[kip51035]

I would like to de-bunk one myth. I have done load tests on bolts. The stronger bolts are not more brittle!!! As the strength goes up so does the bend resistance and the shear strength. What happened here is typical of low strength bolts, hardware store bolts. Stainless steel bolts are generally weaker and bend and stretch easier and do not break easily.

[volksman]

to shear the bolts the center nut is not holding its load well. run down torque is very important in a fastener keeping its applied torgue load. if the run down is not present the fastener will not hold torque unless over torqued. in effect stretching the contact area of the threads. the shear may happen when both plugs fire, the cylinder not supposed to fire has enough vapour in cylinder to give a backfire. the snap will loosen the center nut. the ignition fires both plugs at the same time, unless things have changed

[speedbird]

interesting web site on bolts and torquing
...go past the wood screw bit
http://store.curiousinventor.com/gui...orking/Screws/

also from bolt science.com
If a gap does form between the plates comprising the joint then the bolt or bolts are almost always subjected to non-linear loadings from bending and shear forces acting. This usually quickly leads to bolt failure. Hence it is normal to set a design criteria that the applied forces must not under any circumstances result in a gap forming within the joint.

Variations of bolt preload with torque
the role of friction

What Federal Standard H28 (FED-STD-H2 has this to say on the subject:

Tests in numerous laboratories have shown that satisfactory torque-tension relationships may be established for a given set of conditions, but that the change of any one variable may alter the result. Changes such as indeterminate friction, a change in the surface roughness of the bearing surfaces or of the threads, or a change lubrication will drastically affect the friction and thus the torque-tension relationship. Thus, it must be recognized that a given torque will not always produce a definite stress in the bolt but will probably induce a stress that lies in a stress range that is satisfactory.

Torquing of screws = test of screws - not bolts unfortunately into 6061 aluminum
but very interesting
[url]http://valliecomponents.com/attachments/-01_torque_testing_M5.pdf[/url

[speedbird]

information relocated

[speedbird]

Fastener Grade and strengths
http://www.americanfastener.com/techref/grade.htm

[speedbird]

relocated information

[speedbird]

Very interesting website
vibration loosening of threaded fasteners
http://www.boltscience.com/pages/vibloose.htm

with much more info at
http://www.boltscience.com/

[super08]

I ran into the same problem two years ago about two minutes into a maiden flight. I was using a CF spinner and a CF prop. The prop was torqued with a torque wrench. At first I blamed the prop but later learned of others having failures with CF back plate type spinners used in conjunction with CF props. Whether the reason is heat related or slip related I stopped using spinners with CF back plates and have never looked back.

[groo]

good stuff, lots of great links here to bolt tension information. not a lot of data on bolts under tension(tensile strength) subjected to sudden lateral forces(toughness). also if the bolts have been reused, have they been overstressed in tension resulting in slight plastic deformation( precursor to fracture)?

You Tube

You Tube

tensile test. note the necking down prior to failure, this would occur in the thread spaces. surface of the fractred bolts in the flying giants link(although hard to see) look just like the surfaces of this kind of specimen.

as compared to toughness measured this way;

You Tube

You Tube

toughness is related to shear resistance more so than tensile strength.

i tend to believe the bolts have been overstressed by tensioning, further stressed in tension by the prop load resulting in plastic deformation, causing a loss of bolt strength and clamping force in one or more bolts ultimately leading to shear fracture from sudden changes in throttle or rotational prop forces.
definately complex problem with a combination of factors leading to the end result, failure. could the solution be as simple as using larger diameter bolts yielding an increase in all forms of material strength??

[speedbird]

removed to correctly respond to kip51035 using quote