Bolted joint with diffrent young module

[Verkstad]

Hi
I have screw a4-80 witch is used with a aluminium thread. As i have understood it you reduce the strength of the joint based the difference in yeild strength.

But a structural engineer claims that you also reduce the capacity off the joint based on the diffrence in the young module between the steel and aluminium. The argument is that this effect how the load distribute between the threads. Is this correct?

The reason i ask is that some off the structural engineers hadn't heared off that.

 

[Stress_Eng]

Thread loading along the engaged length differs depending on how the joint is loaded. Three joint loading types come to mind, axial preload due to torque tightening and thread load distribution based on how the external load is applied, e.g. tension / compression through a single member or through both. Thread load peaking factors may change with differing material Moduli. This is an interesting point, I’m going to check on this myself.

 

[SWComposites]

Is the fastener loaded in tension or shear?

Ive never heard of adjusting strength based on modulus values. Makes no sense.

 

[Verkstad]

Both shear and tension.

 

[rb1957]

is the load shared between the steel and the aluminium, or does the loadpath go from the steel to the aluminium ?

if former then some averaging between the two materials makes sense.

If then latter than not so much ... each material would be supporting the load on it's own. no?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Stress_Eng]

Can you give some information on how the joint is loaded in tension. The two examples I can think of is tension in the bolt and compression in the nut due to preloading from torque tightening or from an externally applied load, putting both members into tension. Assuming the joint is seeing both forms of loading?

 

[SWComposites]

Verkstad,
How are you determining/defining the “strength” of the joint?
Bearing?
Fastener shear?
Fastener tension?
Net section?
Shear out?
Other?
With bolted joint strength prediction one MUST be specific about the various failure modes; each has its own strength characteristics.

 

[drawoh]

Verkstad,

Any time I have structurally critical threads in aluminium, I use thread inserts of one sort or another.

--
JHG

 

[Verkstad]

It is a box with aluminium threads. It is attached to a steel plate with a4-80 bolts. The force comes from the aluminium box and can come for all direction inclusive shock loads.

My question us more about understanding why you would reduce the strength of a bolted joint based on diffrences in the material moduli and how large such a reduction could be.

The explaination give was that it affected how the load was distribution. But when i google i cant find it mentioned anywhere.

 

[Stick.]

Is your colleague thinking about the stiffness of the joint, particularly the stiffness of the bolt in relation to the clamped items? Since a bolted joint is basically springs in parallel (the bolt being one spring, and all the clamped items being the other), the stiffness of them relative to each other will determine how any external loads are distributed between the bolt and the clamped items. While Young's modulus is part of the stiffness calculation, it's far from the only part.

This link goes into more detail:

https://mechanicalc.com/reference/bolted-joint-analysis#joint-stiffness

 

[drawoh]

Verkstad,

Do you have long steel bolts passing through thick aluminium flanges? Your machine design textbook should have a section on how to analyze your bolted joint as a spring. If this this is a high strength joint, my primary concerned would be the strength of your aluminium threads.

--
JHG

 

[rb1957]

so you've got a tapped Al hole in lieu of a standard (steel) nut ? I consider that tapped thread like an Aluminium nut (or bolt). You lose a lot of strength, but get some back if you use heli-coils (which have a larger thread diameter than the fastener).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[Stress_Eng]

If you would like to see information on how load per thread is distributed along engagement length, I suggest doing a web search using wording such as “thread joint axial load distribution method”. I came across some information describing methods on how to determine the load acting on the threads under preload. This information was based on the shear lag approach. I also know of the method called strain matching. Obviously, the modulus for the bolt and nut are used in calculating shear, axial and bending stiffnesses. Both methods involve the matching of displacement. I wouldn’t be surprised to find that changing the nut from steel to an Alu alloy will reduce the maximum load on the 1st thread by a small amount? Once you have that all important max thread load, the whole joint capacity can be based on the stress analysis of that Alu alloy thread.