Strain Rate Effect

[vmirat]

Please explain the strain rate effect on steel and why strength increases. Also, how much does the strength increase? And finally, what other materials experience an increase in strength due to strain rate?

 

[arunmrao]

Please read Mechanical Metallurgy by Dieter. A full book will be needed to reply .

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[TVP]

Are you a student posting your homework assignment?

 

[vmirat]

OK, guys, I'm a structural engineer, not a materials scientist. I have a report on crash rating of a barrier and it stated that the effects of strain rate were ignored. I read through my mechanics of materials text book and didn't find anything on strain rate, so I thought I'd ask you all. I'm looking for a little more depth concerning why steel gets stronger and how much stronger it gets (formula?).

 

[arunmrao]

Thanks for the clarification. You may please refer to Ch 6 "Strengtheninh Mechanism" Mechanicxal Metallurgy by Dieter pp184-240.

Perhaps this will give you a beetter insight into strain hardening mechanisms and their effect on properties of steel.

Sorry for my previous curt reply.

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[TVP]

vmirat,

You may be surprised at the number of students who post their homework here at Eng-Tips. Thanks for further clarifying your request. There is a ton of good information freely available on the web on this subject. Try the following:

http://www.a-sp.org/database/pdf/CarsAsdm/Chapter2/section2-13.pdf

http://www.a-sp.org/database/pdf/CarsAsdm/Chapter3/section3-06.pdf

http://was.iconicweb.com/assets/attachments/RptRndRobResults.pdf

http://was.iconicweb.com/assets/attachments/DynTestingRecomPract.pdf

With regards to strain rate sensitivity, the following articles discuss a few of the common structural metals:

http://archive.metalformingmagazine.com/2006/01/Science.pdf

http://archive.metalformingmagazine.com/2006/02/Science.pdf

 

[metengr]

As mentioned above, there is no succinct answer because many other variables will effect mechanical properties. Here is an introductory excerpt from ASM Handbook, Volume 8 regarding high strain rate testing;

Introduction
MECHANICAL BEHAVIOR at high strain rates differs considerably from that observed at quasi-static or intermediate strain rates, and many engineering applications require characterization of mechanical behavior under dynamic conditions. For example, strain rates ranging from 100 s-1 to more than 104 s-1 occur in many processes or events of practical importance, such as foreign object damage, explosive forming, earthquakes, blast loading, structural impacts, terminal ballistics, and metalworking.
This introductory article briefly reviews the dynamic factors and experimental methods for high strain rate tension testing and compression testing methods listed in Table 1. Considerable data on the high strain rate behavior of many materials (including steels, aluminum and copper alloys, titanium, beryllium, magnesium, and zinc) are available in the literature (see the Selected References at the end of this article). Results indicate that for many metals a linear relation exists between flow stress and the logarithm of plastic strain rate in the range from quasi-static rates to about 103 s-1. Above this range, however, the flow stress generally rises far more rapidly with strain rate. Thus, the linear relationship is no longer valid, and extrapolation from the lower strain rate regime becomes unreliable. Fewer data are available at the higher strain rates, particularly at very high strain rates above 103 or 104 s-1, at which testing becomes more difficult. At strain rates in excess of 104 s-1, the lower yield stress is often found to be directly proportional to strain rate, rather than to the logarithm of strain rate. This implies a new region of mechanical response controlled in part by a viscous damping mechanism in contrast to thermally activated processes at strain rates below 104 s-1.

There is plenty of information to review on this subject by simply using Google

http://www.google.com/search?q=effe...ical+properties+of+steels&hl=en&start=10&sa=N

 

[vmirat]

Thanks to everyone. I tried the Google thing first, then tried looking for other posts in here. I found some stuff but a lot of it was related to research test results that I didn't understand, so that's why I posted my question here. Again, thanks for the help.

 

[EdStainless]

Remember that there are two related but different things going on. One is the strain hardening that takes place during deformation. The second is that the increase in strength is also a function of the rate of deformation.
In some materials like high strength stainlesses you can get different results in a tensile test just by changing the strain rate.

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