NAS1097AD Rivets

[Rob130]

Does a table exist to compare the static joint strength of a NAS1097AD rivet to the MS20426AD rivet?
MIL-HDBK-5H on shows the NAS1097E rivet for comparison. I teach a course in structural repair and would find this comparison useful.

 

[Nigel]

I do not know of any reference for NAS1097AD joint strengths. For a basis of comparison I would use tables 8.1.2.2 and 8.1.2.2(s) in MIL-HDBK-5J (note ammendment J is now current). In these tables it can be seen that the NAS fastener joint doesn't fail in shear untill 0.100 in. sheet thickness and the MS202426E fastener fails in shear in 0.090 in.. This tells us the MS202426 is a stronger joint in most cases and therefore a better choice. If, however, the application is in thin skin and the use of a NAS1097 fastener will enable you to avoid dimpling, which has poor fatigue properties, that would be the better choice.

Using this information and the information in tables 8.1.2(b) and 8.1.2.1(b), a reasonable compasion could be acheived.

If I can be of anymore assistance please feel free to drop me a line.

Nigel Waterhouse & Associates
Aeronautical Consulting Engineers

Transport Canada and F.A.A approval & certification of fixed and rotor wing aircraft alterations: Structures, Systems, Powerplants and electrical. FAA PMA, TC PDA.
n_a_waterhouse@hotmail.com

 

[Sparweb]

I've fudged the answer in the past by "scaling" one against the other. Nobody cried bloody murder, so I suppose I wasn't too far off in left field.

The "E" rivets are 7075-T73, F_su=43*ksi (driven)
The "AD" rivets are 2117-T3, F_su=30*ksi (driven)

Therefore a 1097AD rivet is 30/43 times the strength of the 1097E rivet.

(I explicitly stated in the preamble to my analysis what I was doing and the margins were pretty big, try to do the same).


STF

 

[JSF]

To Rob, Nigel, and SparWeb
The NAS1097 rivet is developed by Douglas aircraft with a reduced countersunk head. The MS20427 rivet is a larger countersunk head which can knife-edge the skin (sheet). MIL HB-5 provide the bearing/shear failure of the joint strength. MIL HB-5H (Latest J) table 8.1.2.2(C) shows the MS20426AD (Fsu 30ksi) ultimate shear strength of the rivet in different sheets.

The thinner the sheet you may have to use (multiply) by the rivet reduction facor (table 8.1.2.1(b).

For example: you have a 0.063" thick sheet, using NAS1097AD5 or MS20426AD5 rivet. You calculate what is the countersunk depth and subtracted from the skin thickness (0.063&quot. The hole size for #5 rivet is 0.161/0.166" dia. Countersink dia. for NAS1097 is 100 deg countersink 0.226/0.236" and for MS20426AD5 rivet head dia. 100 deg c'sink 0.267/0.277".

For rivet use Min. csk dia with max hole dia to give you the critical condition. to calculate the remaining thickness at critical condition, t_rem = t_skin-(Max Csk Dia-Min Csk Hole Dia)/2*tan(50).
For NAS1097 head: t_rem =0.027" >0.010 to prevent from Knife-edge (O.K for design)
For MS20426 head: t_rem =0.003&quot; <0.010 to Knife-edge condition (not good for design, fatigue problem)

Using these remaining thickness and calculate your rivet shear strength along with the rivet reduction factor.
Fsu (AD rivet) = 30 ksi, Psu (NAS1097AD5 rivet) = Fsu*PI/4*(Min Hole Dia)^2*RF for 0.025&quot; (from MIL-HB-5 rivet reduction factor 0.925) = 564 lb. Similarly the MS20426AD5 rivet can be calculated for 520 lb (with 0.86 reduction factor estimated). Thsi is how the MIL HB-5 joint allowables are established from test and reduction tables are developed. This is just the rivet shear capability.

You need to calculate the rivet bearing strenth. Which is based on the material ultimate bearing strenth at Min. 2xDia. for the least material property (eg. 2024T3 compare to 7075-T6 AL). Pbru = Fbru_2D*t*Min. Hole Dia.
Let say the 2024-T3 clad Fbru (2D) = 125 ksi and 7075-T6 clad sheet Fbru (2D) = 146 ksi.

If your skin is 2024-T3, then Pbru = 1268 lb and for 7075-T6 Pbru = 1481 lb. You are allowed to use effective hole dia. for your bearing strength accounted for the csk larger area.

The failure of the joint is in rivet shear and not bearing failure. That is why NAS1097AD5 rivet joint failure load on 2024-T3 skin is 564 to 570 lb. and MS20426AD5 rivet is 520/523 lb.

Hence when you design the structure or repair you make sure you are using the min. joint allowable.

If you need any help let me know.
Good luck.
Richard Chwa