Harvard Natural Sciences Lecture Demonstrations

The Surface Treatment of Glass

What it shows:

The strength of a material in tension or compression will be affected by discontinuities in its surface structure. This can be demonstrated for glass using microscope slides, and the comparison of failure stress before and after the removal of surface scratches.

How it works:

The slide rests between two custom built test beds (figure 1), the upper bed supporting the load. We use slotted 1kg and 0.5kg masses placed carefully in their holder, and allowing a short time between additions. We find the breaking...

Everyday objects at low temperature

What it shows:

Mechanical properties of some materials change dramatically with temperature. These changes have entertaining effects on everyday objects by taking them from room temperature 300K to the temperature of liquid nitrogen 77K.

How it works:

Place your everyday objects in a dewar of liquid nitrogen for several minutes (at least until the LN₂ stops boiling). Some examples to use:

1. Rubber gloves freeze solid and shatter on impact with floor.
2. Use a banana to hammer a nail into wood
3. Frozen...

Creep of Lead

What it shows:

A metal under stress will not fracture straight away, but will deform plastically due to the dislocation of crystal boundaries; this is called creep.

How it works:

Here we use lead as the test sample because there is significant creep compared to other metals. The lead is loaded (see fig.1) to a value that is just below the breaking stress of the sample. When creep occurs, the lead is drawn thinner at its weakest point (called 'necking', see fig.2) until its reduced cross-sectional area causes the sample to exceed its breaking...

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).

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Harvard Natural Sciences Lecture Demonstrations

Mechanical Properties

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