PSI - Issue 69

Muhammad Asim et al. / Procedia Structural Integrity 69 (2025) 41–46

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The uniaxial tensile deformation was conducted using Instron Universal Tensile Machine (UTM) along with a digital image correlation (DIC) setup to evaluate the strain evolution inside the material during cyclic loading and unloading. The surface roughness of the sample before and after F-LSP was evaluated using confocal microscopy.

Figure 1 (a) Range of laser power parameters explored. (b) Schematic of F-LSP.

3. Results Figure 2 shows results of the Ti 67 Zr 19 Nb 11.5 Sn 2.5 SMA prior and following F-LSP processing. Superelasticity was clearly observed in both cases. Samples were subjected to a similar applied strain level before and after F-LSP. The laser processing conditions were varied as detailed in Figure 1a. The results reported in Figure 2 represent the optimized conditions (summarized in Table 1) resulting in the best performance. The sample treated with F-LSP at low laser power showed a higher level of recoverability in terms of SE strains compared to untreated specimens. To study the effect of F-LSP treatment on functional fatigue properties, the sample was subjected to 25 SE cycles. Figure 3 (a) shows the evolutions of the recoverability levels as a function of SE loading cycle. During the initial cycles, recoverability drops at the same rate for F-LSP processed and unprocessed samples. However, the level of recoverability was slightly higher in the sample treated with F-LSP. For this particular alloy it was observed that after the initial 6 cycles, the drop in recoverability reduces and after 10 cycles, the sample exhibits limited degradation with continued loading when subjected to 25 cycles of loading. Whereas, in the sample without F-LSP, recoverability continues to drop as the number of loading cycles increases. Figure 3 (b) shows the total degradation of the alloy’s functionality following cyclic loading. The degradation of functionality was reduced by 50% after F-LSP treatment. It should be noted here that the functional fatigue properties were studied after excluding the first loading cycle as the majority of localized plastic deformation is introduced during this stage of deformation. Subsequent cycling results in significantly higher recoverability levels compared to the initial loading cycle. In summary, after cyclic loading, it was observed that the F-LSP treated sample showed better recoverability and better stability in terms of degradation of the SE strains with continued loading. The functional fatigue response of the F-LSP treated sample showed that F LSP treatment restricted the accumulation of plastic strains when compared to the sample without F-LSP which led to enhanced functional properties of the alloy. The laser parameters that showed enhanced functional response are shown in table 1 below. It should be emphasized here that the results and conclusion made here were based on a relatively limited number of cycles. To investigate the full fatigue behavior, including the number of cycles for the material to achieve stability in the response, thousands of cycles should be considered.