Fatigue test results

The results of the fatigue tests are presented in terms of crack growth rate, da/dN, in function of the range of Mode I strain energy release rate within a cycle. The data obtained from the different sets of specimens are showed separately for a better comprehension. Firstly, the results related from the T-samples with a value of hatch distance H=50 µm are proposed. Fig.

5.22 shows the trend of the crack growth rate associated to both the reference specimens and the lowest energy density laser treated sample (ED=0.17 J/mm²).

It is noticeable how the slopes of the curves referred to the degreased and the grit blasted specimens seem very similar to each other, although the value of ∆G required for the crack propagation in the grit blasted specimen, keeping fixed the crack growth rate, is considerably higher than the corresponding value of the degreased sample. Moreover, considering the laser ablated joint curve, when the crack growth rate oversteps 1.9e-4 mm/cycle the values of ∆G become slightly bigger than the ones of the degreased sample, which however achieves crack growth rates much higher. In Fig. 5.23, the results related to the other tested T-specimens belonging to the H=50 µm set are shown.

Although all the data in Fig. 5.23 appear gathered around the grit blasted sample curve and it is not easy to make significant distinctions between the fatigue behaviors of the different specimens, it is however possible to rank the laser configurations employed according to their capability to slightly enhance the value of ∆G with respect to the one exhibited by the grit blasted specimen, being equal the crack growth rate. Analyzing the graph following this approach, it is possible to notice how the highest energy density laser ablated joint presents values of ∆G slightly lower than the ones of the grit blasted specimen and the same trend

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Fig. 5.22 Crack growth rate vs ∆G resulting from a fatigue test over the simply degreased, the grit blasted and the laser ablated T-sample with ED=0.17 J/mm²and H=50 µm [158]

is found for the curve related to the ED=3.81 J/mm²set. Considering the ED=0.34 J/mm² data, they appear to be overlapped to the grit blasted joint curve when the crack growth rate is not too high before increasing with da/dN. The improvement brought by the three lowest energy density treated samples to the ∆G value is quite low but discernible. With the intent of quantitatively validating these remarks, the coefficients of the Paris’ law [159], presented in Eq. 5.1, are evaluated by means of a power law regression carried out for every experimental curve.

da

dN = C(∆G)^m (5.1)

These coefficients, namely the intercept C and the slope m of the aforementioned regres-sion curves, are collected in Tab. 5.2.

The values of the slopes summarized in Tab. 5.2 are not very different to each other, with the exception of the ED=0.17 J/mm²sample which presents a lower value of m. There are more remarkable differences in the value of the intercept C, in particular the intercept C of the curve associated to the simply degreased sample is two order of magnitude higher than the others. The laser ablated joints present values of C which appear consistent with what noticed before about the ranking of the laser parameters configurations according to the comparison with the grit blasted specimen: the two families of joints ablated with ED≤1.71

Fig. 5.23 Crack growth rate vs ∆G resulting from a fatigue test over the simply degreased, the grit blasted and the laser ablated T-sample with ED ranging from 0.34 to 5.71 J/mm²and H=50 µm [158]

Table 5.2 Coefficients of Paris’ law for the sets showed in Fig. 5.22 and 5.23 DCB joint (when laser ablated: T pattern and H=50 µm) C [mm^m+1N^−mcycle] m

Degreased 5.7 10⁻¹ 3.76

Grit blasted 2.0 10⁻³ 3.37

ED=0.17 J/mm² 5.6 10⁻³ 1.79

ED=0.34 J/mm² 1.0 10⁻³ 2.99

ED=0.51 J/mm² 1.0 10⁻³ 3.41

ED=1.14 J/mm² 1.4 10⁻³ 4.24

ED=1.71 J/mm² 1.4 10⁻³ 4.13

ED=3.81 J/mm² 2.7 10⁻³ 3.50

ED=5.71 J/mm² 4.1 10⁻³ 3.29

J/mm² and ED>1.71 J/mm² have values of C lower and higher, respectively, than the grit blasted sample one.

