Impact loads and strain rate dependent yield stress 1

[corus]

I've seen references to yield stress that is dependent upon the strain rate that the load is applied, ie. that the apparent yield stress increases with the rate at which the load is applied, as under impact.

Should the allowable stress for impact loads be increased as compared to an equivalent static load given this change of yield stress, and by what factor for plain carbon steel for example. Are there any reference in design standards for this effect on yield stress?

corus

 

[shin25]

The rate of increase of applied load is not necessarily is an impact load.

I think you are referring to increase in yield stress depending on how fast the load is applied during a tension test. This is not comparable with impact load.

Usally, the impact is account for by increasing the static load with a factor. For bridge design, this is usally 1.33.

Hope this helps.

 

[corus]

Thanks, I'm aware of the factors that are applied to static loads in design standards to account for sudden loads, but that's not the same as the effect on the yield stress that a sudden load (or impact load, or load applied at a high rate) can have. I've seen graphs for 316 SS that shows this effect and wondered if it was documented anywhere for plain carbon steels or use in design of structures generally.

As you say, I am referring to an increase in yield stress depending on how fast the load is applied during a tension test. If the load is applied quickly then how is this different from an impact which is also a fast application of load?

corus

 

[UcfSE]

Typical structural loads are not applied fast enough to effect the yield stress, or modulus of elasticity. It takes such a rapidly applied load that usually only blast loads will cause this. So to answer your question, we don't normally account for this nor is it in typical codes for buildings. You may find some information where is concerns designs for blasts. Other than checking government websites, I don't know exactly where you would find this.

 

[bkal]

ACI 349, Appendix C mentions Dynamic Increase Factors for strain rates associated with impactive and impulsive loads. DIF's are material dependent and range from 1.0 to 1.25 and should be applied to static material strengths.

 

[corus]

Many thanks

corus

 

[corus]

Thanks bkal, but ACI stands for American Concrete Institute, I found, and this application is for steel structures under impact loads, and not concrete.

corus

 

[EddyC]

Hi Corus,

The topic that you are inquiring about seems rather interesting. I found the following link while doing a Google search:

http://links.jstor.org/sici?sici=0080-4630(19480902)194:1038<300:TUOFPF>2.0.CO;2-5

I wonder how much the effects are when applied to machinery? If you find out anything more, please let us know, as its an interesting topic.

 

[EddyC]

Here Corus,

Here's another link on this topic:

http://www.springerlink.com/content/q22841102025q73x/

 

[UcfSE]

You can try searching for "dynamic increase factor for yield stress" or some variation. I came up with several abstracts which seem to give the information you want, if you buy the paper. You may be able to get a collection of articles together and go to the local univeristy and find them.