Hybrid joints

Keywords Anaerobic adhesives, hybrid joints, shrink fit, press fit, eryogenic fit... 

Croccolo, D., De Agostinis, M., Vincenzi, N., 2012. Design and optimization of shaft-hub hybrid joints for lightweight structures analytical definition of normalizing parameters. Int. J. Mech. Sci. 56 (1), 77-85. 

In order to test the behavior of different adhesive systems in the hybrid joint and to analyze the influence of the interference contribution, hub/shaft laboratory samples were created. 

The second set of experiments was carried out to identify the interference contribution in the hub/shaft samples and its interaction in the hybrid joints bonded with the FT-EP adhesive (Gallic et al., 2014). The design of the experiment is depicted in table 2, a minimum of 4 specimens were tested for each sample type. 

Fatigue tests on hybrid joints bonded with the FT-EP adhesive were also carried out, by using a tension-tension fatigue test (Croccolo et al., 2010 Sekercioglu et al., 2005) on a resonant testing machine. 

Figure 5. a) Image and b) SEM magnification of a shaft of a hybrid joint at 35 jm of interference and bonded with the FT-EP adhesive after the decoupling phase. 

The hybrid joint samples with three different classes of interference (10, 22,5 and 35 pm) provided three different additional loads after the break up of the adhesive. Some examples of load-displacement curves related to the decoupling of hybrid joints at different interference levels and of an adhesive joint in clearance condition are shown in figiue 6 (Gallio et al., 2014). 

Figure 6. Decoupling curves of three hybrid joints at different interferences and an adhesive joint in clearance condition. At least 4 samples for each class of interference were tested, only one sample is shown as an example in the graph (with permission of Gallio et al., 2014).

The adhesive joints in clearance conditions were also characterized by a residual load level after the break up of the adhesive, due to the friction forces between the eured adhesive residues present on the fracture surfaces. This load presented a different behavior with respeet to the interference residual load recorded in the hybrid joints, sinee it expired before the eomplete decoupling of the joint. 

The load after break in the hybrid joints mainly depended on the radial contact pressure present between the hub and the shaft due to the interference fit. The important contribution due to the friction and wear phenomena observed in the unbonded samples was absent in these joints, thus the load after break can be most likely referred to the system described by the analytical equations. According to this hypothesis, the load after break can be more easily related to the contribution provided by the interference level. In order to obtain an estimation of this additional load, the mean value between the first peak and first valley of the stick and slip state was calculated. The obtained values are plotted in figure 7 and compared to those of the unbonded interference samples. 

Load after break hybrid joint experimentai data Non-bonded joint experimental data — Theoreticai data [ji = o.J, e = i[Pg.30]

Figure 7. Comparison among the interference contribution detected from the load after break of hybrid joints, the unbonded interference samples, and the results obtained from the analytical equations.

The presence of the interference could also affect the stiffiiess of the hybrid system. The first elastic stretch of the curves of the hybrid joints were compared with the ones of the systems bonded with clearance, considering all the different adhesives tested. The results are reported in figure 8 (Gallio et al., 2013). 

A tension-tension fatigue test was performed on the hybrid joints bonded with the FT-EP at 35 pm of interference and the FT-EP bonded samples with clearance, in order to investigate the influence of the interference on the fatigue behavior of the joints. 

The results obtained for the hybrid joints were not comparable with the clearance adhesive samples in terms of absolute values of the amplitude load. In fact, the hybrid joints presented higher statical resistance (of about 7000 N) than the clearance ones, so they were expected to begin to suffer the effects of fatigue cycles at higher amplitudes. Nevertheless, it was possible to compare the two cases in terms of a percentage of their static resistance. 

Figure 9. Comparison of the attained cycles to failure of hybrid joints and clearance adhesive joints at difference maximum stress amplitudes. The F ax in percentage of their total static resistance is indicated for both case.

Figure 10. The pull-out loads of the hybrid joints, the interference eontribution (detected by the load after break of the hybrid joints) and the adhesive joints in clearance condition as a function of the interference level.

In order to imderstand the importance of the adhesive contribution the total resistanees of the hybrid joints bonded with the FT-EP at different interferenee levels were compared to the interference contributions detected from the loads after break of the same samples (figure 10). In addition the resistanees of adhesive joints in clearance condition are also included. 

From figure 11 it was possible to observe that the different adhesives presented a diverse behavior in the presence of the same interference level. Indeed, a similar interferenee eontribution for every hybrid system could be assumed, taking into account a mean interference value of 17 4 pm among the specimens. The resultant total strength of the hybrid joints changed on the basis of the adhesive type. 

The hybrid joints prepared with the FT-EP, AC and PU adhesives presented an increase in the adhesion strength with respect to the unbonded samples. The use of the FT-EP adhesive provided values of maximum decoupling load fom times higher than those of the interference alone. In the case of the PU adhesive, the interference decoupling load was doubled in the hybrid joints. The AC adhesive presented the highest resistance improvement in the presence of the interference-fit compared to its performances in clearance condition because of its anaerobic curing mechanism (Gallio et al., 2013). 

The resultant strength of the examined hybrid interference fitted-ahdesive bonded joints was not always a result of the sum between the adhesive resistance and the interference contribution. The experiments carried out at laboratory level showed that different adhesive types provided dissimilar behaviors in the hybrid joint. Indeed, a greater influence of the adhesive on the maximum strength attainable for the hybrid joint was observed. Rheology of adhesive played a relevant role in determining how much adhesive was spewed away from the joint. If a few quantity of adhesive remained inside the... 

The exploitation of adhesives with different flexibility did not provide changes in the stiffness of the hybrid systems, which seemed rather dependent on the interference. Moreover, the presence of the interference positively affected the fatigue behavior of the cylindrical joints. Hybrid joints sustained 10 fatigue cycles at load amplitudes equal to an higher percentage of then-static resistance compared to the clearance joints. 

In the application of the hybrid joint in the wheel system, it was observed that the joint geometry played a relevant role on its performances. The presence of the disc flange angle and the deformability of the rim well created zones of clearance inside the joint. The more the interference the less the quantity of adhesive penetrated inside the clearance. The extent of the bonded area strongly affected the total static resistance of the hybrid joint. On the contrary, the fatigue life of the bonded wheel seemed to be independent to its resistance to the decoupling, and to depend on the interference level to a large extent. 

Effect of the Engagement Ratio on THE Shear Strength and Decoupling Resistance of Hybrid Joints... 

Keywords Anaerobic Adhesives, Engagement Ratio, Hybrid-Joints, Slip-Fitted Joints, Shear Strength... 

HJ Hybrid Joint (i.e. press-fitted and adhesively bonded joint)... 

Figure 4. Coupling force of a dry press-fitted joint PFJ) compared to that of a hybrid joint HJ).

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