Shear – Tension interaction
Most fasteners are mainly loaded in shear. That should be the only way for rivets as their tension capability is somewhat low. That is especially true for blind rivets. Therefore, whenever there is a substantial tension load in a fastener, a threaded bolt should be used. A very common type of threaded fastener used throughout the airframe (metallic and composite) is the HI-LITE™ from HI-SHEAR Corp. This fastener is a direct descendant from the original HI-LOK™ which is made out of A-286 high strength corrosion resistant steel (CRES) but this lighter version is made out of 6Al-4V Titanium alloy thus a little less than half the weight for the same strength.
When they are installed, part of the Hi-Lite’s Collar snap off under a certain applied torque. This introduces a pre-determined tension preload into the Hi-Lite pin. Contrarily to ordinary AN type bolts, the Hi-Lites are installed by interference fit (the reamed hole is slightly smaller than the pin shank diameter), bending in the pin is usually not considered.
When analyzing fasteners (and when calculating the bearing in parts), one must use the hole diameter for deformable fasteners (solid and blind rivets) and the nominal shank diameter for nondeformable fasteners (screws and bolts). Refer to Bruhn Table D1.6 and to MMPDS-09 Table 126.96.36.199 for diameters to be used.
The torsion shear due to the installation torque is always neglected as it is believed that it dissipates after some time before it gets loaded when the aircraft is in operation.
The tension preload in the fastener due to the installation torque is usually neglected at ultimate loads. Since that at ultimate loads, the applied load is generally higher that the preload in the bolt. Therefore, the fastened parts are assumed not to be in intimate contact anymore and so the only tension load that the bolt experiences is simply the applied ultimate load.
This is a different story at limit loads where the preload in the fastener must be considered. In fact, their shouldn’t be preferably any gap when the limit loads are applied. So the proper torque shall be applied depending on the application. For instance, for shear applications (small applied tension loads), the bolt preload is smaller than for tension applications (high applied tension loads). Most aircraft companies already have pre-determined installation torque values depending whether it is for shear or tension applications.
To take into account the combined loading of shear and tension in a fastener, different interaction equations are used depending on the fastener type. Table 2 shows the Interaction equations that have been developed for different types of fasteners.
Assuming we’re analyzing the fasteners installed on a wing plank. They are 6AL-4V Titanium HI-LITEs flush (100 deg. countersunk) 3/16” Pins HST11-6 installed along with 7075-T73 Aluminum Collars HST79-6 for shear applications.
The Pin has a shear strength of 95 ksi and a tension strength of 160 ksi.
The ultimate applied loads are Ps = 1883 lbs and Pt = 1000 lbs. Since the shear load is higher than the tension load therefore a shear application was determined.
All the information regarding the Hi-Lite fasteners shown in Table 1 can be found at: http://www.lisi-aerospace.com.
Analysis of the HST11-6 pin
Be careful that the flush pins have a lower tension allowable than the protruding head pins.
Tension Ratio; Rt = 1000 / 2000 = 0.50
Shear Ratio; Rs = 1883 / 2690 = 0.70
Using equation 1) instead of equation 4) for conservatism, from Table 2 we get a value of 0.593 which is lower than 1.0 so it should be fine but we still have to show a proper Margin of Safety.
Now calculating the Margin of Safety using the methodology shown in Ref. NIU Fig. 9.8.17 and a 1.15 Fitting Factor. Figure 1 shows the interaction curve and the Margin of Safety line segment.
The MS is calculated as:
Analysis of the HST79-6 Collar
This is a shear application Aluminum collar. Therefore, although it can take some tension load (like the preload), it is to be used in locations where the applied tension load is not too high. The MS is calculated using a 1.15 Fitting Factor.
Tension allowable; Ptu = 1600 lbs
Applied load; Pt = 1000 lbs
The Margin of Safety including the FF is calculated as:
The Pin and the Collar are found to be adequate. However, at 1000 lbs of ultimate applied tension load, even if the MS is positive, I would begin to think that a tension application collar like the A-286 CRES HST78-6 could have been used. Especially if the fastener is submitted to high GAG cycles.
The curve from Figure 1 was created in a spreadsheet where the exponents could be changed easily. Once that very handy interaction tool is made, it can also be used for calculating MS for other types of structures. For example, it can be used for calculating the Margin of Safety in buckling of flat panels that combines shear and compression, on column buckling that combines bending and compression, etc.
Refer to Bruhn §C4.22 to C4.28, §C4.22 to C4.28 and, §C5.9 to C5.12 for details and examples.
“Two wrongs don’t always make a right!”.
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