Roy, Thank you for pointing out that the stress/strain field may differ under dynamic test, for the moment we will keep it simple so we only obtain the force-displacement (or the stres-strain curves) of the micro-pillar under different strain rates. I will keep you posted if we find anything interesting. -Kejie

Kejie,

Thanks for your encouragement! You know a nano-indentation problem is a two-body contact mechanics problem. If a same indenter tip hits two samples very quickly, the strain fields and strain rates of a soft sample and a hard sample would be very different, so I really cannot tell you a general rule. But you may run a finite element analysis on your micro-pillar compression test to find maximum strain and the rate, if you know the maximum tip speed of your nano-indentation system. Look forward to knowing nice result from you.

Roy

Dear Roy,

I am glad to hear that the proposal works out, brilliant ideas will enventually be funded, congratulations! I understand that indentation would be in the quasi-static range, I was wondering how much we can play.. Look foward to discussing more next time.

-Kejie

Kejie,

Glad to hear from you. I didn’t know any nano-indentation system is able to conduct a “strain rate” test--- because we often use gas guns to conduct high strain rate tests. Think about the test procedure: an indenter tip compresses a sample and can get the indentation depth. But in terms of the strain measurement, we need another length scale…..Moreover, the indenter tip must move as fast as a gas gun projectile (e.g., > 20 m/s) to get a high strain rate during the initial contact with the sample. I think we cannot make such a fast nano-indentation system now.

Good news is after we submitted a rebuttal to the funding agency, our proposal would be funded. Here is an illustration of our major idea, and we’d discuss more details with you. Keep in touch!

A Multi-Scale Approach of Combining Nano-indentation with Computational Mechanics to Predict Impact Behavior of Structural Composite Materials

Dear Roy,

We are using indentation (Keysight G200) to perform micro-pillar compression tests. We are interested in the strain rate performance of the micro-pillars. In your exerience, may I ask the range of strain-rate the indenter can apply? Many thanks,

-Kejie

Dear Ajay and Guru,

Of course, you’re nanoindentation experts but might not conduct low-speed impact tests for engineering materials. I do both for more than seven years. The key issue is the definition of “low-speed impact by projectile”. The impact of speed of our impact tests on composite materials is around 50 m/s. Of course, nanoindentation cannot do it so nanoindentation is a static test method compared to a low-speed impact problem. Here I find the reviewer didn’t understand nanoindentation tests and didn’t conduct low-speed impact tests too.

Ajay --- can you comment “capture delamination” problem using nanoindentation? Do you know delamination in composites?

Roy

Being in the EU, I can't comment much about the funding & reviewer remarks. But I can share my thought on the feasibility of such a project.

I do not agree with Guru completely here. Nanoindentation is not only to measure material properties - though that is what many have been using it for. It has been my side project over the last 6 years & I have done testing for my group members too.

We have a machine from Agilent (or nor Keysight) & if one is using the standar methods provided by the manufacturer, the number of possibilities are limited. However, one can go and program the back end itself to setup tests that one desires. This opens up a world of possibilities that one can only imagine. At the end of the day, NI is a tool & it is as good as the user. I did a bit of programming on the backend & were able to do fascinating experiments that one would not think about. We are presently writing up on them & would be glad to share in some time.

Though NI is a quasi-static testing setup, by default, one can change the method to actually do low-velocity impact too! So if you would say, you want to do low-velocity impact, I wouldn't say it is impossible. I would only be worried about the lifetime of the tip which are rather costly. But theoretically, such a thing is actually possible.

Dr. Chandrashekar,

We didn't call that, this reviewer called. Of course, nanoindentation is a static test method. This reviewer just didn't have nanoindentation experience like you. Is it clear to you now?

Roy Xu

Dear Dr. Xu,

Altough your proposed ideas sound interesting, calling nanoindentation as 'low velocity impact method' might confuse a lot of people. Nanoindentation is primarily applied to measure material properties, where the tip penetrates into the sample at extremely low velocities. I have worked for many years relating computational mechanics with nanoidnentation experiments. My knowledge on studying impact related problems using nanoindentation is very limited. Furthermore, the literature on such problems is sparse. I totally understand the points made by the reviewers.

Thanks,

Dr. Gurudutt Chandrashekar

Gurudutt,

You have perfectly understanding! Nanoindentation and projectile impact sound very different. However, for low-speed impact (unlike bullet penetration), projectile impact is a kind of dynamic indentation so we can use indentation mechanics to connect these two topics. I list our two papers on these two different topics below if you’re interested. The value of our proposal is to combine nanoindentation with modern computational mechanics to predict/simulate complicated impact problem. Our purpose to use Nanoindentation is to measure contact stiffness--- like (static) Young’s modulus. However, our great idea from three faculty members was just killed by a reviewer who even doesn’t know Nanoindentation is a static method or low velocity impact method. My point here: for young researchers, your nice work or treasure might be treated as “trash”. But if you think you’re doing the right thing, never give up because our scientific research is mainly to discover truth, not just for job and money (of course we need them)!

Roy Xu

Dear Dr. Xu,

I have worked on characterization of polymers by nanoindenation for a few years. Based on my experience in this field, I haven't seen any impact problem where nanoindentation has been applied. Nanoindentation is a quasistatic method, and studying high velocity impact/material damage problems could be beyond the scope of this method.