7/28/2019 Structural Joint Failures

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If you are developing a structural analysis of a structural joint,you need to check all possible failure modes of this joint. Butwhat type of analysis must I perform? What type of failure willappears in my joint? Structural joints are made by structuralsplices or structural doublers. These two joint types: doublersand splices are very different regarding design philosophiesand load applications, check future posts to understand thedifference. The joints/connections are made by fastenersbetween the two structural elements. This type of joint that youcan find in every structure of the aircraft can fails bythe following failure modes:

Tension failure of the plates, the net section of theparts/plates must resist to the load applied to the joint. You

must check by calculation the stress applied to the plates andthe margin of safety regarding the net section of the members.

Tension cleavage failure of plates, if we have a doubleload application applied in the two direction of the plate. Thesum of the two loadings gives us the maximum load applied tothe rivet and net section of the plate. We must check if thetension stress applied as adequate margin of safety in the joint.

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Shear-out failure, if the hole of the first fastener line is tooclose to the border of the plate, failure by shear out of the platewill occur, you must always check if this can occur. Do notinstall fasteners to close to the border. To avoid this type of failure obey to the general installation rule that the rivet line

must be at 2D(two times the rivet diameter) minimum. If you obey this rulethis mode of failure will be avoided.

Bearing failure, this is the typical critical failure mode of the joint. When you design the joint try to design to this typeof failure, this will give you the safety that you need. To better understand the failures visit future post regarding the splice or doubler analysis.

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Fastener shear,  this is other typical failure mode of the joint.The shear resistance of the fastener selected is not adequateto use. The application of load will cut the fastener body,causing the failure of the joint.

Fastener tension, if the joint is tension critical the tensionfailure of the bolt can occur. Then you must check the margin of safety of the tension applied to the bolt. Be careful, the bolt

margin of safety must be the result of the combined loadbetween tension and shear for bolt applications.

Fastener pull-through, in tension application mainly in

the countersunk fasteners, the fastener head cannot have the

adequate strength to avoid the pull-through failure. This type of 

failure is only applicable to tension applications. This can onlybe checked by structural tests of the fastener/plate assembly.

Use test tables to check that.

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 All this type of failure modes can occur in our joint. You mustcheck all this type of failure modes and design for the critical

one. Generally the net section, fastener shear, fastener tension

and bearing failure are the critical modes. You must make the

analysis for these four modes of failure and find the margin of 

safety or reserve factor applied. The allowable of the fasteners

and plates must be compared with the applicable load in the

 joint.

From:

http://aeroteaching.blogspot.in/2012/04/aircraft-damage-structural-

 joint.html