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A visual inspection of the fracture surfaces is useful to understand the failure mode occurring depending on the laser parameter configurations. An overview of photographs of the degreased and grit blasted samples, besides of the laser ablated T-specimens with H=50 µ m, is offered in Figs. from 5.24 to 5.32.

Fig. 5.24 Fracture surface of a fatigue tested degreased joint [158]

Fig. 5.25 Fracture surface of a fatigue tested grit blasted joint [158]

Fig. 5.26 Fracture surface of a fatigue tested T-joint (ED=0.17 J/mm²and H=50 µm) [158]

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Fig. 5.27 Fracture surface of a fatigue tested T-joint (ED=0.34 J/mm²and H=50 µm) [158]

Fig. 5.28 Fracture surface of a fatigue tested T-joint (ED=0.51 J/mm²and H=50 µm) [158]

Fig. 5.29 Fracture surface of a fatigue tested T-joint (ED=1.14 J/mm²and H=50 µm) [158]

Fig. 5.30 Fracture surface of a fatigue tested T-joint (ED=1.71 J/mm²and H=50 µm) [158]

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Fig. 5.31 Fracture surface of a fatigue tested T-joint (ED=3.81 J/mm²and H=50 µm) [158]

Fig. 5.32 Fracture surface of a fatigue tested T-joint (ED=5.71 J/mm²and H=50 µm) [158]

In the lowest energy density ablated sample (Fig. 5.26) the fracture surface presents the same appearance of the simply degreased specimen (Fig. 5.24) and analogously the failure occurs in a completely adhesive mode. As ED grows up to 0.34 J/mm²(Fig. 5.27) and 0.51 J/mm²(Fig. 5.28), the failure mode progressively moves from interfacial to cohesive and the fracture surface appears similar to the grit blasted one (Fig. 5.25). Further rising the energy density, it is possible to detect a switch from the occurring of the crack propagation within the adhesive (ED=1.14 J/mm², Fig. 5.29) to an increase of the amount of the zones undergoing adhesive failure (ED=1.71 J/mm², Fig. 5.30) and finally to a crack occurring within the adhesive but very close to the interface (ED=3.81 J/mm² and ED=5.71 J/mm², Fig. 5.31 and 5.32, respectively). All the remarks carried out with regard to the ranking of the laser parameters configurations and the dependence between the configuration employed and the failure mode recorded are consistent with what found for the quasi-static case. Therefore, it is meaningful to assume that even in this case the cause of the differentiation of the mechanical behavior according to the laser parameter configuration employed is to be ascribed to the presence of air bubbles in the laser ablation induced grooves. Even the fracture surfaces observed after the occurrence of a fatigue test are subjected to the entrapment of air between the adhesive and the substrate, generating inclusions which, in this case, seem to behave as tension points from which the crack initiates and propagates, often for small distances, along an interface. This behavior is appreciable in Fig. 5.33, panel a, while on panel b the corresponding fracture surface for the quasi-static case is juxtaposed.

Fig. 5.33 Fracture surface of joints ablated with ED=0.34 J/mm², H=50 µm and T pattern, after fatigue test (panel a) and quasi-static test (panel b). In panel a the arrows are placed to point the air inclusions out [158]

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In order to assess the influence of the hatch distance over the fatigue behavior, H is varied between 50 and 100 µm keeping constant the other process parameters for specific ED treated samples. The results for the T-joints are presented in Fig. 5.34.

The differences between results belonging to samples realized with different H values are not so marked as in the quasi-static case, but some similarities are however recognizable.

Due to the higher amount of untreated area, the samples with H significantly higher than the spot diameter are characterized by slightly lower values of ∆G with respect to the joints realized with H=50 µm. The values of the coefficients of Paris’ law provided in Tab. 5.3 for the H=100 µm set confirm these comments, in particular noticing the higher values of the intercept C with respect to the H=50 µm set case (Tab. 5.2).

