Fracture Strength of Chalk

What it shows:

This demonstration allows you to compare chalk’s compressive strength with its tensile strength.

How it works:

We use railroad chalk, which although being softer and harder to work, is nice and big and easy to see. A sample is placed in each of the two types of testing assembly (details in Setting it Up), and loads carefully applied. Railroad chalk has a tensile strength of 195kNm-2 ± 30kNm-2 (a load of 2.5 to 3.5kg) and a compressive strength of 500kNm-2 ± 65kNm-2 ( a load of 7 to 9kg).


Setting it up:

The Railroad chalk has to be machined into shape and this is not easy! Railroad chalk is extremely soft; to cut it on a lathe takes a very fine and sharp tool bit, and you must work it at a very high speed. The chalk's diameter is such that it fits into the PVC piping (3.3cm outer diameter, 2.2cm inner) as indicated in Fig. 1. The pipe can be tightened down onto the chalk by cutting four slits in each pipe, about 2cm long, and squeezed using hose clamps. The other ends of the pipes have cords through them; one cord allows the assembly to hang from a bench clamp, the other is for loading with slotted weights. The compression bed is simpler, consisting of a clear Plexiglas tube that is long enough to enclose the chalk and most of a 1kg mass that sits on top of the chalk. The main worry is the stability of this setup - the mass should be as wide as possible compared to the Plexi tube so there is as little uneven loading or leaning, which will fracture the chalk. Further slotted weights can then loaded onto this first mass.


Thanks to Prof. Frans Spaepen for development suggestions. Take your machining skills to their limit! The results from this demo are very approximate; you can say that chalk is stronger in compression than tension, but don't take it much further.