An important criteria for aircraft rivets is their ability to resist vibration damage. Smoking rivets pictured above have vibrated loose; the longer they remain in service the more the hole is damaged. The rivet's ability to transmit stress across the lap is hindered.
Joint fatigue life depends on the ability of rivets to resist vibration damage. Below are some generalized factoids on preventing rivet loosening and increasing joint fatigue life.
1. Joint fatigue life is highly dependent on proper rivet installation.
2. Good quality holes improve joint fatigue life. Hole preparation that minimizes roughness, scratches, and other drilling defects improves fatigue life.
3. Especially with blind rivets, expect joint fatigue life to lower as hole size approaches the upper size limit.
4. Tolerance stack up may work against you and decrease fatigue life. A -5 blind rivet has a hole tolerance of .160-.164 inch. #20 drill has a nominal diameter of 0.1610 inch. Its actual size can be from 0.1603 to 0.1610. Drilled holes, even using good techniques, very by -0.0010 to + 0.0030 tolerance. A drill on the high side of tolerance .1610 and a hole on the high side of tolerance .003 takes you to the extreme of .1640.
5. Rivets in tension reduce joint fatigue life.
6. Poor clamp-up of the sheets (more prone with blind rivets) reduces joint fatigue life. Poor clamp-up allows greater joint deflection and higher bearing stresses.
7. Solid rivets have better vibration performance over blind rivets due to their superior hole filling ability, and greater yield strength. There is usually (not always) a big difference in blind rivet ultimate strength and yield strength due to the blind rivet's multi-piece design. Fatigue failure (and smoking) occurs when the rivet yields - not when it breaks (ultimate strength).
8. NAS1398/1399 rivets originally produced by Olympic have a better lock ring than the Cherry design and demonstrate better resistance to loosening in vibration environments.
9. Do not substitute rivets unless you understand the difference between shear critical and bearing critical joints. Poor fatigue life if you get it wrong.
10. Knife edge conditions reduce fatigue life. Per Boeing SRM, the countersink depth must not exceed 60% of the material thickness. (ref 737 SRM 51-40-08 Figure 7). Anything greater, the material is considered knife-edged and a poor fatigue detail. May be fine for static loads but poor fatigue in cyclic loads. Corollary: Avoid knife edge conditions on pressurized skin.
Some interesting quotes on the subject of joint fatigue life:
Colgate-Palmolive Company helicopter that snapped apart at the tail in 1997 accident report:
"Postaccident testing conducted by Textron Aerospace Fasteners (TAF) and earlier testing conducted by Eurocopter consistently demonstrated that the joint fatigue life of materials fastened with blind rivets is less than the joint fatigue life of the same materials fastened with solid rivets.
The Safety Board is concerned that other maintenance personnel may install blind rivets in applications where solid rivets are required, thereby reducing the structural fatigue life of an airframe. Therefore, the Safety Board believes that the FAA should issue a maintenance alert to all certificated airframe mechanics and inspectors to notify them of the circumstances of this accident and to inform them of the hazards associated with the installation of blind rivets."
From the famous Dr. John W. (Jack) Lincoln [Mr USAF DADTA, now deceased]
"failing to meet expectations on a SINGLE fastener installation [out of tens-of-thousands] could put the whole structure at risk for undetected premature fatigue failure at that location."
