (Nebel et al., 2003), are also observed for the brazed joints. Here, the temperature measurement is the most suitable to characterise the cyclic hardening effects. (Schmiedt et al., 2018a)