PSI - Issue 14

Bimal Das et al. / Procedia Structural Integrity 14 (2019) 619–626 Das et al./ Structural Integrity Procedia 00 (2018) 000 – 000

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Table 3. Tests parameters and fatigue life under different loading conditions. Strain amplitude (%) Mean strain (%)

Strain ratio (R)

Number of cycles to failure

0.3 0.3 0.3 0.5

0

-1

4965 3321 3213 1833

0.1 0.2 0.2

-0.5 -0.2

-0.43

3. Experimental results The recorded hysteresis loops at cycle number 1, 0.25 N f , 0.5 N f and N f subjected to asymmetric loading with a mean strain of 0.1 % and strain amplitude of 0.3% are shown in Fig. 2(a). The imposed mean strain resulted in the opening of the hysteresis loop of first cycle as shown in Fig. 2(a). This opening of hysteresis loops can be attributed to the variation in the plastic modulus between the loadings in the forward and reverse direction [5] . The non-closure of the hysteresis loops resulted in the relaxation of the responded mean stress of the material. As the number of cycle progresses the hysteresis loops closes and shifted downward resulting in the decrease in stress amplitude. This variation of stress amplitude can be observed from the plot between stress amplitude vs. number of cycle as shown in Fig. 2(b). This decrease in stress amplitude with number of cycle is a clear indication of continuous cyclic softening undergone by the material. The cyclic softening curve depicted in Fig. 2(b) can be divided into three zones during the whole lifetime of the material. The primary zone indicates significant cyclic softening in the first 10-15 % of the total lifetime. After that, the secondary zone depicted a linear softening behavior up to around 0.7 N f . Thereafter, severe softening of the material takes place in the tertiary zone due to the fatigue damage evolution such as micro crack initiation and propagation etc. Similar nature of cyclic softening curve has been previously reported by several authors like Verma et al.[9] and Guguloth et al. [10] for P91 steel.

The variation of cyclic mean stress with number of cycles for R= -0.5 and strain amplitude of 0.3% is shown in Fig. 3(a). It can be observed that mean stress diminishes rapidly in the initial few cycles thereafter follows a steady variation and finally diminishes sharply. The tensile and compressive peak stresses during asymmetric strain controlled loading are presented in Fig. 3(b). This difference in tensile and compressive peak stresses lead to the non-closure of the hysteresis loops leading to the evolution of mean stress. It can be observed from Fig. 2(a) and 3(a) that the variation of cyclic softening and mean stress relaxation with progression of cycles is quite similar in (b) Fig. 2. Mean stress relaxation test at mean strain of 0.1% and stress amplitude of 0.3%. (a) Selected stress-strain hystresis loops. (b) Cyclic softening curve: Variation of stress amplitude vs number of cycles. (a)