Effect of HRC on Hi-speed Failure load

[rehanjamshed]

Hi
Requesting verification of the following please:

Material: Alloy steel Hardened to 50 HRC
Loading: A pin designed for shear

Will the Shear Failure load be same for ....
a. Quasi Static loading
b. High Speed or Abrupt loading

Is it correct to say that at Low HRC Steel will have different Failure Loads whereas as we increase HRC, the two Failure Values (I.e. Slow and Abrupt) become closer?

A clear and short answer will be appreciated with Supporting Literature

Best regards
Rehan J.

 

[Jughandle]

Regarding a. and b. – Theoretically, no, not the same. Failure loads are functions of strain rate (and also temperature). Lab tests can range from creep (~10^-6 in/in/sec), to quasi static (~10^-1 /sec), to dynamic (10^2 /sec), hi-speed (e.g., Hopkinson ~10^4 /sec), explosive (10^6 /sec). Reference Dieter or any textbook on mechanical metallurgy. Charpy V-Notch impact is simple and can be useful for screening as it measures energy absorption. For hardened alloy steel at ‘room’ temps, not much area under the s-e curve, so yeah, I would expect a hardened pin would break before a more ductile pin. Also you may want to search ‘strain hardening exponent’ on different materials.

 

[EdStainless]

Your assumption is more or less correct. There is a desired range for strength of shear pins. Too soft and they will smear a lot as the shear making them difficult to remove. And with more deformation comes the chance for higher strain hardening. Very soft ones will also tend to fail gradually and not abruptly.
If you go too hard and they are brittle then calibration becomes very difficult.

The shape and size of your groove matters also. We used to use Q&T 4140 always tempered to the same hardness (I believe about 160ksi). Then with each batch we made samples and sheared them in the lab and then adjusted the groove depth for that lot.

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P.E. Metallurgy, consulting work welcomed