A Work in progress - NanoIndentation

By the end of 2012, (and no Apocalyptic events so far) I am trying to put together results of first set of Nano indentation.

As mentioned earlier, 4 specimens from 60°C testing (3 from original mix design + 1 RILEM standard mix design) are used for analyses. Main problem following indentation process is matching mechanical properties to exact locations on specimens. So far success rate of indent attempts are a bit low (around 55-70%). This was mainly due to some last minute revisions on samples and disturbing the flatness of our samples. Yet there are enough data for our purpose. With our new sample holders I believe this rate will raise drastically.

For the last week or so, I have been working on a Matlab code which gathers thousands of indent data, crunches the numbers and groups curves in to several E Modulus values (with 30GPa intervals). I have been working on a standardised way to identify (hypothesise) a way to identify locations of indents. As they are not (or very hardly visible) through light microscope, we needed to randomly distribute a grid on indent area, then check whether it is statistically comparable with NanoIndenter results.

Here is how it works.

\begin{example}

We had original documentation of our samples where interested indent areas are marked.

Micrograph image under stereo microscopy MagX0.63 - Each grid represents 4mm­² area

After indentations is performed another micrograph is taken with a higher magnification (4X) for detailing indent area alone.

Micrograph image under stereo microscopy MagX4 - Each cross is placed 200microns from each other as indentation points were

It is hardly visible from this image but manual point counting is applied on this image where each 200 microns increment is marked digitally. Finally we end up with two images for matching and comparison purposes. First one representing hypothetical  distribution of indent points and their corresponding material phases. Second one a representation of how E-modulus values vary through indentation grid.

Point count results

Surface map diagram of Indent results for E (GPa) values

This methods allows a standard way to overlay test results with real material properties. This can be further investigated by correlating findings with following subgroups of indent results shown in subplots.

\end{example}

Next set of experiments will start in Jan 3rd 2013. Till then,

Merry Christmas and a happy New year to All.

What did I learned in 2012?

It is time to draw a balance for the 2012. For PAT-ASR PhDs this was a great year, full of work and new ideas!
So, what did I learn this year? A lot of interesting knowledge about concrete and concrete matters useful for life!

About concrete matters 

• your background forms your point of view and your point of view hides your hypotheses, which means that some times you have to defend/change/reevaluate your hypothesis!
• it is really important how you present your work and especially how you present yourself, be open to new ideas, new critics and everything new! (it can be not so bad!).

About concrete (numerical point of view):

• I studied Poromechanics Theory, a so amusing theory which can bridge micro and macro modeling level.
• I studied Homogenization Theory.
• I learned how to play with tensors and how to represent them in matrix and column vector.
• I started to use Matlab! After a while you get why there are so many nerd people around!

About concrete (the real one):

• I learned how to perform mechanical tests of concrete specimens in laboratory.
• I learned how to build up a device and how many things can go wrong in once!
• I learned how to make concrete, like construction workers do! (and for a numerical person, this is not so obvious!)
• and I think that make concrete it is like make a cake... do you have some doubts? Than look at this:

I hope you also learned something important this year, and in general that your life and your professional life was full of great experiences! If not, don't worry there is still time!

I wish to our followers a Merry Christmas and an Happy New Year! 

See you in 2013...there is still a lot to do!

Nano indentation test is ...still.. running

With a bit of smallish problems last Tuesday we started our first batch of Nano Indentation tests.

First batch covers samples from Nautesund Bridge Original mix 1 (Prisms 1-2-3) and Nautesund aggregate Standard AAR-4 testing (Prism 5). All samples were subjected to 60°C and %100 humidity for 140 days. A total of 850 indent points selected for 4 samples (roughly 15mm X 15 mm).

On the 6th day of experiments, Indentation still continues... Needle is on the final indent group though.

Hopefully, by the end of today, we will get our samples our for further investigation

Going to the Basis...

Since I enter in the "fabulous" word of  Poromechanics and Homogenization theories I started to see that their formulation is written in tensor notation. (More information of tensor algebra can be found at http://www.foamcfd.org/Nabla/guides/ProgrammersGuidese3.html)

Every engineer remember that one in some lecture the professor told to him/her:

"The stiffness tensor is a  4th order tensor that has 81 constants. Due to symmetry when can reduce the number of this constants and we can write it as matrix 6x6". 

After that the word "tensor" is complete disappeared from our world and we keep write the stress strain relationship as:

or better:

Of course everything is fine till we go from strain to stress and the other way around. But what happen if I need to multiply the stiffness or the compliance tensors for other tensors? Can i still use the matrix notation?

My thought was, maybe yes. But actually is not totally true... I mean we need to pay attention to how we write the matrix form. And we have to remember that matrix operations and tensor operations are not equivalent!

A clear explanation of this issue can be found in the presentation of Klaus Helbig, titled "The Structure of the Elastic Tensor. A study of the possibilities opened up by Kelvin 150 years ago." (if you are interested in, just google the title!)

Conclusion: some time we forgot where some assumptions are coming from, but still they can make a lot of difference!

Self healing concrete and asphalt: Erik Schlangen at TEDxDelft 2012

Nano indentation

Recently I have been working on ways to identify material properties of phases in my concrete samples. As of yesterday we started with our first Nano indentation test. The main aim of this study will be to identify dissolved properties of aggregate and paste in the matrix with known exposure to Accelerated test conditions. Here is a short photo series of how we prepared samples for nano-indenter.

We cut sections from expansion prism (roughly 1.5 cm thick)

Sections are organized named and further selections are marked

Smaller Specimens are prepared to fit the sample holder for Nano Indentation

Time-lapse writing of research paper