Fig. 5.34 Crack growth rate vs ∆G resulting from a fatigue test over two couples of laser ablated samples, treated with the same ED (ED=0.51 J/mm²and ED=1.71 J/mm²) and the same pattern (T), but testing both the hatch distance values H=50 µm and H=100 µm [158]

Table 5.3 Coefficients of Paris’ law for the sets showed in Fig. 5.34 DCB joint (T pattern and H=100 µm) C [mm^m+1N^−mcycle] m

ED=0.51 J/mm² 1.3 10⁻³ 3.46

ED=1.71 J/mm² 6.8 10⁻³ 4.21

A similar trend is detectable even for the C-samples, as shown in Fig. 5.35, especially for the case ED=0.51 J/mm²for which the gap between the curve referred to the H=50 µm sample and the curve referred to the H=100 µm joint is significant. When ED=1.71 J/mm² the behavior seems less sensitive to the hatch distance value, especially when the crack growth rate is quite high.

Fig. 5.35 Crack growth rate vs ∆G resulting from a fatigue test over two couples of laser ablated samples, treated with the same ED (ED=0.51 J/mm²and ED=1.71 J/mm²) and the same pattern (C), but testing both the hatch distance values H=50 µm and H=100 µm [158]

Finally, the effect of a variation of the laser scanning strategy (unidirectional or grid pattern) is evaluated by means of the comparisons provided in Figs. 5.36 and 5.37, performed by keeping the hatch distance fixed to 50 µm and 100 µm, respectively.

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Fig. 5.36 Crack growth rate vs ∆G resulting from a fatigue test over two couples of laser ablated samples, treated with the same ED (ED=0.51 J/mm²and ED=1.71 J/mm²) and the same hatch distance (H=50 µm), but testing both the T and the C textures [158]

Fig. 5.37 Crack growth rate vs ∆G resulting from a fatigue test over two couples of laser ablated samples, treated with the same ED (ED=0.51 J/mm²and ED=1.71 J/mm²) and the same hatch distance (H=100 µm), but testing both the T and the C textures [158]

Regardless of the hatch distance, a decrease of ∆G is recorded in the C-specimens with respect to the unidirectional case, more evident for the ED=1.71 J/mm²case and when the crack growth rate is low (for high da/dN an overtune with respect to the usual hierarchy happens in the ED=0.51 J/mm² and H=100 µm curves), while in the quasi-static test the results belonging to the T and the C-joints appear very close to each other and, even when a difference is apparent, it is very slight. Tab. 5.4 collects the coefficients of the Paris’ law for every C-joint analyzed.

Table 5.4 Coefficients of Paris’ law for the C-joints DCB joint (C pattern) H [µm] C [mm^m+1N^−mcycle] m

ED=0.51 J/mm² 50 1.4 10⁻³ 3.20

ED=0.51 J/mm² 100 1.4 10⁻³ 3.08

ED=1.71 J/mm² 50 2.8 10⁻³ 3.59

ED=1.71 J/mm² 100 1.2 10⁻² 3.89

The value of C relative to the curve associated with the joint treated with ED=1.71 J/mm² and H=100 µm, which is one order of magnitude higher than the others, provides a quantitative validation of the previous remarks. The images of the fracture surfaces of two C-joints treated with ED=0.51 J/mm²for both the cases H=50 µm and H=100 µm are offered in Figs. 5.38 and 5.39, respectively. A switch of the failure locus from the inner adhesive to the interface occurs when using H=100 µm instead of H=50 µm for ablating the surface, at the same energy density level.

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Fig. 5.38 Fracture surface of a fatigue tested C-joint ablated with ED=0.51 J/mm²and H=50 µ m [158]

Fig. 5.39 Fracture surface of a fatigue tested C-joint ablated with ED=0.51 J/mm²and H=100 µ m [158]

5.4 Quasi static characterization of the joints after an