2014.01.30-2nd exam

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Hofi
Beiträge: 107
Registriert: 07.11.2010, 11:23

2014.01.30-2nd exam

Beitrag von Hofi »

This exam was quite a little tougher then the previous one.

1) The first one was a picture of a bone. Questions had been the stresses (& strains?) in the neck of the femur.
He gave the forces (3kN each), the angle (27°), the inner and outer diameter (23mm and 28mm) and the elastic modulus (E=??).
Now the first trick was to split it up in a force parallel to the axis, and one normal to the axis. So you can calculate each stress individual.
Next one would have to combine them to calculate a so called "Risk Factor" (he described it at the exam. If your total stress is 100MPA your RF is 1 (=fracture). Otherwise it´s less then 1 or higher). this implies you have to use the right units to restate your stress in Pa.

I wasn´t sure how to calculate the total stress, but I assume it´s the same like vectors: square both, take the root and you got it. Then devide throw 100 MPa (in this case) and you got the Risk factor.

2) Composition of Blood?

3) Describe the Windkessel modell.

4) A diagramm about the timing of the heart. When do you hear sounds? Sorry, this questions sounds odd, I can hardly remember the stuff which came at the exam, but it was something like this and I´ll give my best to write down hear everything I still know.

5) For sure there was a question like: give mechanical properties and some values for bones (like E=12-17GPa, ductile while compressed, ultimate compressive strength = 180MPa,...)

6) There was this casson fluid plot with a lot of dots and y-axis: sqrt(tau) / x-axis: sqrt(shear rate). Calculate the flow rate for that one. Given dp= 10Pa.

For this you should know that the slope is eual to mü, sketching a para//el (to the x-axis) line throw the last dot you will get the value for tau_c (but the rootvalue!).
So you sqare this value, get tau_c --> R_c --> xsi --> F(xsi) (which was given in a table for 20 different xsi values, so no formular here).
Now the tough nut was more on interpreting the graph in the correct order and then not mixing up the units again. Cause you had to recalculate from dynes to N/something. And not forget to square it! Cause it´s all root values.
If you got this one correct, it was easy to calculate the flow. But getting this during the exam was quite tricky and took me most of the time and still I failed under timepressure with the units. So keep an eye on them!

Guess that was it. If you remember any other question from this exam, or you can spot an error here and there, I would be glad to add your suggestion. Or even better- feel free to add your answers and exam questions yourself to keep this forum alive!

So good luck for the exam,
best wishes,
Manuel
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tschneider
Beiträge: 1
Registriert: 26.01.2015, 18:12

Re: 2014.01.30-2nd exam

Beitrag von tschneider »

Hallo Hofi!

Ich bin mal so frei und schreibe auf Deutsch!

Bei dem ersten Beispiel... Hast du da vl einen genaueren Lösungsansatz im Kopf? (Ich weiß, ist schon ein Zeiterl her)

Ich vermute ja, dass man sowas wie Flächenträgheitsmoment I benötigt und dann mit M=EI/R oder so auf die Spannung kommt?!

lg Thomas

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Hofi
Beiträge: 107
Registriert: 07.11.2010, 11:23

Re: 2014.01.30-2nd exam

Beitrag von Hofi »

:) Not really anymore. It's quite long ago. But nowadays my guess would be to caluculate a von Mises stress. It`s similar to the squared methode I descriped, but more often used. With von Mises you can translate a multiaxial loading case into a compatible tensile loading case. And with the stresses there you can see if something breaks down or not, which might be the basic idea behind this risk factor I assume....

But for von Mises you will need pure shear and pure stresses. That`s why you have to split up the forces anyway. As it`s only two dimensional, it should be easy. Only one stress and one shear case. The formular you described will be needed when you want to get stresses and strains from the external forces as you mentioned. With that in mind it should be easy. But maybe you will find something useful in the books he recommened or using google books for riskfactor?

As far as I can remember he mostly chose his examples from the Biomechanics book he recommened.

Good luck for the exam!

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