Fastener failure mode

[MrHiLok]

I am trying to justify corrosion blends on the external door skin of aft cargo door of a large passenger aircraft. The door is not plug type.

The skin nominal thickness is 0.07 inch. All the blends are close to access panel cutout in the door. All the blends are at 20:1 taper ratio and the maximum depth of the blend is less than 10% of the nominal thickness.
All the blends are within the limit of SRM, only deviation encountered from the SRM is that SRM states to have 1.5D distance from the centre of the fastener hole to the edge of the blend.
Some of the blends are on the fastener holes.

These fastener holes (where blends have been done) are used for securing the panel to the door. The cutouts (oval in shape) are 5.2 inches wide and 3.9 inches deep.
Fastener hole diameter is (0.190 – 0.194 inch).
The fasteners used to secure the access panel to the external skin is
a 100 deg head Bolt, cross recess, 95 KSI shear, short thread (BACB30LH3).
The access panel has nut plates, which are self-locking, floating, self sealing-cap (BACN10JZ3AZ).

From my understanding of the structure and fastener used, I conclude that SRM 1.5D is not relevant to the subject blend locations, as the fastener holes are not structural and are design to secure the access panel cutout.

I am only concerned with the bearing material at the blend locations (at fastener locations). However one of my colleague argues that the failure mode will be fastener tension and not material bearing. He argues that the fastener will take tension load, which I don’t agree, as the fastener is a 95 ksi shear and the subject fastener is not used in tension application.

I would like to know the opinion of this forum regarding if it material bearing critical or the fastener will experience tension load.
Can also some one comment on the nut plate used to secure the access panel, Is the subject nut plate, used in tension application or just for shear application?
Thanks in advance.

 

[rb1957]

a pick would be nice ...

 

[40818]

If your working outside the bounds of the SRM, then you will need to get a "no technical objection" or such like from the OEM.
Just because the fastener type is "shear or tenison type" may not have any bearing (exscuse the pun) on the loads applied to it or its failure mode.
Would you please give some justification as to why you believe the fasteners are non-structural (being 3/16 size on outer pressuer skin).
Regards

 

[SWComposites]

Is there an internal pressure load is this area which could result in tension loads on the fastener? Reduced skin thickness will result in lower pull-through allowable loads (the fastener will not likely fail in tension, but the head may pull through the skin if there is a tension load). The SRM has limitations on blend out locations for a reason; have you asked the OEM whether the proposed (it is proposed, not already done, right?) repair is acceptable?

 

[rb1957]

sure you can ask the OEM, maybe your covered by their customer support agreements (though why'd you ask the question if this was an option ?) ... i suspect you'll need to cover this with your own repair approval.

 

[MrHiLok]

Thanks everyone.
This is going to be under local repair approval.

 

[tbuelna]

MrHiLok,

Sorry for the tardy response, but regarding your nutplate there are specific types of nutplates for shear or tension applications. The nutplates that are designed for use with shear fasteners have a counterbore before the thread. This counterbore is there to ensure that the threaded portion of the fastener is beyond the bearing area of the clamped parts, and thus there are no threads in shear.

I don't have details of that Boeing BACN10JZ3AZ part, but if your panel fasteners are intended to carry shear the nutplates should ideally have a counterbore.

Hope that was helpful.
Terry