causes of fine crack?

Question

i want to ask about the possible causes of fine crack at the intersection of the oversized head and body of the bolt made of phosphor bronze with 10% bronze and made by conventional upset forging bolt manifacturing procees

Answer 1

Intersection of the oversized head and body of the bolt IS the weakest portion
with reference to failure.(Endurance limit).
Do you tightened the bolt with the max torque value given by the manufacturer.
?
It may fail due to overtightening.
Second thing is that you analyse the fracture in met. lab for the microstructure by which the probable cause can be derived.
Check the physical and chemical analysis of the material.
PMI test will tell you within 5 minute that the material is ok or not.
Check the heat treatment carried out by the manufacturer.
If it is not hard and fast to use phosphor bronze with 10% bronze , then you can go for some other MOC like stainless steel /LTCS(for low temperature application).
Also check the bolts are getting evenly tightened or not.

Answer 2

Overtorque stressing as mentioned.

I believe that exposure to sulfur in the heat treating process will cause cracking in CuNi. There may be a similar effect with bronze. So you can specify the use of sulfur free fuels in heat treatment

Possibly stress concentration localized near the tight radius/corner formed by contraction as the bolt is cooled.

Easing the radius between the head and body during manufacture might help. Better control of heat treating profile could also reduce thermal induced stress.

Answer 3

Cracks are initiated by geometrical imperfections. They are propagated by energy.

What is critical is the energy (think strain energy here) divided by the radius at the tip if the crack. Obviously, if there is a lot of energy, or a very small radius - cracking will be a problem, especially in a more brittle material (like your phosphor bronze).

To fix your problem - add a fillet or a chamfer to relieve the "small radius" at the point where the crack is initiating (your fillet is making this radius "bigger"). If a crack starts, you can arrest its growth (sometimes) by drilling a hole at the "point" of the intitiation of the crack (again, you are making the radius at the initiation "big" with this technique).

Answer 4

Over-tightening, maybe?