Joining high impact strength parts with low impact strength parts

[Ian63]

I have been asked a question that seems to have an easy, obvious answer answer, but is it this simple.

If I have two parts that have a reasonable Charpy impact strength ( "A" joules) loaded in tension and wish to connect them with another part with a lower impact strength ( "B" joules), is it sufficient to just proportion cross section the connecting piece by the ratio A/B?

Lets say you had grade 8.8 threads that you wanted to connect with a grey cast iron coupling. if the iron had only a 1/10th of the Charpy value of the thread, can you just made the coupling 10 times the cross section?

Ian63

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[metengr]

For static load applications, yes, this can be done. However, for dynamic load applications (impact-type), this would not work. Your weak link under impact-type loading is still the material which exhibits the lowest notch toughness behavior.

 

[Ian63]

metengr,

thanks for your reply

If I want to ensure that such a system is limited by the threads in my example, surely if the coupling is very big in cross section it would not fail in an impact load that would fail the bolts??

My question s how much bigger in cross section? can you just use the ratio of charpy impact strengths?

Ian63

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[arunmrao]

Large cast iron blocks break easily under hammer blows, if the notch created is adequate. Thus there is no assurance that the large block will not fail under severe impact conditions.

 

[EdStainless]

Now, if you design a joint so that the low toughness material is loaded in compression, then you will have a more reliable joint.
You never want to put threads into a low toughness material if impact is a concern, you are better off clamping that material.

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